geologic map of the north lake tahoe–donner pass · pdf filei geologic map of the north...

STATE OF CALIFORNIAEDMUND G. BROWN JR.

GOVERNOR

THE NATURAL RESOURCES AGENCYJOHN LAIRD

SECRETARY FOR RESOURCES

DEPARTMENT OF CONSERVATIONMARK NECHODOM

DIRECTOR

CALIFORNIA GEOLOGICAL SURVEYJOHN G. PARRISH, Ph.D.

STATE GEOLOGIST

CALIFORNIAGEOLOGICAL SURVEY

Geologic Map of the North Lake Tahoe–DonnerPass Region, Northern Sierra Nevada, California

Scale 1:48,000

2012

MAP SHEET 60

California Geological SurveyCalifornia Department of Conservation

Sacramento, California

CALIFORNIA GEOLOGICAL SURVEYJOHN G. PARRISH, Ph.D.

STATE GEOLOGIST

Copyright © 2012 by the California Department of Conservation,California Geological Survey.All rights reserved. No part of this publication may be reproducedwithout written consent of the California Geological Survey.The Department of Conservation makes no warranties as to thesuitability of this product for any given purpose.

i

Geologic Map of the North Lake Tahoe–Donner

Pass Region, Northern Sierra Nevada, California

Scale 1:48,000

By

Arthur Gibbs Sylvester1, William S. Wise1, Jordan T. Hastings2, and Lorre A. Moyer3

2012

MAP SHEET 60

California Geological Survey California Department of Conservation

Sacramento, California

1 Department of Earth Science, University of California, Santa Barbara, California, 93106 2 Nevada Bureau of Mines & Geology, University of Nevada, Reno, Nevada, 89557 3 U.S. Geological Survey, Mackay School of Earth Sciences, University of Nevada, Reno, Nevada 89557

California Geological Survey - MAP SHEET 60

iii

Table of Contents

Abstract ......................................................................................................................................................... 1

Introduction ................................................................................................................................................... 1

Geology and Geologic History ..................................................................................................................... 2

Paleozoic and Middle Mesozoic Rocks .................................................................................................... 3

Cretaceous Intrusive Rocks ....................................................................................................................... 4

Eocene Rocks and Events ......................................................................................................................... 5

Late Oligocene, Miocene and Pliocene Volcanic Rocks .......................................................................... 6

Valley Springs Formation ..................................................................................................................... 6

Mehrten Formation ............................................................................................................................... 7

Basaltic to andesitic lavas and pyroclastic rocks (Tsb, Tsba, Tspa, Tsha, and Tsp) ............................. 9

Block and Lapilli Tuff Megabreccia of Mt. Disney (Tsblt) ................................................................ 11

Biotite-hornblende andesite lava and lava-domes (Tsbha) ................................................................. 13

Pliocene to Quaternary Rocks and Events .............................................................................................. 13

Truckee River Formation .................................................................................................................... 13

Glacial Deposits .................................................................................................................................. 15

Alluvium ............................................................................................................................................. 17

Structural Geology ...................................................................................................................................... 18

Tahoe-Sierra Frontal Fault Zone ............................................................................................................. 18

Central Graben Faults ............................................................................................................................. 21

West Tahoe-Dollar Point Fault ............................................................................................................... 21

Carnelian Bay and Agate Bay Faults ...................................................................................................... 21

Brockway Fault ....................................................................................................................................... 21

Cryptic Truckee River Fault ................................................................................................................... 22

Folds and Tilted Strata ............................................................................................................................ 23

Some interpretations of volcanic activity .................................................................................................... 23

The Squaw Peak Volcano ....................................................................................................................... 23

Silver Peak to Mt. Watson ...................................................................................................................... 24

Mt. Pluto to Martis Peak ......................................................................................................................... 25

Acknowledgments ....................................................................................................................................... 25

Description of Map Units ............................................................................................................................ 27

Alluvium and Related Deposits; Glacial Deposits .................................................................................. 27

Late Pliocene to Quaternary Rocks ......................................................................................................... 28

Late Oligocene, Miocene, and Pliocene Volcanic Rocks ....................................................................... 30

Cretaceous Intrusive Rocks ..................................................................................................................... 35

Geologic Map of the North Lake Tahoe–Donner Pass Region, Northern Sierra Nevada, California

iv

Paleozoic and Middle Mesozoic Rocks .................................................................................................. 35

References Cited ......................................................................................................................................... 40

Plate

Geologic Map of the North Lake Tahoe–Donner Pass Region, Northern Sierra Nevada, California, 1:48,000 scale ............................................................................................................................... (in pocket)

Figures

Figure 1. Index map depicting 7.5-minute quadrangles and place names in the Lake Tahoe-Donner Pass region ............................................................................................................................................ 3

Figure 2. Oligocene tuff (above) deposited upon a tonalite ridge (below) with a paleosol at the contact. Parking lot cut on Highway 40 at Summit .................................................................................... 5

Figure 3. Block and lapilli tuff breccia on the south side of Mt. Lincoln. Hammer handle is 40 cm long. A. Unsorted angular andesite blocks in matrix of tuffaceous andesitic ash (Tlp). B. Interbedded, normally graded layers of fluvially reworked, sub-rounded andesite pebbles, cobbles, and tuff (Tlf) ............................................................................................................................................... 7

Figure 4. View east from Squaw Peak across upper Truckee River Canyon to Lookout Mountain (left), Mt. Pluto (center with the ski trails), and Mt. Watson (right) all in the center of the Tahoe-Truckee graben. Mt. Pluto is a remnant of an andesitic stratovolcano ......................................... 9

Figure 5. View south of Mt. Lincoln (left) and Mt. Disney (right) with their caps of debris avalanche of Mt. Disney (Tsblt) above dashed line. See also Figure 10 ......................................................... 11

Figure 6. Polygonally fractured, juvenile block of andesite in megabreccia of Mt. Disney (Tsblt) .......... 12 Figure 7. Nubbin of glassy olivine basalt (QTtpm) in William Kent State Park ....................................... 14 Figure 8. View east-northeast across Donner Lake, a U-shaped valley, to Donner Ridge ........................ 16 Figure 9. Giant granite boulder carried into upper Truckee River Canyon by a Squaw Valley glacier.

Jökulhlaups transported many such boulders downstream to Truckee ....................................... 17 Figure 10. West to east structure sections across the Tahoe-Sierra Frontal Fault Zone. Upper section

illustrates about 250 m of displacement of strata (Tlf) on Mt. Judah into adjacent graben. Lower section illustrates about 450-500 m of displacement of Mt. Disney debris avalanche (Tsblt) across the Tahoe-Sierra Frontal Fault Zone ................................................................................ 19

Figure 11. Fractures in granite along Tahoe-Sierra Frontal Fault Zone in railroad cut on northeast flank of Donner Peak ............................................................................................................................... 20

Figure 12. Apparent anticlinal warp in volcaniclastic deposits on east flank of Mt. Lincoln .................... 23 Figure 13. West to east structure section from Squaw Peak to the Truckee River to illustrate inferred

Pliocene form of stratovolcanoes related to volcanic vents on Squaw Peak and KT22 ............. 24 Figure 14. West to east structure section from Silver Peak to Mt. Watson to illustrate inferred relation of

their lava flows to those of Mt. Pluton ....................................................................................... 25 Figure 15. Southwest to northeast structure section from Mt. Pluto to Martis Peak to illustrate

relationship between lava flows and volcaniclastic debris from Mt. Pluto (Tsp) and Martis Peak (Tmp) .......................................................................................................................................... 25

Tables

Table 1. Radiometric ages of Tahoe-Donner Pass area rocks arranged according to age (Ma) ................ 36 Table 2. 40Ar/39Ar ages of volcanic rocks, Tahoe – Donner Pass Region, California ................................ 38


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By Arthur Gibbs Sylvester1, William S. Wise1, Jordan T. Hastings2, and Lorre A. Moyer3

Abstract

The Lake Tahoe-Donner Pass region encompasses the north part of the Tahoe-Truckee graben, which comprises Tertiary and Quaternary andesitic and basaltic lava flows and andesitic volcaniclastic rocks that erupted in four main periods: 13-7 Ma, 7-5 Ma, 5-3 Ma, and 4-1 Ma. Cretaceous granite forms the eastern side of the graben in Nevada, whereas the western side, in the Donner Pass region, consists of late Paleozoic metasedimentary rocks and Mesozoic metavolcanic rocks intruded by Cretaceous granite, all of which constitute the basement for overlying Tertiary volcanic rocks. Seven Oligocene rhyolitic ignimbrite flow units and abundant Miocene and Pliocene andesitic flows and volcaniclastic rocks fill an Eocene paleovalley in the basement near the summit of Donner Pass. Quaternary valley glaciers and streams carved the west shoulder of the graben and deposited till, drift, and alluvium in the graben.

Several major fault zones are in the graben including the Brockway Fault, the West Tahoe-Dollar Point Fault, the Carnelian and Agate Bay faults, and the Polaris Fault. The faults are right-oblique normal faults, with local strike-separations from ¼ to ½ kilometer, and all are considered to be active. The Tahoe-Sierra Frontal Fault Zone bounds the west side of the graben and vertically separates the Pliocene debris avalanche of Mt. Disney about 500 m, thus yielding a maximum age of fault displacement of about 4 Ma. Evidence for younger displacement across any of the zone's discrete faults is scanty and not compelling.

Introduction

The Donner Pass region and the north end of Lake Tahoe lie in the tectonically active Tahoe-Truckee graben (Lindgren, 1897), an asymmetric half graben between the Sierra Nevada microplate on the west and the Basin and Range province on the east (Schweickert et al., 2000; 2004; 2011). Much of the graben is partially filled with andesitic lava flows, lava domes, necks, dikes, and volcaniclastic rocks broadly assigned to the Mehrten Formation of late Miocene and Pliocene age (Gale, in Piper et al., 1939; Curtis, 1953). Other large areas are underlain by basaltic lava flows assigned by us to the Truckee River Formation of late Pliocene and Quaternary age. Cretaceous granite with Mesozoic and late Paleozoic metasedimentary and metavolcanic rock roof pendants form the shoulders of the graben at the north end of Lake Tahoe and are separated from the volcanic rocks by major active, or possibly active, fault zones. Quaternary glaciers, which left U-shaped valleys, glacial till and outwash, affected most of the higher elevations on the west side of the graben.

The area under geologic consideration herein is part of a larger area originally mapped in the mid-1890s at a scale of 1:125,000 for the purpose of identifying mineral resources (Lindgren, 1897). The Southern

1 Department of Earth Science, University of California, Santa Barbara, California, 93106 2 Nevada Bureau of Mines & Geology, University of Nevada, Reno, Nevada, 89557 3 U.S. Geological Survey, Mackay School of Earth Sciences, University of Nevada, Reno, Nevada 89557


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Pacific Land Company also mapped parts of the region in the 1950s to identify potential exploitable resources (Coonrad et al., unpub. 1959; Burnett and Jennings, 1962). Harwood (1980; 1981) mapped a large area west of the Sierran crest. Sundry theses and few local topical studies include Latham's work on the Dry Lake Volcanics (1985) and Kulow's studies of the granitic rocks west of Truckee (1996). Birkeland (1961; 1963; 1964) studied the volcanism and glaciation over much of the present map area. Geology of the entire Tahoe basin was compiled at a scale of 1:125,000 (Matthews (1968); Burnett (1968; 1971), and at a scale of 1:100,000 (Saucedo, 2005). Faults in the Donner Pass area were mapped by Hudson (1948; 1951), with some important recent additions throughout the Tahoe basin by Olig et al. (2005) and Hunter et al. (2010). The main faults beneath Lake Tahoe have been mapped by several investigators (Hyne, et al., 1972; Schweickert et al., 2000; Gardner et al., 2000; and Brothers et al., 2009) and were compiled by Saucedo (2005).

This geologic map represents a compilation of a contiguous area of the Tahoe-Truckee graben from analog mapping by136 undergraduate geology major students done intermittently from 1987 to 2006, and supervised and field checked by Sylvester and Wise. The present map includes all of the Norden, Truckee, Tahoe City, and Kings Beach 7.5' quadrangles, and most of the Granite Chief and Martis Peak 7.5' quadrangles (Figure 1).

The purpose of the mapping was to determine the geologic history of the Lake Tahoe-Donner Pass region and to present a base of geologic information for more detailed studies, for public edification, and for land use planning and decisions.

The students' mapping builds mainly upon mapping by Lindgren (1897) and Harwood (1980; 1981) and focuses primarily on the Tertiary and Quaternary volcanic rocks, glacial deposits, and faults. Mapping was done on the ground surface at a scale of 1:12,000 on photocopied enlargements of 1:24,000 topographic maps. Field locations in the 1980s and 1990s were determined by inspection and sometimes with the aid of an altimeter. Hand-held GPS units were used in the 2000s. The students' maps, together with some additions from Birkeland's maps of the Quaternary geology in the Truckee area (Birkeland, 1961; 1963; 1964), were compiled onto mylar copies of the 7.5' quadrangle maps at a scale of 1:24,000. Each of those quadrangle maps was digitized using ArcMap® 9.1, a commercial GIS software application by Environmental Systems Research Institute (ESRI).

The fault traces on the floor of Lake Tahoe were taken from Saucedo (2005). The subaerial parts of these faults north of the lake were field checked in 2004 and 2005. Previously available isotopic ages for rock units are given in Table 1; newly determined 40Ar/39Ar ages are given in Table 2.

Geology and Geologic History

The geologic map covers about 775 square kilometers (about 300 square miles) of the Lake Tahoe Basin, the Truckee River, and the headwaters of the Yuba River and North Fork of the American River in the northern Sierra Nevada of California (Figure 1). The map area lies largely between the Sierran crest on the west, the California/Nevada border on the east, and the Lake Tahoe shoreline around the northwest part of the lake as far south as Ward Creek near Tahoe City. The northern boundary lies about five kilometers north of Interstate 80.

Geologically, the map straddles the north part of the Tahoe-Truckee graben between the Carson Range of Nevada and the Sierra Nevada crest. The rocks include late Paleozoic and Mesozoic miogeoclinal metasedimentary rocks, Cretaceous granodiorite and tonalite of the Sierra Nevada batholith, Tertiary basaltic, andesitic and rhyolitic volcanic rocks, and Quaternary and Recent glacial, fluvial, lacustrine, and alluvial deposits.


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Figure 1. Index map depicting 7.5-minute quadrangles and place names in the Lake Tahoe-Donner Pass region.

Much of the map area is underlain by andesitic lava flows, lava domes, and volcaniclastic rocks broadly assigned to the Mehrten Formation of late Miocene and Pliocene age (Gale, in Piper et al., 1939; Curtis, 1953), whereas other large areas are underlain by basaltic lava flows assigned by us to the Truckee River Formation of late Pliocene and Quaternary age. Cretaceous granite with Mesozoic and late Paleozoic metasedimentary and metavolcanic rock roof pendants comprise the Sierran crest. Quaternary glaciers, which left U-shaped valleys, glacial till and outwash, affected most of the higher elevations.

Paleozoic and Middle Mesozoic Rocks

The oldest rocks in the map region are scattered patches of Paleozoic metasedimentary rocks in the headwaters of the North Fork of the American River near The Cedars. They have been assigned to the Picayune Valley Formation (DMpv) of Late Devonian and Early Mississippian age (Harwood et al., 1991) and consist of turbiditic sequences of chert- and quartz-rich, quartzose sandstone, and black pelitic rocks, the latter of which were metamorphosed to cordierite-andalusite hornfels during intrusion by Mesozoic plutons. Marble (Onion Creek marble - Mlo) and biotite schist (Serena Creek Formation - Mls) crop out also in a patchwork arrangement of roof pendants in the southwest corner of the Norden quadrangle and the northwest corner of the Granite Chief quadrangle (Harwood, 1980). These Paleozoic

CastlePeak

AndesitePeak

Mt.Disney

Mt. Lincoln

Mt. Judah

PaintedRock

DonnerPeak

Alder Hill

TinkerKnob

Little Chief

Big Chief

AndersonPeak

GraniteChief

SquawPeak

WardPeak

Scott Peak

TwinPeaks

StanfordRock

PaintedRock

Bald Mt.

Boreal Ridge

CrowsNest

Shal lenberger

Ridge

Donner Ridge

MartisValley

LookoutMt.

Mt.Pluto

Mt.Watson

MartisPeak

BurnedHill

JuniperHill

Onion Valley

Squaw Valley

AlpineMeadows

FirCrags

Thunder C l i f f TwinCrags

Rampart

Page Meadows

AntoneMeadows

BrockwaySummit

KlondikeMeadows

Sawtooth Ridge

Summit Lake

LakeMary

LakeAngela

LakeVan Norden

Donner Lake

DryLake

MartisLake

DollarPoint

CarnehanBay

AgateBay

StatelinePoint

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North Fork

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Deep Creek

Pole Creek

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SodaSprings

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Sunnyside

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KingsBeach

TahoeCity

Homewood

William KentState Park

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formations are exposed more extensively in the adjacent quadrangles to the west and are fully described by Harwood (1992).

Jurassic formations of the Lake Tahoe sequence (Harwood, 1992) consist of calc-silicate hornfels, marble, and pelitic hornfels of the Blackwood Creek Formation (Jlb, older), and the Ellis Peak Formation (Jle, younger). These metasedimentary rocks crop out west of the Sierran crest around The Cedars and east of the crest in the Alpine Meadows area in upper Bear Canyon and Ward Creek, and in the Blackwood Canyon area five kilometers west of Homewood. The protoliths of all these metamorphic rocks were deposited originally as marine sediments and volcanic rocks before being deformed and metamorphosed by subsequent intrusions of the Sierra Nevada batholith (Lindgren, 1897; Harwood, 1992).

Cretaceous Intrusive Rocks

Granitic rocks crop out extensively on the shoulders of the Tahoe-Truckee graben. Those on the east side of the graben in the Mt. Rose quadrangle are coarse-grained, titanite-bearing hornblende granodiorite. Those on the west side of the graben in the Norden and Granite Chief quadrangles are largely hornblende-biotite granodiorite and tonalite, which are generally finer-grained and poorer in titanite than those on the east side of the graben.

Following Kulow (1996), we recognize two main bodies of granitic rocks in the Donner Pass area: the hornblende-biotite granodiorite of Summit Lake (Ks) with its K-feldspar megacrystic facies (Ksp), and the tonalite of Lake Mary (Klm). These rocks are subdivisions of the Rattlesnake Creek pluton (Kulow, 1996), which is widely exposed outside the map area west and northwest of Donner Pass. Granitic rocks south of Mt. Lincoln lack age and petrographic information and are mapped as undifferentiated quartz monzonite (qm) and hornblende-biotite granodiorite (hbg) that closely resemble the hornblende-biotite granodiorite of Summit Lake (Ks). It is possible that the Summit Lake pluton crops out more extensively south of Mt. Lincoln than indicated by Kulow or mapped by us.

The Summit Lake pluton (Ks) consists of homogeneous, equigranular, light-gray to white hornblende-biotite granodiorite, locally ranging to granite and tonalite. A sample for zircon dating was collected from the southwest part of the pluton, “on U.S. Highway 40 just west of Donner Lake" (Kulow, 1996, p. 95), and analysis of four zircon fractions yielded a 207Pb/206Pb age of 117 ± 7 Ma (Kulow, 1996; Map #126; Sample MK158; Table 1).

The tonalite of Lake Mary (Klm) crops out in the vicinity of Donner Pass, extending westward along State Highway 40 and the north shore of Lake Van Norden for a distance of about two kilometers west of Norden. Although primarily tonalite, the pluton ranges from quartz diorite to monzodiorite, so that some exposures are melanocratic, especially at the crest of Donner Pass near Lake Mary. The tonalite is overlain nonconformably by rhyolite tuff in railroad cuts between Donner Pass and Norden. Beneath the rhyolite, the tonalite is weathered to grus to a depth of 1-2 m. Locally, as observed in a parking lot exposure at Sugar Bowl on Highway 40, the weathered tonalite is overlain by a one-half meter-thick paleosol (Figure 2). The tonalite yielded Rb/Sr whole-rock isochron ages of 117.5 ± 4.2 Ma and 102.7 ± 3.7 Ma (John et al., 1994), and recalculated K/Ar ages between 94.8 and 112.1 Ma (Evernden and Kistler, 1970; Map #s 123, 122, 125, 124 and 121, respectively, for Samples KA1512-HBD, KA1512, KA1512-QTZ, KA1512-KF, and KA1512-PL; Table 1), and a zircon 207Pb/206Pb age of 120 Ma (Kulow, 1966; Map #127; Sample MK90; Table 1).

Judging by field relations and the presence and nature of enclaves in contact zones between the two plutons, the tonalite of Lake Mary and the hornblende-biotite granodiorite of Summit Lake not only mutually intruded one another, but they also mingled and locally mixed with fine-grained mafic rocks.


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The field relations are especially well displayed in glacially polished exposures 100-200 m west of the dam across Lake Angela as well as in roadcuts along the old Highway 40 at the crest of Donner Pass.

Figure 2. Oligocene tuff (above) deposited upon a tonalite ridge (below) with a paleosol at the contact. Parking lot cut on Highway 40 at Summit.

A small outcrop of mafic hornblende-biotite granodiorite (hbg), of unknown age and affinity, crops out beneath the bridge across Onion Creek at the base of the thick succession of rhyolitic ash flow tuff units in the Onion Creek drainage. The granitic outcrop is stratigraphically significant because it is at the base of the Valley Springs Formation in Onion Creek and thereby provides a lower boundary on the thickness of that formation in this area.

At the north end of Lake Tahoe at Brockway is a coarse-grained titanite-bearing, biotite-hornblende granodiorite (Kg) characterized by lath-shaped hornblende phenocrysts up to 15 mm long. It is extensively exposed north and east of Incline Village. Because of textural and mineralogic differences with the Summit Lake and other related plutons on the west side of the Tahoe-Truckee graben, we consider it a separate and distinct pluton whose age and extent have yet to be determined.

Eocene Rocks and Events

Following intrusion of the Cretaceous batholithic rocks, the ancestral Sierra Nevada was uplifted so that streams and rivers flowing west out of central Nevada cut canyons as deep as 300 m into the batholithic and pre-batholithic metamorphic rocks (Lindgren, 1911; Garside et al., 2005). Most of these paleocanyons trend E-W or NNE-SSW. Such a major paleocanyon, now filled with Oligocene ignimbrite deposits, trends southward from Independence Lake, beneath Castle Peak, down Onion Creek and to The


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Cedars before resuming its E-W course outside the map area south of French Meadows Reservoir and the Middle Fork of the American River.

The floors of many of the paleocanyons have a variably thick veneer of gold-bearing gravel (Garside et al., 2005). We have not observed such deposits except beneath rhyolite tuff in the paleocanyon on the west edge of the map about two kilometers south of Lake Van Norden. There the gravel contains sandstone cobbles bearing Eocene leaf fossils (University of California, Berkeley, Museum of Paleontology Locality Number PA692) and black chert pebbles, presumably derived from the Perdido Formation in western Nevada (Garside et al., 2005).

Late Oligocene, Miocene and Pliocene Volcanic Rocks

Two lithologically and temporally distinct groups of volcanic and related rocks were recognized by Lindgren (1897) in the Tahoe/Donner Pass map area: Oligocene rhyolite tuff of the Valley Springs Formation (Gale, in Piper et al., 1939), which is restricted to the Norden quadrangle in the present map, and widespread Miocene and Pliocene andesitic lavas and volcaniclastic deposits of the Mehrten Formation (Gale, in Piper et al., 1939). The rhyolite tuffs constitute a major part of the early Tertiary system of ignimbrite deposits that once filled pre-Oligocene paleocanyons. The tuffs in the Donner Pass region and the implications of their geologic setting were recognized by Lindgren (1897), mapped by Hudson (1951) and Rood et al., (2003), and studied regionally by several investigators (Deino, 1985; Garside et al., 2005; Faulds et al., 2005, Henry and Faulds, 2010). The sources of several, if not all, of the tuff units in Onion Valley are volcanic centers in central Nevada (Garside et al., 2005; Faulds et al., 2005; Henry and Faulds, 2010). Their deposition in paleocanyons alternated with erosion, which left depositional remnants that were buried by subsequent ash flow tuffs and then eventually overlain by locally erupted andesitic lava flows and volcaniclastic deposits of the Mehrten Formation.

Valley Springs Formation

An amalgamated succession of rhyolitic ash-flow tuffs (Tvrt) lies nonconformably upon pre-Tertiary granitic and metamorphic rocks in the Onion Creek drainage at the headwaters of the North Fork of the American River, a few kilometers south of Lake Van Norden and north of The Cedars. These ash-flow tuffs may be traced down the west flank of the Sierra Nevada to the Great Valley where they are assigned to the Valley Springs Formation (Gale, in Piper et al., 1939). We mapped eight separate tuff units in and around the Onion Creek drainage in the southwest part of the Norden quadrangle and the southeast corner of the adjacent Soda Springs quadrangle (Rood et al., 2003). Their aggregate thickness indicates the Onion Creek paleocanyon was as deep as 300 m (Rood et al., 2003). Vegetation and colluvium obscure most contacts with underlying metamorphic rocks in steep canyon walls along Onion Creek, whereas well-exposed contacts with granitic rocks are located in highway and railroad cuts between Donner Pass and Norden. A thin paleosol is present between granite basement and the rhyolite on the north side of Boreal Ridge and in a large parking lot cut on U.S. Highway 40 at Summit (Figure 2).

K/Ar ages are late Oligocene and early Miocene for rhyolite tuff samples from Beacon Peak (Map #s 120 and 119; Samples KA1131 and KA1235; Table 1). C.D. Henry obtained a similar range of 40Ar/39Ar ages for petrologically and mineralogically equivalent rock units in Nevada (Samples H99-22, H99-27, H99-29, and H99-31; Table 2). The basal tuff in Onion Valley is petrographically similar to the tuff of Rattlesnake Canyon (31.03 ± 0.03 Ma) in Nevada (C.D. Henry, personal communication, 2007; age data in Faulds et al., 2005). The uppermost tuff in Onion Valley has been correlated with the Nine Hill Tuff (Deino, 1985; Henry and Faulds, 2010), which is 25.18 ± 0.06 Ma in Onion Valley (Sample H99-22; Table 2), whereas the average of many samples of the Nine Hill tuff is 25.3 Ma (Faulds et al., 2005).


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Mehrten Formation

Curtis (1953) assigned all andesitic flows and volcaniclastic rocks along the northern Sierran crest to the Mehrten Formation, a rock stratigraphic unit, the distribution of which was previously defined in and restricted to the Mokelumne area on the lower western slopes of the Sierra Nevada (Gale, 1939). All of Curtis’ extended Mehrten Formation and its associated volcanic and fluvial rocks in the Tahoe-Donner Pass area erupted over a period of 10 Ma and can be subdivided into three informally named, time stratigraphic members: Mt. Lincoln (13-7 Ma), Martis Peak (7-5 Ma), and Squaw Peak (5-3 Ma).

Andesitic lava flows and volcaniclastic rocks at the north end of Lake Tahoe around Martis Peak define a major volcanic center of late Miocene age. Similarly, large volcanic centers existed at Silver Peak, Mt. Pluto, and KT22 near Squaw Peak in Pliocene time. Major lava dome extrusions formed Mt. Watson and Lookout Mountain, as well as the ridges at Big Chief, Little Chief, and Sawtooth Ridge also in Pliocene time. Smaller eruptive centers, mainly basaltic and of similar age, are located a few kilometers northwest of Tahoe City and north along the Truckee River to Truckee.

Mt. Lincoln Member The Mt. Lincoln member of the Mehrten Formation is an informal name we have assigned to andesitic lava flows, fluvially reworked volcanic deposits, and fluvial deposits between 13 Ma and 7 Ma in the map area. This member unconformably overlies the batholithic and pre-batholitic metamorphic rocks and the Valley Springs Formation. It is widely distributed from Mt. Lincoln north to Castle Peak, westward at least as far as the Serene Lakes, southward at least as far as the North Fork of the American River, and eastward to the head of Coldstream Valley where its relation to lithologically similar rocks of the Squaw Peak member has not been clarified.

The bulk of the member comprises andesitic volcaniclastic rocks (Tlp) and block and lapilli tuff breccia (Tllt) with scattered patches and intercalated flows of hornblende andesite (Tlha) north and west of Donner Lake, and with fluvial, reworked volcaniclastic beds of sandstone and conglomerate on Mt. Lincoln (Tlf) (Figure 3).

Figure 3. Block and lapilli tuff breccia on the south side of Mt. Lincoln. Hammer handle is 40 cm long. A. Unsorted angular andesite blocks in matrix of tuffaceous andesitic ash (Tlp). B. Interbedded, normally graded layers of fluvially reworked, sub-rounded andesite pebbles, cobbles, and tuff (Tlf).

Additional fluvial, lacustrine, and interbedded volcaniclastic units (Tlf) are exposed in railroad cuts east of Mt. Judah at Eder where they rest nonconformably upon deeply weathered granite. They are probably part of the Mt Lincoln member, but the presence of intercalated coal and lacustrine units in the Eder


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exposures contrasts with presumed age-equivalent strata on Mt. Lincoln, on Boreal Ridge, and on the flanks of Castle Peak.

A tadpole-shaped intrusion of a plagioclase-phyric andesite (Tlhai) dike, 700 m long by 400 m wide that intruded hornblende-biotite granodiorite of Summit Lake (Ks), is exposed in an extensive roadcut on the north side of I-80, about two kilometers east of the summit of Donner Pass and one kilometer northeast of Billy Mack Flat. The rocks in the roadcut are massive, non-brecciated and non-vesicular. Smaller, isolated exposures of the same rock crop out another kilometer north of I-80.

The Mt. Lincoln member also includes stubby, aphyric basalt flows, plugs, and dikes, especially on Andesite Ridge and on the west shoulder of Castle Peak. One of these flows is a vitric, non-vesicular basalt (Tlb) that caps the crest of Boreal Ridge. Remnant linear vents are arranged in a right-stepping, en echelon pattern between the highest point on Boreal Ridge westward about 500 m to Crater Lake, a small lake at the west end of the ridge. Most of vents have been modified or nearly obliterated by earth-moving equipment. About two kilometers north of Boreal Ridge is a prominent outcrop of basaltic scoria near the crest of Andesite Ridge. Because of its similar elevation, petrography, short distance from Boreal Ridge, and its similar relation to underlying porphyritic andesite, we correlate the Andesite Ridge rocks with the basalt on Boreal Ridge.

The age of the Mt. Lincoln member, 13-7 Ma, is problematic. We infer its age from only two K/Ar age determinations on andesitic clasts near its contact with granite on the shoulder of Castle Peak (Map # 117; Sample 1 9[CP-4]; 13.5 ± 0.4 Ma; and Map # 116; Sample 2 9[CP-4]; 12.9 ± 0.4 Ma; Table 1), and by the fact the top of the member is capped by the basalt flow on Boreal Ridge with its K/Ar age of 7.6 ± 0.2 Ma (Map # 115; Sample KA1109; Table 1). An isolated plug of andesitic lava, having an anomalous KAr age of 26.4 Ma (Map # 118; Sample 8(BH-T)), crops out at the west end of Donner Lake.

The source of the Mt. Lincoln volcaniclastic rocks is also problematic. Unambiguous vents have not been found in the map area, except for the large andesite intrusion in the I-80 roadcut (Map # 139; Sample H00-113; 6.37 ± 0.25 Ma; Table 2). Much of the andesitic breccia may have erupted from many relatively small lava domes, necks, and fissures as autobreccia (Anderson, 1933; Durrell, 1944; Curtis, 1953), and the products of each eruption may have been covered by the tephra and breccia from succeeding eruptions or fluvial events. Alternatively, some or all of the rocks may have sources outside the map area, such as the major volcanic centers northeast of Lake Tahoe near Mt. Rose.

Martis Peak Member The Martis Peak member is our informal name for all lava flows, intrusive rocks, and volcaniclastic deposits emplaced during the latter part of the Miocene Epoch, between 7 and 5 Ma (Samples WW30 and WW67B; Table 2), and exposed in the vicinity of Martis Peak and eastward into the Carson Range. Because we lack field criteria and detailed age data to distinguish these rocks from those of the Mt. Lincoln and Squaw Peak members, assignments of rocks are provisional among the three members of the Mehrten Formation south of Martis Valley and west of upper Truckee River Canyon.

The Martis Peak member lavas and clastic rocks are andesitic, ranging from basaltic andesite (Tmba) with olivine microphenocrysts, to pyroxene andesite (Tmpa) with two pyroxene and plagioclase phenocrysts, to hornblende andesite (Tmha) with hornblende, hypersthene, and plagioclase phenocrysts. Phenocrysts comprise less than 5 percent to as much as 20 percent of the rock and are generally less than 5 mm in length. Lava flow outcrops are discontinuous and are intercalated with volcaniclastic debris. Volcaniclastic rocks (Tmp) are poorly exposed, but the unsorted and unbedded nature of the deposits was


9

clearly revealed in road cut exposures a few hundred meters southeast of Brockway Summit before they were obscured by rock walls built to protect the roadcuts from erosion.

The intrusion of hornblende andesite at the summit of Martis Peak is pipe-like, but it broadens to a plug shape on the north slope of the peak and hills within one kilometer north of the peak. The intrusive rock is more porphyritic than the surrounding remnant lava flows, with phenocrysts of hornblende and plagioclase reaching 10 mm in length and 30 percent in abundance. Many outcrops are white, yellow, or pale green from extensive hydrothermal alteration to argillic and propylitic assemblages. The plug area of altered intrusive andesite appears to represent the conduit zone of a large volcanic center that produced most of the volcanic rock mapped as Martis Peak member. We interpret the distribution of various volcanic rock types over the outcrop area as the remnants of a composite andesite volcano as much as fifteen kilometers in diameter and reaching a height of at least 1500 m higher than the present Martis Valley, or 2000 m above its base on the floor of the graben (see cross section Figure 15).

Squaw Peak Member The Squaw Peak member is our informal name for all the lava flows, lava domes, intrusive rocks, and volcaniclastic deposits emplaced during the Pliocene Epoch, between 5 and 3 million years ago. These rocks are distributed primarily along the upper Truckee River Canyon and are divided into two groups: 1) the large domes of biotite-hornblende andesite (Tsbha) that underlie the Mt. Watson, Lookout Mountain, Sawtooth, Big Chief, and Little Chief ridges (3.41 to 3.7 Ma; Map #s 135 and 136; Samples AGS90-72 and AGS90-71; Table 2) and various basaltic to andesitic lavas and volcaniclastic units related to 5-3 Ma old volcanic centers located along the present Sierra Nevada crest (Samples 112 Sq-1, 109 Sq-3, 113 Sq-4, 108 Sq-5, 111 Sq-6, and 114 GC-94; Table 1) and probably also the Mt. Pluto volcano (Figure 4).

Figure 4. View east from Squaw Peak across upper Truckee River Canyon to Lookout Mountain (left), Mt. Pluto (center with the ski trails), and Mt. Watson (right) all in the center of the Tahoe-Truckee graben. Mt. Pluto is a remnant of an andesitic stratovolcano.

Basaltic to andesitic lavas and pyroclastic rocks (Tsb, Tsba, Tspa, Tsha, and Tsp)

The map units of the Squaw Peak member are parts of andesitic eruption centers along the present Sierra Nevada crest and possibly Mt. Pluto. A major source was a volcano centered at KT22, one kilometer east


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of present-day Squaw Peak. The large intrusions at KT22 and two kilometers farther east on Juniper Ridge may represent the conduits for a stratovolcano and satellite vent similar to- but smaller than, Mount Shasta. The large exposures forming Squaw Peak appear to be the distal ends of thick andesite flows that are now exposed in cirque walls, rather than the plug of a volcano (see cross section, Figure 13).

Andesitic lava flows and dikes in the basin below Squaw Peak range in age from 4.7 to 3.7 Ma (Samples Sq-1, Sq-4, Sq-5, and Sq-6; Table 1); the basaltic lava flow (Tsb) at High Camp, about one kilometer north of Squaw Peak, is 4 Ma (Map # 109; Sample Sq-3; Table 1). Smaller volcanoes, judging from the smaller sizes of their conduits, were located at and south of Silver Peak as well as Tinker Knob. Anderson Peak is part of a thick basaltic lava flow remnant, possibly erupted from the Tinker Knob volcano, that overlies coarse fluvial boulders and gravel.

We lack direct data or field observations that would allow us to assign an age to the andesitic lavas and pyroclastic debris exposed on the north and east slopes of Mt. Pluto. These rocks and deposits could belong to either the Martis Peak or Squaw Peak members. If we assume that Mt. Pluto is the approximate site of a stratovolcano (Figure 15), however, then it could be the source of pyroclastic deposits and flows that overlie an eastward dipping flow (possibly from the Tinker Knob volcano) exposed in the north wall of Deep Creek Canyon.

The various andesitic lava flows in the Squaw Peak member are petrographically similar in the field, that is, dark- to light-gray andesite with various small phenocrysts, predominantly plagioclase accompanied by one or two pyroxenes (Tspa), or hornblende (Tsha), comprising about 25 percent of the rock. Some of the andesitic rocks are nearly aphyric, and some are extremely porphyritic with plagioclase and prismatic hypersthene phenocrysts up to 10 mm in length.

Volcaniclastic debris (Tsp) that originated by explosive eruptions was deposited on the volcano slopes by fallout, avalanches, gas-driven debris flows, lahars, and mudflows. Remnants of those deposits and intercalated lava flows are generally poorly consolidated and do not form outcrops, but instead underlie areas of gentle to moderate slopes with much surficial soil, vegetation, and scattered sub-angular boulders, similar to glacial till. Road cut exposures exhibit various sizes of volcanic rock clasts, ranging from fine silt to large boulders. Stratification is apparent only in those deposits created from fluvially reworked flows or ash falls.

A N30°W-trending swarm of mafic andesite dikes (Tsi) radiates northward and southward from Squaw Peak and cuts the metamorphic rocks on Ward Peak as well as volcanic rocks from Squaw Peak almost to Granite Chief. The dikes are parallel to the prevailing structural trend of the Tahoe-Sierra Frontal Fault Zone, which probably also localized the Squaw Peak, Tinker Knob, and Silver Peak volcanic centers.

Large volumes of rocks are hydrothermally altered in the Silver Creek drainage, including in the ridge north of Squaw Valley, and along the upper Truckee River canyon on both sides of Deer Creek. The alteration was caused by the reaction of hot water with wall rocks along faults or other fractures, resulting in formation of quartz, calcite, and kaolinite, typical argillic assemblages, and local recrystallization of the rock. Alteration is particularly extensive where the hot water was strongly acidic, leaving strongly leached, silica-rich rocks. White, yellow, and rusty orange exposures along State Highway 89 near Deer Creek are typical examples of these altered rocks, in which trace amounts of gold were found in the abandoned Knoxville area in the 1860s (Lindgren, 1897). Where the water was less acidic but still hot, the reactions caused the breakdown of plagioclase and the glassy groundmass of the volcanic rock to form clay, zeolite, and silica minerals. Deposition of these hydrothermal minerals indurated the volcaniclastic debris, yielding fairly resistant outcrops, such as those along the Silver Creek spur of the Pole Creek road. This type of alteration imparted a greenish or purplish cast to the volcanic rocks. Granitic rocks are also


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extensively altered along faults in the upper end of Squaw Valley, perhaps related to the intrusion and extrusion of biotite-hornblende andesite magma on the flanks of Mt. Pluto, a few kilometers northeast of the alteration zone.

Block and Lapilli Tuff Megabreccia of Mt. Disney (Tsblt)

The Squaw Peak member contains a special type of volcaniclastic deposit that is especially well exposed on some peaks and ridge slopes in the Norden quadrangle, including Mt. Disney, Mt. Lincoln, Mt. Judah, and ridges in Coldstream and Emigrant valleys (Figure 5). It is an unsorted, unstratified, heterolithic, matrix-supported megabreccia comprising rock fragments ranging from fine ash and lapilli up to mappable mega blocks commonly many tens of meters in largest dimension. Internal layering within the large blocks is randomly oriented in both strike and dip.

Figure 5. View south of Mt. Lincoln (left) and Mt. Disney (right) with their caps of debris avalanche of Mt. Disney (Tsblt) above dashed line. See also Figure 10.

The megabreccia contains distinctive dark-blue-gray, phaneritic hornblende andesite blocks, 0.5 m to 4 m in diameter that are subrounded to rounded, with penetrative columnar cooling fractures (Figure 6). Similar andesitic blocks have been reported in the debris avalanche of Barker Pass (Berkebile et al., 2004), and similarly fractured volcanic boulders in debris avalanches elsewhere in the world are considered to be relicts of the juvenile magma, the eruption of which triggered debris avalanches (Stoopes and Sheridan, 1992). Alternatively, the blocks may have formed from avalanches on the stratovolcano slope prior to the cone collapse that triggered the debris avalanche.

The megabreccia is underlain by gray, commonly monomict andesitic lapilli tuff that forms the bulk of the Mt. Lincoln member, but on the northeast shoulder of Mt. Lincoln, it is underlain by a basalt flow. The megabreccia varies in thickness from 3 m on the west end of Boreal Ridge to 140 m on the north face of Mt. Lincoln, to 280 m on Mt. Disney, where it cuts across and down into all underlying strata as if filling a channel or steep-sided canyon.

Like the nearby debris avalanche of Barker Pass (Harwood, 1981; Berkebile et al., 2004), we regard the megabreccia as a remnant of a very large landslide triggered by either the gravitational destabilization of over-steepened volcano slopes by rapid growth during volcanic eruptions, or by earthquake shaking. Thus we have informally named this unit, which was first described by Fosdick et al. (2004), "the debris avalanche of Mt. Disney."


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Figure 6. Polygonally fractured, juvenile block of andesite in megabreccia of Mt. Disney (Tsblt).

The age of the megabreccia is bracketed between two basaltic lava flows. In Roller Pass between Mt. Lincoln and Mt. Judah, the megabreccia overlies a basalt flow with an 40Ar/39Ar age of 5.25 ± 0.18 Ma (Map # 133; Sample 4G-13-1; Table 2), whereas it is overlain by a basalt flow near Horseshoe Bend in Coldstream Valley having a 40Ar/39Ar age of 4.24 ± 0.16 Ma (Map # 132; Sample 4G-25-1; Table 2). The age of the Barker Pass debris avalanche is almost identical: "The critical ages obtained by Harwood include a K-Ar date of 5.4 +/- 0.07 Ma for an andesite lava flow lying beneath the debris-avalanche deposit on the west flank of Anderson Peak, and K-Ar dates of 4.1 ± 0.1 to 3.6 ± 0.1 Ma for andesite lava flows resting on top of the debris avalanche deposit in the Barker Pass area" (Richard Hanson, personal communication, 25 August 2011).

The source of the debris avalanche of Mt. Disney is speculative. Because it thickens and coarsens southeastward from Boreal Ridge through Mt. Disney and Mt. Lincoln, to the ridges in Coldstream and Emigrant valleys, we suspect its source was several kilometers southeast of Mt. Lincoln in the approximate location of the Mt. Pluto stratovolcano.

If the Mt. Pluto stratovolcano were the source of westward-directed debris avalanche lobes crossing the present Sierran crest, as they seem to have done, then that crest did not exist, at least in the Lake Tahoe area when those avalanches erupted. The crest must have been uplifted, or, more likely, the Tahoe-Truckee graben sank after 4 Ma, but before the 3.5 million year eruptions dammed the graben and ponded Lake Tahoe.


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Biotite-hornblende andesite lava and lava-domes (Tsbha)

The biotite-hornblende andesite, which forms the large, homogeneous masses that underlie the Mt. Watson, Lookout Mountain, Sawtooth, Big Chief, and Little Chief ridges, and flank the west slope of Mt. Pluto, is unique in the Lake Tahoe-Donner Pass region in its phenocryst and xenocryst mineralogy, chemical composition, and eruptive characteristics. These exposures consist of large, massively rounded, resistant outcrops (Figure 4) resembling the morphology of weathered granitic outcrops. The irregularly shaped outcrop area is about eight kilometers across, and the coalesced lava-domes and thick flows probably formed a structure much like that of Mammoth Mountain in Mono County, California.

The biotite-hornblende andesite is very coarse-grained with abundant fritted plagioclase crystals up to 1 cm, and biotite and hornblende crystals up to 5 mm in length. Olivine and quartz xenocrysts with pyroxene reaction rims, although rare (less than 1 percent), prevail throughout the outcrop area. Because the SiO2 content of these rocks (about 61 percent) is typical of andesite, and the persistent occurrence of biotite is unusual, we use the term biotite-hornblende andesite to distinguish these rocks from those of the Martis Peak and Mt. Pluto stratovolcanoes. Partially assimilated and hybridized mafic magmatic enclaves ranging from 1 to 50 cm in size are common. These lithologic characteristics, as well as the olivine and quartz xenocrysts, are typical of rocks formed by the mixing and mingling of a mafic magma and partially melted granitic rocks (e.g., Wiebe, 1991; Barbarin and Didier, 1992; Pitcher, 1993; Sylvester, 1998).

Plagioclase and biotite in the biotite-hornblende andesite at the top of Lookout Mountain yielded 40Ar/39Ar ages of 3.4 and 3.7 Ma (Map #s 136 and 135; Samples AGS90-71 and AGS90-72; Table 2). Eruption of these large lava domes and thick flows blocked the ancestral Truckee River, so that Lake Tahoe formed behind the consequent dam (Birkeland, 1963). The lake rose and its outflow stream subsequently carved a new outlet and canyon around the west side of these resistant rocks, approximately the locus of the present Truckee River

Pliocene to Quaternary Rocks and Events

Local basaltic eruptions replaced the widespread andesitic volcanism that predominated in late Miocene time. Basaltic lava fields, flows, and plugs are common in, but not restricted to, a five kilometer-wide, north-south trending zone that parallels the upper Truckee River between Tahoe City and Alder Hill, one kilometer north of Truckee. Quaternary glaciers cut deeply into the highly elevated part of the Sierra, leaving U-shaped canyons, scoured and polished rock surfaces, and abundant moraine deposits as evidence of their passing.

Truckee River Formation

We assign to the Truckee River Formation all basaltic lavas and older alluvial units in the Tahoe/Truckee region that have erupted and been deposited, respectively, since 2.5 Ma, and which Birkeland (1963) correlated with the Lousetown Formation in western Nevada. The youngest age obtained to date, 0.92 Ma, is for a trachyandesite lava flow (QTttt) one to four kilometers northwest of Tahoe City (Kortemeier et al., 2009). With the exception of the Dry Lake, Bald Mountain, and Alder Hill lava fields, each basalt flow and associated cinder cone represents a single eruption. Undated flows and cones are assigned to this formation based on the absence of significant erosion. The formation lacks andesitic products and so would not have been included in Curtis' (1953) extension of the Mehrten Formation in this area. Geochemical data for many of these volcanic rock units are given by Cousens et al. (2011).

Individual lava flows can be distinguished from one another by the size, type, and abundance of phenocrysts. Thus, the main basalt types in the Truckee River Formation are: olivine basalt (>5 percent olivine phenocrysts, 1-2 mm), trachybasalt (very small olivine phenocrysts >5 percent), trachyandesite


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(rare, very small olivine phenocrysts), and shoshonite (rare hornblende and olivine phenocrysts). Some of the small basaltic exposures, such as the basalt nubbins in the mouth of Ward Creek and in William Kent State Park (QTtpm), are almost glassy, consist only of plagioclase microlites, and represent the upper parts of quickly chilled dikes (Figure 7).

Figure 7. Nubbin of glassy olivine basalt (QTtpm) in William Kent State Park.

Truckee River Formation lava flows form dark- to medium-gray outcrops. Flow thicknesses range from 2-20 m, averaging about 5 m. Basalt outcrops commonly contain somewhat irregular vertical cracks, yielding crude columns. Almost all trachyandesite outcrops are flow-jointed into closely spaced, sub-horizontal slabs.

Noteworthy basaltic lava fields in this formation include those at Bald Mountain (QTtbm) and Hirschdale (QTth). The former occupies a large area of outcrop centered four kilometers south of Truckee. The latter is notable for its youthful appearance and by the fact that one of its flows formed a lava dam that blocked the Truckee River, causing deposition of the Prosser Creek alluvium (QTtpc) over much of Martis and Prosser Valleys (Birkeland, 1963). Both of these lava fields have similar K/Ar ages of 1.2 and 1.3 Ma, respectively (Map #s 101 and 102; Samples KA1078 and KA1094; Table 1), whereas the Bald Mountain basalt has three 40Ar/39Ar ages averaging 1.41 Ma (Map #s 128, 129, and 130; Samples 97-LT-65, 97-LT-75, and 97-LT-62B; Table 2).

The Dry Lake lava flows (QTtdl) are trachyandesitic, containing either hornblende or pyroxene microphenocrysts. We have subdivided Dry Lake lava flows into four mappable units, whereas Latham


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(1985) recognized six. We obtained a 40Ar/39Ar isochron age of 1.35 Ma for our unit 2 (Map # 134; Sample WW34; Table 2).

Three sedimentary units named by Birkeland (1963) are included within the Truckee River Formation, which, from youngest to oldest, are: The Prosser Creek alluvium (QTtpc) and Cabin Creek alluvium (QTtca) in the Truckee quadrangle, and the Fir Crags gravels (QTtfc) in the Tahoe City quadrangle. The Prosser Creek alluvium represents sediment deposited in a lava-dammed lake that formed when flows from the Hirschdale volcano blocked the Truckee River, 1.2 million years ago (Birkeland, 1963). The Hirschdale lavas are among the youngest of the basaltic units in the Truckee River Formation in spite of the K/Ar data, because the Prosser Creek alluvium overlies the Polaris (QTtp) and Bald Mountain basalts. The Cabin Creek alluvium overlies Squaw Peak volcaniclastic deposits and underlies the Bald Mountain basalt in the upper Truckee River canyon three to four kilometers south of Truckee. It probably represents a riverbed deposit of the ancestral Truckee River. The Fir Crags gravels (QTtfc), consisting of up to 80 m of sand and boulders, rest on andesitic bedrock and are overlain by Page Meadow basalt (QTtpm; 40Ar/39Ar age of 2.2 Ma, Kortemeier et al., 2009). Birkeland (1963) postulated that Ward Creek deposited the gravels behind a lava dam formed by the Big Chief basalt (our trachyandesite flow of Deer Creek, QTtdc).

Local deposits of basaltic lapilli tuff and pillow lava (QTttr) crop out on the north shore of Lake Tahoe at Commons Beach and in Lake Forest at Skylandia (Kortemeier et al., 2005, Cousens et al., 2007). Much of the tuff has been converted to palagonite. Cousens et al. (2011) obtained a 40Ar/39Ar age of 2.2 Ma on the deposits (Map # 131; Sample 04-LT-59; Table 2). We regard the deposits as the remnants of a littoral cone or a near-shore vent possibly related to the eruption of Lake Forest lava flows (QTtlf) into Lake Tahoe, although a direct connection between the volcaniclastic deposits and the lava field is now obscured by soil, roads, and houses. Alternatively, the deposits may have been extruded from a near shore vent (Kortemeier, et al., 2005; Cousens et al., 2007).

The trachybasalt of Tahoe City (QTttb) represents the youngest dated volcanic event in the Tahoe-Donner Pass region with its 40Ar/39Ar age of 0.92 Ma (Kortemeier et al., 2009). This rock unit was once considered to be twice as old (1.9 ± 0.1; Dalrymple, 1964, Sample KA1097, Table 1, recalculated to 2.0 ± 0.1 according to Dalrymple, 1979). The youthfulness of these lava flows, together with the earthquake activity north of Kings Beach (von Seggern, et al., 2008), indicate that volcanic activity is more recent and on-going than previously thought.

Glacial Deposits

Glacial deposits blanket large parts of the western half of the map area, especially the major U-shaped canyons that drain eastward from the Sierran crest (Figure 8). The glacial deposits include all types of glacial detritus, generally termed "glacial till", which is mostly a jumble of unconsolidated, unstratified rock fragments with a wide range of sizes and shapes from angular to rounded. The surfaces of glacial deposits are typically covered with various sub-angular boulders, not unlike the eroded and weathered surfaces of andesitic volcaniclastic deposits.

Careful judgment of the weathered state of those granitic boulders within till is required to distinguish among the four ages of till that Birkeland (1963; 1964) described in the Tahoe-Donner Pass area. The youngest tills are products of the Tioga glaciation (Qti; Birkeland, 1964) and are about 14,000 – 26,500 years BP old (Birkeland, 1964; Yount and La Pointe, 1997, Kaufmann et al., 2004; Howle et al., 2005; 2012; Rood et al., 2007; Schweickert et al., 2011). Granitic boulders are very fresh even in subsurface exposures, whereas those in Tahoe tills (Qta), about 70,000 years BP (Rood et al., 2007; Howle, et al., 2005; 2012; Schweickert et al., 2011), have 1-2 mm weathering rinds, and surface ridges are less distinct


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than those of Tioga tills (Birkeland, 1964). The prominently exposed granitic boulders, which lie at an elevation of 2200 m in many of the glacial valleys and canyons on the west side of Lake Tahoe and the north side of Donner Lake, are remnants of Tahoe tills.

Figure 8. View east-northeast across Donner Lake, a U-shaped valley, to Donner Ridge.

The Donner Lake till (Qd) (Birkeland, 1964), of unknown age but probably greater than 131,000 years BP (Yount and LaPointe, 1997), is difficult to distinguish from underlying volcaniclastic deposits; the presence of deeply weathered granitic boulders is required for any deposit to be mapped as Donner Lake till. The Donner Lake till and corresponding outwash (Qdo) have been recognized in Martis Valley and in upper Truckee River canyon (Birkeland, 1963). The oldest till recognized by Birkeland (1964), is the Hobart till, also of unknown age, which Birkeland maintained is commonly difficult to distinguish from the Donner Lake till; he mapped Hobart till only where he could clearly separate it from the Donner Lake till. We found no compelling evidence of Hobart till in the map area.

Three generations of glacial outwash from glacier melting are distinguished by the terraces they form, especially in Martis Valley (Birkeland, 1964). The earliest outwash (Qdo) derived from the largest glaciers of the Donner Lake glaciation and forms the most widespread deposits east of the Sierran crest. The next most voluminous deposits (Qtao) are those from the Tahoe-aged glaciers in the large, U-shaped valleys east of the Sierran crest, and the least extensive (Qtio) are from the smallest and most recent glaciers of the Tioga advance and subsequent minor advances.

During one or several of the glaciations, glaciers extended eastward from Squaw Valley and Bear Creek, blocking the course of the Truckee River and causing Lake Tahoe to rise as much as 200 m higher than its present elevation (Birkeland, 1968; Crippen and Pavelka, 1972, p. 15; Kortemeier et al., 2009). When the ice dams broke, erratics formerly entrained in the glaciers were carried by jökulhlaups down the Truckee River as far east as Reno, Nevada (Birkeland, 1968). Where the upper Truckee River Canyon broadens abruptly at Truckee, moreover, several hundred granite boulders, as large as 8-10 m in diameter and derived from the head of Squaw Valley (Burt et al., 2007), were deposited (Figure 9). At least one such


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huge boulder was carried to Verdi, Nevada, seventy-five kilometers downriver from its source in Squaw Valley.

Figure 9. Giant granite boulder carried into upper Truckee River Canyon by a Squaw Valley glacier. Jökulhlaups transported many such boulders downstream to Truckee.

Alluvium

All of the alluvial units are deposits of rock fragments, silt, sand, gravel, and boulders that formed within the past one million years. Some of these deposits may have formed during the last ice advance but are not glacial outwash deposits. All deposits are unlithified and form parts of the present land surface.

On the map, alluvium (Qa) is sediment actively or recently deposited by streams, including floods. Older alluvium (Qoa) is similar but formed up to about 20,000 years ago (Birkeland, 1963). A fan deposit (Qf) is alluvium deposited at the mouth of a steep canyon. Colluvium (Qc) has been mapped where loose, heterogeneous, and incoherent masses of soil and/or rock fragments have accumulated on gentle to moderate slopes and cover the underlying bedrock. Such slopes may include some remnant glacial detritus (Qc+d) and are commonly heavily vegetated. The north slope of Shallenbarger Ridge is a particularly good place where such alluvial detritus may be observed and mapped.

A headwall scarp, a hummocky surface, and a steep toe front characterize landslide deposits (Qls). Internally, a landslide may comprise large, randomly distributed blocks in a fragmented, lithologically similar matrix. Landslides are prevalent in the upper Truckee River Canyon, and several have deflected the river, such as the one on the east side of the canyon between the mouths of Squaw Valley and Bear Creek (Alpine Meadows drainage). Another large slide on the west slope of Mt. Judah consists primarily


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of fragmented, interbedded fluvial and reworked volcaniclastic deposits. Talus (Qt) is mapped where rock falls are so extensive that underlying rock units cannot be discerned.

Sand dunes (Qsd) are active to stabilized deposits on the north shore of Lake Tahoe in the community of King’s Beach. Scattered lacustrine deposits (Ql) on shores of Lake Tahoe consist of stratified silt and fine sand and represent former high stands of the lake (Birkeland, 1964; Kortemeier and Schweickert, 2007).

Structural Geology

The general structure of the map area is a north-trending graben that is bounded by steep north- and north-northwest-striking normal faults, along which Cretaceous granite and older metamorphic rocks were juxtaposed against Tertiary volcanic rocks that cover the floor of the graben (Lindgren, 1897). Vertical separation across bounding faults is at least 300 m, based on tenuous stratigraphic correlations of Tertiary volcanic units across the faults, and because Pre-Tertiary granitic and metamorphic rocks are nowhere exposed within the north part of the graben, whereas they are extensively exposed in the high country on the flanks of the graben.

Tahoe-Sierra Frontal Fault Zone

The west side of the Tahoe-Truckee graben is bounded by a NNW-trending, one to two kilometer-wide zone of faults and sheared basement rocks, called the Donner deformation zone (Hudson, 1948), the Tahoe-Sierra Frontal Fault Zone (Schweickert et al., 2000; Howle et al., 2012), and the Donner Pass Fault zone (Henry and Perkins, 2001). By any name this zone of faults stretches north northwestward across the west part of the Tahoe basin from Carson Pass through Donner Pass and probably continues through Mt. Lola, ten kilometers north of Castle Peak, and into the Mohawk Valley Fault Zone northwest of Sierraville, judging from photo lineaments and fault maps of the northern Sierra Nevada (Olig et al., 2005). The Tahoe-Sierra Frontal Fault Zone is less clearly demarcated in the map area from Squaw Valley northwestward to Castle Peak. Along that locus of discontinuously mapped fault segments, Tertiary volcanic rocks are juxtaposed against Cretaceous granite for several hundred meters at a stretch, but the faults themselves are nowhere exposed owing to soil and vegetation cover and two-dimensional exposures. We infer their presence primarily from the apparent downthrow of the nonconformity between granitic rocks and overlying Tertiary volcanic rocks into the Tahoe-Truckee graben, as clearly recognized by Lindgren (1897) and Hudson (1948; 1951). The locus of downthrow coincides with the location of some photo and vegetation lineaments, and with a zone of intensely sheared granitic rocks along the projected strike of the fault zone, one to two kilometers north and northeast of Donner Peak. Although we diligently searched the lineaments and locus of downthrow, we were unable to find unequivocal exposures of major faults.

On the east side of Mt. Lincoln the fault zone includes a poorly expressed master fault, the eastern block of which is from 250 m to 400 m lower than the block to the west as estimated from vertical separation of an apparent paleovalley. There, Nine Hill tuff (Tvrt7) has been separated vertically a minimum of 30 m across a line, which when extrapolated south-southeastward, coincides with a reasonably well-defined fault in the Tahoe-Sierra Frontal Fault Zone. The fault zone also juxtaposes older units of the Tertiary volcanic rocks on the west against younger units on the east in the southern part of the Norden quadrangle, especially in the southwest corner of Section 35 at the west end of the ridge between the north and south forks of Coldstream Valley. There Oligocene rhyolite tuff unit Tvrt6 on the west is juxtaposed against Pliocene debris avalanche deposits of Mt. Disney (Tsblt), which are separated stratigraphically by only 20 m in Sugar Bowl, but by 140 m across the fault zone.


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The displacement may also be estimated from the stratigraphic separation of an isolated patch of flat-lying Tertiary volcanic and fluvial rocks exposed on a ridge between Donner Peak and Eder east of the Tahoe-Sierra Frontal Fault Zone, and a similar succession of rocks on Mt. Judah on the west (Figure 10, upper). In fact, the Tertiary rocks either fill a paleovalley, or more probably have been down-dropped into a graben whose bounding faults must be inferred, because they are not exposed. The fault on the west side of the patch of Tertiary rocks coincides with the locus of the master fault of the Tahoe-Sierra Frontal Fault Zone. We infer that the fault on the east side of the patch dips steeply west, antithetic to the main fault. The vertical separation between presumed stratigraphically equivalent units in the patch and those on Mt. Judah is nearly 250 m. In our opinion this amount of separation requires an excessively deep paleovalley and more likely reflects fault displacement.

Figure 10. West to east structure sections across the Tahoe-Sierra Frontal Fault Zone. Upper section illustrates about 250 m of displacement of strata (Tlf) on Mt. Judah into adjacent graben. Lower section illustrates about 450-500 m of displacement of Mt. Disney debris avalanche (Tsblt) across the Tahoe-Sierra Frontal Fault Zone.

The debris avalanche of Mt. Disney gives evidence for even greater displacement. The debris avalanche's thalweg, which has a northwest-directed gradient of 7.5 percent determined from the respective elevations of its lowest contact on Mt. Disney (7,200 feet) and uppermost contact on Mt. Lincoln (8,000 feet), may be extrapolated southeastward to where it would intersect the Tahoe-Sierra Frontal Fault Zone fault at 10,000 feet. The southeastern contact of the debris avalanche is at 6,400 feet elevation in Coldstream Valley, yielding an apparent vertical separation of 3,600 feet (1,200 m).

Abundant shear fractures in granite are well exposed in abandoned railroad tunnels and snow sheds, where they cross the locus of the Tahoe-Sierra Frontal Fault Zone on the north side of Shallenbarger Ridge south of Donner Lake (Figure 11). They strike between 150°-170° dip steeply west to vertical across a zone about one kilometer wide. Fracture spacing is about 5 to 150 cm. Typically a 1 to 10 mm thick zone or two of greenish-gray clay gouge is smeared along each fracture. Determination of displacements across any given shear fracture is precluded in the homogeneous granite. In a Highway 40 roadcut part way up the Donner Pass grade, an unsheared basaltic dike, about 35 cm-thick, intruded a NNW-striking shear zone that dips 85° W (Hudson, 1951). Henry et al. (2004) obtained an 40Ar/39Ar age of 8.5 ± 0.4 Ma for the dike (Map # 140; Sample H00-63; Table 2). We investigated the outcrop and


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found plenty of fractures but little evidence of a fault. The attitude of the unsheared dike is the same as prevalent shear fractures in granite in the railroad tunnel cuts.

Locally within the Tahoe-Sierra Frontal Fault Zone, basalt dikes intruded minor faults in granitic and metamorphic basement rocks. In a natural exposure half way between Beacon Peak and Lake Angela, a 10 cm-wide, 20 m-long mafic dike intruded a fault in granite. The dike has chilled margins and is not sheared, whereas an older, gently dipping aplite dike in the granite is separated 2.5 m right-laterally by the fault. However, if the displacement is purely dip-slip, then about 1.5 m of reverse displacement occurred before the mafic dike intruded the 85° W dipping fault.

Figure 11. Fractures in granite along Tahoe-Sierra Frontal Fault Zone in railroad cut on northeast flank of Donner Peak.

Conjugate sets of shear fractures are well-exposed in flat-lying volcaniclastic deposits in the railroad cuts between Mt. Judah and Donner Lake, one kilometer ENE of Donner Peak. The more strongly developed set of the fractures strikes NNW, parallel to the inferred master fault, and is vertical or dips steeply east, whereas the other set has a similar strike but dips moderately west. The bisector of the conjugate sets strikes NNW and dips steeply west, implying that the direction of principal extension was about ENE/WSW in accord with extension inferred for the eastern edge of the northern Sierra Nevada during Basin and Range deformation (Henry and Perkins, 2001).

Quaternary displacement is known on faults along the southern continuation of the Tahoe-Sierra Frontal Fault Zone west of Lake Tahoe and south of Squaw Valley (Schweickert et al., 2000) but not in the Donner Pass area.


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In summary, the Tahoe-Sierra Frontal Fault Zone comprises several discontinuous and overlapping fault strands that locally bound elongate graben in the map area. Vertical separation across the deformation zone is on the order of 250-1200 m and is inferred to be younger than 4 Ma, judging from displacement of the debris avalanche of Mt. Disney.

Central Graben Faults

Several fault strands have been mapped by various methods on the floor of Lake Tahoe (Hyne et al., 1972; Schweickert et al., 2000; Gardner et al., 2000; Brothers et al., 2009). Each fault is inferred to be a normal fault, judging from the sense of lake floor separation and from onshore trench exposures. Three faults extend onto land at the north end of the lake: The West Tahoe-Dollar Point Fault and two splays from it termed the Carnelian Bay and Agate Bay faults. At the south end of the lake, the West Tahoe-Dollar Point Fault displaces Tioga-aged glacial moraines.

West Tahoe-Dollar Point Fault

The onshore locus of the Dollar Point Fault (Brothers et al., 2009) has been determined largely from photo and topographic lineaments. It displaces but little the trachyandesite flows of Cedar Flat. It continues NNW across vegetated volcaniclastic deposits and thence into the Bald Mountain basalt five kilometers south of Truckee. A splay off this fault may strike north through vegetated volcaniclastic deposits, across Sawmill Flat, and then steps along the east edge of Martis Valley, as depicted by Saucedo (2005), to connect with an inferred fault two kilometers west of Union Mills that coincides with short, discontinuous faults mapped by Olig et al. (2005).

Carnelian Bay and Agate Bay Faults

The Carnelian Bay Fault splits off the Agate Bay Fault on the floor of Lake Tahoe about two kilometers southeast of Dollar Point (Schweickert et al., 2000). Onshore, the Carnelian Bay Fault juxtaposes disparate rock units about two kilometers west of Flick Point and two kilometers north of Cedar Flat. The fault cannot be traced continuously farther northward owing to heavy vegetation and colluvium north of a point one kilometer west of Brockway Summit, but a possible continuation is the Polaris Fault. Sag ponds, fault scarps, displaced fluvial terraces, and a mole track in Martis Valley are evidence of Quaternary activity on the Polaris Fault (Howle et al., 2009; Brown, 2010; Hunter et al., 2009; 2010; 2011).

Onshore, the Agate Bay Fault sharply juxtaposes trachyandesite flows of Agate Bay with volcaniclastic rocks of the Martis Peak member of the Mehrten Formation. We have not been able to trace the fault more than four kilometers north of the Lake Tahoe shore.

A prominent and apparently youthful, 10 m-high, west-facing fault scarp displaces older alluvium in the residential area of Kings Beach. Its spatial and genetic relations to other faults in the area remain to be determined.

Considerable seismic activity has been monitored recently in the area between the north end of Lake Tahoe and Hirschdale, activity thought to be caused by the upward rise of magma (Smith et al., 2004; von Seggern et al., 2008).

Brockway Fault

The east side of the onshore part of the Tahoe-Truckee graben is bounded by what has been termed the State Line-North Tahoe (SLNT) Fault (Brothers et al., 2009), but which we term the Brockway Fault in


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view of the precedence of "State Line" for a fault in southern Nevada. The Brockway Fault may be the landward extension of the SLNT Fault, which has been prominently imaged on the floor of the lake (Brothers, et al., 2009). Granite of the Carson Range on the east shoulder of the fault is uplifted relative to the various Miocene and Pliocene volcanic rocks of the graben. From its quite evident position on the Lake Tahoe shore at Brockway, the fault has not been mapped northward in any detail, probably because it lies almost exactly on the California/Nevada geopolitical boundary. It may bend or splay north northwestward about five kilometers north of the lake and join with faults we have mapped incompletely north of Murphy Meadows, through Burnt Flat and Juniper Flat, to Hirschdale.

Cryptic Truckee River Fault

The nearly straight, northward trace of the upper Truckee River Canyon, from Fir Crags to Truckee, strongly suggests that it is structurally controlled. Neither Lindgren (1897), nor Birkeland (1963), nor we have found any evidence of a controlling structure, such as a fault that could have been etched by the river. Birkeland (1963) proposed that intrusions of the porphyritic biotite-hornblende andesite (Tsbha) underlying Big Chief, Sawtooth Ridge, and Mts. Watson, Pluto, and Lookout dammed up the ancestral Truckee River, probably covered any fault, and caused the formation of Lake Tahoe. Isotopic ages of those rocks suggest that event happened 3.5 Ma ago. We postulate that Tahoe City and Bald Mountain lava flows also diverted the Truckee River westward, thus burying any cryptic fault, if, indeed, one ever existed.

We postulate that a cryptic fault did exist and that it still exists. It was buried by lava flows from the several volcanic eruptions but was subsequently exploited as a conduit for basaltic magma that formed eleven small plugs and cinder cones (QTcc) aligned in a north-south belt about five kilometers wide from Sunnyside north to Truckee and Alder Hill. Thus, near the mouth of Ward Creek in Sunnyside, and one kilometer west, are two, nearly linear arrangements of isolated basaltic outcrops that are labeled QTtpm on the map, but they are much younger than the 2.2 Ma basalt of Page Meadow. These basaltic outcrops intruded water-saturated glacial till and outwash, so that the magma quenched and formed several unique nubbins, 2-3 m high and about 3-4 m in diameter (Figure 7).

Five kilometers farther north and one kilometer south of Rampart, is a deposit of red brown cinders, the remnant of a young basaltic vent. Five kilometers north of Rampart and three kilometers from the junction of upper Truckee River and Bear Creek is a large, cinder cone, sometimes called the Tahoe City cinder cone, the youngest basalt at 0.92 Ma in the map area (Kortemeier et al., 2009). About three kilometers north of that cone and opposite the mouth of Squaw Creek, is another smaller basaltic cinder cone. Farther north, between Sawtooth Ridge and the Little Chief/Big Chief ridge is the Deer Creek lava flow (QTtdc), which, although undated, has the surficial aspect of being one of the youngest basalt flows in the map area. Its main vent lies at the topographically highest part of the flow, about six kilometers east of Bullshead on the Truckee River. The mapped form of the flow suggests that it filled a river course, perhaps an ancestral locus of the Truckee River itself. The three Bald Mountain cinder cones south of Truckee mark the northward continuation of the postulated fault. It is not certain if those cinder cones mark the location of vents that fed the Bald Mountain basaltic lava flows (1.41 Ma), or if they are much more recent cones merely perched upon older flows. Last in the line are two cinder cones north of Truckee, one on the north edge of the town, and the other on the top of Alder Hill. Several more youthful basaltic cones and plugs in the next quadrangle to the north, the Independence Lake 7.5' quadrangle, were identified during reconnaissance mapping. They are aligned along the extrapolation of the same cryptic lineament.


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Folds and Tilted Strata

Folds are not present in the mapped region except for some broad undulations in volcaniclastic strata on the east flank of Mt. Lincoln (Figure 12). The undulations are neither extensive nor pervasive, and they lack regularity of form and style. Whereas Hudson (1948) interpreted the deformation of these strata to be tectonic, it is more likely that the undulations of lithologic layering represent initial dips on volcano slopes or channels, sags, and subsidence features related to deposition, dewatering, and compaction of the volcaniclastic sediments.

Figure 12. Apparent anticlinal warp in volcaniclastic deposits on east flank of Mt. Lincoln.

Most of the sedimentary strata in the mapped area are flat lying, with an important exception being the east dipping fluvial and volcaniclastic strata exposed in railroad cuts on the north flank of Shallenbarger Ridge, and the similar strata exposed across Donner Lake on Donner Ridge. Strata at both locations dip about 20-25°E. The strata on Shallenbarger Ridge rest depositionally on deeply weathered granitic rocks, whereas those on Donner Ridge are intercalated with andesitic lava flows, breccia, and lapilli tuff. Parting cleavage in the Donner Ridge lava flows also dips 25° E, implying that the entire succession has been tilted eastward on a hypothetical, west-dipping, listric normal fault one kilometer or so east of Donner and Shallenbarger ridges and Donner Lake. Neither we nor Hudson (1948) found any evidence for such a fault, but that may be because it is obscured in the forest that prevails around Donner Lake and on Shallenbarger Ridge.

Some interpretations of volcanic activity

Three cross sections drawn through some of the major mountains in the Tahoe-Donner area illustrate our ideas of the development of the volcanoes across the north end of what is now the Lake Tahoe Basin.

The Squaw Peak Volcano

The first section is drawn from Squaw Peak to KT22 and along Juniper Ridge across the Truckee River and onto the slopes east of the river. This section (Figure 13) gives one interpretation of the complex assemblage of faults, intrusive bodies, and remnants of lava flows and pyroclastic rocks exposed in the vicinity of the Squaw Valley ski area. In this and the following two cross sections the shapes of individual


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volcanoes are inferred from the stratovolcano morphology in the present Cascade Range. The cones of these volcanoes generally have a height/radius ratio of 1/5 (see for example, Wise, 1969, pl. 2). Each Tahoe volcano was scaled sufficiently large to cover lavas and pyroclastic debris thought to be attributable to that volcano. Therefore, the extent of the cone governed the original height of the volcano, which we have sketched to show the appearance of the volcano during or some time following its activity.

It is noteworthy that Squaw Peak most likely is not a remnant of the neck for a volcano, because good field observations indicate that the peak consists of two thick lava flows. The two large intrusive masses at KT22 and about one kilometer east probably are related to conduits and could be interpreted several ways. We have chosen to model the volcanic structure after that of Mt. Shasta, although a little smaller. The pink color represents all pre-Tertiary granitic and metamorphic rocks. The faults and elevations of the granite-volcanic contact are well exposed in the canyon south of Juniper Ridge (Alpine Meadows). To the east we arbitrarily draw the contact at the approximate depth of the Lake Tahoe. Of course the elevation of this surface at the time of the eruption of the volcano about 4 Ma is unknown.

The subsurface intrusion of Tsbhai at the east end of section is only hypothetical but is shown to represent several lava domes that did erupt in that area.

Figure 13. West to east structure section from Squaw Peak to the Truckee River to illustrate inferred Pliocene form of stratovolcanoes related to volcanic vents on Squaw Peak and KT22.

Silver Peak to Mt. Watson

This section, about five kilometers north of the Squaw Peak section, was chosen to represent the western part of the Tahoe-Truckee graben (Figure 14). The intrusion at Silver Peak was chosen as the conduit of a moderate sized stratovolcano. It is likely that this volcano was active at about the same approximate time as the KT22 volcano, but we lack age control and so can only speculate on possible interfingering of deposits of the two cones.

At about 3.5 Ma, biotite-hornblende andesite lava domes erupted from many sites on the western slopes of the Mt. Pluto stratovolcano. The remnants of one large dome and lava flow (Tsbha) form the present


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Mt. Watson ridge. A large area between Silver Peak and Mt. Watson has been hydrothermally altered, possibly from a source several thousand feet below the present surface (Figure 14).

Mt. Pluto to Martis Peak

These two stratovolcanoes (Figure 15) clearly form the “lava dam” limiting the north end of Lake Tahoe. The Martis Peak volcano (about 6.5 Ma) is centered on the intrusions at the summit of Martis Peak.

We lack age control for the Mt. Pluto lavas, but if we assume an age of about 4.5 Ma, then this volcano could be the source for lavas and debris avalanche deposits to the northwest. Pyroxene andesite intrusions have not been recognized on Mt. Pluto, so that placement of the cone over the present summit is somewhat arbitrary. Complicating this interpretation is a small circular intrusion of biotite-hornblende andesite at the summit. As in the other sections, the base of the volcanic pile is drawn at the approximate elevation of the present day Lake Tahoe.

Figure 14. West to east structure section from Silver Peak to Mt. Watson to illustrate inferred relation of their lava flows to those of Mt. Pluton.

Figure 15. Southwest to northeast structure section from Mt. Pluto to Martis Peak to illustrate relationship between lava flows and volcaniclastic debris from Mt. Pluto (Tsp) and Martis Peak (Tmp).

Acknowledgments

The following 136 students and graduate teaching assistants did the detailed mapping and kindly permitted us to use their work for the compilation: Christiane Andrews, Erica Frantz, Mike Porter, Jeff


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Malaihollo, Phil Anderson, Mike Richards, Pat Brienen, Randy Mills, Joel Metcalf, Nellena Beedle, Barbara Belding, Joe Weidman, Karla Sanders, Don LeSage, Scott Williams, Freedom Johnson, Wendy Arima, Brian Bjorkland, Penny Bloodhart, Eileen Bailiff, Cole Davisson, Paul Bigelow, Lowell Woodbury, Diane Brooks, Ron Clemens, Jeff Johnson, Alan King, Catherine McKissock, Dave Parkinson, Peter Sapienza, John-Eric Schroeder, Keith Stone, Peter Hill, Andy Barnes, Brendan Laurs, Bill Baugh, Christopher Hitchcock, Ron Mendivel, Tom Bruscotter, Brian Steele, Gerald Bawden, Danny Boyer, Andy Stone, Jamene Pinnow, Rich Graf, Andreas Stollar, Stephanie Barber, Nicole Nelson, Brian Wertheim, Deborah Underwood, David Valentine, John Feltman, Jordan Kear, Allison Reed, Peter McDonald, Ted Heinse, Doug Korengold, Ken Stelman, Les Hasbargen, Richard Sturn, Janine McAdams, Mike Pavelko, Nancy Riggs, Michael Ort, Ben Adams, Allison Reed, Nathan King, Geoff Stokes, Sue Dougherty, Maggie Olson, Richard Sturn, Matt Jackson, Julie Fosdick, Gabriel Rotberg, Katie Hickling, Falene Petrik, Winnie Yeh, Christie Till, Nick De Sieyes, Ryan Galeria, Thomas Neely, Dylan Rood, Noah Garrison, David Young, Emily Fudge, Tim Middleton, James Jacobson, Aaron Puente, Cohn O’Donnell, Jeff Miller, Lurena Huffman, Ian Cutler, Mike Sherwood, Sarah Richmond, Rob Pennington, Trystan Herriott, Juan Ocampo, Reijo Rataleinen, Malcolm Swan, Reid Drossos, Lindy McCullough, Mike Maguire, Garret Bean, Grace Giles, Jon Sanks, Jeff Pfost, Daisy Pataki, Scott Herman, Michael Busby, Adrian Almanza, Keith Tsudama, Sean Loyd, Terrence Topits, Evan Price, Adam Wade, Katie Anderson, Cindy Broderick, Andy Russ, Summer Miller, Jennifer Van Pelt, Chris Burt, Nate Berner, Christine Orlowski, Shaun Burree, Elise Hale, Leah Levine-Goldberg, Mitchell Prante, Ryan Wopschall, Grant Yip, Duane DeVecchio, Harmony Tomsun, Amber Minami, Mike Hoshiyama, Jamie Henkle, Luis Busso, Michael Ryan, Stephanie Satoorian, and Ryan Weidert.

We are grateful to owners, managers, and staff of the Squaw Valley USA, Northstar at Tahoe, Alpine Ski Corporation, Sugar Bowl Ski Corporation, Royal Gorge, Donner Ski Ranch, and Boreal Ridge ski areas, as well as Fibreboard Corporation, Sierra-Pacific Corporation, the Chickering Reserve of the University of California, and the North Fork Association for permission to map on their respective properties. The University of California's Sagehen Creek Field Station provided logistical base support in 1987 and 1988, and the university's Cal Lodge in the years thereafter. The Truckee District of the U.S. Forest Service provided use permits, some maps, loan of some aerial photographs, and local access.

We express considerable appreciation to David Harwood (USGS) who provided preliminary copies of his geologic maps together with much advice and encouragement in the early stages of the mapping. Gary Raines and Peter Vikre, both of the US Geological Survey Reno Office, provided support and encouragement during the digitization of this work. We thank Vikre, Don Causey, Richard Hanson, an anonymous reviewer, and Chris Wills for their helpful reviews of early versions of the map and accompanying documents, and David Harwood and George Saucedo for their thorough reviews of a later version. We also thank Chris Henry, Nevada Bureau of Mines and Geology, for obtaining and providing 40Ar/39Ar ages of several volcanic rock units that are critical to establish the chronology of geologic events in the area. We are deeply grateful to Carlos Gutierrez for his skillful work to bring the map into conformity with California Geological Survey standards and into digital rigor.


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Description of Map Units

Alluvium and Related Deposits; Glacial Deposits

Qa Recent alluvium (Holocene) – Unconsolidated, moderately- to poorly-sorted silt, sand, gravel, and boulders in active stream and river bottoms.

Qc Colluvium (Holocene) – Unsorted, poorly consolidated, granitic colluvium, decomposed granite, soil, matrix-supported debris flow material, sand and cobble to boulder gravel.

Ql Lacustrine deposits (Holocene) – Thin-bedded clay, silt, and sand deposits.

Qsd Sand dunes (Holocene) – Unconsolidated wind-blown sand deposits.

Qoa Older alluvium (Holocene and Pleistocene) – Unconsolidated, moderately- to poorly- sorted sand, silt, gravel, and boulders in river terraces; locally may include glacial outwash deposits, debris flows and alluvial fans.

Qf Fan deposits (Holocene and Pleistocene) – Unconsolidated, moderately- to poorly-sorted silt, sand, gravel, and boulders, locally at the mouths of small canyons.

Qc+d Colluvium and glacial drift, undivided (Holocene and Pleistocene) – Unsorted, poorly consolidated granitic colluvium, decomposed granite, soil, matrix-supported debris flow material, sand and cobble to boulder gravel.

Qls Landslide (Holocene and Pleistocene) – Unsorted, angular, boulder- to clay-sized debris.

Qt Talus (Holocene and Pleistocene) – Accumulations ranging from coarse angular blocks to gravelly and sandy granitic and volcanic debris.

Qol Older lacustrine deposits (Pleistocene) – Moderately sorted, gravelly, coarse arkosic sand near Tahoe City.

Tioga glacial deposits (Holocene to Pleistocene) – Approximate age 14,000 – 26,500 years BP (Birkeland, 1964; Yount and La Pointe, 1997; Kaufmann et al., 2004; Howle, et al., 2005; 2012; Rood et al., 2007; Phillips et al., 2009; Schweickert et al., 2011).

Qti Till – Unconsolidated, unstratified, gray to light-tan, bouldery, polymict till characterized by large, generally unweathered, granitic boulders; weak soil development; preserved as sharp-crested moraines. Locally may contain small undifferentiated outwash deposits.

Qtio Outwash deposits – Unconsolidated boulder and cobble gravel, sand, and silt.

Qtip Ponded deposits – Poorly stratified silt and sand.

Tahoe glacial deposits (Pleistocene) – Tahoe maximum ~ 70,000 years BP (Howle, et al., 2005; 2012; Schweickert et al., 2011).

Qta Till – Unconsolidated, unstratified bouldery till with distinct yellow-brown weathered matrix and weak to moderate soil development; preserved as larger moraines with rounded and broad crests. Locally may include small, undifferentiated outwash deposits.

Geologic map of the north Lake Tahoe–Donner Pass region, northern Sierra Nevada, California

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Qtao Outwash deposits – Unconsolidated, polymict boulder to cobble gravel, sand, and silt; distinguished from Tioga outwash deposits by higher terrace position and greater degree and depth of soil development. May include jökulhlaup deposits in Truckee area (Birkeland, 1964; 1968; Burt et al., 2007).

Donner Lake glacial deposits (Pleistocene) – Approximate age greater than 130,000 years BP (Birkeland, 1964; Yount and LaPointe, 1997).

Qd Drift – Deeply weathered bouldery deposits, generally without morainal form; surface granitic boulders are weathered with stained, pitted and knobby surfaces; granitic boulders within deposit are decomposed.

Qdo Outwash deposits – Poorly sorted boulder and cobble gravel, sand, and silt. Locally may include jökulhlaup deposits in Truckee area (Birkeland, 1964; 1968; Burt et al., 2007).

Late Pliocene to Quaternary Rocks

Truckee River Formation (late Pliocene and Pleistocene, 2.8 – 1.0 Ma) – Predominantly olivine basalt to trachyandesitic lava flows, lava fields, cinder cones, and associated volcaniclastic and sedimentary deposits. Derived largely from central vents defined by associated tephra. Includes Lousetown Formation of Birkeland (1963).

QTcc Cinder cone deposits – Unconsolidated to partly amalgamated basaltic cinders, ash, and blocks.

QTttt Trachyandesite of Tahoe City – Trachyandesite lava flows northwest of Tahoe City erupted from cinder cone vent in Sec. 34, T16N, R16E. 40Ar/39Ar age of 0.92 Ma on lava flow two kilometers northwest of Tahoe City (Kortemeier et al., 2009).

QTttb Tahoe City basalt (Tahoe City olivine latite of Birkeland, 1963) – Dark gray olivine basalt flows, basalt and palagonitic basaltic flows with local pillow structures at the base; underlies Tahoe City and forms a bench along the north side of the Truckee River; erupted from unknown vents. Whole rock K/Ar age of 1.9 ± 0.1 Ma (Dalrymple, 1964, Sample KA1097, Table 1, recalculated to 2.0 ± 0.1 Ma according to Dalrymple, 1979).

QTtdc Trachybasaltic andesite flow of Deer Creek (Big Chief basalt of Birkeland, 1963) – Dark-gray, hypocrystalline basalt with sparse olivine phenocrysts, erupted from cinder cone vent in Sec. 14, T16N, R16E north of Deer Creek.

QTtlf Olivine basalt of Lake Forest – Dark-gray hypocrystalline basalt with sparse olivine phenocrysts, erupted from cinder cone vent in sec. 30, T15N, R17E, three kilometers northwest of Lake Forest.

QTtjf Juniper Flat alluvium (Birkeland, 1963) – Interfingering lenses of andesitic fluvial silt, sand, and gravel.

QTtpc Prosser Creek alluvium (Birkeland, 1963) – Indurated polymict, boulder to pebble gravel, sand, and silt; partly alluvial and partly lacustrine. Deposited when Truckee River drainage was blocked by Hirschdale olivine basalt lava flows.


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QTth Hirschdale basalt (Birkeland, 1963) – Dark-gray to dark-green porphyritic olivine basalt lava flows with pillow structures near base; erupted from cinder cone west of Juniper Flat, Martis Peak Quadrangle. Whole rock K/Ar age of 1.3 ± 0.1 Ma (Map # 102; Sample KA1094; Table 1).

Dry Lake lava domes and flows (Birkeland, 1963; Latham, 1985) – At least four hornblende- or pyroxene-bearing trachyandesite and dacite lava flows derived from domes south and southeast of Dry Lake in Martis Peak Quadrangle:

QTtdl4 Unit 4 (youngest)

QTtdl3 Unit 3

QTtdl2 Unit 2 – Whole rock 40Ar/39Ar age of 1.37 ± 0.04 Ma; isochron 40Ar/39Ar age of 1.35 ± 0.14 Ma (Map # 134; Sample WW34; Table 2).

QTtdl1 Unit 1 (oldest)

QTtbm Bald Mountain basalt (Birkeland, 1963) – Dark-gray olivine basalt erupted from cinder cone vents in Sections 27 and 34, T17N, R16E. Other flows are trachybasalt and shoshonite (K20 > 3.0 wt percent). Whole rock K/Ar age of 1.2 ± 0.1 Ma (recalculated from Dalrymple, 1964; Map # 101; Sample KA1078; Table 1). Isochron 40Ar/39Ar age of 1.41 ± 0.07 Ma (Map #s 128, 129, and 130; Samples 97-LT-65, 97-LT-75, and 97-LT-62B; Table 2).

QTtob Olivine basalt – Microporphyritic olivine basalt lava flows of unknown age and affinity.

QTtp Polaris basalt (Birkeland, 1963) – Olivine basalt from cinder cone vent two kilometers east of Truckee. Overlain by Prosser Creek alluvium. Whole rock K/Ar age of 1.68 ± 0.05 Ma (recalculated from Doell et al., 1966; Map # 103; Sample S19; Table 1).

QTtpm Page Meadow basalt – Olivine basalt flows and dikes along south side of Truckee River Canyon and underlying Page Meadows; also dikes in mouth of Ward Creek near Sunnyside. Possible vent in Sec. 11, T16N, R16E beneath moraine.

QTtbc Olivine basalt of Burton Creek – Flows of olivine basalt lava north of Tahoe City.

QTtca Cabin Creek alluvium (Birkeland, 1963) – Weathered andesitic cobble gravel and interstitial andesitic alluvium, fluvial sand and gravel in Truckee Quadrangle; may represent old Truckee River channel deposits overlain by the Bald Mountain lava flow.

QTtah Alder Hill basalt (Birkeland, 1963) – Three olivine basalt flows erupted from Alder Hill (and other unknown vents?) in Truckee Quadrangle. Whole rock K/Ar age of 2.4 ± 0.1 Ma (recalculated from Dalrymple, 1964; Map # 106; Sample KA1102; Table 1).

QTtab Trachyandesite flows of Agate Bay – Dark-gray olivine trachyandesite lava flows at the west end of Agate Bay.

QTttr Volcaniclastic rocks of Skylandia – Basaltic ash, cinders, and angular blocks in yellowish, palagonitic matrix that comprise a small littoral cone remnant along the shoreline at Lake Forest. Plateau 40Ar/39Ar age of 2.2 ± 0.26; Map # 131; Sample 04-LT-59; Table 2.


30

QTtcf Trachyandesite of Cedar Flat (Watson Creek basalt of Dalrymple, 1964) – Trachyandesite lava flows north and west of Cedar Flat; erupted from cinder cone vents at northwest ends of flows. Whole rock K/Ar age of 2.2 ± 0.10 Ma (Dalrymple, 1964; Map # 107; Sample S20; Table 1; recalculated to 2.6 ± 0.07 Ma by Doell et al., 1966).

QTtap Trachyandesitic plug – Center of Sec. 27, T16N, R17E.

QTtfc Fir Crags gravel (Birkeland, 1963) – Weathered andesitic gravel covered by Page Meadow lava flows along south side of Truckee River Canyon; may be old fluvial alluvium of ancestral Truckee River.

Late Oligocene, Miocene, and Pliocene Volcanic Rocks

Mehrten Formation (Miocene and Pliocene)

Squaw Peak member (late Miocene and Pliocene) – Light- to dark-gray, fine-grained porphyritic, massive to locally flow-banded basaltic to andesite lava flows and domes, plugs, and dikes, and andesitic volcaniclastic deposits. Includes discontinuous flows and isolated remnants of larger flows deposited in channels in older Tertiary volcanic deposits. Presumed to be 3-5 Ma and related to volcanic centers around and including Squaw Peak and Mt. Pluto, as well as the biotite-hornblende andesitic domes of Mt. Watson and Lookout Mountain.

Tsb Basalt flows – Undivided olivine basalt flows of unknown age and affinity.

Tsba Basaltic andesite flows – Undivided basaltic andesite flows of unknown age and affinity with phenocrysts of olivine and augite.

Tsd Diabase intrusion – Dark-gray, hypocrystalline, olivine-bearing diabase.

Tsi Andesitic intrusions and plugs – Dark-gray and black, aphyric and plagiophyric andesite with plagioclase and hornblende phenocrysts.

Tsbha Biotite-hornblende andesite lava domes and flows – Distinctive coarse-grained, porphyritic biotite-hornblende andesite lava-domes and lava flows with phenocrysts or xenocrysts of hornblende, hypersthene, biotite, quartz, olivine, and plagioclase. Plagioclase 40Ar/39Ar age of 3.75 ± 0.25 Ma; biotite 40Ar/39Ar plateau age of 3.41 ± 0.02 Ma and isochron age of 3.40 ± 0.06 Ma (Map #s135 and 136; Samples AGS90-72 and AGS90-71; Table 2) from rocks on Lookout Mountain.

Tsbhai Biotite-hornblende andesite intrusions – Distinctive coarse-grained, porphyritic biotite-hornblende andesite intrusions with phenocrysts or xenocrysts of hornblende, hypersthene, biotite, quartz, and plagioclase.

Tsblt Block and lapilli tuff of Mt. Disney (Fosdick et al., 2004) – Polymict, red-brown, matrix-supported megabreccia with blocks up to several tens of meters. Clasts are diverse in size and type and include: resistant, dark red-brown, monomict plagioclase andesite breccia blocks up to 20 m in diameter with ashy andesitic matrix; basaltic andesite lava boulders and blocks 1 to 3 m in diameter; stratified, variably-oriented, interbedded sandstone and polymict conglomerate of fluvial origin; rare hornfelsic metamorphic rocks; granitic clasts, 0.1 to 1 m in diameter, especially near the base of the breccia; reworked white, buff, grayish-pink, monomict, dacitic tuff blocks, 1 to10 m in size; hydrothermally altered welded rhyolite tuff. The polylithic matrix


31

is distinctive yellow-brown to red-brown, poorly sorted sand- and pebble-sized, consisting primarily of andesitic detritus and 1 cm-long pumice fragments. Underlain by basalt lava flow in Roller Pass (Tlb) with plateau 40Ar/39Ar age of 5.25 ± 0.18 Ma (Map # 133; Sample 4G13-1; Table 2) and overlain by basalt lava flow (Tsb) at Horseshoe Bend in Coldstream Valley with plateau 40Ar/39Ar age of 4.24 ± 0.16 Ma (Map # 132; Sample 4G25-1; Table 2).

Tss Stratified volcaniclastic deposits – Andesitic tuff, lapilli tuff, and tuff breccia; moderately well sorted. May represent fluvially reworked deposits.

Tsp Volcaniclastic deposits – Andesitic tuff, lapilli tuff, and tuff breccia, mainly unsorted and unstratified; generally poorly exposed.

Tspa Pyroxene andesite – Gray to dark gray, porphyritic andesite lava flow remnants with small blocky phenocrysts of augite.

Tsha Hornblende andesite – Tan to gray, porphyritic, andesitic lava flow remnants with acicular phenocrysts of hornblende.

Martis Peak Member (Miocene) – Predominantly light- to dark-gray andesitic flows, volcaniclastic deposits, and hypabyssal rocks, and reworked sedimentary and debris avalanche deposits, and lesser basaltic lava flows and intrusions all presumed to be 5-7 Ma and evidently derived from discrete or cryptic volcanic centers in the vicinity of Martis Peak.

Tma Andesite – Remnants of medium-gray, aphyric lava flows, microporphyritic lava flows, and plagiophyric lava flows.

Tmi Andesitic intrusions, plugs, dikes – Dark-gray and black, porphyritic andesite with plagioclase and hornblende phenocrysts.

Tmp Andesitic volcaniclastic deposits – Andesitic tuff, lapilli tuff, tuff breccia, mostly unsorted and unstratified, generally poorly exposed. Hornblende 40Ar/39Ar age of hornblende andesite block 6.38 ± 0.06 Ma, isochron 40Ar/39Ar age of 6.35 ± 0.16 Ma (Map # 138; Sample WW67B; Table 2).

Tmba Basaltic andesite flows – Dark-gray lava flow remnants with olivine and pyroxene microphenocrysts.

Tmpa Pyroxene andesite flows – Dark-gray andesitic lava flow remnants with small hypersthene and blocky augite phenocrysts.

Tmha Hornblende andesite flows – Gray andesitic lava flow remnants with subhedral hornblende phenocrysts.

Tmhai Hornblende andesite intrusion – Gray, porphyritic andesite intrusion, locally hydrothermally altered to argillic and propylitic assemblages, especially on the north flank of Martis Peak. Hornblende 40Ar/39Ar age of 6.32 ± 0.04 Ma and isochron 40Ar/39Ar age of 6.59 ± 0.08 Ma (Map # 137; Sample WW30; Table 2).

Mt. Lincoln Member (Miocene) – Predominantly andesitic flows, volcaniclastic deposits, and hypabyssal rocks, reworked sedimentary rocks and debris avalanche deposits, and lesser basaltic lava flows and intrusions; presumed to be 7-13 Ma and related to local cryptic volcanic centers in the vicinity of, but not on, Mt Lincoln.


32

Tlb Basalt – Lava flows, undivided, consisting of olivine basalt with olivine and plagioclase phenocrysts. Basalt lava flow in Roller Pass (Tlb) has plateau 40Ar/39Ar age of 5.25 ± 0.18 Ma (Map # 133; Sample 4G13-1; Table 2). Lava flow atop Boreal Ridge has whole rock K/Ar age of 7.6 ± 0.2 Ma (recalculated from Dalrymple, 1964; Map # 115; Sample KA1109; Table 1).

Tlbi Basaltic intrusions – Plugs and dikes consisting of olivine basalt with olivine and plagioclase phenocrysts.

Tlbc Basaltic tephra – Red-brown deposits of ash, cinders, and blocks, locally indurated.

Tlf Fluvial strata, undivided – Reworked volcaniclastic deposits, including tuffaceous sandstone, conglomerate, and minor lacustrine deposits including coal, claystone, and sandstone. Railroad cuts east of Eder expose 5 m of pale-gray claystone, 1 m of fine-grained, finely laminated, well-sorted white sandstone and polymict conglomerate, and a 2 m thick, low-grade coal bed. Railroad cuts west of Eder contain mostly fluvially reworked volcaniclastic strata consisting of interbedded, medium- to coarse-grained, well-sorted sandstone and gravel- to cobble-sized, moderately well-sorted, clast-supported polymict conglomerate and breccia with a sandy tuffaceous matrix. The beds are discontinuous laterally and have gradational to sharp contacts with underlying and overlying units.

Tlf3 Unit 3 (youngest) – Gray interbedded sandstone and conglomerate. Well-sorted, well-graded sandstone. Conglomerate clasts are aphyric andesite, light and dark gray hornblende andesite, and andesitic tuff. Tlf3 is 12 m thick on west face of Mt. Lincoln.

Tlf2 Unit 2 – Alternating pebble and conglomerate beds interlayered with less common sandstone beds. Contacts between pebble and cobble beds are gradational; sharp contacts are between sandstone and conglomerate. The matrix is gray volcanic ash and feldspathic sand. Clasts are rounded hornblende andesite, aphanitic andesite, scoriaceous andesite, cinders, granite, and reworked tuff. Flame structures and imbricated pebbles indicate deposition by a northwest-directed paleocurrent. Tlf2 varies from 9 m thick on Mt. Lincoln to 22 m on Mt. Disney.

Tlf1 Unit 1 (oldest) – Interstratified conglomerate, mudstone, and tuffaceous sandstone, tan to buff; well-stratified, normally graded, moderately to well-sorted, volcaniclastic sedimentary rock comprising rounded to sub-rounded clasts of granite, reworked tuff, light-gray hornblende andesite, and dark-gray aphanitic andesite. Matrix is feldspathic volcanic ash and coarse feldspathic sand. Beds are winnowed with local low-angle cross beds. Flame structures and imbricated pebbles indicate a southward paleocurrent direction. Tlf1 varies from 8 m thick on Mt. Lincoln to 11 m on Mt. Disney.

Tla Andesite lava flows, undivided – Typically gray to dark-gray, porphyritic andesite with plagioclase and hornblende phenocrysts.

Tlaa Aphyric andesite lava flows – Gray, fine-grained andesite lacking phenocrysts.

Tlha Hornblende andesite lava flows – Gray to red-brown lava flows and flow remnants with hornblende and plagioclase phenocrysts.

Tlhai Hornblende andesite intrusion – Red-brown to lavender andesitic intrusion with white plagioclase phenocrysts. Comprises large hypabyssal masses well exposed in interstate roadcuts above Donner Lake.


33

Tlpa Pyroxene andesite flows – Andesitic lava flows and flow remnants containing augite, hypersthene, and plagioclase phenocrysts.

Tlba Basaltic andesite flows – Dark-gray and black lava flow remnants with olivine and augite, and plagioclase phenocrysts.

Tld Diabase intrusion – Dark-gray and black, fine-grained diabase, age and affinity unknown. Locally mingled with granodiorite (Ks) in sec. 5, T17N, R15E.

Tlp Volcaniclastic rocks, undivided – Locally amalgamated gray andesitic tuff breccia, lapilli tuff, block and lapilli tuff; no apparent stratification; generally poorly exposed. The volcaniclastic rocks are gray to dark-gray, ashy, matrix-supported, poorly sorted, unstratified lapilli tuff, block and lapilli tuff, and breccia. Typically the member comprises 40 percent clasts and 60 percent matrix. The matrix is dominantly andesitic, sandy, and well cemented. Clasts are generally angular to subangular plagioclase- or hornblende-plagioclase andesite and range from 0.02 m to 1 m in diameter; common large is 0.2 m. Clast size, angularity, and abundance increase eastward across the map area (Hudson, 1951, p. 940). Much of the member has the internal fabric of mudflows and lahars, both hot and cold. Some units consist of amalgamated subunits that locally intruded the overlying unit as clastic dikes. Andesitic blocks near the base of the tuff breccia on the west flank of Castle Peak have whole rock K/Ar ages of 13.5 ± 0.4 Ma and 12.9 ± 0.4 Ma (Map #s 117 and 116; Samples 1 9(CP-4) and 2 9(CP-4); Table 1).

Tllt Lapilli tuff – Andesitic lapilli tuff locally interlayered with fluvial deposits, generally poorly exposed.

Valley Springs Formation (late Oligocene and early Miocene)

Tvrt Rhyolite tuff, undivided – Pale-yellow, white, pale-pink, and buff weathering rhyolite tuff, locally vitrophyric, locally welded, locally pumiceous.

Tvrt7 Nine Hill Tuff (Bingler, 1978; Deino, 1985, 1989) – Rhyolite tuff consists of pink to purple, unwelded to welded, pumice-rich, crystal-poor vitrophyre. Contains 2-3 percent sanidine, 2-3 percent quartz, about 1 percent biotite phenocrysts, and trace plagioclase, and vapor phase-altered pumice flattened into disk-shaped fragments from 0.1 to 5 cm long. Vapor phase-altered, pumice cavities are filled with zeolite minerals. Fiamme are common in the vitric top of the exposed section. Basal layer is a non-vitrophyric ash fall unit that grades upward into a densely welded gray vitrophyre. Entire unit contains several distinct zones of welding, indicating that it is a compound unit of several separate depositional events. Prominent cliff-forming unit whose thickness varies from 5 m to over 100 m. Sanidine 40Ar/39Ar age of 25.18 ± 0.06 Ma in Onion Creek (Sample H99-22; Table 2). Nine Hill Tuff is laterally extensive throughout northern California and Nevada (Deino, 1985, 1989) and is considered to have erupted and flowed west from volcanic centers in the Carson Sink of Nevada (Faulds et al., 2005).

Tvrt6 Rhyolite tuff – Pink to white (weathers tan/brown), unwelded to moderately welded, pumice-rich, crystal-poor tuff containing 7 percent sanidine, 1-2 percent vermicular quartz, 1-2 percent biotite, and gray pumice fragments ranging from < 5 mm to lapilli-sized lithophysae. Locally contains sparse granitic clasts of unknown affiliation, ranging from 5 to 75 cm in diameter at its base in railroad cut exposures between Donner Pass and Norden.


34

Pumice affected by vapor phase alteration. Tuff is crystal-rich near the top of the unit. Thickness ranges from 20 to 55 m.

Tvrt5 Mickey Pass Tuff (Proffett and Proffett, 1976; 1984) – Rhyolite tuff: White, crystal-rich, unwelded, vitric tuff that weathers tan/brown; distinguished by “popcorn” weathering. Contains 5-10 percent rose smoky quartz, 3 percent sanidine, trace biotite, and small pumice fragments ranging from 5 to 50 mm. Thickness ranges from 30 to 80 m. Sanidine 40Ar/39Ar age of 26.98 ± 0.07 Ma in the Onion Creek area (Sample H99-31; Table 2). Erupted from Mt. Jefferson in the Toquima Range of Nevada (Faulds et al., 2005).

Tvrt4 Tuff of Campbell Creek (Henry et al., 2004) – Rhyolite tuff: Pink to white, coarse ash and crystal-rich vitrophyre that grades downward from welded to densely welded. Contains 10 percent sanidine, 7 percent vermicular quartz, 1 percent biotite, and trace dark-gray to greenish-gray lithic fragments. Glass shards and fiamme are deformed around grains. Vitrophyric base. Its thickness ranges from 40 m to 65 m in the Onion Creek section. 40Ar/39Ar age of 28.79 ± 0.08 Ma (Faulds et al., 2005). Believed to have erupted and flowed south from volcanic centers in the Desatoya Mountains of Nevada (Henry et al., 2004).

Tvrt3 Tuff E (Henry et al., 2004) – Crystal-rich to crystal-poor tuff that grades downward from unwelded to densely welded. The upper section is crystal-rich with 10 percent quartz, 3 percent sanidine, 1-3 percent biotite, and small pumice fragments with vapor-phase alteration. The lower section is crystal-poor with 2 percent quartz, 1-2 percent biotite, 1 percent sanidine, small gray pumice fragments, and small glass shards. Compound cooling unit with several zones of welding render this tuff variably resistant to weathering and erosion. It crops out only on the west side of Onion Valley. Thickness ranges from 65 to 100 m. May be equivalent to tuff E (Brooks et al., 2003). Sanidine 40Ar/39Ar age of 29.02 ± 0.08 Ma (Map # 141; Sample H99-29; Table 2).

Tvrt2 Rhyolite tuff – White, moderately crystal-rich, unwelded tuff, containing 5 percent sanidine, 3 percent biotite, 1 percent quartz, small pumice fragments, and sparse lithic fragments. Crops out only on the west side of Onion Valley. Thickness ranges from 25 to 45 m.

Tvrt1 Tuff of Sutcliffe (Henry et al., 2004) – Rhyolite tuff characterized by high content of biotite. Light-gray to light-tan, crystal-rich, biotite-rich welded tuff with 10 percent biotite, 5 percent sanidine, trace quartz, and very small pumice fragments with vapor phase alteration. Thickness ranges from 9 to 70 m. Sanidine 40Ar/39Ar age of 30.48 ± 0.08 Ma (Map # 142; Sample H99-27; Table 2). May have erupted from Job Canyon in the Stillwater Range, Nevada (Faulds et al., 2005).

Tvrt0 Tuff of Rattlesnake Canyon (Faulds et al., 2005) – Rhyolite tuff characterized by large sanidine phenocrysts. Rosy pink, crystal-rich, moderately-welded with 10-15 percent euhedral sanidine crystals that range in size from 1 to 4 mm. Contains trace amounts of quartz, and small, dark-gray, plagioclase andesite lithic fragments from 5 to 10 mm in diameter, and dark-gray basaltic andesite lithic fragments about 3 mm in diameter. Lacks pumice fragments. Thickness ranges from 13 to 80 m. Informally called Big Sanidine Tuff. 40Ar/39Ar age of 31.01 ± 0.09 Ma (Faulds et al., 2005).


35

Cretaceous Intrusive Rocks

Ks Hornblende-biotite granodiorite of Summit Lake – Light-gray to light-tan, holocrystalline, hornblende-biotite granodiorite. Contains 43-60 percent plagioclase, 17-37 percent quartz, 5-23 percent orthoclase, 2-15 percent microcline, and less than 20 percent biotite and hornblende (Kulow, 1996). It has less than one percent light-brown titanite. The megacrystic facies, which crops out in a three square kilometer area adjacent to the east end of Boreal Ridge and has gradational contacts with the main body of granodiorite, contains about 20 percent K-feldspar megacrysts up to 2 cm-long. Zircon 207Pb/206Pb age of 117 ± 7 Ma (Kulow, 1996; Map # 126; Sample MK158; Table 1); Rb/Sr ages of 118 and 103 Ma (John et al., 1994).

Ksp Porphyritic hornblende-biotite granodiorite of Summit Lake – Light-gray to light-tan, holocrystalline, hornblende-biotite granodiorite, contains K-feldspar megacrysts up to 20 mm.

Klm Tonalite of Lake Mary – Dark-blue-gray, holocrystalline tonalite. Rb/Sr whole-rock isochron ages of 117.5 ± 4.2 Ma and 102.7 ± 3.7 Ma (John et al., 1994); K/Ar ages between 94 and 110 Ma (Table 1; Evernden and Kistler, 1970); zircon 207Pb/206Pb age of 120 Ma (Kulow, 1996; Map # 127; Sample MK90; Table 1).

Kg Granodiorite

hbg Hornblende-biotite granodiorite, undivided

qm Quartz monzonite, undivided

Paleozoic and Middle Mesozoic Rocks

Lake Tahoe Sequence of Harwood (1992)

Jle Ellis Peak Formation – Alternating beds of medium- to fine-grained quartz arenite and dark-gray quartzitic metasiltstone.

Jlb Blackwood Creek Formation – Black sulfidic argillite and pelitic hornfels interbedded with light-gray sandy limestone and feldspathic quartzite in variable proportions.

Mlo Onion Creek Formation – Light-blue-gray marble and calc-silicate hornfels.

Mls Serena Creek Formation – Biotite schist and light-gray to tan metachert with thin partings of black siliceous argillite.

Metamorphic Rocks

MDpv Picayune Valley Formation – Turbiditic sequence of chert- and quartz-rich conglomerate, quartzose sandstone, and black cordierite-andalusite hornfels.

gn Gneiss of unknown age and affinity – Consists of light-gray quartzo-feldspathic and dark-gray biotite-rich bands; may be relict-flow banded metavolcanic rock. Single locality on ridge crest two kilometers north of Brockway Summit.

Tabl

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3362.021- 7691.93

pma

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901 0542.021-

7641.93 kaeP dra

W fo S m 001

011 7172.021-

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wauqS fo W83

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111 0562.021-

7181.93 ruotnoc '0068 no kP

wauq S fo E46N

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kaeP wauqS fo E42

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5233.021- 6813.93

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0513.021- 3813.93

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mk 1 ,04 yw

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0513.021- 3813.93

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ekaL rennoD fo tse

w m 004 ,04 y

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36

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1 02.0

6.7 r

AK

esalcoigalp aval tlasab

blT 511 11

6 Tl

lt an

desi

te b

lock

and

ash

flow

ho

rnbl

ende

K

Ar

12.9

0.

40

1 6

117

Tllt

ande

site

blo

ck a

nd a

sh fl

ow

plag

iocl

ase

KA

r 13

.5

0.4

1 6

118

Tla

ande

site

lava

pl

agio

clas

e K

Ar

26.4

0.

8 1

6 1 1

5.0 72

rA

K enidinas

ffut etiloyhr

7trvT 911

1 1

7.0 43

rA

K enidinas

ffut etiloyhr

7trvT 021

3 1

1.0 8.49

rA

K esalcoigalp

etilanot

mlK

121 3

1 4.0

1.99 r

AK

etitoib etilanot

ml

K 221

3 1

5.0 2.001

rA

K ednelbnroh

etilanot

mlK

321 3

1 1.0

6.201 r

AK

esalcohtro etilanot

ml

K 421

3 an

nevig ton 1.211

rA

K ztrauq

etilanot

mlK

521 5

2 7

711 bP/

U nocriz

etiroidonarg s

K 621

5 2

3.3 021

bP/U

nocriz etilanot

ml

K 721

R

efer

ence

s:

1. D

alry

mpl

e, G

.B.,

1964

. 2.

Doe

ll, R

.R.,

Dal

rym

ple,

G.B

., an

d C

ox, A

llen,

196

6.

3. E

vern

den,

J.F.

, and

Kis

tler,

R.W

., 19

70.

4. H

arw

ood,

D.S

., un

publ

ishe

d da

ta, i

n Sa

uced

o, G

.J., a

nd W

agne

r, D

.L.,

1992

. 5.

Kul

ow, M

.J., 1

996.

6.

Mor

ton,

J.L.

, and

oth

ers,

1977

.


37

Tabl

e 2.

40

Ar/

39A

r ag

es o

f vo

lcan

ic r

ocks

, Tah

oe –

Don

ner

Pas

s R

egio

n, C

alifo

rnia

Map

#

edutignoL edutitaL

noitacoL

5581.021-

0082.93 htuos niatnuo

M dlaB

821 9902.021-

1782.93 tse

w niatnuoM dla

B 921

6881.021- 9313.93

htron niatnuoM dla

B 031

3011.021- 3381.93

eohaT ekaL ,hcaeB aidnalykS

131 2572.021-

6392.93 yella

V maertsdlo

C 231

2223.021- 0192.93

ssaP relloR

331 134

0550.021- 7603.93

aera ekaL yrD

1341.021- 7462.93

natnuoM tuokooL

531 1341.021-

7462.93 niatnuo

M tuokooL 631

7920.021- 9692.93

kaeP sitraM fo E°01

N mk 6.0

731 4100.021-

8772.93 kaeP sitra

M fo E°06S mk 2.3

831 8403.021-

0043.93 ekaL renno

D fo htron 931

7213.021- 2323.93

ssaP rennoD fo tsae

041

Map

#

edutignoL edutitaL

ecruoS aredlaC

na 5

76183.021- 38292.93

?kniS nosraC

na

5 71573.021-

76672.93 egna

R amiuqoT ,nosreffeJ t

M

33173.021- 05762.93

nwonknu

141 07763.021-

04172.93 ?egna

R retawllitS ,noyna

C boJ 241

1. A

ll sa

mpl

es e

xcep

t WW

-34,

AG

S90-

, WW

30, a

nd W

W67

B w

ere

anal

yzed

at t

he N

ew M

exic

o G

eoch

rono

logi

cal R

esea

rch

Labo

rato

ry (m

etho

ds in

McI

ntos

h et

al.,

200

3). N

eutro

n flu

x m

onito

r Fis

h C

anyo

n Tu

ff sa

nidi

ne (F

C-1

). A

ssig

ned

age

= 28

.02

Ma

(Ren

ne e

t al.,

199

8). W

eigh

ted

mea

n 40

Ar/39

Ar a

ges c

alcu

late

d by

the

met

hod

of S

amso

n an

d A

lexa

nder

(198

7). O

ther

sam

ples

wer

e an

alyz

ed

by P

hilli

p G

ans a

t the

Uni

vers

ity o

f Cal

iforn

ia S

anta

Bar

bara

. Dec

ay c

onst

ants

and

isot

opic

abu

ndan

ces a

fter S

teig

er a

nd Jä

ger (

1977

); l b

= 4.

963

x 10

-10 y

r-1; l

e+e’ =

0.5

81 x

10-1

0 yr-1

; 40K

/K =

1.1

67 x

10

-4

2. A

ges i

n bo

ld a

re b

est e

stim

ates

of e

mpl

acem

ent a

ge. B

oth

plat

eau

and

isoc

hron

age

s are

val

id fo

r thr

ee sa

mpl

es.

3. %

39A

r = p

erce

ntag

e of

39A

r use

d to

def

ine

plat

eau

age.

4.

n =

num

ber o

f sin

gle

grai

ns a

naly

zed;

%39

Ar =

% o

f tot

al 39

Ar i

n pl

atea

u. M

iner

als w

ere

sepa

rate

d fr

om c

rush

ed, s

ieve

d sa

mpl

es b

y st

anda

rd m

agne

tic a

nd d

ensi

ty te

chni

ques

; san

idin

es w

ere

leac

hed

with

dilu

te H

F to

rem

ove

mat

rix a

nd h

andp

icke

d.

5. S

ampl

e lo

cate

d ou

tsid

e m

ap e

xten

t.


38

Sam

ple

1

56-TL-79 57-TL-79

B26-TL-79 95-TL-40

1-52G4

1-31G4 WW

34

27-09SG

A 17-09S

GA

03W

W

B76W

W 311-00

H 36-00

H Sam

ple 1

22-99H

13-99H

92-99H

72-99H

tinU pa

M

mbtT

Q

mbtTQ

mbtT

Q rttT

Q bsT blT Q

Ttdl

2 ahbsT ahbsT iah

mT p

mT iahlT

iblT

tinU pa

M

7trvT

5trvT

3trvT

1trvT

Tabl

e 2

(con

tinu

ed).

40A

r/39

Ar

ages

of

volc

anic

roc

ks, T

ahoe

– D

onne

r P

ass

Reg

ion,

Cal

iforn

ia

Map

#

Map

Uni

t R

ock

Type

M

ater

ial

Age

s (M

a) 2

St

ep h

eatin

g Pl

atea

u ±2

s %

39A

r 3 St

eps

Isoc

hron

±2

s 40

Ar/36

Ar i

±2s

MSW

D

Tota

l Gas

±2

s 12

8 Q

Ttbm

B

asal

t lav

a m

atrix

1.

23

0.05

82

.1

5/9

1.17

0.

11

299

7 2.

6 1.

24

0.08

12

9 Q

Ttbm

B

asal

t lav

a m

atrix

1.

44

0.03

67

.4

5/10

1.

41

0.07

29

7 8

0.84

1.

53

0.05

13

0 Q

Ttbm

B

asal

t lav

a m

atrix

1.

58

0.04

10

0 9/

9 1.

54

0.07

30

3 11

3.

7 1.

60

0.04

13

1 Q

Tttr

Bas

altic

and

esite

bo

mb

mat

rix

2.20

0.

26

76.8

4/

10

2.5

0.7

260

80

3.8

2.33

0.

65

132

Tsb

Bas

alt l

ava

mat

rix

4.24

0.

16

96.8

8/

11

4.3

1.2

290

70

3.1

4.30

1.

6 13

3 Tl

b B

asal

t lav

a m

atrix

5.

25

0.18

86

8/

12

5.4

0.2

281

12

9.3

5.06

0.

12

134

QTt

dl2

And

esite

lava

m

atrix

1.

34

0.03

1.

35

0.14

135

Tsbh

a B

iotit

e-ho

rnbl

ende

an

desi

te

biot

ite

3.41

0.

01

3.40

0.

06

136

Tsbh

a B

iotit

e-ho

rnbl

ende

an

desi

te

plag

iocl

ase

3.

75 0.

01

na

137

Tmha

i H

ornb

lend

e an

desi

te

dike

ho

rnbl

ende

6.

32

0.04

6.

59

0.08

138

Tmp

Hor

nble

nde

ande

site

bl

ock

in p

yroc

last

ic

depo

sit

horn

blen

de

6.38

0.

06

6.35

0.

16

139

Tlha

i H

ornb

lend

e an

desi

te

plug

ho

rnbl

ende

6.

37

0.25

85

.8

4/10

N

C

6.39

0.

22

140

Tlbi

B

asal

tic d

ike

mat

rix

8.5

0.4

65.5

5/

9 6.

8 0.

4 30

2.6

1.4

5.7

9.7

3.7

Map

#

Map

Uni

t R

ock

Type

M

ater

ial

Age

(Ma)

±2

s K

/Ca

±1s

n 4

Rhy

olite

ash

-flo

w

tuff

s:

Sing

le

crys

tal

Wei

ghte

d M

ean

na 5

Tvrt 7

Nin

e H

ill T

uff

sani

dine

25

.18

0.06

12

.2

3.2

27

na 5

Tvrt 5

Mic

key

Pass

Tuf

f, G

uild

Min

e M

embe

r sa

nidi

ne

26.9

8 0.

07

68.3

6.

7 15

141

Tvrt 3

tuff

E

sani

dine

29

.02

0.08

72

.1

10.2

15

14

2 Tv

rt 1tu

ff o

f Sut

cliff

e sa

nidi

ne

30.4

8 0.

08

32.0

2.

4 14

R

efer

ence

s:

Dal

rym

ple,

G.B

., 19

64.

Doe

ll, R

.R.,

Dal

rym

ple,

G.B

., an

d C

ox, A

llen,

196

6.

Ever

nden

, J.F

., an

d K

istle

r, R

.W.,

1970

. H

arw

ood,

D.S

., un

publ

ishe

d da

ta, i

n Sa

uced

o, G

.J., a

nd W

agne

r, D

.L.,

1992

.


39


40

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