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172 Ma

121°0'0''38°30'0"

38°22'20"121°0'0''

38°22'20"120°52’30”

120°52’30”38°30'0"

STATE OF CALIFORNIA – EDMUND G. BROWN JR., GOVERNORTHE NATURAL RESOURCES AGENCY – JOHN LAIRD, SECRETARY FOR NATURAL RESOURCES

DEPARTMENT OF CONSERVATION – DAVID BUNN, DIRECTOR CALIFORNIA GEOLOGICAL SURVEYJOHN G. PARRISH, Ph.D., STATE GEOLOGIST

This geologic map was funded in part by the USGS National Cooperative Geologic MappingProgram, Statemap Award no. G15AS00006

PRELIMINARY GEOLOGIC MAP OF THE IRISH HILL 7.5' QUADRANGLE,AMADOR COUNTY, CALIFORNIA

VERSION 1.0By

Peter J. Holland

Digital Preparation by

Peter J. Holland and Carlos I. Gutierrez

2016

PRELIMINARY GEOLOGIC MAP OF THE IRISH HILL 7.5’ QUADRANGLE, CALIFORNIA

Copyright © 2016 by the California Department of ConservationCalifornia Geological Survey. All rights reserved. No part ofthis publication may be reproduced without written consent of theCalifornia Geological Survey.

"The Department of Conservation makes no warranties as to thesuitability of this product for any given purpose."

Coordinate System:Universal Transverse Mercator, Zone 10NNorth American Datum 1927.

Topographic base from U.S. Geological SurveyIrish Hill 7.5-minute Quadrangle, 1962. Shaded relief image derived from USGS 1/3 arc-second National Elevation Dataset (NED).

Professional Licenses and Certifications: P. Holland - PG No. 7994, CEG No. 2400

Preliminary Geologic Map available from:http://www.conservation.ca.gov/cgs/rghm/rgm/preliminary_geologic_maps.htm

13.6 °

MA

GN

ETI

C N

OR

TH

TRU

E N

OR

TH

Approximate MeanDeclination, 2016

0

0

0

1

1

1.5

.5 2

2

2Thousand Feet

Kilometers

Miles

Scale 1:24,000

Contour Interval 40 feetNational Geodetic Vertical Datum of 1929

Sa l t S p r i n gs Re s

Lake Amado r

Pardee Res

C o mm a n c h e R e sB e a r d s l e y L a k e

N e w H o g a n L a k e

Salt SpringValley Res.

N e w M e l o n e s R e s

Avery

BearValley

Columbia

Copperopolis

Jamestown

MokelumneHill

RailRoadFlat

AltavilleAngels Camp

Arnold

TwainHarte

ValleySprings

West Point

4

4

124

49

49

16

104

88

12

108

26

5Kilometers

5Miles

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104

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26

IONE

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MOKELUMNE

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ROAD

FLAT FORT

MOUNTAIN

DORRINGTON

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LIBERTY

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WALLACE

VALLEY

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COLUMBIA

COLUMBIA SE

TWAIN

HARTEHULL

CREEK

IRISH

HILL AMADOR

CITY PINE

GROVEWEST

POINT

DEVILS

NOSEGARNET

HILLCALA

VERAS

DOME

TAMARACK

Mapping completed under STATEMAPCURRENT YEAR FY 2015-16FY 2014-15

SELECTED REFERENCES

Bartow, J.A., and Marchand, D.E., 1979, Preliminary Geologic Map of Cenozoic Deposits of the Sutter Creek and Valley Springs Quadrangles, California: U.S. Geological Survey Open File Report 79-436, scale 1:62,500.

Behrman, P.G., 1978, Paleogeography and Structural Evolution of a Middle Mesozoic Volcanic Arc-Continental Margin, Sierra Nevada Foothills, California: University of California, Berkeley, PhD Dissertation, 301 p.

Clark, L.D., 1964, Stratigraphy and Structure of the Western Sierra Nevada Metamorphic Belt, California: U.S. Geological Survey Professional Paper 410, 70 p.

Douglass, R.C., 1967, Permian Tethyan fusulinids from California: U.S. Geological Survey Professional Paper 593-A, 13 p.

Duffield, W.A., and Sharp, R.V., 1975, Geology of the Sierra Foothills Melange and Adjacent Areas, Amador County, California: U.S. Geological Survey Professional Paper 827, 30 p.

Loyd, R.C., 1983, Mineral Land Classification of the Sutter Creek 15 Minute Quadrangle, Amador and Calaveras Counties, California: California Department of Conservation, Division of Mines and Geology Open-File Report 83-36, 42 p., scale 1:48,000.

Piper, A.M., Gale, H.S., Thomas, H.E., and Robinson, T.W., 1939, Geology and Ground-Water Hydrology of the Mokelumne Area, California: U.S. Geological Survey Water Supply Paper 780.

Saleeby, J.B., 1982, Polygenetic Ophiolite Belt of the California Sierra Nevada: Geochronological and Tectonostratigraphic Development: Journal of Geophysical Research, v. 87, p. 1803-1824.

Sketchley, H.R., 1965, Soil Survey of the Amador Area, California: U.S. Department of Agriculture Soil Conservation Service in cooperation with the California Agricultural Experiment Station, U.S. Government Printing Office, scale 1:20,000.

Taliaferro, N.L., 1950, Geologic map of the Sutter Creek quadrangle: University of California, Berkeley, unpublished map, scale 1:62,500.

Turner, H.W., 1894, Jackson, California: U.S. Geological Survey Geological Atlas, Folio 11, 6 p., scale 1:125,000.

25

35

MAP SYMBOLS

?

?

? Contact between map units - Solid where accurately located; short dash where inferred; long dash where approximately located; dotted where concealed; queried where identity or existence is uncertain

Fault - Solid where accurately located; long dash where approximately located; short dash where inferred; dotted where concealed; queried where identity or existence is uncertain.

Thrust Fault - Barbs on upper plate; solid where accurately located; short dash where inferred; dotted where concealed; queried where identity or existence is uncertain.

Strike and dip of sedimentary beds. Number indicates dip angle in degrees:

Inclined bedding

Vertical bedding

Strike and dip of inclined metamorphic foliation. Number indicates dip angle in degrees

Strike and dip of inclined joints. Number indicates dip angle in degrees.

Sample locality showing radiometric age date; may be shown with leader line

25

22.9 Ma

CORRELATION OF MAP UNITS

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SURFICIAL UNITS

Artificial fill (historic)—Consists of man-made deposits of earth materials that may be engineered or non-engineered.

Tailings (historic)—Consists of man-made deposits of earth materials that are the by-product of mining processes.

Landslide deposit (historic)—Consists of landslide mapped on slope of abandoned mine pit.

Stream channel deposits (modern to latest Holocene)—Fluvial deposits within active, stream channels, which consist of loose, unconsolidated silt, sand, gravel, and cobbles, with minor clay.

Alluvial deposits (Holocene to Pleistocene)—Alluvium deposited in fan, terrace, or basin environments. Typically consists of poorly to moderately sorted sand, silt and gravel with occasional cobbles.

Modesto Formation, undivided (early Holocene to late Pleistocene)—Alluvial deposits of arkosic gravel, sand, and silt. Deposited as a series of coalescing alluvial fans extending continuously from the Kern River drainage in the south, to the Sacramento River tributaries in the north. Compositionally, deposits of the Modesto Formation are often indistinguishable from those of the Riverbank Formation. Identity is usually determined by topographic position, geomorphic expression, or (most commonly) by soil development (Marchand and Allwardt, 1981).

Modesto Formation, upper unit—The upper unit has been differentiated from the lower unit (not present in map area) on the basis of topographic expression and position, and the presence of a weakly developed buried soil between the lower and upper units (Marchand and Allwardt, 1981).

Riverbank Formation, undivided (Pleistocene)—Consists primarily of arkosic sediment derived mainly from the interior of the Sierra Nevada but also includes locally derived sediment from small drainage basins along the foothills (Marchand and Allwardt, 1981). Marchand and Allwardt (1981) subdivided the Riverbank Formation into three informally designated units on the basis of superposition, paleosols and buried soils, and on geomorphic evidence. The upper unit occurs within the map area:

Riverbank Formation, upper unit—The upper unit has been differentiated from the middle and lower units (not present in map area) on the basis of topographic expression and position, and the presence of a weakly developed buried soil between the units (Marchand and Allwardt, 1981).

North Merced Gravel (Pleistocene to Pliocene)—Lag gravel deposits tentatively correlated by Bartow and Marchand (1979) to the North Merced Gravel of Arkley (1962). In the map area, the gravels typically consists of well-rounded gravel and cobbles in a reddish brown sandy matrix.

TERTIARY SEDIMENTARY AND VOLCANIC UNITS

Mehrten Formation (early Pliocene to Miocene)—Volcanic mudflow deposits interbedded with sandstone and conglomerate. Compositionally distinct – dominated by andesite clasts. Lahar beds laden with andesite cobbles are particularly resistant and often form a cap and corresponding cliff faces.

Valley Springs Formation (Miocene to middle Oligocene)—Tuffaceous sandstone, siltstone, and conglomerate interbedded with tuff and minor clay. Deposits are moderately mature compositionally which reflects the peritropical environment at the time of deposition. The formation may be distinguishable by rhyolitic ash component. Conglomerates are compositionally heterogeneous and feature significant proportions of Jurassic metamorphic rocks, quartzite, and chert cobbles. Tuff beds are often present as resistant, cliff-forming outcrops. Green clay rock, which consists of silica-cemented pebbles of expansive smecticic clay, is present near the base of unit and poses significant geologic hazards related to ground swelling and slope stability (California Geological Survey, 2009; Wood and Glasmann, 2013; Wood, 2015).

Ione Formation (middle Eocene)—Clay, sandstone, and siltstone. Distinctly light in color; dominated by white and light pastel shades of buff, rust and lavender on the weathered surface. Compositionally, this unit is very mature and is distinctly dominated by various forms of quartz, especially vein quartz. Clays are predominantly kaolinitic and are interpreted as having formed in a tropical climate (Wood, 1995). Sandstones are often silica-cemented. Conglomerates typically feature abundant white vein quartz and quartzite cobbles in a matrix of reddish oxidized silt and sand. Finer beds typically feature striking sedimentary structures such as cross-bedding and convolute laminae crossed by more recent subparallel and subvertical joints typically filled with iron or silica cement. Clays of the Ione Fm. are commercially valuable and have been actively mined for many decades. Early prospectors also suspected the Ione conglomerates of being gold-bearing and where exposed, they are often disturbed by historic exploratory strip mining.

JURASSIC TO PALEOZOIC UNITS

Copper Hill Volcanics (Late Jurassic)—Dark to medium green meta-andesite and metabasalt. Strongly sheared and crenulated near Bear Mountains Fault Zone. Minor porphyritic rhyolite or dacite with quartz phenocrysts. Named by Clark (1964) with the type section on the Cosumnes River north of the quadrangle. Interfingers with and overlies the Salt Spring Slate and is likely of Kimmeridgian age (Clark, 1964). Includes felsic facies (Jchf) that was originally mapped as quartz porphyry by Turner (1894) and Taliaferro (1943).

Salt Spring Slate (Late Jurassic)—Black Sericite slate is the dominant lithology but greywacke and tuff are widespread, and thin conglomerate layers occur locally (Clark, 1964). Prior to Clark (1964) naming the formation, these rocks were mapped as Mariposa Formation (Turner 1894; Taliaferro, 1943). Late Oxfordian to early Kimmeridgian in age based on invertebrate fossils collected along Consumnes River. Plesiosaur fossil reported from Salt Spring Slate at Lake McClure south of the Quadrangle (Clark, 1964).

Gopher Ridge Volcanics (Late Jurassic)—Named by Clark (1964) for exposures along the Calaveras River south of the quadrangle. Consists mostly of massive mafic and intermediate tuff, breccia, and occasional greywacke and agglomerate. Likely Oxfordian in age (Clark, 1964). Includes felsic facies (Jgof) that was originally mapped as quartz porphyry by Turner (1894) and Taliaferro (1943).

DESCRIPTION OF MAP UNITS

JURASSIC TO PALEOZOIC UNITS (continued)

Logtown Ridge Formation, Goat Hill member (Middle to Late Jurassic)—Thinly- to thickly-bedded very fine to medium grained tuff, coarse pumice lapilli tuff in graded beds and thickly-bedded fine to coarse volcanic breccia that grades upward into medium-fine grained tuff. The age of the Goat Hill Member is well established as Callovian to late Jurassic by the occurrence of the ammonite Pseudocadoceras (Duffield and Sharp, 1975).

Mélange belt (Jurassic to Paleozoic)—mapped by Duffield and Sharp (1975) in the eastern portion of the quadrangle consists of chaotically intermixed blocks whose lithologies primarily consist of clastic meta-sedimentary rock types. Virtually none of the map units in the mélange consists of a single lithology. Subdivided by Duffield and Sharp (1975) into the following mappable units:

Porphyritic greenstone (Jurassic)—Very coarse plagioclase, plagioclase-augite, and augite porphyry greenstone.

Greenstone (Jurassic)—Aphanitic and finely porphyritic greenstone. Phenocrysts include plagioclase, hornblende and biotite.

Augite-hornblende gabbro (Jurassic)—Fine to medium grained.

Pyroxene gabbro (Jurassic)—Massive to brecciated, altered, medium grained gabbro.

Biotite granodiorite (Jurassic)—Fine grained, brecciated, and locally altered to dolomite-magnesite-quartz rock.

Hornblende quartz diorite (Jurassic)—Brecciated, medium grained. Duffield and Sharp (1975) report a K/Ar hornblende age of 172 ± 4.3 Ma.

Serpentinite (Jurassic)—Black to bluish gray, sheared to blocky, often with well developed slaty cleavage. Locally contains talcose rocks, rodingite, and tremolite rocks. Where possible, the follwoing units are mapped seperately:

Talcose rocks

Reddish, antigoritic carbonate rock

Hornfels siltstone inclusion—Mapped in the northeastern corner of the quadrangle.

Hornblende amphibolite—Dark gray to black, fine- to coarse-grained; usually associated with serpentinite bodies

Greywacke and clay slate—Thinly and rhythmically interbedded. Locally include minor shale-clast granule and pebble conglomerate.

Conglomerate—Includes granule to pebble shale-clast conglomerate and pebble to cobble and boulder volcanic-clast conglomerate.

Slate—Interbedded with greywacke, quartzite, and siliceous shale-clast conglomerate. Occasional sparse beds of quartzitic sandstone and chert.

Clay slate—Siliceous, interbedded with chert, quartzite and micaceous sandstone.

Quartzite—Locally gradational into siliceous conglomerate. Occasionally calcareous.

Siliceous conglomerate—Siliceous granule to cobble conglomerate and quartzitic conglomerate. Locally gradational into quartzite.

Mudstone—Nonfissile to weakly fissile, sparsely interbedded chert, blocky weathering.

Limestone—Gray, fine to coarse grained and massive, locally brecciated. Occasionally contains abundant chert nodules.

Chert marker bed—Laminated to thinly bedded.

Quartzite bed—Phyllitic, laminated, contains abundant white mica. Interbedded with clay slate (ssl).

White mica phyllonite—Laminated, quartzitic; locally intersheared with chert.

Tectonic breccia—Strongly sheared, locally contains monolithologic clasts.

Bear Mountains Ophiolite, metavolcanic rock—Schistose with minor amounts of massive meta-igneous rock consist of green hornblende, epidote, and plagioclase. Whole rock chemistry of the equivalent rock unit within the adjacent Latrobe 7.5 minute quadrangle to the north, suggests the protoliths were probably basaltic in composition (Behrman, 1978).

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