Llano Uplift: Paleozoic and Younger

•  A roughly oval-shaped area where Precambrian and Paleozoic aged rocks have been exposed by erosion of the Cretaceous rocks of the Edwards Plateau.

Matt Engle, Timothy Fedor, Patrick Glass, Mary Hangen, Kurt Hellmich

Enchanted Rock

• Large pink granite pluton stock (<40 sq mi)

• Exfoliated Dome• Vernal pools• Top ~ 1800’ ASL• People have been coming

for ~10,000 yrs.

Inks Lake State Park• Home base• On banks of Colorado River • Majority of mapping done here

History of Llano Uplift from Cambrian to Recent

Llano Uplift

Background

• Created during the Grenville Orogeny from 1.25GY to 980MY

• Was upthrown/abducted over 270MY• Located in the eastern region of the Edwards

Plateau • Marked by abundant disconformities driven by

divergent plate boundaries

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Sedimentary Deposition

• The majority of the sediment was deposited by the Western Interior Seaway and the Sundance sea

• Time of deposition took place from the Jurassic to the Oligocene

• This seaway opened due to the subduction of the Farallon plate

• Sedimentary structures deposited consisted of: limestone, shale, and sandstones

Paleoclimatology

• Western Interior Seaway and The Sundance Sea were warm empiric seas that supported abundant marine life such as Mosasaurs, Sharks, and many invertebrates

• This seaway was over 2,500m deep, 600mi wide and 1000mi long

Climate Today• Temperature:

– Annual average high temperature 78.3 , Low 54.0, Average 66.1 – Average annual precipitation 31.7 in

Cambrian Strata of the Llano Uplift

Riley Formation

• Base Formation of the Cambrian Strata• Contains three members:– Hickory Sandstone Member– Cap Mountain Limestone Member– Lion Mountain Sandstone Member

Hickory Sandstone Member• Base unit• Unconformably overlies Pack Saddle Schist• Thickness: 276’ to 470’

• Lower Hickory:• Friable• Poorly sorted• Rounded to sub-rounded• Fine Grained• Contains feldspars from the Precambrian

rocks beneath• Upper Hickory

• Dark red• Friable• Well rounded• Medium to coarse grained

Cap Mountain Limestone Member

• Thickness: 90’ to 411’• Gradational boundary from Hickory Sandstone– Marked by displacement of quartz and hematite

cement by calcite cement• Lower Cap Mountain– Limy and sandy

• Middle Cap Mountain– Silty

Lion Mountain Sandstone Member

• Thickness: 29’ to 69’• Characteristics

– Coarse grained– Dusky-green to grayish

olive-green– Cross bedded– Glauconitic– Contains lenses of white,

glauconitic trilobite coquinite, and phosphatic brachiopods

– Contains hematite nodules

Wilberns Formation

• Contains four members– Welge Sandstone Member– Morgan Creek Limestone Member– Point Peak Member– San Saba Member

• Upper portions contain algal reefs

Welge Sandstone Member

• Thickness: 11’ to 30’• Basal unit of Wilberns Formation• Unconformably overlies Lion Mountain

Sandstone• Characteristics:– Medium to coarse grained– Dark yellow-brown– Well sorted quartz sandstone

Morgan Creek Limestone Member• Thickness: 114’ to 143’• Gradational contact with the Welge Sandstone beneath• Lower characteristics:

– Coarse grained– Green-gray to light olive-gray – Glauconitic limestone

• Middle characteristics: – Thin to medium bedded– Dark green-gray– Silty– Argillaceous – Fine grained limestone inter-bedded with coarse grained limestone

• Upper characteristics: – Coarse grained glauconitic limestone inter-bedded with thick bedded, dark green-gray

silty, fine grained limestone

Point Peak Member• Thickness: 150’• Gradational contact with both the

Morgan Creek beneath, and San Saba above

• Characteristics:– Thinly bedded– Light olive-gray– Argillaceous– Glauconitic

• Consists of both calcareous siltstone and fine grained silty limestone– Siltstone is predominate lower in

member– Limestone predominates higher in

member• Contains varicolored conglomerate of

thin, flat, sub-rounded limestone clasts

San Saba Member• Thickness: 280’ to 325’• Consists of limestone and dolomite• Limestone

– Thinly to thickly bedded– Fine grained– Glauconitic– Varying shades of gray

• Dolomite– Either medium bedded and fine grained, or thickly bedded and coarse grained– Contains chert– Various shades of gray– May be mottled with red or purple– Generally occurs higher in the section

Llano Region

Ordivician: Ellenberger Group-limestone and dolomite

Tanyard Formation (oldest)•Threadgill Member

Gray dolomite with limestone lenses

•Staendebach MemberLight gray, finer grained cherty dolomite

Gorman FormationVariable mixture of limestone and

dolomite

Honeycut FormationLimestone near top and bottom, brown dolomite in the center

Longhorn Cavern

Limestone Cave in Burnet Couty, Ellenberger Group

Devonian strata

• Houy Formation• Stribling Formation

Outcrops rare in Llano region

Mississippian Strata

• Chappel LimestoneCrinoidal biosparite and biomicrite

Usually only a few feet in thickness

• Barnett ShaleBlack to dark gray petroliferous shaleMicrosparite concretions common near the topContains major natural gas reserves

Pennsylvanian and Cretaceous Stratigraphy

WELL DATA SOLUTIONS http://wellsite-ds.com/?p=2296

Pennsylvanian Groups

• Bend Group– Marble falls Limestone– Smithwick Shale

• Strawn Group

• Canyon Group

• Cisco Group

Bend Group• Marble Falls limestone: Interbedded cherty

and non-cherty limestone with shale. Commonly believed to be an unconformity at the Miss/Penn Boundary.

• Lower: light to dark chert limestone and thin shale beds. About 30m thick but ranges from 21-45m in some areas.

• Upper: light to dark algal biomicrite and shale. Facies oriented in N-S in contrast to lower marble falls. About 82m thick.

Bend Group

• Smithwick Shale: 400 feet thick of dark gray claystone, grades into interbedded sandstone and claystone.

• Claystone is composed of illite, quartz and muscovite silt.

• The sandstone indicates the source area was composed of sedimentary and granitic plutonic rocks with low-grade metamorphic and volcanic rocks. (American Geological Institute).

Strawn Group• Composed of massive conglomerate

sandstone, and alternating sandy shale.

http://aapgbull.geoscienceworld.org/content/47/10/1840.short

Canyon Group

• Massive limestone with alternating shale• Can be up to 250ft thick• Limestone described as thinly bedded, fine

grained and cherty• Shale is yellow to grey and described as clayey.

Cisco Group• Composed of sandy shale, sandstone, thin

limestone beds, and some coal• Thin limestone beds described as fine grained

yellow to grey

Harpersville Formation

http://northtexasfossils.com/harpersville.htm

Journal of Geology http://www.jstor.org/stable/30063569?seq=7

Cretaceous Groups

• Trinity Group

• Fredericksburg Group

• Washita Group

Trinity Group

• Upper Trinity: Upper Glen Rose

• Middle Trinity: Lower Glen Rose, Hensel Sand, Cow Creek Limestone

• Lower Trinity: Sycamore, Hosston, Sligo

Glen Rose Formation

• Most well know for its dinosaur fossils

http://www.seriouslyfunnyscience.com/node/69

Limestone trace fossils

http://www.seriouslyfunnyscience.com/node/69

Fredericksburg Group

• Walnut Formation: 70 to 80 feet of marly limestone, alternating with harder more crystalline limestone and limy clay.

• Comanche Peak: white, irregularly bedded, nodular limestone interbedded with marl.

• Edwards: massive limestone beds with bands of chert nodules and rudistid biostromes (tube shaped bivalves).

• Kiamichi: a light brown to gray, argillaceous (resembles clay) limestone.

Washita Group

• Georgetown Formation: light grey chalky limestone and marl

• Del Rio Formation: greenish gray to tan, soft, plastic, ‐laminated and gypsiferous (containing gypsum) mudstone or shale.

• Buda Formation: tan to brown, very hard, medium to ‐massive bedded, coarse grained, slightly glauconitic ‐ ‐crystalline limestone.

Buda Formation

http://www.sunstar-solutions.com/sunstar/geology/CristoRey/mid-Cretaceous.htm

Buda Formation

http://www.sunstar-solutions.com/sunstar/geology/CristoRey/mid-Cretaceous.htm