Llano Uplift: Paleozoic and Younger A roughly oval-shaped area where Precambrian and Paleozoic aged...
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Transcript of Llano Uplift: Paleozoic and Younger A roughly oval-shaped area where Precambrian and Paleozoic aged...
![Page 1: Llano Uplift: Paleozoic and Younger A roughly oval-shaped area where Precambrian and Paleozoic aged rocks have been exposed by erosion of the Cretaceous.](https://reader036.fdocuments.net/reader036/viewer/2022062515/56649c805503460f949382f9/html5/thumbnails/1.jpg)
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
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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.
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Inks Lake State Park• Home base• On banks of Colorado River • Majority of mapping done here
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History of Llano Uplift from Cambrian to Recent
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Llano Uplift
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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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You are headed here!
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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
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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
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Climate Today• Temperature:
– Annual average high temperature 78.3 , Low 54.0, Average 66.1 – Average annual precipitation 31.7 in
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Cambrian Strata of the Llano Uplift
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Riley Formation
• Base Formation of the Cambrian Strata• Contains three members:– Hickory Sandstone Member– Cap Mountain Limestone Member– Lion Mountain Sandstone Member
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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
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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
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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
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Wilberns Formation
• Contains four members– Welge Sandstone Member– Morgan Creek Limestone Member– Point Peak Member– San Saba Member
• Upper portions contain algal reefs
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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
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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
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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
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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
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Llano Region
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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
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Longhorn Cavern
Limestone Cave in Burnet Couty, Ellenberger Group
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Devonian strata
• Houy Formation• Stribling Formation
Outcrops rare in Llano region
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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
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Pennsylvanian and Cretaceous Stratigraphy
WELL DATA SOLUTIONS http://wellsite-ds.com/?p=2296
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Pennsylvanian Groups
• Bend Group– Marble falls Limestone– Smithwick Shale
• Strawn Group
• Canyon Group
• Cisco Group
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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.
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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).
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Strawn Group• Composed of massive conglomerate
sandstone, and alternating sandy shale.
http://aapgbull.geoscienceworld.org/content/47/10/1840.short
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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.
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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
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Journal of Geology http://www.jstor.org/stable/30063569?seq=7
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Cretaceous Groups
• Trinity Group
• Fredericksburg Group
• Washita Group
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Trinity Group
• Upper Trinity: Upper Glen Rose
• Middle Trinity: Lower Glen Rose, Hensel Sand, Cow Creek Limestone
• Lower Trinity: Sycamore, Hosston, Sligo
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Glen Rose Formation
• Most well know for its dinosaur fossils
http://www.seriouslyfunnyscience.com/node/69
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Limestone trace fossils
http://www.seriouslyfunnyscience.com/node/69
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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.
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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.
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Buda Formation
http://www.sunstar-solutions.com/sunstar/geology/CristoRey/mid-Cretaceous.htm
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Buda Formation
http://www.sunstar-solutions.com/sunstar/geology/CristoRey/mid-Cretaceous.htm
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