Llano Uplift: Paleozoic and Younger
Matt Engle, Timothy Fedor, Patrick Glass, Mary Hangen, Kurt Hellmich
•
A roughly oval-shaped area
where Precambrian and
Paleozoic aged rocks have
been exposed by erosion of
the Cretaceous rocks of the
Edwards Plateau.
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
You are
headed here!
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 Member
Light gray, finer grained cherty dolomite
Gorman Formation
Variable mixture of limestone and
dolomite
Honeycut Formation
Limestone 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 Limestone
Crinoidal biosparite and biomicrite
Usually only a few feet in thickness
• Barnett Shale
Black to dark gray petroliferous shale
Microsparite concretions common near the top
Contains major natural gas reserves
Pennsylvanian and Cretaceous
Stratigraphy
http://wellsite-ds.com/?p=2296
WELL DATA SOLUTIONS
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