Rocks,
minerals, and fossils found on the surface of Dallas and Tarrant
Counties were deposited in the Mesozoic Era and Cenozoic Era. (See
Time
Chart) The resulting formations are either of
Cretaceous
Age, 66 to 144 million years ago (ma) or Quaternary Age,
present day to 10,000 years ago (Dates are approximations.) In Dallas
County, the Cretaceous ranges from about 75 ma to 90 ma. In Tarrant
County, the Cretaceous ranges from 90 ma to 115 ma. In other parts of
Texas more of the Cretaceous Era is represented. For instance, the
Cretaceous/Tertiary
(K/T) boundary.
Cretaceous
deposits in North Texas are mostly marine in origin. The Western
Interior Seaway at its greatest extent spanned the the North American
continent from the Artic Circle to the present day Gulf of Mexico.
Although this was the Age of the Dinosaur, their habitats were sparse
in Tarrant County and non-existent in Dallas County. The most abundant
habitat was marine. The depth of the ocean rose and fell many
times. In doing so, the sediments deposited varied from
generally sandy shorelines and deltaic complexes to low energy, deeper
water.
Quaternary
sediments were deposited in areas adjacent to the Trinity River and its
tributaries. As the Trinity winnowed over time, it would leave terraces
covered with sand and gravel. These river deposits range in
age from 1.8 ma. to recent. Mammoth and bison bones are sometimes found
along the Trinity and its tributaries.
To understand
the sedimentary rock record there are three fundamental ideas to grasp:
Time units, Rock units, and Time-rock units.
Formation
nomenclature is colloquial in nature. Formation names commonly come
from their general make-up, the location where it was first described,
or the person whom first discovered the layer. This can lead to some
confusing names until you learn your way around - for instance, we have
the "Austin Chalk" in Dallas County! Recognizing formations will assist
in finding fossils, identifying them, and recording them for study.
Construction of most geologic maps is at the formation level. In
addition to maps, observations of the rock types, the fossils, and even
the vegetation can help identify the formation. For example, the cedar
trees around Cedar Hill prefer limy soil; Post Oak trees prefer sandy
soil. More detailed information on the formations comes later in this
document.
Here is an
example of Rock units, Time-rock units and Time units.
Arlington,
Texas was once the setting for a delta on the eastern shore of the
Western Interior seaway. Sands, and clays are the individual beds
that comprise the three Members
, Rush Creek, Dexter, Lewisville
and Arlington of the Woodbine Formation . They are
in the Woodbine Group. These are the rock units. Time-rock units and
Time units can be confusing because the same name often applies to both.
The Time-rock
units of the Woodbine Formation place it in the Cenomanian Stage. The
stage tells you the Woodbine is in the lower part of the Gulf Series in
the Cretaceous System. These are all relative times because they differ
according to geographic location. Deposition of the Woodbine did not
occur at the same time in all the same places. It took millions of
years for the delta to advance out into the sea way.
Time units
give us a specific measurement in millions of years. The Cenomanian Age
(91 to 97.5 ma) is in the Late Epoch (66.4 to 97.5 ma) of the
Cretaceous Period (66.4 to 144 ma).
TOPOGRAPHY AND
FORMATION EXPOSURES
During the
Cretaceous Period, the seas rose and fell leaving multiple layers of
deposits with various compositions. These formations represent several
deposition environments: shallow marine, deltaic, and coastal. Ancient
land deposits to deeper-water sediments provide a rich variety of
fossils: vertebrate specimens include marine reptiles, sharks, turtles,
fishes and rarely dinosaurs. There are also plenty of invertebrate
fossils: sea urchins, shells of clams, snails, and ammonites (related
to the Nautilus).
After the
Cretaceous Period the area tilted slightly resulting in a 1/2 degree
dip due east. This slight dip across the thin deposits causes many
formations to outcrop on the surface - much like slicing an onion. The
exposures form bands that run generally north-south. Since the various
rock units are deposited one on top of another, the oldest is on the
bottom and youngest on top. The tilting exposes the oldest rocks in the
western part of the area and the youngest in the east.
During the
Quaternary Period, the Trinity River carved out terraces through the
Cretaceous deposits. The river left behind clays, sands, and gravel
that are of economic value. Today, the Trinity River headwaters form in
western Tarrant County, fed by the many creeks flowing from Parker,
Jack and Denton Counties. The river causes a
dendritic
drainage pattern across most of western Tarrant
Country. Fossil mammoth, bison and other mammal bones can be
found along the Trinity River.
Dallas County
has six formations represented on the surface. Tarrant County has
fifteen formations on the surface. The list below is from most recent
to oldest.
FORMATION
DESCRIPTIONS and INFORMATION
Quaternary
alluvium and terrace deposits - Dallas and
Tarrant Counties
The
Quaternary period has two subdivisions. The Holocene Epoch ranging the
present to about 12,000 years ago and the Pleistocene from 12,000 to
about 1.8 ma. Although the Pleistocene is referred to as the Ice Age,
none of the ice sheets extended as far south as Fort Worth or Dallas.
The maximum extent, 18 to 20 thousand years ago, of the
Laurentide
ice sheet was barely touching northeast Kansas. The ecology
of DFW area was somewhere in the transition zone with respect to
vegetation. A broadleaf forest (included: chestnut,
Paw Paw or "Prairie Banana", Osage Orange) was present in eastern North
Texas (Is that why they call it the Eastern Cross Timbers?) which gave
way to open woodlands (include: juniper and mesquite) to the
west. The boundary is likely close to where it is today
because, in part, the soils becomes more rocky to west and the climate
is dryer.
Alluvial
deposits of clay, sand, and gravel are found in the flood-plain areas.
They are the youngest deposits in the area. Some deposits are present
day as the result of storms moving large volumes of water requires high
energy. Subsequently there is enough energy to erode and move sediments
as large as gravel and boulders. Gravel bars are a good place
to look for fossils.
Terrace
deposits outcrop adjacent to and outside of the immediate channels the
Trinity River and its tributaries. Their composition is that of
alluvium. Terraces were formed when the river changed course,
abandoning the previous flood-plain, and the new channel cuts deeper
into the earth.
Fossils
of mammoth (Mammuthus
columbi), Bison (Bison bison
and
Bison
antiquus), camel, horse and other mammal bones
are can be found in these Quaternary deposits.
Quaternary
deposits unconformably overlie the Ozan formation. The hiatus lasted
about 70 million years.
Ozan Formation - Dallas
County
The Ozan is
also referred to as the Lower Taylor Marl. The Ozan unconformably
overlies the Austin Chalk. Descriptions of color and composition vary
widely because often the descriptions are either state wide summaries
or of particular localities. The Geology of Texas Volume 1,
Stratigraphy by Sellards, Adkins, and Plummer describe the Ozan as a
dark gray marl. The marl can be sandy, chalky or glauconitic and can be
mostly sand. Its generally a calcareous micaceous clay that coarsens
upward though the section with increasing proportions of calcareous
silt and sand. Occasionally present are glauconite, hematite and pyrite
in nolular form, and phosphate pellets. When weathered, it is
a brownish gray color. The clay in this formation contains the clay
mineral montmorillonite
that expands when wet. Montmorillonite is descused in more detail in
the section on the Eagle Ford. The Ozan was deposited in a deep marine,
low energy, environment.
The bivalve
Inoceramus
is much smaller than those found in
the Austin Chalk. Other fossils include shark's teeth and
microfossils. Trace fossils being burrows of
Ophiomorpha (shrimp),
Thalassinoides
(arthropod) and
Planolittes (worm) can be found while vertebrate fossils are
close to nonexistent. Although not in Dallas or Tarrant Counties,
probably the best exposure and fossil collecting is in the North
Sulphur River. The Society's Occasional Papers Volume 4,
Fossil Collector's
Guidebook to the North Sulphur River, by Mark
McKinzie, Ron Morn and Ed Swiatovy, 2001 is an excellent resource.
Austin Chalk - Dallas
County
The Austin
Chalk is considered by many to be a group of formations. The Austin
Chalk has five members. From bottom to top is the Atco Member, the
Austin Chalk, Blossom Sand, Brownstown Marl and Gober Chalk. In Dallas
County, the Geologic Atlas of Texas, Dallas Sheet has not
differentiated mapped area. Specifically, the condensed section at the
base of the Atco Member and known as the Fish Bed Conglomerate is
present where the Austin Chalk-Eagle Ford Group contact
outcrops. The Fish Bed Conglomerate is a light greenish sandy
calcareous clay. Phosphate nodules are abundant. However, the reason
for the name is the extreme abundance of small to microscopic fish
teeth. The easiest way to collect from the Fish Bed Conglomerate is to
submerse large chunks of material in a weak acid bath like vinegar for
a few days then wash the material through a fine sieve to catch the
teeth. The acid bath and wash may have to be repeated several
time. the There used to be a great exposure along
FM 1382 west of Clark Road in Cedar Hill, Texas.
Unfortunately, a retaining was was erected prevent erosion and rock
falls along the sharp and tall roadcut.
The western
extent of the Austin Chalk is sometimes referred to as the White Rock
Escarpment. The contrast between the chalk and the underlying Eagle
Ford is striking. Looking east from locations in Grand Prairie and
Irving, the Austin Chalk raises out of the plains and appears white
overlying the dark gray or black Eagle Ford. Composition of the chalk
is primarily coccolithophores from
foraminifera.
Coccoliths
are calcareous plates found on the surface of some flagellate
microorganisms. Austin Chalk covers one-third to one-half of the
surface of Dallas County. Cedars prefer limy soil and are prolific.
The chalk dips
at a rate between 15 and 40 feet per mile to the east or southeast.
has many normal faults and fractures, all inactive so don't worry. This
is the same formation that made horizontal drilling famous. Oil
companies use the faults and fractures to their advantage, especially
in Brazos and Burleson Counties, Texas. Oil companies bore vertically
to the target depth then bore horizontally through the chalk. A
horizontal boring encounters more oil filled faults and fractures a
vertical boring. So, the horizontal well produces much more oil or gas.
Several quarries located near Midlothian, Texas mine the chalk for use
in cement. The quarries occasionally grant permission for members of
this Society to enter the quarries and collect fossils. Fossils are
generally sparce in the Austin Chalk. A shark and a few mosasaur
skeletons have been found.
Eagle Ford
Formation or Eagle Ford Group - Dallas and Tarrant
Counties
The gently
rolling hills are referred to as the "Black Prairie" and the residual
soil as "black gumbo." The Eagle Ford contains bentonite, an impure
clay that is a product of weathered volcanic ash. Bentonite consists
mainly of the clay mineral montmorillonite. Differential
swell/contraction of montmorillonite as it absorbs or looses water can
be as much as seven inches causing problems for foundations and
roads. Bentonite occurs in the Eagle Ford in layers up to
ten inches thick. The good news is
that the bentonite layers are good sources of samples for radiometric
age dating.
The Eagle Ford is a bituminous shale with
calcareous concretions and large septaria; sandstone and sandy
limestone in the upper parts; and bentonitic in the lower part. The laminated
bedding structure and dark color makes the Eagle Ford easy to identify.
The
Eagle Ford has four members: the basal unit is Tarrant Beds or Six
Flags Limestone, Britton Shale, the Kamp Ranch Limestone and the
Arcadia Park Shale, top member. The Tarrant Beds is dark gray shale
that weathers to brownish or bluish-gray. It also includes small lenses
of either calcareous sandstone or fossiliferous limestone covering a
few feet laterally and approximately two inches thick.
Overlying the basal unit is the Britton Formation consisting of dark
gray, silty shale that grades laterally into gray and tan
shale. A thin limestone, flaggy, layer above the Britton is
named the Kamp Ranch Limestone. Depositional environment of the Kamp
Ranch was in shallow water with moderate to high energy. The abundance
of shell fragments aid in the determination of the energy level.
Overlying the Kamp Ranch is the Arcadia Park shale. The Arcadia Park
and the Britton are very similar in appearance and were deposited in
low energy waters with depths between 120 and 600 feet which is part of
the controversy as whether the Eagle Ford is a single formation or a
group.
The Eagle
Ford produces quality specimens: ammonites with shells composed of the
original mother of pearl; shark teeth of Leptostyrax and
Ptychodus, fish and other
marine vertebrate skeletons. Dallas Paleontological Society member
Arlene Pike was honored for her fossil crab find which was named
Homolopsis
pikeae. Society members Van Turner was also honored
for his find Dallasaurus
turneri, and Mark Cohen for Russelosaurus Coheni, D.
turneri and R. Coheni are the two the
oldest known specimens of mosasaur-like marine reptiles in
North America.
Woodbine Formation or group
- Dallas and Tarrant Counties
As a group, the Woodbine consists of the Rush Creek
Member, Dexter member, Lewisville Member and the Arlington Member.
The
Woodbine was a fluvial deltaic system consisting of both marine and
non-marine facies. The Woodbine Formation consists of mostly fine
grained sandstone with clay and shale. The
basal unit, Rush Creek Member, is composed of marine prodelta clays,
silts and sands. The overlying Dexter Member is predominately
sandstone with clay and sandy or carbonaceous shales
interbedded. Features
include large scale cross-bedding, ripple marks, and large discoid
concretions. The Dexter Member is interpreted to
be deposited from both marine and non-marine sediments. The
Lewisville Member is a marine shale with various quantities of sand,
silt and carbonaceous clays. The Arlington Member is composed
of sandstone and calcareous sand lenses interbedded with sandy
shales. Beds
of ironstone and ironstone conglomerate also occur in the lower part. Sandstones
are occasionally conglomeritic and contain occasional to abundant
oyster shells. The Woodbine unconformably overlies the Grayson Marl.
The Woodbine
provides soil for a wooded region called the Eastern Cross Timbers
where Post Oaks are common. The Woodbine is a sandy soil, rich with
iron. Iron nodules litter the ground. Iron nodules form when solutions
deposit around a "seed" or nucleus of sand or organic material. These
concretion nodules look like iron meteorites but are distinguishable
because they do no have the magnetic properties. Iron mineral mostly
limonite and hematite give the Woodbine a characteristically
reddish-brown any yellow hues.
The Woodbine Formation, a secondary aquifer in
North Texas. To the east, the Woodbine
produces hard-water as a result of the iron. Many water wells produce
from the Woodbine aquifer. Pesticides, herbicides and petrochemicals on
spilt onto surface exposures have the potential to seep down into this
aquifer and pollute water wells down dip (to the east.) Tan colored
clays are interbedded in the Woodbine, and used by companies in Denton
County to make brick. In east Texas, the Woodbine is the largest oil
and gas producing reservoir rock in the state.
Fossils from
the Woodbine include the ammonite Acanthoceras, the
oyster Gryphaea,
dinosaurs include an unnamed nodosaurid, an unnamed nodosaurid and an
unnamed hadrosaur and
various mollusks. Society member Bill Lowe was honored by
having a trace fossil footprint of a theropod dinosaur named
after him, Magnoavipes lowei. Also, member Gary
Bird was honored with his discovery, Protohadros byrdii.
Grayson Formation -
Tarrant County
The Grayson
Formation is a chalky, yellow-white to gray marl with gray shale beds.
The transition from Grayson to overlying Woodbine represents a
withdrawal or regression of Cretaceous seas.
Fossils
include ammonites: Scaphites
subevolutus, Turrilites
bosquensis, T. brazoensis, Adkinsia
sp.; pelecypods,
Exogyra arientina (also known as "rams horn") and
E.plexa. Trace fossils include
Thalassinoides (arthropod) and
Serpula
(worm tubes).
Main Street
Formation - Tarrant County
The Main
Street formation overlaying the Paw Paw is a resistant limestone formed
from supersaturated lime mud. The contact between the two formations
represents a transgression of the Cretaceous oceans. The Main Street
weathers to a white or grayish white color. was a deeper water
environment than the Paw Paw.
Paw
Paw Formation, Denton Clay, Weno Limestone, Fort Worth Limestone and
the Duck Creek Formation - Tarrant County
In Tarrant
County, these formations are mapped undivided on the Geologic Atlas of
Texas.
The Paw Paw's type locality
is Paw Paw Creek which begins in Dennison and flows generally northeast
to the Red River. The clay is dark gray or blackish and somewhat
lustrous when fresh and weathers red-brown to yellowish.
Shales and clays with some sandstone ledges comprise the formation.
Particularly in northern Tarrant County, concretions and iron nodules
are abundant. The Paw Paw is a ferruginous clay of shallow marine
origin. Rocks here represent the transgression of Cretaceous seas.
Fossils
include a unique miniature fauna such as ammonites less than one inch
in diameter; frequently pyritized with intricate detail. Other genera
also exhibit dwarfism but the reasons for the dwarfism are unknown.
Shark's teeth are common fossil in the Paw Paw.
The area
north of Loop 820 and North Beach Street and the community of Fossil
Creek was productive until development covered the site. Typical
fossils include shark teeth and vertebrae and cartilage, crab claws and
carapaces fragments, and dwarf ammonites, and the pelecypod, ostrea
quadriplicata. This area has also produced some unusual finds
for a marine depositional environment. A
nodosaur,
Nodosauridae indet, was found by then 12-year old
Johnny Maurice
the son of Society member. John Maurice. Unfortunately, it was not
named in his honor. Another nodosaur,
Pawpawsaurus campbelli
was found by then 19-year old Cameron Campbell. Society affiliation
unknown. Member Robert Reed found a (unnamed)
turtle associated with nodosaur bones.
Weno Limestone has
three units. The upper unit is chiefly limestone. It varies
from hard and massive to soft and chalky. When fresh it is
light gray to yellowish gray and weathers gray to yellowish brown. Hard
limestone will form topographic benches. The middle unit is
generally a calcareous clay with occasional lenses of sand-sized shell
fragments. Color varies from olive brown to olive gray. The lower unit
is predominantly limestone with some sand. This fossiliferous formation
is light gray, except where it is sandy it is medium gray. The lower
unit weathers to a yellowish brown.
Echinoids
Macraster,
Tetragramma;
worm
tubes Serpula;
gastropods
Gyrodes
Conrad, Leptomaria
austinensis can be found.
Denton Clay has
alternating layers of calcareous clay with marl and limestone weathers
dusky brown. At the time of the Denton Clay, deeper water covered the
area. The calcium carbonate deposits hardened into a micritic
limestone. Clay units have abundant shell debris and some burrowing is
evident. Marl beds vary from argillaceous limestone to calcareous clay.
Limestone units are dark gray when fresh and can have thin (<0.6
ft.) Gryphea beds. Fossils include pelecypods: Anomia,
Pectin, Gryphea and Alectryonia;
echinoids:
Heteraster and Macraster;
ammonite: Pervinquieria
The Fort
Worth Limestone and the Duck Creek
Limestone are both grayish to yellow-gray or yellow-brown.
Both formations are limestone and difficult to differentiate. Fort
Worth Limestone is a limestone with alternating thin layers of marly
clay. The upper 40 feet are cream-colored. The lower 10 to 15
feet has alternating layers of grayish-blue clay/marl with
yellowish-white limestone. Pecten are more
abundant in the Fort Worth Limestone while Gryphea are
more abundant in the Duck Creek Fm. They are generally prairie land.
Fossils include
echinoids:
Holaster simplex, Macraster;
ammonites: Pervinquieria, Mortoniceras; pelecypods: Exogyra,
Gryphea and Alectryonia;
brachiopod: Kingena wacoensis, and nautiloids
Goodland
Limestone Formation - Tarrant County
The Goodland
is a white, fossiliferous, micritic, coarsely nodular, limestone.
Bedding is massive with some thin clay beds. The upper 5 feet is a
fossiliferous, massive, limestone with oolites. Depositional
environment is interpreted as shallow,
warm water, marine (not necessarily near shore), and within the forces
and actions of waves. Oolites are formed around a nucleus where aragonite crystals grow in layered rings as a result of the
back and forth action of the waves. Since aragonite is a polymorph of
and is less stable than calcite, over, geologic, time will
convert to calcite. The fragmentation of
the shells indicates an erosional energy source (waves) and the
intraclasts also indicate waves as an erosional agent. The
formation is medium to dark gray when fresh and weathers to a yellowish
brown. This unit has a low clay content making it a good support for
buildings.
Fossils
include the ammonite Oxytropidoceras acutocarinatum,
pelecypods Protocardia,
Pinna
and Lima
wacoensis, heart-shaped urchins in abundance,
and miscellaneous gastropods.
Kiamichi
Formation - Tarrant County
The Kiamichi
has alternating layer of clay and limestone with units generally not
exceeding 5 feet in thickness. Sandstone is a minor component. olive
brown flaky shale that weathers to tan. Unconformities bound
the Paluxy above and below. A transgressing sea deposited clays to form
the Walnut Clay formation. Oyster reefs of Gryphea mucronata create
resistant, cliff forming benches over the underlying Walnut Clay.
Walnut Clay Formation
- Tarrant County
Calcareous
clay and limestone interspersed with Gryphea beds.
Some of the beds form topographic benches. Clay units are
fossiliferous. Limestone units when fresh, the color is dark gray and
weathers to a yellowish brown. Clay units are olive brown and weathers
to a yellowish brown.
Paluxy Formation -
Tarrant County
Characteristically,
the Paluxy Formation is a reddish sandstone and shale unit that
supports Post Oaks. The environments of deposition were coastal streams
and beach environments. Ancient beach deposits form localized dunes of
white, well rounded, friable sand. Some iron stains are present in
these dunes. These clean beach sands are unmistakable - miles away from
the present day ocean. Although not from Tarrant County, dinosaurs like
the Tenontosaur at the Dallas Museum of Natural
History are found in the Paluxy Formation in Parker and Jack Counties.
REFERENCES
(not referenced
by hyperlinks)
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Leaders, Field Trip, Fort Worth Geological Society Geology of
Tarrant County. (Publisher not listed.)
Dawson, W. C., McNulty, C. L.,
Reaser, D. F., Leaders, 1979, Fieldtrip Guidebook for
Stratigraphy and Structure of Selected Upper Cretaceous Rocks in
Northeast Texas. (Texas Academy of Science 82nd Annual
Meeting, Arlington, Texas.)
Fisher, W. L., Director, 1972,
"Geologic Atlas of Texas, Dallas Sheet":
Bureau of Economic Geology, (The University of Texas at Austin, Austin,
Texas.)
Graham, A. 1999. Late
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Mexico. (Oxford Univ. Press, New York.)
Hagland, J., McKinzie, M.G. and
Wilson, M., 2001, Field Guide to the Stratigraphy of the
Dallas Fort Worth Area. (Self published by the Dallas
Paleontological Society.)
Mason, B., Berry, L. G., 1968, Elements
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Matthews, R. K., 1974, Dynamic
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Mckinzie,
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Welton, B. J., PHD, Farish, R. F.,
1993, The Collector's Guide to Fossil Sharks and Rays from
the Cretaceous of Texas. (Before Time, Lewisville, TX.)
Winkler, D. A., Murry, P. A.,
Jacobs, L. L., Editors, 1989, Field Guide To The Vertebrate
Paleontology Of The Trinity Group, Lower Cretaceous Of Central Texas.
(Institute for the Study of Earth and Man, Southern Methodist
University, Dallas, Texas.)
Winton, W. M., Adkins, W. S. 1919, The
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