Rocks, minerals, and fossils found on the surface of Dallas and Tarrant Counties were deposited in the Mesozoic Era and Cenozoic Era. (See Geologic Time Chart) The resulting formations are either of Cretaceous Age, 145 to 66 million years ago (ma) or Quaternary Period, 2.58 ma to present. In Dallas County, the Cretaceous Period 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, different time spans of the Cretaceous Period are 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 North American continent from the Arctic Circle to the present day Gulf of Mexico. Although this was the Age of the Dinosaur, their habitats were sparse in Tarrant County and possibly non-existent in Dallas County. 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 2.58 ma to recent. Mammoth and bison bones are sometimes found along the Trinity and its tributaries.
The Sedimentary Record
To understand the sedimentary rock record there are three fundamental ideas to grasp: Time units, Rock units, and Time-rock units.
Time units are names given to intervals of time: The designation for Time units is in years before the present. Designations are abbreviated, often as mya (millions of years ago) or ma (mega-annum or millions ago). Time units define the chronological sequence of the events in rocks. Their ranking from longest to shortest is Eon, Era, Period, Epoch, then Age. Age dating by radiometric and/or correlation using fossils are two methods to define the age of rocks.
Rock units are the initial stratigraphic ranking of rocks. Ranking from largest to finest the order is Group, Formation, Member, bed.
The most common use of the rock unit classification is the Formation. A formation is a layer of rock that is uniform in its makeup, corresponds to other layers deposited during that period, and can be defined (by recognition, fossil correlation or other means) over an area large enough to be traced and mapped. Geologists call this lithographic and stratigraphic homogeneity. Members subdivide a formation when there are distinctive changes in some of the minor component. These changes can be an increase or decrease in the number of shale, limestone or sandstone beds in a relatively short vertical section of the formation.
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 mostly at the formation level with groups sometimes included. Occasionally, geologic maps do not separate two or more formations. The term undivided is applied when differentiation is not possible due to insufficient map data, unexposed sections, or topographic relief. 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.
Time-rock units, System, Series, and Stage, provide the concept of a sequence in time. Time-rock units do not equate with a specific time interval. Instead, they define the duration of a geologic event in a geographic region.
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 (97.51 to 91 ma) is in the Late Epoch (97.5 to 66.4 ma) of the Cretaceous Period ( 144 to 66.4 ma).
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.
|Quaternary:||Alluvium and terrace deposits||Dallas and Tarrant|
|Cretaceous:||Ozan (Lower Taylor Marl)||Dallas|
|Austin Chalk (often referred to as a Group)||Dallas|
|Eagle Ford (often referred to as a Group)||Dallas and Tarrant|
|Woodbine (often referred to as a Group)||Dallas and Tarrant|
|Main Street Limestone||Tarrant|
|Paw Paw Formation||Tarrant|
|Fort Worth Limestone||Tarrant|
|Duck Creek Limestone||Tarrant|
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.
Quaternary deposits unconformably overlie the Ozan formation. The hiatus lasted about 70 million years.
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 nodular 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 discussed 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, second edition, Fossil Collector's Guidebook to the North Sulphur River, by Mark McKinzie, Ron Morn and Ed Swiatovy, 2001 and 2019 (second edition) are excellent resources.
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.
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 andPtychodus, 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.
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.
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).
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, Grypheaand Alectryonia; brachiopod: Kingena wacoensis, and nautiloids
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.
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.
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.
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 Perot Museum of Nature and Science are found in the Paluxy Formation in Parker and Jack Counties.