The Dallas–Fort Worth Metroplex sits above Cretaceous-age strata ranging from ≈145-66 Ma (million years ago). These Cretaceous-aged sediments lie above the eroded Ouachita Mountains and the Fort Worth Basin, which was formed by the Ouachita Orogeny. Going from west to east in the DFW Metroplex and down towards the Gulf of Mexico, the strata get progressively younger. The Cretaceous sediments dip very gently (about 1°) to the east.
Structural and tectonic history of the DFW Metroplex
The geology of the DFW Metroplex consists of gently tilted sediments of mostly Cretaceous age, which also obscures a much older geologic record. Sediments older than Cretaceous can only be found at the surface in the extreme western part of the DFW Metroplex, in the area around Weatherford, Texas. Ancient folded mountains formed by the Ouachita orogeny existed in the eastern part of the Metroplex 300 million years ago. These ancient mountains were reduced by erosion and rifting associated with the opening of the Gulf of Mexico in the Jurassic and then were buried beneath younger Cretaceous sediments. Although the Ouachita Mountain roots are not visible in the DFW Metroplex since they are buried, they can still be recognized through boreholes and other data. In west Texas near Marathon, the mountain range appears on the surface, and is known as the Marathon Uplift. To the north of the DFW Metroplex, the roots of the mountains can be identified in SE Oklahoma as the Arbuckle mountains, heavily eroded from the original vast mountain range.
The Marathon-Ouachita-Appalachian-Variscan cordillera, which stretched through central Texas, around Arkansas, up through the Appalachian Mountains and eventually into eastern Europe, occurred when the supercontinents Pangea and Laurussia collided to form Pangea in the late Paleozoic ≈300 Ma. The zone of deformation known as the Ouachitas marks a zone of weakness that was exploited when the Gulf of Mexico opened about 165 Ma, in Jurassic time.
The oldest rocks in Texas date from the Precambrian, specifically the Mesoproterozoic and are about 1,600 million years old, and mark the southern limit of the North American craton. These rocks are mostly buried beneath Phanerozoic sediments, but are exposed in the Llano area, where previous Precambrian igneous and metamorphic rocks were uplifted and exposed at the surface.
The Fort Worth Basin which lies beneath Cretaceous sediments west of Dallas formed as a foreland basin during the Ouachita orogeny. Horizontal shortening caused flexural isostasy to bend the lithosphere. The bent lithosphere to the west of the Ouachita mountains caused a bowl-shaped depression to form, known as a foreland basin, preserving the Mississippian sediments of the Barnett Shale and other Paleozoic sediments; these sediments mostly formed before the Pangeic collision. Significant deposits of hydrocarbons such as natural gas have economic importance as is seen in formations like the Barnett Shale.
Pangea started to break up during the Triassic ≈225Ma. Rifting affected regions which became the central Atlantic (between North America and Africa) and the Gulf of Mexico at about the same time. This rifting created a divergent plate margin that would play an integral role of the future geologic processes to follow. Rifting, which involves the stretching of pre-existing crust and mantle lithosphere, was initiated by the existence of sufficient horizontal deviatoric tensional stress that broke the lithosphere. Eventually rifting gave way to sea floor spreading in the Atlantic and Gulf of Mexico in the mid Jurassic, around ≈165 Ma. Sea floor spreading is where new oceanic lithosphere is being created by upwelling of material, unlike rifting where it only involved the stretching of the crust. Convection currents in the sub-lithospheric mantle are the driving mechanisms that caused sea floor spreading to occur. New lithosphere is made when hot material beneath ocean ridges is brought to the surface by these cells. As the new lithosphere moved horizontally away from the ridges, the new crust added to the Gulf of Mexico and the Atlantic caused the continents of North America and South America to move apart. Seafloor spreading in the Gulf of Mexico ceased by the beginning of the Cretaceous and spreading shifted to the proto-Caribbean.
Around 110-85 Ma, there was worldwide oscillatory increases in ocean floor spreading rates. The increase in the amount of basalt being injected into the ocean caused a displacement of water from the ocean basins, which resulted in sea level rise, flooding the coasts of the Texas margin and other bordering continents around the world. The major sea level rise that took place due to an occurrence of an oscillation is known as the Cenomanian transgression, which is the most well known and last major transgression in the Cretaceous. The dispersal of extra magma warmed the water in the ocean, and was a conducive environment for calcareous-shelled organisms, which after death sank to the bottom of the ocean floor creating thick deposits of limestone. In addition to the displacement of water, an increase in injected magma raised carbon dioxide levels to around 2-6 times the current level. This increase along with the extra production of crust caused global temperatures to rise, which also played an integral role in the future development of different Cretaceous formations. When the sea floor spreading rates slowed around ≈85 Ma, so did the amount of basaltic material being thrown into the ocean which caused the initial water displacement. As seen around the DFW Metroplex, the Cretaceous rocks deposited during this time were directly influenced by increased sea floor spreading rates.
Depositional environment for the rocks of the DFW Metroplex
The DFW Metroplex sprawls across a wide N-S trending belt of outcropping Cretaceous sediments. Fort Worth in the west is neatly built on Early Cretaceous (Comanche Series) and Dallas in the east is built on Late Cretaceous (Gulf Series) sediments. The Cretaceous rocks of the Comanche Series were deposited over a 20 million year interval. The sediments deposited during these 20 million years are bound within a sequence boundary, and are defined by a major regression at the end. The time frame of the Comanche Series span between ≈118-98mya, and are responsible for the deposition of the Trinity, Fredsrickberg, and Washita Groups. These three Groups all lie west of the Balcones Fault Zone, and span from slightly west of Weatherford to the east side of Fort Worth. The Trinity Group is best known for the Glen Rose Formation that lies within it. The 40–200 ft thick beds of the Glen Rose formation are composed of a limestone with alternating units consisting of clay, marl, and sand. The depositional environment of the Glen Rose was a shallow marine to shoreline environment. This shoreline environment would eventually bring notoriety to the Glen Rose since it would eventually preserve dinosaur tracks. This process would occur when living terrestrial creatures would roam about and look for food near the shoreline. As they would do this, they would leave footprints and trackways that would eventually be preserved by mud depositing in and on top of the footprints. Eventually more formations would be deposited on top of the mud layers, and build essentially a 100 million year time capsule of the trace fossil. As time passed, weathering from water and wind caused the overlying sediments to erode and expose the footprints, and hence the reason why dinosaur tracks are present in the Glen Rose Dinosaur Valley State Park.
Cretaceous formations that lie from eastern Fort Worth to east of Dallas are part of the Gulfian Series. Easterly progration of the formations go from west to east which also slowly get younger in age, ranging from ≈97Ma to 66 Ma. The Gulfian Series is known for the Cenomanian-Turonian transgression which deposited the mid-Cretaceous formations in the DFW Metroplex. The Gulfian Series consists of three groups, from oldest to youngest being the Dakota, Colorado and Montana. First of the Gulfian Series is the Woodbine formation which lies in the Dakota Group, and formed in a high energy depositional environment since it is composed mostly of large rounded grain quartz sediments. Present in the Woodbine Formation is marine and terrestrial sediments and fossils, including the last known dinosaurs in this part of Texas, dating back around ≈96mya. The Woodbine ranges in thickness of 175–250 ft, and thickens northward. Terrigenous sediments that eroded from Paleozoic rocks, and weakly metamorphosed sedimentary rocks from the Ouachita Mountains in Oklahoma constitute a large part of the Woodbine.
Transgression continued to occur after the deposition of the Woodbine, and created the Colorado Group which first created the Eagle Ford Shale which lies directly beneath west Dallas. The Eagle Ford Shale had sea level depths of around ≈100m or 330 ft, and occurred around 20–50 km from the shore. The depositional environment in the lower beds was low-energy and a slightly anoxic setting. This anoxic setting of the deeper waters in the ocean is a result several factors including increased global levels, higher sea levels, increased organic productivity and decay,Schlanger, S.O. and Jenkyns, H.C., 1976, Cretaceous oceanic anoxic events: Causes and consequences: Geologie en Mijnbouw, v. 55, p. 179-184. Milankovitch cyclicity,Mitchell et al. (2008) and restricted thermohaline currents in the Western Interior Seaway.Arthur, M.A., Schlanger, S.O., and Jenkyns, H.C., 1987, The Cenomanian-Turonian Oceanic Anoxic Event, II. Palaeoceanographic controls on organic-matter production and preservation: Geological Society of London Special Publications, v. 26, p. 401-420. The lower section of the Eagle Ford consists of organic-rich, pyritic, and fossiliferous marine shale's which marks the maximum flooding surface, or peak of deepest water in its deposition. The different fauna that was present in the Eagle Ford suggests the waters were calm and within the photic zone. A small member of the Eagle Ford that consists of a thin limestone unit between shales is known as the Kamp Ranch. A small regressive highstand occurred to have formed this carbonate layer towards the top of the Eagle Ford, which is known due to the high energy traits it displays such as ripple marks from storm generated waves and interbedded carbonaceous siltstones. The overall thickness of the Eagle Ford Group undivided is around 200–300 feet thick.