Geology of the Dallas–Fort Worth Metroplex

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.

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.

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.

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.

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.

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.

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.

[3] 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.

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 Austin Chalk formation is filled with micro-organisms known as coccoliths, and are a product of the warm waters that were displaced from the increased sea-floor spreading at the time.

Navarro beds reflect anoxic waters at the time due to the shale present, and are a result of increased volcanic activity in the south-western part of the United States.

Also during this time around 66 mya a major extinction including dinosaurs took place, and is believed to have been caused by a meteorite hitting Mexico off the Yucatan Peninsula.

All of these occurrences mark the end of an important time for Texas, especially the DFW Metroplex, and brought in a new Era and Period known respectively as the Cenozoic and Paleogene.

Directly east of the Woodbine is the Eagle Ford, where fossilized shark teeth, plesiosaurs, crabs, and small marine lizards called Coniasaurus can be found.

Directly east of 408 on Kiest Blvd is a large section of the Eagle Ford Shale outcropping beneath the Austin Chalk, where fossilized shark teeth are often found.

Shark teeth are present and primarily easy to find in condensed zones, along with clam shells (Inoceramus) which may also be found in the Austin Chalk.

Dallas was situated at the best ford, downstream from where the Elm Fork joins the main stream, where the river flows southeast over the chalk.

The future site of Dallas was selected by John Neely Bryan as the place for his trading post to overlook the ferry that he operated at the crossing.

The Pleistocene terraces affected the development of Dallas, providing a rich alluvial soil and a perched aquifer, very useful indeed during the early years.

Life is better and easier near sweet water, and this simple fact helped DFW prosper relative to settlements on the larger rivers to the north and south.

(See: History of Dallas, Texas (1874–1929)) The Dallas-Fort Worth Metroplex is thus truly a modern metropolitan area, because it could not have grown so large until mechanical transportation systems made the Trinity disadvantage in river navigation insignificant.