Geology of the Dallas-Fort Worth Metroplex
Encyclopedia
Dallas–Fort Worth sits above Cretaceous
-aged strata
, dates ranging from ~145-65 Ma (million years ago). These Cretaceous-aged sediment
s lie above the erode
d 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 gets progressively younger. The Cretaceous sediments dip very gently (about 1°) to the east.
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 Jurassic
time and 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 by boreholes and other data. In west Texas near Marathon, the mountain range makes an appearance to the surface, and is known as the Marathon Uplift
. To the north of the DFW Metroplex, we can see the roots of these mountains in SE Oklahoma. We know of these today as the Arbuckle mountains, despite the fact that they are far from what the untrained eye would consider a former vast mountain range.
The Marathon
-Ouachita
-Appalachian
-Variscan
cordillera, which stretched through central Texas, around Arkansas, up through the Rockies and eventually into eastern Europe
occurred when the supercontinent
s 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 where uplifted and exposed at the surface. These billion year old rocks can only be seen several thousand feet in the subsurface by boreholes and other data in the DFW Metroplex.
The Fort Worth Basin which lies beneath Cretaceous sediments west of Dallas formed as a foreland basin
during the Ouachita orogeny. Horizontal shortening caused flexual 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
moves 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 be moved 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 world wide 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 eventually died and sunk 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 CO₂ levels to around 2-6 times the current level. The increase in CO₂ levels along with the extra production of crust caused global temperatures to rise, which would also play 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.
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 40-200ft 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 65Ma. 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 lye’s 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 175-250ft, 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 330ft, 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 of the increased amount of CO₂ during the time of its deposition. Occurrence of an oceanic anoxia event which contributed to the development of the Eagle Ford was the last in the Cretaceous. 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 shale’s 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.
Sea level rose even higher in this massive transgressive event, which helped form the 300-500ft thick Austin Chalk
, around ~89-85mya. The Austin formation consists of recrystalized, fossiliferous, interbedded chalks and marls. Exposures of Austin chalk are mainly seen in quarries, roadcuts, and stream beds where the water eroded the top soil. Austin chalk is the well known white rock that the city of Dallas sits on. Volcanic ashes are present in the Austin chalk, and were deposited by wind from distant erupting volcanoes and erupting igneous intrusions around 86 Ma. These eruptions occurred along a 250 mile long by 50 mile wide belt of submarine volcanoes, which are located in present day south-central Texas. This belt of volcanoes coincides with the trend of the Balcones Fault zone and is known as the Balcones volcanic province. Of what consisted of these ancient volcanoes is only visible in a few places since most were buried by the Austin and Taylor Groups, and now are in the subsurface. The presence of this volcanism during deposition of the Austin Chalk is correlated with the Laramide orogeny
. Sea level rose for conditions to be right for the deposition of the Austin Chalk, which also coincides with the maximum extent of the Cretaceous Interior Seaway. The depths of the deposition of the Austin Chalk occurred in ~250m or 820ft of water. 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.
On top of the Austin Chalk are several different layers of beds known as the Taylor formation. Deposition of the Taylor beds marks the point of eustatic regression which continued until the end of the Cretaceous period. Ozan Marl is the first bed overlying the Austin chalk and can be found underneath the city of Richardson and Garland. The Ozan Marl consists of calcareous micaeous clay with increasing silt and sand towards the top. The environment that hosted the deposition of the Ozan marl was still in a relatively deep marine and low energy environment, but began to see sedimentation dominated by mud since the sea level was falling. The Ozan marl is around 500ft thick, and marine megafossils can be found. On top of the Ozan marl is two thin beds known as the Wolfe City Sandstone and Pecan Gap Chalk. The Wolfe City Sandstone is known in the city of Rockwall, Texas
for its sandstone dikes that protrude into the surface, and gives a surface expression of a wall built of rocks. The very top of the Taylor has a 300 foot section of marls known as the Marlbrook Marl.
The last beds of the Cretaceous, which are also deposited directly over the Taylor formation and found east of Dallas are the Navarro beds. 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. During the end of deposition of the Taylor formation, the eustatic regression
had brought sea-level down to the present day shoreline of the Gulf of Mexico. This world-wide regression marks the slowing of sea-floor spreading and bounds the Gulfian Series in a sequence boundary. In the western part of the country the Laramide Orogeny
that was building the Rockies Mountains started to accommodate horizontal shortening by uplift versus the previous folding and thrusting accommodation. Also during this time around 65mya 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
.
are found. One species of Mosasaur was named after the city: Dallasaurus turneri.
Dinosaur Valley State Park
is located in Glen Rose Texas off 67 and 144 which is south west of Fort Worth. Dinosaur Valley State Park has some of the worlds best preserved dinosaur tracks. The types of tracks came from the carnivorous Theropod, and the vegetarian Sauropod.
Woodbine Formation located between Dallas and Fort Worth consists of marine and terrestrial fossils such as concretions and trace fossils, including the last known dinosaurs in this part of Texas. Directly east of the Woodbine which is directly west of Dallas is the Eagle Ford, which consists of sharks teeth, Plesiosaurs, crabs, and little marine lizards called Coniasaurus. The North-West qaudrant of the I-20 and 408 loop is abundant in sharks teeth. Directly east of 408 on Kiest Blvd is a large section of Eagle Ford Shale outcropping beneath the Austin Chalk. This location right before you reach the top of the hill contain good amounts of sharks teeth in the Eagle Ford.
Directly beneath Dallas you can find Ammonites, although they are rare. Sharks 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. Although not viewable with the naked eye, Austin Chalk is derived and consists of millions of coccoliths, which are calcite derived from coccolithophores.
has been important in shaping the DFW Metroplex. 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. This provided a place where travelers need only cross the river once, at a place with relatively firm landings and bottoms. This was the best place to cross the Trinity from the earliest days, best for fordings, ferries, and bridges. During the days of the Republic of Texas
, the DFW Metroplex was mostly uninhabited by Europeans, but settlers began to find their way north in the 1840s. The route north naturally followed the low hills and gentle ridges of Austin chalk hills to the river ford that soon became Dallas. 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.
Dallas was also affected subtly by much younger geologic formations deposited by an older, more vigorous Trinity River. The northern hemisphere Ice age
occurred in Pleistocene
time, when a continental ice sheet
reached as far south as Kansas
during the Wisconsin glaciation
. The ice age climate had two effects on the Trinity River: It caused downcutting (few people know that there is a 100 metres (328 ft) deep buried canyon beneath the Trinity in Dallas) , and a wetter climate caused much more water to flow in the river. The greater river flow generated great sedimentary river terraces. From time to time these terrace deposits reveal bones of extinct giant mammals, such as Mastodons and Mammoths . The Pleistocene terraces affected the development of Dallas, providing a rich alluvial soil and a perched aquifer
, very useful indeed during the early years. Downtown Dallas
is built on a series of these terraces, rising subtly eastward from the Trinity river.
. The larger rivers are longer and flow over salt-bearing Permian
sediments, well west of the Trinity headwaters. The Trinity is consequently sweeter water than either the larger Brazos or Red rivers. 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. Because the Trinity is not suitable for navigation, the Metroplex could not have grown to be a large city until the railroad arrived, which happened early in the Metroplex's history, in the early 1870s. (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.
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
-aged strata
Stratum
In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...
, dates ranging from ~145-65 Ma (million years ago). These Cretaceous-aged sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
s lie above the erode
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
d Ouachita Mountains
Ouachita Mountains
The Ouachita Mountains are a mountain range in west central Arkansas and southeastern Oklahoma. The range's subterranean roots may extend as far as central Texas, or beyond it to the current location of the Marathon Uplift. Along with the Ozark Mountains, the Ouachita Mountains form the U.S...
and the Fort Worth Basin, which was formed by the Ouachita Orogeny
Ouachita orogeny
The Ouachita orogeny was a mountain building event that resulted in the folding and faulting of strata currently exposed in the Ouachita Mountains...
. Going from west to east in the DFW Metroplex and down towards the Gulf of Mexico
Gulf of Mexico
The Gulf of Mexico is a partially landlocked ocean basin largely surrounded by the North American continent and the island of Cuba. It is bounded on the northeast, north and northwest by the Gulf Coast of the United States, on the southwest and south by Mexico, and on the southeast by Cuba. In...
, the strata gets 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 CretaceousCretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
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
Weatherford, Texas
Weatherford is a city in Parker County, Texas, United States, and a western suburb of Fort Worth. The population was 19,000 at the 2000 census. It is the county seat of Parker County and is part of the Dallas-Fort Worth Metroplex.-Geography:...
. Ancient folded mountains formed by the Ouachita orogeny
Ouachita orogeny
The Ouachita orogeny was a mountain building event that resulted in the folding and faulting of strata currently exposed in the Ouachita Mountains...
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
Gulf of Mexico
The Gulf of Mexico is a partially landlocked ocean basin largely surrounded by the North American continent and the island of Cuba. It is bounded on the northeast, north and northwest by the Gulf Coast of the United States, on the southwest and south by Mexico, and on the southeast by Cuba. In...
in Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
time and 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 by boreholes and other data. In west Texas near Marathon, the mountain range makes an appearance to the surface, and is known as the Marathon Uplift
Marathon Uplift
The Marathon Uplift is a Tertiary-age domal uplift, approximately in diameter, in southwest Texas. The Marathon Basin was created by erosion of Cretaceous and younger strata from the crest of the uplift...
. To the north of the DFW Metroplex, we can see the roots of these mountains in SE Oklahoma. We know of these today as the Arbuckle mountains, despite the fact that they are far from what the untrained eye would consider a former vast mountain range.
The Marathon
Marathon, Texas
Marathon is a census-designated place in Brewster County, Texas, United States. The population was 470 in 2007, after growing from 455 in 2000, but had decreased to 430 by 2010.-Geography:Marathon is located at ....
-Ouachita
Ouachita Mountains
The Ouachita Mountains are a mountain range in west central Arkansas and southeastern Oklahoma. The range's subterranean roots may extend as far as central Texas, or beyond it to the current location of the Marathon Uplift. Along with the Ozark Mountains, the Ouachita Mountains form the U.S...
-Appalachian
Appalachian Mountains
The Appalachian Mountains #Whether the stressed vowel is or ,#Whether the "ch" is pronounced as a fricative or an affricate , and#Whether the final vowel is the monophthong or the diphthong .), often called the Appalachians, are a system of mountains in eastern North America. The Appalachians...
-Variscan
Variscan orogeny
The Variscan orogeny is a geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica and Gondwana to form the supercontinent of Pangaea.-Naming:...
cordillera, which stretched through central Texas, around Arkansas, up through the Rockies and eventually into eastern Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
occurred when the supercontinent
Supercontinent
In geology, a supercontinent is a landmass comprising more than one continental core, or craton. The assembly of cratons and accreted terranes that form Eurasia qualifies as a supercontinent today.-History:...
s 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
Gulf of Mexico
The Gulf of Mexico is a partially landlocked ocean basin largely surrounded by the North American continent and the island of Cuba. It is bounded on the northeast, north and northwest by the Gulf Coast of the United States, on the southwest and south by Mexico, and on the southeast by Cuba. In...
opened about 165 Ma, in Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
time.
The oldest rocks in Texas date from the Precambrian
Precambrian
The Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...
, specifically the Mesoproterozoic
Mesoproterozoic
The Mesoproterozoic Era is a geologic era that occurred between 1600 Ma and 1000 Ma . The Mesoproterozoic was the first period of Earth's history with a respectable geological record. Continents existed in the Paleoproterozoic, but we know little about them...
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
Phanerozoic
The Phanerozoic Eon is the current eon in the geologic timescale, and the one during which abundant animal life has existed. It covers roughly 542 million years and goes back to the time when diverse hard-shelled animals first appeared...
sediments, but are exposed in the Llano
Llano
Llano is the Spanish word for plain. It may refer to:* Llano, California* Llano Estacado, a region in northwest Texas and eastern New Mexico* Llano, Texas, a small city in Llano County, Texas* Llano County, Texas* Llano River, a Texan river...
area, where previous Precambrian igneous and metamorphic rocks where uplifted and exposed at the surface. These billion year old rocks can only be seen several thousand feet in the subsurface by boreholes and other data in the DFW Metroplex.
The Fort Worth Basin which lies beneath Cretaceous sediments west of Dallas formed as a foreland basin
Foreland basin
A foreland basin is a depression that develops adjacent and parallel to a mountain belt. Foreland basins form because the immense mass created by crustal thickening associated with the evolution of a mountain belt causes the lithosphere to bend, by a process known as lithospheric flexure...
during the Ouachita orogeny. Horizontal shortening caused flexual isostasy to bend the lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...
. The bent lithosphere to the west of the Ouachita mountains caused a bowl shaped depression to form, known as a foreland basin
Foreland basin
A foreland basin is a depression that develops adjacent and parallel to a mountain belt. Foreland basins form because the immense mass created by crustal thickening associated with the evolution of a mountain belt causes the lithosphere to bend, by a process known as lithospheric flexure...
, preserving the Mississippian sediments of the Barnett Shale
Barnett Shale
The Barnett Shale is a geological formation located in the Bend Arch-Fort Worth Basin. It consists of sedimentary rocks of Mississippian age in Texas...
and other Paleozoic
Paleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
sediments; these sediments mostly formed before the Pangeic collision. Significant deposits of hydrocarbons such as natural gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...
have economic importance as is seen in formations like the Barnett Shale
Barnett Shale
The Barnett Shale is a geological formation located in the Bend Arch-Fort Worth Basin. It consists of sedimentary rocks of Mississippian age in Texas...
.
Pangea started to break up during the Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
~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
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
, around ~165 Ma. Sea floor spreading is where new oceanic lithosphere is being created by upwelling
Upwelling
Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water. The increased availability in upwelling regions results in high levels of primary...
of material, unlike rifting where it only involved the stretching of the crust. Convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
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
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...
moves 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 be moved apart. Seafloor spreading
Seafloor spreading
Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. Seafloor spreading helps explain continental drift in the theory of plate tectonics....
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 world wide oscillatory increases in ocean floor spreading rates. The increase in the amount of basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
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
Transgression
Transgression may be:*a Biblical transgression, violation of God's ten commandments; sin *a legal transgression, a crime usually created by a social or economic boundary*a social transgression, violating a norm...
, 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
Calcareous
Calcareous is an adjective meaning mostly or partly composed of calcium carbonate, in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines.-In zoology:...
-shelled organisms, which eventually died and sunk to the bottom of the ocean floor creating thick deposits of limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
. In addition to the displacement of water, an increase in injected magma raised CO₂ levels to around 2-6 times the current level. The increase in CO₂ levels along with the extra production of crust caused global temperatures to rise, which would also play 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 100 kilometres (62 mi) wide N-S trending belt of outcropping CretaceousCretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
sediments. Fort Worth in the west is neatly built on Early Cretaceous
Early Cretaceous
The Early Cretaceous or the Lower Cretaceous , is the earlier or lower of the two major divisions of the Cretaceous...
(Comanche Series) and Dallas in the east is built on Late Cretaceous
Late Cretaceous
The Late Cretaceous is the younger of two epochs into which the Cretaceous period is divided in the geologic timescale. Rock strata from this epoch form the Upper Cretaceous series...
(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
Marine regression
Marine regression is a geological process occurring when areas of submerged seafloor are exposed above the sea level. The opposite event, marine transgression, occurs when flooding from the sea covers previously exposed land....
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
Balcones Fault
The Balcones Fault Zone is a tensional structural system in Texas that runs approximately from the southwest part of the state near Del Rio to the north central region near Waco along Interstate 35. The Balcones Fault zone is made up of many smaller features, including normal faults, grabens, and...
Zone, and span from slightly west of Weatherford
Weatherford
Weatherford may refer to:Towns and cities*Weatherford, Texas, medium city in Parker County*Weatherford, Oklahoma, small city in Custer CountyPeople...
to the east side of Fort Worth. The Trinity Group is best known for the Glen Rose
Glen Rose
-External links:* at College Basketball at Sports-Reference.com...
Formation that lies within it. The 40-200ft 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
Terrestrial animal
Terrestrial animals are animals that live predominantly or entirely on land , as compared with aquatic animals, which live predominantly or entirely in the water , or amphibians, which rely on a combination of aquatic and terrestrial habitats...
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
Trace fossil
Trace fossils, also called ichnofossils , are geological records of biological activity. Trace fossils may be impressions made on the substrate by an organism: for example, burrows, borings , urolites , footprints and feeding marks, and root cavities...
. 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
Dinosaur Valley State Park
- History :Dinosaur Valley State Park, located just northwest of Glen Rose in Somervell County, is a scenic park set astride the Paluxy River. The land for the park was acquired from private owners under the State Parks Bonds Program during 1968 and opened to the public in 1972...
.
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 65Ma. The Gulfian Series is known for the Cenomanian-Turonian transgression
Transgression
Transgression may be:*a Biblical transgression, violation of God's ten commandments; sin *a legal transgression, a crime usually created by a social or economic boundary*a social transgression, violating a norm...
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 lye’s 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
Woodbine
Woodbine may refer to:* Woodbine , advertising agency in North Carolina* Woodbine , one of various species* Woodbine , a tobacco product - a buoy tender of the United States Coast Guard-Places:Australia...
ranges in thickness of 175-250ft, and thickens northward. Terrigenous
Terrigenous
In oceanography, terrigenous sediments are those derived from the erosion of rocks on land; that is, that are derived from terrestrial environments. Consisting of sand, mud, and silt carried to sea by rivers, their composition is usually related to their source rocks; deposition of these sediments...
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
Woodbine
Woodbine may refer to:* Woodbine , advertising agency in North Carolina* Woodbine , one of various species* Woodbine , a tobacco product - a buoy tender of the United States Coast Guard-Places:Australia...
, 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 330ft, and occurred around 20-50 km from the shore. The depositional environment in the lower beds was low-energy and a slightly anoxic
Hypoxia (environmental)
Hypoxia, or oxygen depletion, is a phenomenon that occurs in aquatic environments as dissolved oxygen becomes reduced in concentration to a point where it becomes detrimental to aquatic organisms living in the system...
setting. This anoxic setting of the deeper waters in the ocean is a result of the increased amount of CO₂ during the time of its deposition. Occurrence of an oceanic anoxia event which contributed to the development of the Eagle Ford was the last in the Cretaceous. 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
Fauna
Fauna or faunæ is all of the animal life of any particular region or time. The corresponding term for plants is flora.Zoologists and paleontologists use fauna to refer to a typical collection of animals found in a specific time or place, e.g. the "Sonoran Desert fauna" or the "Burgess shale 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 shale’s is known as the Kamp Ranch. A small regressive highstand occurred to have formed this carbonate
Carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....
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.
Sea level rose even higher in this massive transgressive event, which helped form the 300-500ft thick Austin Chalk
Austin Chalk
The Austin Chalk is an upper Cretaceous geologic formation in the Gulf Coast region of the United States. It is named after type section outcrops near Austin, Texas...
, around ~89-85mya. The Austin formation consists of recrystalized, fossiliferous, interbedded chalks and marls. Exposures of Austin chalk are mainly seen in quarries, roadcuts, and stream beds where the water eroded the top soil. Austin chalk is the well known white rock that the city of Dallas sits on. Volcanic ashes are present in the Austin chalk, and were deposited by wind from distant erupting volcanoes and erupting igneous intrusions around 86 Ma. These eruptions occurred along a 250 mile long by 50 mile wide belt of submarine volcanoes, which are located in present day south-central Texas. This belt of volcanoes coincides with the trend of the Balcones Fault zone and is known as the Balcones volcanic province. Of what consisted of these ancient volcanoes is only visible in a few places since most were buried by the Austin and Taylor Groups, and now are in the subsurface. The presence of this volcanism during deposition of the Austin Chalk is correlated with the Laramide orogeny
Laramide orogeny
The Laramide orogeny was a period of mountain building in western North America, which started in the Late Cretaceous, 70 to 80 million years ago, and ended 35 to 55 million years ago. The exact duration and ages of beginning and end of the orogeny are in dispute, as is the cause. The Laramide...
. Sea level rose for conditions to be right for the deposition of the Austin Chalk, which also coincides with the maximum extent of the Cretaceous Interior Seaway. The depths of the deposition of the Austin Chalk occurred in ~250m or 820ft of water. 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.
On top of the Austin Chalk are several different layers of beds known as the Taylor formation. Deposition of the Taylor beds marks the point of eustatic regression which continued until the end of the Cretaceous period. Ozan Marl is the first bed overlying the Austin chalk and can be found underneath the city of Richardson and Garland. The Ozan Marl consists of calcareous micaeous clay with increasing silt and sand towards the top. The environment that hosted the deposition of the Ozan marl was still in a relatively deep marine and low energy environment, but began to see sedimentation dominated by mud since the sea level was falling. The Ozan marl is around 500ft thick, and marine megafossils can be found. On top of the Ozan marl is two thin beds known as the Wolfe City Sandstone and Pecan Gap Chalk. The Wolfe City Sandstone is known in the city of Rockwall, Texas
Rockwall, Texas
Rockwall is a city in Rockwall County, Texas, United States, and a suburb of Dallas. It is part of the Dallas/Fort Worth Metroplex. The population was 37,490 at the 2010 census. It is the county seat of Rockwall County. Rockwall County is the second wealthiest county in the state of Texas in terms...
for its sandstone dikes that protrude into the surface, and gives a surface expression of a wall built of rocks. The very top of the Taylor has a 300 foot section of marls known as the Marlbrook Marl.
The last beds of the Cretaceous, which are also deposited directly over the Taylor formation and found east of Dallas are the Navarro beds. Navarro beds reflect anoxic waters at the time due to the shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
present, and are a result of increased volcanic activity in the south-western part of the United States. During the end of deposition of the Taylor formation, the eustatic regression
Marine regression
Marine regression is a geological process occurring when areas of submerged seafloor are exposed above the sea level. The opposite event, marine transgression, occurs when flooding from the sea covers previously exposed land....
had brought sea-level down to the present day shoreline of the Gulf of Mexico. This world-wide regression marks the slowing of sea-floor spreading and bounds the Gulfian Series in a sequence boundary. In the western part of the country the Laramide Orogeny
Laramide orogeny
The Laramide orogeny was a period of mountain building in western North America, which started in the Late Cretaceous, 70 to 80 million years ago, and ended 35 to 55 million years ago. The exact duration and ages of beginning and end of the orogeny are in dispute, as is the cause. The Laramide...
that was building the Rockies Mountains started to accommodate horizontal shortening by uplift versus the previous folding and thrusting accommodation. Also during this time around 65mya a major extinction including dinosaurs took place, and is believed to have been caused by a meteorite hitting Mexico off the Yucatan Peninsula
Yucatán Peninsula
The Yucatán Peninsula, in southeastern Mexico, separates the Caribbean Sea from the Gulf of Mexico, with the northern coastline on the Yucatán Channel...
. 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
Cenozoic
The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...
and Paleogene
Paleogene
The Paleogene is a geologic period and system that began 65.5 ± 0.3 and ended 23.03 ± 0.05 million years ago and comprises the first part of the Cenozoic Era...
.
Fossils
People enjoy searching for fossils in the rocks around Dallas. Remnants of dinosaurs and Late Cretaceous marine reptiles such as MosasaurMosasaur
Mosasaurs are large extinct marine lizards. The first fossil remains were discovered in a limestone quarry at Maastricht on the Meuse in 1764...
are found. One species of Mosasaur was named after the city: Dallasaurus turneri.
Dinosaur Valley State Park
Dinosaur Valley State Park
- History :Dinosaur Valley State Park, located just northwest of Glen Rose in Somervell County, is a scenic park set astride the Paluxy River. The land for the park was acquired from private owners under the State Parks Bonds Program during 1968 and opened to the public in 1972...
is located in Glen Rose Texas off 67 and 144 which is south west of Fort Worth. Dinosaur Valley State Park has some of the worlds best preserved dinosaur tracks. The types of tracks came from the carnivorous Theropod, and the vegetarian Sauropod.
Woodbine Formation located between Dallas and Fort Worth consists of marine and terrestrial fossils such as concretions and trace fossils, including the last known dinosaurs in this part of Texas. Directly east of the Woodbine which is directly west of Dallas is the Eagle Ford, which consists of sharks teeth, Plesiosaurs, crabs, and little marine lizards called Coniasaurus. The North-West qaudrant of the I-20 and 408 loop is abundant in sharks teeth. Directly east of 408 on Kiest Blvd is a large section of Eagle Ford Shale outcropping beneath the Austin Chalk. This location right before you reach the top of the hill contain good amounts of sharks teeth in the Eagle Ford.
Directly beneath Dallas you can find Ammonites, although they are rare. Sharks teeth are present and primarily easy to find in condensed zones, along with clam shells (Inoceramus
Inoceramus
Inoceramus is an extinct genus of fossil marine pteriomorphian bivalves that superficially resembled the related winged pearly oysters of the extant genus Pteria....
) which may also be found in the Austin Chalk. Although not viewable with the naked eye, Austin Chalk is derived and consists of millions of coccoliths, which are calcite derived from coccolithophores.
Trinity River
The Trinity RiverTrinity River (Texas)
The Trinity River is a long river that flows entirely within the U.S. state of Texas. It rises in extreme north Texas, a few miles south of the Red River. The headwaters are separated by the high bluffs on the south side of the Red River....
has been important in shaping the DFW Metroplex. 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. This provided a place where travelers need only cross the river once, at a place with relatively firm landings and bottoms. This was the best place to cross the Trinity from the earliest days, best for fordings, ferries, and bridges. During the days of the Republic of Texas
Republic of Texas
The Republic of Texas was an independent nation in North America, bordering the United States and Mexico, that existed from 1836 to 1846.Formed as a break-away republic from Mexico by the Texas Revolution, the state claimed borders that encompassed an area that included all of the present U.S...
, the DFW Metroplex was mostly uninhabited by Europeans, but settlers began to find their way north in the 1840s. The route north naturally followed the low hills and gentle ridges of Austin chalk hills to the river ford that soon became Dallas. The future site of Dallas was selected by John Neely Bryan
John Neely Bryan
John Neely Bryan was a Presbyterian farmer, lawyer, and tradesman in the United States and founder of the city of Dallas, Texas.- Early life :...
as the place for his trading post to overlook the ferry
Ferry
A ferry is a form of transportation, usually a boat, but sometimes a ship, used to carry primarily passengers, and sometimes vehicles and cargo as well, across a body of water. Most ferries operate on regular, frequent, return services...
that he operated at the crossing.
Dallas was also affected subtly by much younger geologic formations deposited by an older, more vigorous Trinity River. The northern hemisphere Ice age
Ice age
An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...
occurred in Pleistocene
Pleistocene
The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
time, when a continental ice sheet
Continental Glacier
Continental Glacier is located in Bridger-Teton and Shoshone National Forests, in the U.S. state of Wyoming and straddles the Continental Divide in the northern Wind River Range. Continental Glacier is in both the Bridger and Fitzpatrick Wildernesses, and is part of the largest grouping of glaciers...
reached as far south as Kansas
Kansas
Kansas is a US state located in the Midwestern United States. It is named after the Kansas River which flows through it, which in turn was named after the Kansa Native American tribe, which inhabited the area. The tribe's name is often said to mean "people of the wind" or "people of the south...
during the Wisconsin glaciation
Wisconsin glaciation
The last glacial period was the most recent glacial period within the current ice age occurring during the last years of the Pleistocene, from approximately 110,000 to 10,000 years ago....
. The ice age climate had two effects on the Trinity River: It caused downcutting (few people know that there is a 100 metres (328 ft) deep buried canyon beneath the Trinity in Dallas) , and a wetter climate caused much more water to flow in the river. The greater river flow generated great sedimentary river terraces. From time to time these terrace deposits reveal bones of extinct giant mammals, such as Mastodons and Mammoths . The Pleistocene terraces affected the development of Dallas, providing a rich alluvial soil and a perched aquifer
Aquifer
An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
, very useful indeed during the early years. Downtown Dallas
Downtown Dallas
Downtown Dallas is the Central Business District in Dallas, Texas USA, located in the geographic center of the city. The area termed "Downtown" has traditionally been defined as bounded by the downtown freeway loop: bounded on the east by I-345 Downtown Dallas is the Central Business District...
is built on a series of these terraces, rising subtly eastward from the Trinity river.
Water quality
The DFW Metroplex had an additional, if subtle, geologic advantage. The Trinity is not good for navigation by boats but is great for drinking. Trinity River water is better than either of the larger rivers to the north and south, the Red River and the Brazos RiverBrazos River
The Brazos River, called the Rio de los Brazos de Dios by early Spanish explorers , is the longest river in Texas and the 11th longest river in the United States at from its source at the head of Blackwater Draw, Curry County, New Mexico to its mouth at the Gulf of Mexico with a drainage...
. The larger rivers are longer and flow over salt-bearing Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
sediments, well west of the Trinity headwaters. The Trinity is consequently sweeter water than either the larger Brazos or Red rivers. 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. Because the Trinity is not suitable for navigation, the Metroplex could not have grown to be a large city until the railroad arrived, which happened early in the Metroplex's history, in the early 1870s. (See: History of Dallas, Texas (1874-1929)
History of Dallas, Texas (1874-1929)
The history of Dallas, Texas, United States from 1874 to 1929 documents the city's rapid growth and emergence as a major center for transportation, trade and finance. Originally a small community built around agriculture, the convergence of several railroads made the city a strategic location for...
) 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.