Austin Chalk stratigraphic column in Texas

The Austin Chalk is an upper

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

geologic

formation Formation may refer to: Linguistics * Back-formation, the process of creating a new lexeme by removing or affixes * Word formation, the creation of a new word by adding affixes Mathematics and science * Cave formation or speleothem, a secondary ...

in the Gulf Coast region of the United States. It is named after type section outcrops near

Austin, Texas Austin is the capital city of the U.S. state of Texas, as well as the county seat, seat and largest city of Travis County, Texas, Travis County, with portions extending into Hays County, Texas, Hays and Williamson County, Texas, Williamson co ...

. The formation is made up of

chalk Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor. Cha ...

and marl.

Fossils

The putative galloanseran bird '' Austinornis lentus'' has been found in the Austin Chalk.Weishampel, et al. (2004). "Dinosaur distribution." Pp. 517-607. The general absence of dinosaurs is a reflection of the Austin limestone being marine in origin, primarily composed of microscopic shell fragments from floating sea organisms known as "

coccolithophore Coccolithophores, or coccolithophorids, are single celled organisms which are part of the phytoplankton, the autotrophic (self-feeding) component of the plankton community. They form a group of about 200 species, and belong either to the kingdom ...

s" (the same organisms that contributed to the

White Cliffs of Dover The White Cliffs of Dover is the region of English coastline facing the Strait of Dover and France. The cliff face, which reaches a height of , owes its striking appearance to its composition of chalk accented by streaks of black flint, deposi ...

, on the south coast of England). Nevertheless, the Austin Chalk will occasionally produce fossils of larger creatures, such as ''

Inoceramus ''Inoceramus'' (Greek: translation "strong pot") is an extinct genus of fossil marine pteriomorphian bivalves that superficially resembled the related winged pearly oysters of the extant genus '' Pteria''. They lived from the Early Jurassic t ...

'' clams, ammonite cephalopods, and large marine vertebrates such as ''

Xiphactinus ''Xiphactinus'' (from Latin and Greek for "sword-ray") is an extinct genus of large (Shimada, Kenshu, and Michael J. Everhart. "Shark-bitten Xiphactinus audax (Teleostei: Ichthyodectiformes) from the Niobrara Chalk (Upper Cretaceous) of Kansas." ...

'', a predaceous fish.

Geology

The rocks of the Austin Chalk consist of recrystallized, fossiliferous, interbedded

s and marls. Exposures of Austin Chalk are mainly seen in quarries, roadcuts, and stream beds where water eroded the soil. Austin Chalk outcrops can be seen throughout

Dallas Dallas () is the List of municipalities in Texas, third largest city in Texas and the largest city in the Dallas–Fort Worth metroplex, the List of metropolitan statistical areas, fourth-largest metropolitan area in the United States at 7.5 ...

, and extend south underneath I-35 down into Austin and

San Antonio ("Cradle of Freedom") , image_map = , mapsize = 220px , map_caption = Interactive map of San Antonio , subdivision_type = Country , subdivision_name = United States , subdivision_type1= State , subdivision_name1 = Texas , subdivision_t ...

Volcanic ash Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, created during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to refer ...

layers are present in the Austin chalk, and dates obtained from radioactive minerals in these ash layers indicate they were deposited by wind from distant erupting volcanoes around 86 mya. These eruptions occurred along a 250-mile long by 50 mile wide belt of

submarine volcanoes Submarine volcanoes are underwater vents or fissures in the Earth's surface from which magma can erupt. Many submarine volcanoes are located near areas of tectonic plate formation, known as mid-ocean ridges. The volcanoes at mid-ocean ri ...

, 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. Evidence of these ancient volcanoes is only visible in a few places as most were buried by the Austin and

Taylor Group The Taylor Group is a geologic group in Arkansas. It preserves fossils dating back to the Cretaceous period. See also * List of fossiliferous stratigraphic units in Arkansas * Paleontology in Arkansas Paleontology in Arkansas refers to paleon ...

, and now are in the subsurface. The presence of this volcanism during deposition of the Austin Chalk is correlated with the

Laramide orogeny The Laramide orogeny was a time 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 o ...

. 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 Western Interior Seaway (also called the Cretaceous Seaway, the Niobraran Sea, the North American Inland Sea, and the Western Interior Sea) was a large inland sea that split the continent of North America into two landmasses. The ancient sea ...

. The depths of the deposition of the Austin Chalk occurred in ~250 m or 820 ft of water, well-suited to the deposition of

coccoliths Coccoliths are individual plates or scales of calcium carbonate formed by coccolithophores (single-celled phytoplankton such as ''Emiliania huxleyi'') and cover the cell surface arranged in the form of a spherical shell, called a ''coccosphere''. ...

Footnotes

References

* Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.): The Dinosauria, 2nd, Berkeley: University of California Press. 861 pp. .

External links

Cretaceous Arkansas Cretaceous geology of Texas Chalk Austin, Texas Limestone formations of the United States {{Texas-geologic-formation-stub