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Uncle Siphuncle

A Comprehensive Guide To The Cretaceous Strata Of The Greater San Antonio Area; How To Read The Rocks

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Uncle Siphuncle

Fossils of the Del Rio Formation of Bexar County

post-22-0-20444100-1423535268_thumb.jpg Index fossil Ilymatogyra arietina

post-22-0-08987300-1423535271_thumb.jpg Echinoids, perhaps Palhemiaster calvini

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Uncle Siphuncle

Shark Teeth of the Eagle Ford Group of Bexar County - Ptychodus anonymus

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post-22-0-30282900-1423628504_thumb.jpg Note Carcharias tooth far right

Edited by Uncle Siphuncle

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Shark Teeth of the Eagle Ford Group of Bexar County

Cretoxyrhina mantelli

post-22-0-80493500-1423628997_thumb.jpg

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Uncle Siphuncle

Shark Teeth of the Eagle Ford Group of Bexar County

Squalicorax falcatus

post-22-0-18982300-1423629269_thumb.jpg Note oyster hash

post-22-0-51325700-1423629289_thumb.jpg

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post-22-0-71522900-1423629389_thumb.jpg with fish vertebra

Edited by Uncle Siphuncle

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Fish Teeth of the Eagle Ford Group of Bexar County

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Uncle Siphuncle

Ammonites of the Eagle Ford Group of Bexar County

post-22-0-49529400-1423630025_thumb.jpg Unidentified

post-22-0-01527700-1423630029_thumb.jpg

post-22-0-19411500-1423630020_thumb.jpg "Acanthoceras"?

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post-22-0-91429100-1423630232_thumb.jpg Calycoceras?

Edited by Uncle Siphuncle

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Uncle Siphuncle

Austin Group, a.k.a. Austin Chalk

Age: Santonian , Coniacian and perhaps basal Campanian Stages, Austin Group, 83.0-89.8 MYA

Map Abbreviation: Kau

Thickness: 135 to 200 feet thick in Bexar County

Geology: The type area of the Austin Chalk surrounding Austin, Travis County consists of 7 members, listed here from oldest to youngest: Atco, Vinson, Jonah, Dessau, Burditt, Pflugerville, and McKown. In Bexar County, only the Atco, Vinson and Dessau are present.

The Austin Group represents shelf deposits, and its gray to white to yellow chalks and marls are composed of microgranular calcite and foraminifera. It is said to be 85% calcium carbonate. Pyrite nodules are common.

As for the 3 members present in Bexar County, I’m not overly confident in distinguishing them unless I can see contact zones. However, the Atco is the lowest member, and is easily confirmed if the lower contact with the Eagle Ford is exposed. I have not found many fossils in this interval, nor in the overlying Vinson. The Dessau is a gritty, yellowish limestone containing dark specks, giving it a dirtier appearance than the Atco. There is a distinct lithological break at its upper contact with the Pecan Gap Chalk.

Area Exposures: The San Antonio Zoo and Japanese Tea Gardens are built in an old, repurposed Austin Chalk quarry just north of downtown. The large, yellow road cuts along Hwy 281 at Hildebrand Road nearby are also Austin Chalk. I have included GPS coordinates and images of where these exposures lie.

post-22-0-19930600-1423711070_thumb.jpg Overview of Brackenridge Park, San Antonio Zoo, Sunken Gardens, Japanese Tea Gardens area (Google Earth)

post-22-0-20005500-1423751550_thumb.jpg Austin Chalk exposure along 281 northbound, just south of Hildebrand Road (Google Street View)

post-22-0-09005600-1423751553_thumb.jpg Austin Chalk exposure along 281 southbound entrance ramp, entering from Hildebrand Road (Google Street View)

Paleontology: Most of the local Austin Chalk fossils in my collection hail from the Dessau Formation. Fossils other than oysters can be elusive in this interval, but are worth the effort. While the Austin can be exciting to collect, please note that this sequence takes time and dedication to collect successfully, in part due to the slow weathering nature of this rather hard limestone.

Ammonites in particular seem to be concentrated in discrete zones. Baculites, Texanites, Peroniceras, Behavites, Submortoniceras, Prionocycloceras and Puzosia are some of the Austin ammonites found in Bexar County, and some can be rather large. Bowling ball sized Eutrephoceras campbelli are perhaps more common than ammonites in the Austin. While some of these genera can be collected with patient dedication, for someone simply looking to find their first ammonites fast, it would be much more expedient to visit the prolific Washita exposures of North Texas.

Echinoids of the local Austin include Globator vaughni, Conulus stephensoni, Mecaster texanus, Phymosoma sp., and an odd holasteroid currently under study, perhaps Cardiaster sp. or Crassiholaster sp. I have found plates and spines of cidarid echinoids as well, but never a whole test. Echinoids are most often found in the marlier zones.

Other fossils include shark teeth such as Scapanorhynchus and Squalicorax, however these are most often found as hollow blades with the roots dissolved away. Perhaps this explains why I haven’t found some of fish and reptile remains present in equivalent exposures in Central and North Texas.

Crustacean appendage segments occur rarely in marl zones. Inoceramus and other bivalves are rather common, the former often present as fragments. Gastropods are present, but not terribly common. Oysters Exogyra laeviuscula serve as a positive zone marker for the Dessau.

Edited by Uncle Siphuncle

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Ammonites of the Austin Chalk of Bexar County

I still need to ID these.

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Edited by Uncle Siphuncle

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More Austin Chalk ammonites from Bexar County

I need to ID these too.

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Edited by Uncle Siphuncle

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More Austin Chalk Ammonites of Bexar County

post-22-0-10715400-1423716082_thumb.jpg Baculites anceps

post-22-0-19565700-1423716084_thumb.jpg Pachydiscus?

post-22-0-49251100-1423716087_thumb.jpg Peroniceras

post-22-0-60581000-1423716093_thumb.jpg Submortoniceras?

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More ammonites of the Austin Chalk of Bexar County

Texanites or Behavites

post-22-0-38965400-1423716427_thumb.jpg

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Edited by Uncle Siphuncle

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Eutrephoceras c.f. campbelli nautiloids from Bexar County

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More Eutrephoceras c.f. campbelli nautiloids from Bexar County

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post-22-0-89007000-1423716848_thumb.jpg

post-22-0-69515700-1423716886_thumb.jpg

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More echinoids from the Austin Chalk of Bexar County

post-22-0-81404700-1423717548_thumb.jpg Saleniid or Cidarid spines

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post-22-0-21026900-1423717561_thumb.jpg Mecaster texanus

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post-22-0-49558900-1423717572_thumb.jpg Phymosoma?

post-22-0-50137700-1423717575_thumb.jpg

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More echinoids from the Austin Chalk of Bexar County

Phymosoma?

post-22-0-48979000-1423717806_thumb.jpg

post-22-0-58961000-1423717814_thumb.jpg

Edited by Uncle Siphuncle

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More Globator vaughni

post-22-0-90205900-1423719542_thumb.jpg

post-22-0-00501700-1423719583_thumb.jpg

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Anacacho Formation, a.k.a. Lower Taylor Group

Age: Campanian Stage, Taylor Group, about 77.5-81.5 MYA

Map Abbreviation: Kan

Thickness: Up to 200 feet in western Bexar County, thinning rapidly to the east

Geology: This beige to yellow to tan unit has variable lithology. The lower Anacacho is dominated by wackestones and packstones. The middle Anacacho is marl and limestone, and the upper Anacacho is represented by packstones and grainstones. These strata represent shallow water (roughly 50 meters) shelf deposits. Limestones weather yellow to brown. Forms calcareous soils. Type section in the Anacacho Mountains of Kinney County.

Area Exposures: The upper part of this formation interfingers with the Pecan Gap Chalk in western Bexar County, where excessive faulting and lithological similarities with other Taylor horizons make accurate mapping difficult. Many exposures are too small to show up at 1:250,000 map scale, and thus appear where other formations are mapped, such as the Austin. This formation has a stronger presence to the west approaching Del Rio, but exposures tend to be locked up on private ranches, so access can take some effort. Unfortunately, I don’t have a suitable locality to share at this time.

Paleontology: The Anacacho Formation holds an interesting and varied fauna, much of it not seen personally by the author in the limited exposures of Bexar County. Ammonites include Menabites delawarensis, Bostrychoceras polyplocum, Eubostrychoceras reevesi, Placenticeras placenta, Trachyscaphites spiniger, Pachydiscus travisi, Baculites taylorensis, and others.

Echinoid genera include Echinocorys texanus, Coenholectypus, Petalobrissus cubensis, Salenia pseudowhitneyi, Proraster dalli, Hemiaster wetherbyi, Mecaster texanus, Hardouinia , Codiopsis, Cidaris texanus and others. Oysters (Pycnodonte mutabilis, Exogyra ponderosa erraticostata, etc.), bivalves (Spondylus, Lima, Inoceramus, Neithea quinquecostata and others) and gastropods (Gyrodes, Turritella, Pyropsis, etc.) abound.

On rare occasions, shark teeth (Scapanorhynchus texanus, Squalicorax kaupi, etc.) and mosasaur remains are found. But as sparse as the vertebrate remains are, for those wanting a quicker return on effort, and better preservation, the Ozan Formation outcropping in the North Sulphur River near Ladonia, Texas is a worthwhile destination.

Edited by Uncle Siphuncle

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Ammonites of the Anacacho Limestone of Bexar County

Placenticeras placenta

post-22-0-10473200-1423798510_thumb.jpg

post-22-0-90890900-1423798531_thumb.jpg

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Ammonites of the Anacacho Limestone of Bexar County

Puzosia sp.

post-22-0-99538200-1423798911_thumb.jpg

post-22-0-10726500-1423798922_thumb.jpg

post-22-0-48975900-1423799072_thumb.jpg The Kid smoked the Old Man this day.

Edited by Uncle Siphuncle

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