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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

Ammonites of the Anacacho Limestone of Bexar County

post-22-0-51539900-1423800011_thumb.jpg Pachydiscus sp.

post-22-0-70806900-1423800154_thumb.jpg

post-22-0-81090200-1423800180_thumb.jpg Don't let this double fool you. Ammonites are not terribly common finds around San Antonio.

Edited by Uncle Siphuncle

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Uncle Siphuncle

Ammonites of the Anacacho Limestone of Bexar County

post-22-0-88991800-1423800382_thumb.jpg Pachydiscus travisi

post-22-0-40619700-1423800407_thumb.jpg Pachydiscus travisi

post-22-0-61199700-1423800411_thumb.jpg Pachydiscus travisi surrounding Menabites sp. top center

post-22-0-39564300-1423800455_thumb.jpg Pachydiscus left, unidentified right

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Uncle Siphuncle

Miscellaneous Fossils of the Anacacho Limestone of Bexar County

post-22-0-60546900-1423800761_thumb.jpg Baculites top left, Eutrephoceras nautiloid left, Pachydiscus ammonite right

post-22-0-11399700-1423800812_thumb.jpg Unidentified bivalves

post-22-0-51515000-1423800830_thumb.jpg Pyropsis? gastropods

post-22-0-38982200-1423801001_thumb.jpg Trigonia bivalve

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Uncle Siphuncle

Echinoids of the Anacacho Formation of Bexar County

post-22-0-21007900-1423801180_thumb.jpg Petalobrissus cubensis

post-22-0-91246300-1423801173_thumb.jpg

post-22-0-79748000-1423801234_thumb.jpg Hemiaster wetherbyi

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Uncle Siphuncle

Pecan Gap Formation

Age: Campanian Stage, Taylor Group, about 77 MYA

Map Abbreviation: Kpg

Thickness: Up to 200 feet in Bexar County

Geology: The type section for this mid Campanian, fine grained, soft, conchoidally fracturing, off white to gray chalk is in the town of Pecan Gap, Delta County, Texas very close to the North Sulphur River. Shelf conditions deposited thick bedded chalks in the lower section grade into softer marls in the upper section. The lower contact with the Austin Chalk is marked by a thin bed of abraded shark teeth (most preserved as blades, lacking roots), Baculites sections, bivalves, gastropods, and a few small planispiral ammonites and nautiloids. This phosphate can range from black to orange in Bexar County. The Pecan Gap is approximately equivalent to the top of the Anacahco Formation.

Area Exposures: Since the Pecan Gap Chalk is so soft, it doesn’t lend itself to widespread natural exposures. But since the mapped outcrop area cuts a wide swath from NE to western Bexar County, opportunities to study the Pecan Gap come and go with the vagaries of the construction industry. Most of the sites that produced finds for me over the years have since been consumed by construction.

One current exposure where the avocational geologist/paleontologist can get a good look is at the site of Morgan’s Wonderland in NE Bexar County, just NE of the intersection of Wurzbach Freeway and Thousand Oaks Drive. This is the site of the old Longhorn Quarry, left vacant for years. Before it was repurposed as an amusement park and sports complex, it was a weathering pit exposing tens of meters of the Pecan Gap section. At that time I took numbers of ammonites, bivalves, gastropods, echinoids, nautiloids, even a few shark teeth, a partial fish, and a crustacean tail.

These days, the collectible area has been greatly reduced, but the white, blocky bluff at the edge of the parking lot near the sports fields affords a look at an excellent exposure which still produces fossils at times. This area is located just east of the Thousand Oaks bridge.

Best technique is to exploit weathering fractures with a hammer and chisel, blindly and randomly exposing new, unweathered surfaces that may contain fossils. The chalk splits very easily, and large sections can be separated with minimal effort.

As an aside, faulting has exposed a large section of Austin Chalk, probably Dessau Fm, just west of the Thousand Oaks bridge. Last time I visited, however, that area was posted, so I haven’t gone for a close up look at the Austin there in a few years.

post-22-0-91204300-1424225250_thumb.jpg Morgan's Wonderland Site

post-22-0-39656900-1424225228_thumb.jpg A few shots of the Longhorn Quarry before development

post-22-0-20028000-1424225235_thumb.jpg

post-22-0-80525600-1424225241_thumb.jpg

Paleontology:

The Pecan Gap has a varied fauna with often compressed fossils preserved in exquisite detail. The ammonites, for instance, are often compressed in random orientations, resulting in exotic, distorted presentations that make for interesting displays, especially on matrix pedestals. However, this is not a formation for impatient collectors as vast expanses of rock must often be carefully studied and often blind mined in order to produce good finds. I enjoy this formation not because it is an expedient pursuit, but because it is rather challenging.

Once found, the fossils are fragile, and since there is often not a cleft between the fossil and surrounding matrix, aggressive preparation is not advised as it often results in damage. A “what you see is what you get” approach is often best with these fossils.

A mention of fauna is in order. First, the cephalopods.

Planispiral ammonites: Pachydiscus travisi most common. Placenticeras rare.

Heteromorphic ammonites: Baculites mclearni and B. taylorensis fairly common, followed by Trachyscaphites spiniger porchi. Eubostrychoceras reevesi, Solenoceras, other species of Trachyscaphites rare.

Nautiloids: Eutrephoceras sp. are usually preserved compressed and nondescript, with sutures usually not distinct.

Echinoids: Echinocorys texanus, Hemiaster sp., and a few unidentified, large spatangoids seem to dominate the spectrum.

Oysters: Exogyra ponderosa is most common.

Bivalves: Inoceramus is most common, followed by Neithea, Lima, and other unidentified forms.

Gastropods: Gyrodes, Anchura, and others are often very ornately preserved.

Crustaceans: All are rare occurrences, but I’ve found one nice Necrocarcinus carapace, one decent unidentified claw, and one apparent lobster tail comprised of several segments, but no telson (tail fan) is exposed.

Shark Remains: I’ve picked up mainly Squalicorax kaupi and Carcharias sp. teeth locally, with perhaps a few Scapanorhynchus blades. Again, teeth seem to be preserved as hollow blades in the basal contact zone, but rooted teeth can be found higher in the formation. I’ve seen one shark vertebra found locally by a friend. On rare occasions, shark and ray cartilage are preserved in the Pecan Gap. I have found exactly one specimen, identified as the rostrum (snout) of a large shark or ray.

Fish Remains: Enchodus and pycnodont teeth (perhaps Anamoedus) occasionally turn up, as do unidentified fish vertebrae and scales, the latter two generally presenting in shades of pink or tan against the white chalk. I once found a partial fish in the old Longhorn Quarry, the skull crushed vertically and presenting dorsally, with articulated scales preserved.

Reptile Remains: I’ve only seen one mosasaur vertebra from the Pecan Gap, found by a friend. If articulated mosasaur remains are ever encountered in this formation locally, I would imagine preservation to be spectacular. Regrettably, the similarly aged Ozan Formation, famous for its mosasaur remains, pinches out well north of Bexar County.

Edited by Uncle Siphuncle

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Uncle Siphuncle

Planispiral Ammonites of the Pecan Gap Formation of Bexar County

post-22-0-78989600-1424225818_thumb.jpg Pachydiscus travisi

post-22-0-61040700-1424225830_thumb.jpg

post-22-0-30434500-1424226068_thumb.jpg

post-22-0-60532900-1424226069_thumb.jpg

post-22-0-99175500-1424226070_thumb.jpg The biggest Pachydiscus I ever took at Longhorn Quarry, about 10"

post-22-0-49511400-1424226273_thumb.jpg

post-22-0-19014900-1424226269_thumb.jpg A few delightfully distorted Pachydiscus

post-22-0-00652500-1424226271_thumb.jpg

post-22-0-30189100-1424226275_thumb.jpg

post-22-0-30735600-1424226390_thumb.jpg My son's Menabites, better than I've personally taken in Bexar County

Edited by Uncle Siphuncle

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Heteromorphic Ammonites of the Pecan Gap Chalk of Bexar County

post-22-0-10371400-1424226763_thumb.jpg Trachyscaphites spiniger porchi

post-22-0-31626800-1424226771_thumb.jpg

post-22-0-70287300-1424226785_thumb.jpg Trachyscaphites sp.

post-22-0-50632300-1424227014_thumb.jpg Eubostrychoceras reevesi

post-22-0-01370100-1424226741_thumb.jpg

post-22-0-71459500-1424226750_thumb.jpg

post-22-0-19744700-1424227949_thumb.jpg Solenoceras sp.

post-22-0-11111700-1424227005_thumb.jpg Baculites sp.

post-22-0-50873800-1424226791_thumb.jpg

post-22-0-21252100-1424226779_thumb.jpgTrachyscaphites, Pachydiscus, and Baculites

post-22-0-88966800-1424226788_thumb.jpg Trachyscaphites and Baculites

Edited by Uncle Siphuncle

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Eutrephoceras sp. nautiloids of the Pecan Gap Chalk of Bexar County

post-22-0-70433200-1424228112_thumb.jpg My kid found the best Kpg nautiloid I've seen to date

post-22-0-88945200-1424228116_thumb.jpg

post-22-0-30086400-1424228157_thumb.jpg

post-22-0-51540000-1424228259_thumb.jpg

Edited by Uncle Siphuncle

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Echinoids of the Pecan Gap Chalk of Bexar County

post-22-0-00076600-1424228780_thumb.jpg Echinocorys texanus

post-22-0-90894900-1424228781_thumb.jpg

post-22-0-90532800-1424228788_thumb.jpg

post-22-0-31141800-1424228792_thumb.jpg

post-22-0-18956500-1424228998_thumb.jpg

post-22-0-11426300-1424229023_thumb.jpg Mecaster texanus?

post-22-0-71529800-1424229157_thumb.jpg Isomicraster sp.?

post-22-0-08940900-1424229051_thumb.jpg

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Oysters of the Pecan Gap Chalk of Bexar County

post-22-0-29172900-1424229579_thumb.jpg Exogyra ponderosa

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Bivalves of the Pecan Gap Chalk of Bexar County

post-22-0-71010300-1424229779_thumb.jpg Inoceramus sp.

post-22-0-79944800-1424229942_thumb.jpg Neithea sp.

post-22-0-09426100-1424229946_thumb.jpg

post-22-0-09102100-1424229786_thumb.jpg Lima sp.

post-22-0-99947800-1424229808_thumb.jpg Pecten sp.

post-22-0-08988800-1424229802_thumb.jpg

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Gastropods of the Pecan Gap Chalk of Bexar County

post-22-0-20711700-1424230153_thumb.jpg Gyrodes sp.

post-22-0-61322000-1424230222_thumb.jpg Napulus sp.

post-22-0-91121800-1424230159_thumb.jpg Anchura sp.

post-22-0-80327400-1424230167_thumb.jpg

post-22-0-81558600-1424230174_thumb.jpg

post-22-0-31204800-1424230231_thumb.jpg Unidentified

post-22-0-39527900-1424230238_thumb.jpg

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Crustaceans of the Pecan Gap Chalk of Bexar County

post-22-0-71030100-1424230618_thumb.jpg Necrocarcinus sp.?

post-22-0-89413200-1424230627_thumb.jpg Unidentified crab claw

post-22-0-41099200-1424230662_thumb.jpg Unidentified lobster tail

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Shark Remains of the Pecan Gap Chalk of Bexar County

post-22-0-71084600-1424231131_thumb.jpg Cretalamna sp.

post-22-0-51539800-1424231145_thumb.jpg Rare shark or ray cartilage rostrum (snout)

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Fish Remains of the Pecan Gap Chalk of Bexar County

post-22-0-60776300-1424231380_thumb.jpg Cool Enchodus association

post-22-0-79686100-1424231397_thumb.jpg Fish scale

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Uncle Siphuncle

Corsicana Formation

Age: Maastrichtian Stage, Navarro Group, about 68.0-69.0 MYA

Map Abbreviation: Kco

Thickness: About 90 feet in Bexar and Guadalupe Counties

Geology: The type locality for the formation, now reclaimed, was an old pit in Corsicana, Navarro County, TX. Outcrop area is a thin, intermittent ribbon that follows the eastern edge of the Balcones Fault Zone for approximately 350 miles from San Antonio to the Red River. While mapped discretely in North Texas, in South Texas it is mapped undivided with the Kemp Fm, Neylandville Marl, and Marlbrook Marl due to lithological similarities, faulting, and lack of natural surface expression. Man made exposures tend to revegetate and become overwashed quickly.

In Bexar County, this tan to orange sequence of predominantly marl with occasional claystone horizons represents neritic (shallow offshore) deposition in seas less than 450 feet deep. Some horizons indurated (burrowed). Marl montmorillinitic.

Area Exposures: Interpolating from the Atlas of Texas San Antonio Sheet, this formation runs directly through downtown. An old publication noted an exposure along Salado Creek just downstream of Houston Street on the east side of town, but I found little of paleontological interest there. This is not in the safest part of town, so enter at your own peril. The exposures that produced the bulk of my personal Corsicana collection have been virtually wiped out by construction, so I don’t have better locality information to share at this time.

post-22-0-29438100-1424718341_thumb.jpg Corsicana exposure, now covered

post-22-0-41326700-1424718323_thumb.jpg Indurated marl found in some layers of the Corsicana Formation

Paleontology: The Corsicana Formation holds an extremely diverse fauna, mainly invertebrate.

Oysters: The index macrofossil of this formation is the oyster Exogyra costata, although horizons of large Pycnodonte mutabilis oysters and scattered Ostrea mesenterica are present as well.

Ammonites: A host of local Corsicana ammonites includes Gaudryceras kayei, Glyptoxoceras sp., Pachydiscus sp., Anapachydiscus fresvillensis, Discoscaphites conradi, and Baculites c.f. undatus, Neophylloceras surya, Trachybaculites columna, and Sphenodiscus c.f. pleurisepta.

Nautiloids: Dominated by Eutrephoceras dekayi

Echinoids: 90% by occurrence are Hemiaster wetherbyi (formerly H. bexari). About 8% of the echinoid fauna is made up of Diplodetus americanus, Schizaster variabilis and Proraster dalli. The remaining 2% are comprised of Codiopsis sp., Petalobrissus sp., Catopygus sp., Rachiosoma hondoensis, Cardiaster leonensis, and an unidentified cidarid.

Vertebrates: Shark finds, while infrequent, are dominated by Cretalamna maroccana, with occasional Squalicorax pristodontus. Fish teeth include Enchodus ferox, Protosphyraena permicosa and Anamoedus sp. Unidentified fish and ray vertebrae are found at times. The most rare vertebrate finds I’ve personally made or witnessed include a sawfish or ray vertebra, a poorly preserved mosasaur caudal vertebra, and 2 small eel neurocrania.

Crustaceans: The muffin crab Dakoticancer australis dominates this category in the Corsicana. Rare crustacean occurrences include a single Prehepatus chela and some unidentified crab claw dactyls.

Gastropods: These mollusks flourished in Corsicana seas. Gyrodes rotundus, Turritella c.f. vertebroides, and Anchura sp. make up the bulk of the fauna, but Pyropsis, Ellipsoscapha, Gyrodes petrosus, Bellifusus, Volutoderma, Lupira, Napulus, and Xenophora had a presence as well. Perhaps my favorite gastropod of the Corsicana is the very ornately adorned Striaticostatum bexarense.

Bivalves: These mollusks had great diversity in the Corsicana as well. Pterotrigonia castrovillensis is perhaps one of the most aesthetically appealing bivalve of the formation. Plicatula mullicaensis was present in great numbers as well. Other bivalves include Lima acutilineata, L. guadalupae, L. sayrei, Liopistha protexta, Inoceramus, Neithea bexarensis, Plicatula tetrica, Tenea parilis and Trachycardium sp.

Edited by Uncle Siphuncle

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Ammonites of the Corsicana Formation of South Texas

post-22-0-40241600-1424309685_thumb.jpg Gaudryceras kayei

post-22-0-19431700-1424309668_thumb.jpg

post-22-0-41060700-1424309650_thumb.jpg

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Ammonites of the Corsicana Formation of South Texas

post-22-0-79508300-1424310350_thumb.jpg Neophylloceras surya

post-22-0-61525700-1424310346_thumb.jpg

post-22-0-29927800-1424310337_thumb.jpg

post-22-0-31212000-1424310326_thumb.jpg Anapachydiscus fresvillensis

post-22-0-89512600-1424310315_thumb.jpg

post-22-0-39581700-1424310308_thumb.jpg

post-22-0-60931400-1424310295_thumb.jpg

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Ammonites of the Corsicana Formation of South Texas

post-22-0-29527100-1424717126_thumb.jpg Anapachydiscus sp.

post-22-0-01632100-1424717133_thumb.jpg

post-22-0-31358500-1424717219_thumb.jpg Pachydiscus sp.

post-22-0-89022700-1424717247_thumb.jpg Discoscaphites c.f. conradi

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Ammonites of the Corsicana Formation of South Texas

post-22-0-71423800-1424717497_thumb.jpg Trachybaculites columna

post-22-0-89713900-1424717510_thumb.jpg Baculites c.f. undatus

post-22-0-29257900-1424717507_thumb.jpg Baculites sp.

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Ammonites of the Corsicana Formation of South Texas

post-22-0-51256300-1424717779_thumb.jpg Pachydiscus egertoni

post-22-0-20367800-1424717777_thumb.jpg

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