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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

Hermit crab claws Paleopagurus banderensis from the L. texana zone

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Uncle Siphuncle

Bivalves and gastropods of the L. texana zone

I don't spend much time collecting or studying these classes of mollusks, so this section is limited.

Neithea irregularis

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Tylostoma gastropod

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Other bivalves and gastropods

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Edited by Uncle Siphuncle

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Algal balls Porocystis globularis from the L. texana zone

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Fossils of the Upper Glen Rose (above the L. texana zone), beginning with the common regular echinoid

Loriolia rosana in situ

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Post prep. Note well defined buccal notches circumscribing peristome (mouth) in adoral view (bottom of test)

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Uncle Siphuncle

Ammonite Protengonoceras from the Upper Glen Rose Fm

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(Credit Anthony Talutto)

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A couple miscellaneous gastropods from the Upper Glen Rose Fm

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Edited by Uncle Siphuncle

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Uncle Siphuncle

Walnut Formation

Age: Albian Stage, Fredericksburg Group, About 105.4 to 105.8 MYA

Map Abbreviation: Kwa

Thickness: 30-50 feet

Geology: Gray to yellow limestone, marl and dolomite, nodular at certain levels, generally mapped undivided with the Edwards Group, considered by some as the basal nodular member of the Edwards. Gradational contact with the underlying Glen Rose Formation. The Walnut is thought to reflect normal marine to brackish water deposits. Bull Creek and overlying Bee Cave Marl members of the Walnut may both be represented in South Texas, the Bee Cave being the more fossiliferous of the two.

Area Exposures: I don’t know of a publicly accessible locality close to San Antonio, but a motivated enthusiast could find this zone by searching the extensive mapped contact between the Glen Rose and Edwards Formations, as the Walnut does not have sufficient aerial exposure to warrant separate mapping at the 1:250,000 map scale utilized in the Atlas of Texas series San Antonio Sheet. I’ve seen other maps at 1:100,000 scale, however, which map the Walnut separately, with the caveat that these thin ribbons of outcrop are inferred and largely subsurface.

The Walnut should, in theory, be exposed in the road cuts along Hwy 281 north of town, specifically about 2 miles south of Cibolo Creek where the geo map shows the contact between the Ked and Kgr. I have not investigated this area and leave further study to more industrious collectors.

Paleontology: While the Walnut may be tens of feet thick, I tend to focus on a recessive, nodular marl zone about 2-4 feet thick, which I believe to be equivalent of the Bee Cave Marl of Central Texas. The index fossil is the distinctive oyster Ceratostrean texanum, quite abundant in this zone. But the fossils I focus on are well preserved echinoids, listed in descending order of occurrence: Coenholectypus planatus, Heteraster texanus, Loriolia sp., Phymosoma texanum. On rare occasions I’ve found fish vertebrae and complete ammonites Engonoceras c.f. pierdenale, with fragments thereof being much more common.

Edited by Uncle Siphuncle

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Walnut Formation echinoids Phymosoma texanum

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Uncle Siphuncle

More South Texas Walnut Fm C. planatus

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One more Walnut C. planatus in situ

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Walnut Formation echinoids Phymosoma texanum

This species is more common in Central than in South Texas

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Walnut Formation Loriolia echinoids

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Walnut Formation irregular echinoids Heteraster texanus

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Edited by Uncle Siphuncle

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South Texas Walnut Formation ammonite Engonoceras pierdenale

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Edited by Uncle Siphuncle

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Oyster Ceratostrean texanus, index fossil of the Walnut Formation in South Texas

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Uncle Siphuncle

Edwards Group

Age: Albian Stage, Fredericksburg Group, About 103.4 to 105.4 MYA

Map Abbreviation: Ked = Edwards Group undivided; Kk = lower unit, Kainer Formation; Kp = upper unit, Person Formation

Thickness: Ked 380-400 feet thick total, Kk about 250 feet thick, Kp 130-150 feet thick

Geology: The Edwards Group of South Texas correlates with the Goodland Formation of North Texas, although they are lithologically quite different.

Kainer Fm: Cyclic subtidal to tidal flat limestone and dolomite. Wackestones and packstones in lower part of this unit, localized leached evaporate and breccia in middle part of the unit, grainstones and packstones in upper part of unit. Honeycomb and variable amounts of chert nodules common.

Person Fm: Shallow subtidal to tidal flat cyclic limestone and dolomite. Recrystallized dolomitic limestone, argillaceous limestone, leached and collapsed zones (some infilled with red “terra rosa” clay), interconnected honeycomb porosity, vuggy intervals, chert locally abundant. The lowest 20-30 feet of this unit, the Regional Dense Member, is thought to correlate to the Kiamichi Fm of North Texas.

Area Exposures: The Edwards is perhaps the best exposed limestone unit in Bexar County in part due to its resistance to weathering. More important than its paleontology, this is the primary local building stone, and its honeycombed subterranean catacombs comprise the primary aquifer for the San Antonio area. My house is built out of Edwards limestone, and the Edwards also provides my house with a very stable foundation that is not subject to shifting, as can happen in softer formations.

The karst topography of the Edwards Limestone lends itself to cave formation north of San Antonio. Of notable interest, Friesenhahn Cave a few miles north of Loop 1604 near Hwy 281 and Stone Oak Parkway, when excavated in the early 1950s, was found to contain the remains of several juvenile and adult Homotherium saber cats in association with numbers of juvenile mammoth teeth and other remains. Apparently that cave was used as a den for an extended period 20,000 years ago. Web searches turn up spectacular images of these well preserved cats.

The Edwards is quarried extensively just to the north of Loop 1604 North as well as west of IH-35 heading north toward New Braunfels. The road cuts on Loop 1604 on the north side of town between Gold Canyon Road and IH-10 afford an extensive drive-by view of the Edwards, and it also extends several miles to the north on Hwy 281, approaching within 2 miles of Cibolo Creek.

post-22-0-39695100-1423250563_thumb.jpg Edwards Limestone on Loop 1604 at Blanco Road

Shopping centers cut into local hills tend to leave this resistant limestone exposed without retaining walls. Near the intersection of Loop 1604 and IH-10, two shopping areas (The Rim and The Shops at La Cantera) are detailed in attached images to show excellent Edwards exposures affording easy access. For those seeking a greater thrill, nearby Fiesta Texas is built in a repurposed quarry in the Edwards.

post-22-0-90442200-1423250615_thumb.jpg Google Earth image showing GPS coordinates to exposures at The Rim and The Shops of La Cantera

post-22-0-71672000-1423250636_thumb.jpg Exposure along NW side of The Rim

post-22-0-01525100-1423250577_thumb.jpg One exposure at La Cantera

post-22-0-70919500-1423250595_thumb.jpg Second exposure at La Cantera (many more present there)

Paleontology: Kainer Fm: Rudists, gastropods, oysters, milliloids (a type of microfossil)

Person Fm: Rudists, gastropods, bivalves

Generally speaking, the Edwards is a very hard limestone, grainy and crystalline in places, making it rather difficult to extract its fossils unbroken with hand tools. While Central and West Texas exposures of the Edwards and its equivalents are known to produce breathtakingly preserved echinoids and silicified gastropods, the San Antonio area seems to have been dealt a bad hand in terms of its Edwards fossil content, preservation, and extractability of fossils. Some of the most interesting local Edwards fossils that I've seen afield are external molds of gastropods, leaving behind cavities in the limestone festooned with elaborate surface ornamentation not seen on the steinkerns (internal molds) of gastropods so prevalent in other local formations.

post-22-0-50021200-1423250525_thumb.jpgpost-22-0-39023900-1423250545_thumb.jpg (Goniopygus echinoids from the Devil's River Limestone (Edwards equivalent, Del Rio area)

Perhaps I have not searched enough amongst the local rudist reefs for desirable echinoids, such as Goniopygus. Rudists seem to dominate the Edwards faunal spectrum. These obscure, reef building bivalves are often mistaken for a number of fossils, including dinosaur teeth. Admittedly, my time afield has been biased toward other formations, and further study would be required to give a more balanced assessment.

Edited by Uncle Siphuncle

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Georgetown Formation

Age: Albian Stage, Washita Group, about 97.0-97.4 MYA

Map Abbreviation: Kgt

Thickness: Sporadic exposures to 30 feet in Bexar County. Personally observed exposures only a few feet thick.

Geology: The middle and upper Albian Stage is a variably fossiliferous marine rock unit that persists along the entire Balcones Fault from NE Texas, through Fort Worth, Temple, Austin, pinches very thin over the San Marcos Arch, is very sparsely exposed around San Antonio, and reappears with greater thickness in far West Texas along the IH-10 corridor.

Members of the mid to upper Albian are most prominently exposed in far North Texas in the area between Lake Texoma and Fort Worth. In that area, the Albian formations within the Washita Group include, from oldest to youngest, Duck Creek, Fort Worth, Denton, Weno, Pawpaw and Mainstreet. Heading southward along the Balcones Fault, all of these formations gradually thin and harden, the transition from marls and limestones to predominantly harder limestones denoting increased water depth in a classic offshore sequence. At the Brazos River, the Pawpaw becomes so thin that it pinches out and is not mapped separately southward.

Around Belton, the remaining formations are sufficiently thin and of such narrow aerial outcrop that they are lumped together and mapped undivided as Georgetown Formation in areas farther south, with formations of the north referred to as members of the Georgetown. In latitudes as far south as San Marcos, the Duck Creek, Fort Worth, and Weno members have pinched out. The San Marcos area is the southernmost occurrence of Mortoniceras ammonites and Macraster echinoids encountered by the author. Both reappear in West Texas.

Most of these lower and middle members of the Washita and their wonderful fossils are absent in the surface and subsurface in the San Antonio area. However, sparse exposures of the Mainstreet member represent the Georgetown Formation locally.

This open shelf interval occurs as a thin, tan, nodular bedded limestone disconformably overlying the Person Formation of the Edwards Group, and underlying the Del Rio Clay. It is generally mapped undivided as an upper member of the Edwards, and is lithologically so similar near the Edwards contact that the two can be difficult to distinguish unless fossils are present. The upper contact, in contrast is characterized by a distinct and abrupt break from nodular limestone to clay. I find the Del Rio/Georgetown contact to be the most fossiliferous zone in the Georgetown of Bexar County and interestingly, this contact zone represents the contact of the Comanchean (Lower Cretaceous) and Gulfian (Upper Cretaceous) series. This zone is studded with rusty pyrite, which adds character when adhered to the fossils, and It is possible that much of this pyrite on Georgetown fossils is derived from intimate contact with the overlying Del Rio clay.

Area Exposures: At 1:250,000 scale, the Kgt and Ked are mapped undivided in Bexar County. At 1:100,000 scale, the Kgt is mapped separately along slopes which map Person Formation at the base and Del Rio Formation above. However, most of the outcrop area is covered by soil and vegetation, so mapped outcrop is inferred, and could be more intermittent than implied by the map.

One site that exposes this unit is the bed of Leon Creek in NW Bexar County. This area was recently annexed into the Leon Creek Greenway, and is therefore off limits to collecting. A Google Earth image has been included of the creek section exposing the Georgetown. The image is a bit out of date, as there is now a hike and bike bridge built over the creek, and a neighborhood just to the east. The narrow stretch of Kgt limestone is directly under this bridge and extends perhaps 50 yards upstream and downstream of it.

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Paleontology: The local Georgetown limestone is distinguished from the Edwards by presence in the former of the small brachiopod Waconella wacoensis. Bivalves include large Neithea (possibly N. texana) scallops and steinkerns of Protocardia clams. But fleeting occurrences of ammonites and echinoids provide more excitement for the adventurous collector. Mariella brazoensis is helically spired heteromorphic ammonite which resembles a large, ornamented gastropod. Graysonites is a rare planispiral ammonite easily identified by its prominent ventral tubercles. Paracymatoceras texanum is a coiled nautiloid occasionally encountered, and it is easily distinguished by the thin, sinuous lines on its flanks. The echinoid fauna is dominated by the large, brown, dome shaped Coenholectypus, generally 1.5-2.5 inches diameter. None of these ammonites, echinoids, nor nautiloids are especially plentiful in San Antonio, so for those especially interested in finding them, a weekend trip 200-350 miles north would greatly increase odds of success.

Edited by Uncle Siphuncle

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Ammonites of the Georgetown Formation of Bexar County

A couple decent examples of Mariella brazoensis. Usually found as broken, partial whorls.

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A rare Graysonites. This matrix presentation includes Neithea scallops and Waconella brachiopods. A great piece for diagnostic zonation.

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Echinoids of the Georgetown Formation of Bexar County

Coenholectypus sp.

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post-22-0-89523700-1423250144_thumb.jpg Note rusty pyrite encrustation.

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Edited by Uncle Siphuncle

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Del Rio Formation

Age: Cenomanian Stage, Washita Group, About 96.5-96.9 MYA

Map Abbreviation: Kdr

Thickness: 15 to 50 feet in Bexar County

Geology: The Grayson Formation is mapped from Lake Texoma in far north Texas to the Waco area. The names Grayson and Del Rio Formations are used interchangeably between Waco and Belton. From Belton to San Antonio, through the type locality at Del Rio in Val Verde County, and on into far west Texas, it is referred to as Del Rio Formation.

This interval marks the beginning of the Upper Cretaceous, and represents a shelf mud/prodelta deposit, predominantly a recessive, peanut butter colored calcareous, montmorillonitic (shrink /swell during wet/dry cycles) clay with minor amounts of mudstone. Gypsum and pyrite are common. Del Rio Clay is perhaps the least permeable clay in Bexar county, so it holds water well in stock tanks etc. However, its shrink/swell nature and instability on slopes makes for shifting building foundations.

Area Exposures: The soft, recessive nature of the Kdr makes for weak surface expression. It is encountered mainly on slopes capped by the more resistant Buda Limestone, as well as in valleys and flats. When excavated on hillsides during construction projects, it is quickly supported by retaining walls to prevent slumping. It can be viewed in the flats behind Costco and Discount Tire at the west corner of IH-10 and DeZavala Road, where I. arietina oysters are quite common.

post-22-0-19013200-1423533682_thumb.jpg Del Rio site along University Heights Blvd just west of Discount Tire

post-22-0-39014000-1423534901_thumb.jpg Street level view of Del Rio site

Paleontology: The index fossil of the Del Rio Formation is the ram’s horn oyster Ilymatogyra arietina, quite common in the lower 2/3 of the formation. Less abundant is the oyster Gryphea mucronata, which seems to have the strongest presence in the upper 1/3 of the formation locally. These oysters persist in great numbers through the entire outcrop area noted in the Geology section. The brachiopod Waconella wacoensis, abundant in the Kgt, has phased out by the end of the Albian. San Antonio Del Rio exposures lack the extensive micromorphic ammonite fauna noted in the Waco area. I have encountered one or two spatangoids in the Del Rio, either Heteraster or Washitaster, too poorly preserved to key out with confidence. A few Palhemiaster calvini echinoids have also been found locally.

Edited by Uncle Siphuncle

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