The Geology of Tarrant County

"This formation consists of 23 feet of brown arenaceous clay, lying below the hard, white Mainstreet limestone, and above the softer chalky limestone which forms the cap of the Weno formation. This brown clay contains sandy slabs in its basal half and carries a characteristic assortment of fossils.

Due to its increment in thickness northwards it is about 27 feet thick at the Denton-Tarrant County Line, while at the Tarrant-Johnson County line it is about 12 feet thick.

The prevailing aspect of the formation, aside from this red material, is brown sandy-flaggy. The Pawpaw contains characteristic and unmistakable fossils.

3-4. Nautilus sp. and Hemiaster sp.
The upper portion of the formation, especially the upper 5 feet contains a small species of Nautilus and a rotund bulky species of Hemiaster. The middle part of the formation contains Nautilus sp. near texanus, Pecten subalpina, Plicatula, etc.

2. Metopaster sp.
A thin flaggy stratum about 4 feet above the base of the Pawpaw contains the starfish Metopaster sp., which so far has not been found elsewhere and may prove to be narrowly limited. The zone also contains other species of starfish.

1. 'Pyrite Fossil Zone':
Approximately the lower 8 feet of the clay contains an assortment of iron pseudomorphs and other fossils which is distinctive and includes the following:  Turrilites sp.,  Schloenbachia sp., Scaphites sp. A., Hamites sp., Area sp., Engonoceras sp., Cinulia sp. [gastropod], Enallaster sp., Salenia sp., Fish teeth and vertebrae, Baculites sp., Flickia [Adkinsia] sp., Acanthoceras sp., Mortoniceras spp.  For brevity we call this a pyrite fauna; some fossils are pyrite, some hematite and some limonite."

The Geology of Denton County

" Turrilites worthensis (plate 6). This is the most striking of the dwarfed species found preserved in pyrite in this formation. This species is a spirally coiled ammonite.

Scaphites hilli (plate 6) . This is another of the dwarfed forms and is in the same subgroup of animals to which the ammonites belong. This obscure little species is of considerable scientific importance, as it is one of many used by geologist and paleontologist in linking the rocks of the Texas Comanchean with those of corresponding age in other parts of the world.

Pyritized ammonites, clams and corals (plate 3) of many species occur in the Pawpaw and are interesting mainly because of their dwarfage and will not be detailed here.

Fish teeth and vertebrae (plate 3) occur occasionally in rocks of all the formations in Denton County but are distinctly more abundant in the Pawpaw formation than in the others.  "

The Geology of Texas - Vol. 1

PAWPAW FORMATION

Nomenclature.—The "Pawpaw shales" or clays were first named by Hill (788, pp. 303, 328, 330) from the type locality Pawpaw Creek, starting in the city of Denison and running in a north-northeast direction to Red River. Here Taff considers the formation, mostly sand, to be 55 feet thick.

Facies.—(1) Typically, the Pawpaw is compacted, loosely cemented, ferruginous, cross-bedded, red-brown sandstone, with sandy clay and ironstone concretions. This is the marginal facies.

In Cooke and northern Denton counties, the formation consists of alternating red clay seams and ferruginous red-brown sandstone. (2) In southern Denton County the Pawpaw becomes more clayey and calcareous, and the sandstone ledges fewer, thinner, and lighter colored. The ironstones and jasper-like concretions persist, and in northern Tarrant County locally cover the surface. Here it is a blackish, partly lustrous clay, weathering reddish-brown and yellowish. The same lithology persists into Johnson County. (3) In Hill County the Pawpaw is more marly, having increased in lime content. Toward the Brazos it consolidates as a marly layer in the Georgetown. South of Hill County it is unrecognized. From central Denton to southern Hill County, therefore, it is of the neritic facies. At El Paso it is a similar, but more sandy, marl.

Areal outcrop; local sections. — In the Red River region, the Pawpaw is prevailingly sandstone and ferruginous sand, with interbedded dark-colored shales or clays. At Sugar Loaf Mountain, eastern Bryan County, Oklahoma (Winton, in 10, p. 28), it is 55.8 feet thick, and consists of fossiliferous ferruginous or ironstone ledges, alternating with cross-bedded red sand and sandstone. The fossils are typical Pawpaw ammonites, Alectryonia quadriplicata, Arca, and the usual assortment of small pelecypods and gastropods. In a cut near Sulphur Creek, on the Saint Louis and San Francisco Railway, 2 miles west of Bennington, beneath Main Street limestone, there is exposed 36 feet of Pawpaw marls, sands, and sandstone with nacreous and ironstone fossils: Arca, Nucula, Corbula, Cerithium, Turritella, and Anchura.

North of the railroad, 1½ miles east of Bennington, about 50 feet of massive brown Pawpaw sandstone underlies the Main Street. This sandstone contains limy layers bearing Alectryonia quadriplicata, A. marcoui, A. subovata (?), Trigonia, Gryphaea and Pecten. On the west bluff of Washita River (Bullard, Okla. Geol. Surv., Bull. 39, page 43), due east of Woodville, 60 feet of Pawpaw consists of a basal clay with ironstone concretions, and an upper soft yellowish-brown sandstone containing numerous veins of limonite (174, p. 43). The Pawpaw contains several thin lenses of highly fossiliferous, ferruginous, oxidized, soft sandstones similar to those in the Weno, but differing in containing few Turritella. The Pawpaw weathers to a very sandy, ferruginous soil, often covered with iron concretions and segregations, some hills being capped by a limonite residue. The outcrop is a typical "cross-timbers," like the Woodbine.

In Grayson County, typical Pawpaw is exposed in the cuts on East Main Street and in the upper part of Pawpaw Creek. In Denison it is a massive, poorly cemented but firm, veined, cross-bedded, brick-red sandstone. In the underpass south of the Missouri, Kansas and Texas Railway station, its contact with the overlying thin, gray Main Street limestone is exposed. Its thickness is given as 45 to 55 feet by various writers. Calcareous clay predominates in the lower half of the formation, and sand and interbedded, laminated sand and clay in the upper half. Stephenson records, 4 feet above the base of the Pawpaw clay, a limestone stratum 0.5 to 1 foot thick, and sandy limestones are known in the basal Pawpaw farther north. In an upper sandstone of the Pawpaw, he observed some comminuted plant fragments. In Big Mineral Creek, 1½ miles east of Cedar Mills, Taff records 40 feet of Pawpaw: basally, 35 feet of blue, laminated, arenaceous clay marl, containing lenses and nodules of clay ironstone in disconnected layers, with Pachydiscus and Protocardia; and at its top, a 5-foot bed with Alectryonia quadriplicata.

On the Pilot Point road east of Gainesville, Hill (803, p. 275) records 35 feet of Pawpaw, laminated, arenaceous, and marly clays, yellow at top, blue below. On Clear Creek, Denton County, Taff recorded 34 feet, the basal 9 feet being clay with numerous thin seams of soft sandstone or laminated, friable, brown sand, the upper 25 feet being marl. This locality shows the outfingering of the marginal facies southwards. The formation thins across Denton County: in the northern part it is 50 feet thick, in the southern part 30 feet.

At Blue Mound, 1½ miles south of Haslet, northern Tarrant County, the Pawpaw is 27.3 feet thick; on Sycamore Creek east of Fort Worth, 24.6 feet; near the Tarrant-Johnson County line, 12 feet. The transitional zone from the ironstone to the prevailing clay facies occurs in northern Tarrant County; north of the Trinity there is much ironstone scattered over the outcrop, the residue of numerous thin ironstone ledges, such as are very conspicuous on Red River. South of the Trinity, these are reduced to thin seams or scattered nodules, and blackish clay makes up the bulk of the formation. From Johnson County southwards, the clay becomes decidedly marly, in transition to the limy southern section. From Riovista southwards the Pawpaw is entirely marl, with little clay and no ironstone; the fauna consists of echinoids in abundance oysters, pelecypods, and gastropods, and the pyritic micromorph: are entirely missing. On Nolands River, 10 miles west of Cleburne. the Pawpaw consists of 10.8 feet of reddish clay containing fragments of pyritized Turrilites and Arca (Winton, in 10, p. 33). On the Waco road, one mile south of Riovista, the Pawpaw consist of calcareous yellow marl with a few ironstone and limestone fragments, and a mixture of the pyritic micromorphs of the geosynclinal facies (Turrilites, Arca, Pervinquieria, type locality of Flickia bösei) and neritic fossils (abundant Heteraster, Holaster, Washitaster, Hemiaster calvini).

At Fort Stockton and Kent, the Pawpaw, if represented, is sandy marl in the top of the Weno. At Cerro de Muleros, above the east end of the tunnel, is black clay with pyritic micromorphs (Neokentroceras, Pervinquieria), overlain by sandy clays with Alectryonia quadriplicata and other oysters. The sandy clays are probably Pawpaw; the lower black clays may be Pawpaw also, but it is just as probable that they are Weno, and that Neokentroceras, supposedly a Pawpaw marker, shows only its partial range at other localities where the upper Weno is limestone and where therefore the facies is unfavorable, and that here, where a favorable facies occurs in the Weno, its true range is exhibited.

Paleontology.—The three main facies of the Pawpaw each show a different aggregation of fossils. In the marl, the usual neritic, type, most of the fossils are widely ranging upper Washita types, with a predominance of irregular echinoids. Near Riovista whole nests of these occur. No good markers have been proven in this fades. In the synclinal (black clay) facies there occurs one of the four striking interfingerings of the habitat favorable for pyritic micromorphs which are known in the Washita group. Therefore, as in the interfingerings of rudistid and neritic facies, care must be taken to avoid confusion between total range (given a favorable facies) and partial (local) range. Under these conditions, no zone markers for the Pawpaw are known, but dilligent and prolonged collecting has practically demonstrated that some fossils occur in the Pawpaw and not in the nearby Denton and Grayson pyritic micromorph zones. The known ranges of these micromorphic ammonites are as follows:

The distribution of the pyritic micromorph faunas is significant for paleogeography: they are sparse in the Red River region, abundant in the Fort Worth-East Texas embayment (best Pawpaw in Tarrant and Johnson counties, best Grayson in McLennan and Bell), absent over the Austin high, and abundant again (facies permitting) in the Rio Grande embayment (as in the Grayson at Del Rio). They are absent on the Terrell arch, and again abundant in the Terlingua region, where, off the Quitman-Malone arm of the Kimmeridge-Valanginian sea, the Comanchean formations thickened enormously.

Even in the synclinal areas, the usual neritic limestones and marls interfingered, and their faunas of oysters, echinoids, and molluscs are present (as in the Pawpaw at Fort Worth).

In the Pawpaw, near Fort Worth, at a pyritic fossil locality, several large vertebrae and ribs of a plesiosaur were found.

Miss M. J. Rathbun has kindly identified the following Pawpaw crustacea from the collections of this Bureau: