The Geology of Denton County

" Plictula Dentonesis (plate 21). This small clam, commonly called "Cats paw Clam" by the writer's students, occurs in all of the formations of the Comanchean but is dis­tinctly more abundant in the upper ones. It is first notably abundant in the Denton formation and reaches no definite zone of abundance as do many of the other fossil species in these rocks; but, in general, specimens are more common in certain members of the Pawpaw and in the Grayson. This clam is closely related to a number of small modern marine clams of the genus Anomia. This genus gets its name from a Greek word meaning "unlike" because of the strong dissimilarity or lack of uniformity between different individuals of the species. All bivalve clams are sensitive to their environment. The modern fresh water "button clam," for example, when grown in quiet water has a smooth shell and in rough water has a corrugated or ribbed shell. The modern Anomia grown by various experiment­ers on the Pacific Coast has been found to be so sensitive to differences in environment that the young from a single female grown under different conditions of temperature, oxygen„ turbidity, pressure, food, and other conditions, will produce several dozen different individual types, some of which were actually considered species by older workers. As natural laws are unchanging it is possible to transfer some of this data to our fossil Plicatula. As much of the water was shallow in the Comanchean sea and the shore line was oscillating somewhat, these Plicatulas were subjected to many different conditions of environment.

  Starting with the simplest kind of facts we can safely say that those forms with smooth shells grew in quiet water, those with ribbed shells in rough water, and, in rare cases, as examination of the plate will show some individuals were subjected to rough water during part of their lives and to smooth water during the rest of the time. The demarcation of the annual rings is fairly clear in most specimens and observation shows that about three years represents the average life. With a lens much finer rings between the annual rings are observable. These are rings of growth each resulting from single so called "meals" or feeding periods, as similar rings are observable in nearly all modern bivalves. An examination of a num­ber of specimens under a lens will exhibit irregularities in the smaller growth rings and these irregularities some-times have a distinctive enough sequence so that the same sequence can be detected on different individuals. In this way it is possible to observe that the third year of one individual, for example, was the first year of another. In this way and by using similar rings on various pectens, the writer has been able to build up continuous chronolo­gies for considerable lengths of time. In this work he hopes to be able after much more detailed work to deter-mine in terms of absolute time the rate of deposition of at least some of the marl seams of the Texas Comanchean.

  Ostrea carinata (plate 8). This angular ribbed oyster is an important fossil in the upper part of the Washita division. Unfortunately the name assigned to it by Texas geologists is not precise as this name belongs to a European fossil species originally named by Linnaeus ; and the Texas species resembles this only superficially, but the application of this name has become quite general and it serves after a fashion as a fully agreed upon handle.

  Leiocidaris hemigranosus (plate 13). This sea urchin occurs in both the Denton and the Weno and is represented mainly by large numbers of the thick, ribbed spines with occasional fragments of the test. Perfect individuals similar to that figured are extremely rare and the writer knows of the existence of only five others besides the specimen figured here. "

The Geology of Texas - Vol. 1

DENTON FORMATION

Nomenclature.—The Denton marl of Taff (1575, p. 272), the Marietta beds of Hill (788, pp. 328-329), and the Denton sub-group of Hill (803, pp. 271-273), apparently refer to the same strata. The type locality by implication is on Denton Creek, west of Justin, Denton County, where Taff records a thickness of 38 feet.

Facies.—In central Texas, the formation is a marl, shell marl, and shell aggregate, with shelly and limestone seams. In the Fort Stockton and Kent sections, a rudistid limestone caprock is referred to the Denton.

Areal outcrop; local section.—Taff included the Denton, Weno, and Pawpaw formations in his "Bokchito" in southern Oklahoma. The lower 73 feet is recorded (Bullard, Okla. Geol. Surv., Bull. 33. page 36, 1925) as a clay with a thin medial sandstone, and some or possibly all of it is Denton. In Marshall County (174, p. 36) 45 to 55 feet is recorded, and in Bryan County 46 feet. In Grayson County (844), p. 4), the Denton is 45 to 52 feet thick. Down the strike the following thicknesses are on record: Cooke County, 45 to 60 feet; Denton County, 25 to 35 feet; Tarrant County, 25 feet; Johnson County, 20 to 25 feet; McLennan and Bell counties, about 5 feet; Travis County, about 5 feet. At Fort Stockton the beds of the middle rudistid caprock, assigned to the Denton, are about 38.5 feet thick, but the basal part of this may be Fort Worth Limestone. At El Paso the Denton is a marly, nodular limestone about 50 feet thick (upper part of Bose's subdivision 5).

This is one of the most widespread horizons in the Washita, and some representative of its shell horizon may be found in the southern limy fades of the Georgetown. In the Red River region, Denton lithology is distinguished by these features: the body of the formation is clay or marl; the base, transitional to the Fort Worth limestone, is more calcareous, and locally has thin limestone seams; near the middle of the formation there is a persistent, thin, fissile, laminated, ripple-marked, light-colored sandstone, which breaks into large slabs and covers the lower clay slopes; and at the top of the Denton there is a Gryphaea washitaensis shell aggregate, shell seams, or shelly limestone, one foot or more thick.

In Bryan County, Oklahoma (174, p. 37), the sandstone, brownish-yellow, hard, thinly laminated, ripple-marked, one foot thick, overlies 26 feet of basal Denton clay. In Grayson County, the yellow-brown, thinly laminated, ripple-marked sandstone is 20 to 30 feet and exceptionally more, above the base of the Denton. In Hampton Hollow, Cooke County (177, p. 35), the Denton consists of 67 feet of clay, with a 1.5 foot ripple-marked sandstone 30 feet above the base. To the south the sandstone shortly disappears. In Denton County the upper 6 feet of the formation is Gryphaea shelly limestone and shell marl, and the remainder is clay. In Tarrant County the medial sandstone is absent; the base of the formation above the thin limy seams, transitional to the underlying Fort Worth limestone, contains a small thickness of sandy marl and sandy ledges, followed by blue shelly clay; the top contains two harder Gryphaea shell strata in a very shelly clay. In Bell County the thinned Denton is a uniform shelly marl, which forms a connecting slope between the top of the Fort Worth limestone, exposed beneath as a terrace or narrow shelf, and the overlying Weno limestone. This marl break is distinguishable generally in the topography. Farther south, as the terranes lose the terraced expression characteristic of north-central Texas, the Denton outcrops become very narrow and inconspicuous. At Mexia, the Denton, Weno, and Pawpaw are very reduced in thickness. In some wells in the outcrop counties, the Denton may be recognized from logs; apparently on passing gulfwards it rapidly disappears as a marl.

In Trans-Pecos Texas, Denton is present in its usual facies at El Paso, Kent, and Sierra Prieta. At Cerro de Muleros it forms the upper part of Böse's subdivision 5. and is a marl and nodular dark bluish-gray, soft limestone, containing a fauna of beautifully preserved echinoids (Pyrina, Phymosoma, Holectypus, and others). This echinoid level appears at Kent, where the Denton forms a cap rock of nodular, non-rudistid limestone. At Fort Stockton the upper part of this limestone cap rock is in the rudistid reef facies and contains Eoradiolites n. sp., caprinids, Chondrodonta, reef Nerinea and Pecten, corals, sponges, Actaeonella, and other distinctive fossils. The same features occur near Gap Tank and in eastern Pecos County.

Paleontology.—The top of the Denton is a persistent mass of shell marl and shelly limestone, becoming almost entirely shell marl in south-central Texas. It contains innumerable Gryphaea washitaensis (all growth stages), which are present but less abundant lower in the Denton, and Alectryonia cf. carinata, Leiocidaris hemigranosus (plates and spines), Heteraster, and some less frequent, wide-ranging Washita species. One of the most prominent features is a great development of the large echinoid, Macraster, represented by several species. Several kinds of Pervinquieria occur, one of which (P. aff. kiliani) may prove to be a restricted zone fossil. Numerous ammonites (Pervinquieria and others), preserved as pyritic dwarfs, occur in the Red River section. Rarities are Rhabdocidaris n. sp., Turrilites spp., "Vola" [Lima?] catherina Cragin, and some ammonites. The Denton in the clay facies is notable for small crustacea. Miss M. J. Rathbun has kindly listed the following from Bureau collections!