The Geology of  Denton County

" Paleontology:  

Nautilus texanus (plate 5). This species occurs occasionally in the upper parts of the Fort Worth although its zone of abundance is properly in the Weno and its vertical extent is very great, as specimens are not rare even in the highest portions of the Grayson. This is a characteristic and readily recognized species and its strong external resemblance to the modern chambered nautilus makes this an interesting museum piece, although it is rather poor as a stratigraphic marker in the field. In this connection, about all that may be said of it is that this nautilus marks the upper portions of the Washita division.

Schloenbachia leonensis (plate 4). This conspicuous ammonite, exceeded in size only by the Desmoceras, marks the Fort Worth formation and so far as the writer knows no observed occurrence is known later or earlier than the Fort Worth.

 Schloenbachia aff. inflata (plate 6). This small ammonite is fairly common near the middle of the Fort Worth and because of this is figured and listed here. As yet this species has not been described by American paleontologists, although it has several times been referred to in various publications, the reference being by a letter or a number.

 Hemiaster elegans (plate 13). This is the largest of the biscuit urchins occurring in these rocks. In certain localities, for example the Denton Creek bluffs east of Justin, individuals are very common. This fossil has a number of interesting local names. The most common perhaps being"Texas Star" although "Texas Heart" is also common.

 Pecten bellula (plate 10). This small scallop is marked by a large number of very fine ribs which makes it characteristic even in fragments. It is found near the middle of the Fort Worth formation.

 Exogyra americana (plate 11). This species has one of the shortest lives of any species known in the Comanchean. The occurrence is through a vertical extent of only a few inches and individuals are not common but are worth searching for in doubtful cases as a specimen in place definitely marks the top of the Fort Worth formation.

  Fucoid plant masses (plate 2). These large irregular masses are fairly common in the Fort Worth and occur more rarely in higher formations. No structural details are exhibited and Texas geologists have provisionally considered that these masses are the remains of giant seaweeds. Fragments may be broken off which take peculiar and interesting shapes which are often mistaken by untrained persons for new and rare fossils.   "

• Stratigraphy of the Fort Worth Limestone formation by Winton and Adkins.

The Geology of Texas - Vol. 1

FORT WORTH FORMATION

Nomenclature.—In early literature, the term Fort Worth lime-stone was used as a synonym of Washita (= Georgetown) limestone. In 1891, Hill (772, p. 516) restricted the term to apply to the limestone above the Duck Creek and below the Denison beds. The term is still used in that sense. The localities mentioned include, near Fort Worth, the bluff of Trinity River just north of the Court-house, the quarries near the Union depot, the Texas and Pacific Railway cuts in the city, and the railway cut at Hodge station, 3 miles north of Fort Worth. Some of these localities are now badly over washed and overgrown.

Facies. Neritic, mostly thin- to medium-bedded limestone, with a smaller amount of interbedded marl, on the outcrop.

Areal outcrop; local sections.—The Fort Worth formation is peculiar in retaining a nearly uniform thickness of 30 feet over much of its outcrop. In southern Oklahoma, the Duck Creek and Fort Worth formations together were called Caddo limestone by Taff. In Marshall County. Oklahoma 174. pp. 33-35). the Fort Worth consists of about 40 feet of hard, cream-colored limestone, lithologically somewhat like the Duck Creek limestones, alternating with thinner strata of limy marl. The lower 10 to 15 feet is com-posed of alternating beds of yellowish-white limestone and grayish to blue clay-marl. In the middle 10 to 15 feet thicker marl strata alternate with thinner limestone seams. The upper 10 feet is pre-dominantly limestone, separated by thin marl seams. In Grayson County, Bullard (177, p. 29) records 45 to 50 feet of Fort Worth, the lower 15 feet being alternate layers 1 to 3 feet thick, of shale, or marl and limestone. The next 19 feet consists of clay-shale containing thin limy seams, with the following recorded fossils; Pervinquieria leonensis (Conrad), Holaster simplex, and Pecten. The upper 19 feet is thicker limestone strata alternating with subordinate marl beds; it contains, according to Bullard, Exogyra americana Marcou, Macraster elegans (Shumard), Holaster simplex Shumard, Pervinquieria leonensis (Conrad), and Gryphaea washitaensis Hill. The Tarrant County section of Fort Worth limestone is 30 to 35 feet thick. The beds are uneven and lenticular, and the limestones grade into the marls by a flaky transition zone. The limy strata differ from Duck Creek limes in being generally a foot or less thick, harder, and more regularly alternating. The soil weathers into a rolling, untimbered blackland prairie suitable for grazing. The upper part of the formation is locally marly, and contains abundant fossils, such as Macraster pseudoelegans, M. texanus, M. aguilerae, M. nodopyga, Holaster simplex, the rare but widespread Washitaster longisulcus, Exogyra americana, Pervinquieria maxima, and other species. From Bell County southwards, the Fort Worth is well compacted with the other members of the Georgetown limestone. It is overlain by the attenuated, soft-weathering Denton shell marl, a thin, persistent, soft ledge containing many Gryphaea washitaensis, Alectryonia cf. carinata and Macraster (several species), which weathers to a distinct bench and serves to separate the halves of the Georgetown in the southern section. Here the Fort Worth is a chalky, argillaceous, nodular limestone, dark to light bluish-gray on fresh exposure, whitish and somewhat disintegrated on prolonged exposure. It generally forms an upland prairie, bordered to the west by the narrow Duck Creek outcrop, and in stream cuts, it forms small bluffs.

Paleontology and zonation.—The Fort Worth probably represents only a brief interval in the zonation, and at present is assigned to the sole zone of Pervinquieria maxima, which in the Fort Worth section extends down into the top of the Duck Creek marl. After the explosive appearance of Pervinquieria in the basal Duck Creek, the genus became less abundant, but is represented in the Fort Worth limestone by several distinct lines, which have not been yet sufficiently studied, among them the groups of P. nodosa, P. kiliani, P. trinodosa, and some new species. Prohysteroceras persists from the Duck Creek to at least the Weno. Macraster has a thin zone in the Duck Creek, and reaches a considerable development, with several species, in the Fort Worth-Denton. Washitaster, so far as now known, first appears in the Fort Worth. Several rarities occur in the Fort Worth: Turrilitoides n. sp., a genus like Ancyloceras but much larger (Tropaeum-like), and micromorphs. The best guide fossils are Pervinquieria maxima and Exogyra americana, which occur in abundance.