The Geology of  Denton County

" The fauna of the Grayson formation is so rich and so highly varied that even a list of the species would be cumbersome. In this paper mention will be made only of a few common and diagnostic fossils.

  Gryphea mucronata (plate 7). This oyster, the last of the Grypheas of the lower Cretaceous in this region reached an astonishing development in numbers and the species lived for a considerable length of time, as it is found in more or less abundance throughout the approximately seventy feet of the Grayson. As is the case with all oysters, both living and fossil, slight environmental changes are reflected in the shell and no two specimens are exactly alike. In fact identification of the Grypheas in the field usually calls for the collection of a dozen or more specimens and the search for general characteristics shared by all the specimens in the group. The examples of the different species which are figured here were carefully selected in order to exhibit typical appearance of each species. In the case of this species because of the fine preservation and the great abundance of individuals, striking variations are common.

  Exogyra arietina (plate 11). This is an extreme example of the fossil oysters of this group having twisted beaks. This species occurs in Denton County mainly in the lower part of the Grayson, although in central and western Texas in the corresponding time zones of the Del Rio formation, the vertical extent of the species is considerably greater than in Denton County. Also specimens are usually larger than those encountered in the Grayson in Denton County.

  Small Turrilites superficially resembling Turrilites brazoensis (plate 5) but much smaller occur in the Grayson of this region; and, in at least one important publication, in­dividuals of this species have been figured with T. brazoensis as belonging to that species.

  Echinoids occur occasionally in the Grayson and are different species (which will not be listed here) of some of the same genera previously described in connection with some of the lower formations. The most important of these is a species of Enallaster.

  Pyrite fossils in some variety occur in the Grayson in Denton County. As is always the case in these rocks the species are all dwarfed.

  Pinna sp. (plate 9). This angular and thick clam oc­curs here and less often in the Goodland. There are sev­eral species, all superficially alike.  "

The Geology of Texas - Vol. 1

GRAYSON FORMATION

Nomenclature. --- The formation was first called Grayson by Cragin (325, pp. 40, 43) in 1895. In 1898 Hill and Vaughan (795, p. 236) applied the name Del Rio to its southward extension in the Rio Grande Valley near Del Rio.

Taff (1575, pp. 277-283), like some later writers, considered the scattered limy ledges in the upper Grayson in north Texas to be the equivalent of the Buda limestone farther south. Some earlier writers correlated the Del Rio clay in south Texas with the Main Street limestone in north Texas, and the Buda in south Texas with the Grayson marl in north Texas. These early correlations led to the separate naming of the Grayson and the Del Rio. However, both the basal Grayson flaggy zone, with "Acanthoceras" aff. cunningtoni Böse, Stoliczkaia spp., and abundant Turrilites brazoensis, and the overlying clay, seem essentially identical from Red River to the Rio Grande; and in this event Cragin's name has priority. Taff has expressed a similar opinion; in his reports the Grayson was called "Vola clay" and the Buda "Vola limestone."

Roemer, without describing the clay, had collected from it in southwestern Hill County west of Aquilla, and had described Exogyra arietina (1331, p. 68, pl. 8, figs. 10 a–e) from the Waco Indian camp west of New Braunfels. B. F. Shumard first described the "Exogyra arietina marl" (1464a, pp. 583, 586) from the base of Mount Bonnell near Austin, and correctly placed it beneath the Austin limestone with Eagle Ford at the base and above the "Washita" (= Georgetown) limestone. Very curiously, he over-looked the Buda. In the older literature the Grayson is often called the Exogyra arietina clay" or "marl."

Literature. --- Oklahoma: Bullard, 174; Honess, 839, 840. Central Texas: Cragin, 325; Carpenter, 197; Hi]], 772, 788, 803, 822; Winton, 1791; Roemer, 1330, 1331; Taff, 1575; Shumard, 1464a. Rio Grande Embayment: Hill and Vaughan, 795. Trans-Pecos Texas: Böse, 129; Hill, 722; Udden, 1626, 1648. Paleontology: Alexander, 31; Adkins and Winton, 9; Adkins, 10; Böse, 133, 1341 Cragin, 325; Plummer, 1237, 1238; Roemer, 1331; Udden, 1628.

Synonyms. --- Gelbliche Kalkmergeln, Roemer, 1331, p. 16; Roemer, 1330, p. 184. Exogyra arietina marl, Shumard, 1464a, pp. 583, 586; Marcou, 1042, pp. 86-97. Exogyra arietina clays: Hill, 755, p. xiv, xxiii; Hill, 772, p. 517. Exogyra arietina beds: Hill, 788, pp. 318, 319, 321-322. Exogyra arietina sub-division: Taff, 1575, pp. 275-277. Vola limestone and marl: Taff, 1575, pp. 277-283. Grayson marl: Cragin, 325, p. 43 (type locality: cuts of D. B. & N. 0. Ry., in southeastern part of Denison) ; Hill, 803, pp. 245 (fig. 29), 246 (fig. 30), 247, 286-288. Del Rio clay: Hill and Vaughan, 795, p. 236 (type locality: a conical butte, Loma de la Cruz, and surrounding clay lowlands 2 miles south of Del Rio).

Stratigraphy and contacts. --- Throughout Texas the Grayson is underlain, concordantly and apparently conformably, by the Main Street formation or member. In southeastern Oklahoma the thinned Grayson lies above the Main Street ("Bennington") limestone and below the Woodbine ("Silo"). Some confusion in correlation has arisen over the thin transitional zone, consisting of alternating marls and marly limestones, at the base of the Grayson in central Texas. These strata are paleontologically most like the underlying Main Street limestone, because of an abundance of Kingena and Turrilites brazoensis Roemer. They are also marked by Stoliczkaia spp., "Acanthoceras"⁴³ aff. cunningtoni Bose (a fossil practically restricted to this level), and Exogyra arietina, which is abundant in both Main Street and basal Grayson.

In north-central Texas, north of latitude 31° 30', near Waco, the Grayson is overlain, apparently unconformably, by Woodbine. South of McLennan County, it is overlain by Buda, the nature of the contact being unknown. In McLennan and Bell counties, at the outcrop, the Buda is of intermittent occurrence, and, where it is absent, the black shales at the base of the Eagle Ford flags rest unconformably on the Grayson, with a pebble and phosphatic conglomerate at their base. At a locality near Cedar Mills, Grayson County, the Grayson has been reported absent and the Woodbine rests directly on the Main Street limestone. In southern Oklahoma, east of longitude 95° 30', the Grayson is apparently absent, and the Woodbine overlies a beveled Comanche surface ranging from Grayson down to Goodland.

In Trans-Pecos Texas and northern Coahuila, the Grayson is absent over a considerable area, in which the Buda rests directly en the Georgetown. In Reeves and western Pecos counties, the Grayson thins northwards and disappears. In the southern Edwards Plateau, between San Antonio and Del Rio it thins northward and disappears, as it probably does also in southern Oklahoma.

Throughout Texas, the Grayson is underlain, concordantly and apparently conformably, by the Main Street formation or member of the Georgetown limestone. The "Acanthoceras" cunningtoni zone appears to be transitional between the two formations.

Facies.--- (1) Throughout its extent in southern Oklahoma, central Texas, and western Trans-Pecos, Texas, the Grayson is essentially of the clay facies, with subordinate amounts of friable sandstone flags and siltstone locally, especially near its top. (2) In western Val Verde and Terrell counties and in northern Coahuila, the Gray-son (particularly in its upper part, on crossing the Terrell high axis) consists of interbedded, thin, calcareous flagstones and limy marl, containing ammonites and other fossils. In this region and in the Big Bend, many of these flags are siliceous, and contain vast numbers of the large arenaceous foraminifer, Haplostiche texana. Over the Terrell arch and southwards into northern Coahuila (as near Remolino, El Macho, La Bahia), the Grayson is mostly fossiliferous limestone flags (compare also: 578, p. 1427).

Areal outcrops and local sections.--- Honess (840, vol. III, p. 92) states that in Bryan and western Choctaw counties, Oklahoma, the Woodbine overlaps onto Comanchean formations, and is in unconformable contact with the Main Street ("Bennington") limestone and with the Goodland limestone at the eastern line of McCurtain County. The Grayson is therefore absent at the outcrop in southeastern Oklahoma. In Bryan County, Oklahoma (Taff, Atoka folio No. 79, 1902) the Grayson marl is present but thins to the east; but, at a complete exposure 1 mile north of Durant, it is 50 feet thick (1790, p. 28, footnote). It is exposed at localities 2 to 3 miles east and northeast of Woodville, Marshall County, Oklahoma (174, p. 47).

Red River valley.---The easternmost outcrop of the Grayson formation in Texas is in northeastern Grayson County. On Pawpaw Branch, 3 miles east of Stillhouse Ferry, the Grayson, about 27 feet thick, consists of blue-gray marl alternating, particularly near the top, with thin seams of grayish nodular limestone (177). The topmost limestone seam is overlain by thin blue-gray marl, and black, bituminous clay, which grades into yellow weathering Woodbine sand. At the road crossings of Sandy Creek just east and north of Cedar Mills, Bullard records the Woodbine in direct contact with the Main Street limestone, the Grayson marl being completely missing. However about a half-mile upstream from Cedar Mills, on Sandy Creek, Bullard records a normal thickness (25 to 30 feet) of Grayson, with an irregular upper contact. In a branch of Little Mineral Creek about a mile south of Fink, Bullard records about 29 feet of Grayson, consisting basally of clay marl, and, at the top, marl with a few thin Iimestone seams. At a locality 1 mile south of Bloomfield, Cooke County, the Grayson is 32 feet thick and consists of gray and white marl with some thin strata of white chalky limestone (189, p. 40).

Red River to Brazos River.---The best Denton County exposure is in a tall bluff 3 miles northeast of Roanoke at the edge of Denton Creek valley, where the Grayson is complete with both contacts exposed (1789, p. 73; 1791, p. 30). The section consists of about 80 feet of gray or yellowish marl, containing in the top eleven limestone ledges each less than a foot thick. These upper ledges in the Grayson are correlated with the Buda by Winton, and at a locality only a few miles south of the Grayson bluff, the uppermost hard limestone ledge is called Buda by Hill (822). About 50 feet of middle and upper Grayson is exposed near the highway 2 miles southeast of Burleson; it consists of gray and yellowish marl. At the Aquilla Creek crossing 1.6 miles west of Peoria, the Grayson-Woodbine contact is exposed. Here the Grayson is a gray-blue clay with Pecten and Gryphaea mucronata Gabb, and the basal Woodbine is a lustrous black shale with ferruginous nodules, and, basally. a thin sand and iron layer. At the R. E. Finley well 1.5 miles west of Aquilla, southern Hill County, the complete Grayson, about 75 feet thick. is exposed as a clay containing pyritic micromorphs and, near its top, thin limy layers. In McLennan County, north of the Brazos, there are numerous exposures of the upper Grayson in con-tact with the basal black shale beneath the Acanthoceras flags of the Eagle Ford (11, pp. 52-58).

Brazos River to Colorado River.---Along the Bosque escarpment in southern McLennan County, there are several exposures of the Grayson-black shale contact. These show the upper half of the Grayson, with abundant fossils. At Bosqueville, and from the Bell-McLennan line southwards to a point east of Salado, the Buda is intermittently present at the outcrop between the Grayson and the basal Gulf black shale; in the intervening portions along the contact, the Buda is absent at the contact (11, pp. 58-67; 16, pp. 48-59). A significant locality 2 miles south-southwest of Moody shows rolled and decomposed Buda limestone boulders, but no solid strata of Buda, at this contact (16, p. 52) ; a short distance farther south, solid Buda appears. The intermittent Buda is clear evidence of unconformity at this level. In southern Bell County, about 75 feet of Grayson is exposed; a complete section exists on the south bank of Leon River, 2.5 miles southeast of Belton. On the east bank of Lampasas River, about 4¾ miles east of south of Belton, most of the Grayson, including the basal transition zone, is exposed. From Salado southwards, the main body of the Buda is present at the outcrop, overlying the Grayson. At localities along Smith Creek, 1 to 2 miles east to northeast of Georgetown, a complete section of Grayson is exposed. The entire formation is clay, with flaggy layers near the top. The clay contains ferruginous nodules, pyrite, gypsum, and some ironstone. A complete section is exposed at the concrete bridge just above the mouth of Barton Creek in South Austin. The upper part of the formation and the Grayson-Buda contact occur at several localities along Shoal Creek. In Travis County the Grayson practically lacks the pyrite micromorphs found in the Bell-McLennan county area and the Haplostiche texana which is abundant in West Texas, but has abundant Exogyra arietina in the basal transition zone and the lower half of the formation, and abundant Gryphaea mucronata in the upper part.

Colorado River to Del Rio.--- In Bexar County (888, p. 770; 1402, p. 110) the thickness of the Grayson is 65 to 78 feet; in Medina County, 60 feet (992, p. 39) ; in Uvalde County (1686, p. 7; 1681, p. 253), 50 to 137 feet; in the Brackettville region, 75 feet; in Maverick County, up to 279 feet (1681, p. 253) ; and at Del Rio, about 200 feet (1324, p. 15). Near San Antonio the formation consists largely of clay; on drilling it caves badly, requires casing, and is therefore known to drillers as the "mud hole." Its basal part contains some thin limy flags. The clay also contains gypsum, pyrite, and ferruginous masses; it weathers to a narrow strip of mesquite-covered, hilly topography, or else to valleys.

Thinned Grayson in lower Pecos Valley.--- From about 200 feet in the clay flats south and east of Del Rio, the Grayson thins rapidly in all directions. In the hills north of Del Rio it is 50 feet thick or less. To the west it thins to about 30 feet at Comstock, to 15 feet at localities southwest of Shumla, and to nothing at a railway cut 4 miles west of Shumla and in exposures near Langtry. North of Del Rio the Grayson thins rapidly, and in the Rock Springs area is represented by 15 feet or less of calcareous seams lying between the Georgetown and the thinned Buda limestones. West-ward from Langtry the Grayson thickens.

From Del Rio it outcrops in a straight band running a little west of south for 45 miles to Tinaja Azul, about 6 miles south-southwest of Remolino, Coahuila, where' it disappears. Over this area it gradually thins southward, until the Georgetown and Buda come to lie in concordant contact. The upper zone of the Grayson, marked by an abundance of Exogyra cartledgei, as seen near El Sauz, Goodwin Ranch, and Remolino, persists, and the basal zones drop out, whether by overlap or by change of facies is unknown. In Dumble's Zorro Creek section (480, p. 377), in Coahuila near the Rio Grande, the Grayson, about 110 to 140 feet thick and containing Pecten, Exogyra arietina Roemer, Exogyra drakei Cragin, and Haplostiche texana (Conrad) in blue, yellow, and red clays, with bands of sandy flagstones and concretionary limestones, is like that at Del Rio. The thinning is a result of the northwest-trending Terrell arch, which runs from the Burro Mountains uplift (dome) toward the Marathon basin. West of this high axis, in Brewster County, the Grayson rapidly thickens.

In the Grayson (Del Rio) of the Guajes Valley, east side of the Sierra de Encantada about 100 km. south -southeast of Boquillas in northern Coahuila, C. L. Baker and R. B. Campbell have collected abundant rhynchonellid brachiopods together with Kingena cf. wacoensis, Exogyra arietina (?), and limonitic Turrilites.

Big Bend.--- In Dryden Canyon, just north of the town of Dryden, the Grayson, consisting of 9 feet of loose, brownish-yellow clay, full of Exogyra arietina Roemer, can be seen to pinch out to the east between the Georgetown and Buda limestones. Farther west, near Nichols Pump, the Grayson consists of 25 to 30 feet of hard, calcareous sandstone flags with abundant Haplostiche texana and ripple marks. indicating shallow water deposition (248, pp. 13-14). Many Haplostiche in this district show orientation, presumably by current action. Between Dryden and Sanderson, the thinned Grayson contains Exogyra whitneyi and Hemiaster calvini. In eastern Brewster County, thin but typical Grayson is overlain by Buda. Along the Alpine-Terlingua road, exposures show less than 50 feet of Grayson. East of the Chisos Mountains it is about 20 feet thick. In the Terlingua district (1626, p. 27) it has thickened to 120 to 180 feet at different places. The formation is a clay, with laminated flags, siliceous flags, and calcareous nodules in its top.

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The basal and middle parts contain an abundance of echinoids (mostly Heteraster), Gryphaea mucronata, pyritized micromorphs (ammonites, gastropods), and Haplostiche texana in silicified flags. The top contains the zone of abundance of Exogyra cartledgei Böse. Excellent exposures are in the Mariposa and California Mountain district, and on the Reed Plateau south of Terlingua. In Cibolo Canyon south of Shafter, the Grayson has a thickness of 80 feet, and consists of clay with some thin ledges of fossiliferous limestone (1623, p. 39) . The entire outer circumference of the Solitario dome includes a narrow outcrop of Grayson, about 125 feet thick, forming the cuesta face of the outwardly-dipping Buda limestone. This formation is a clay) with a few limy seams, and has the same fossils as at Terlingua.

Near Chispa Summit, western Jeff Davis County, the Grayson is a clay with considerable nodular or flaggy limestone. In the southern Quitman Mountains, 300 feet of Grayson has been reported, consisting of ferruginous, laminated and flaggy calcareous sand, argillaceous limestone and clay, with Haplostiche texana. In the north-western Eagle Mountains, near the east end of Devils Ridge, the Grayson with Haplostiche consists of less than 100 feet of sandy and somewhat marly limestone. Similar rock occurs in the small hills just southwest of Etholen station.

North of Southern Pacific Railway.--- The Grayson rapidly thins northward, and disappears in western Pecos County. At a locality a mile north of Hovey, it is represented by about 5 feet of limy marl with Exogyra arietina (F. B. Plummer, personal comm.) . A similar situation has been reported from a locality north of Strobel (J. A. Udden, personal comm.). It is absent beneath the Buda at the north end of the Davis Mountains. Likewise at Kent no Grayson has been reported, although thin limy banks carrying Exogyra arietina and referred to the Main Street occur here. Farther north there are no records of Grayson.

El Paso section.--- The small ravines at the west end of the tunnel at Bowen, N. M., contain exposures of about 75 feet of Grayson, which are the No. 8 beds of Böse's Cerro de Muleros section, containing Exogyra whitneyi Bose and Hemiaster calvini Clark.

Subsurface extent.--- The Grayson has not been definitely identified in most parts of the East Texas embayment. A clay at its stratigraphic position occurs in many wells in Red River, Lamar, and Fannin counties.

Mode of deposition.--- The Grayson, as a whole, is a marl or clay, containing oysters and other shells. Its site of deposition was doubt-less epicontinental and neritic, but at what depth or distance from the shore is not known.

Lithology; microscopic appearance; mineral content. --- The formation contains gypsum, pyrite, and hydrated oxides of iron. The uppermost limestones, correlated by Winton and others with the Buda, are coarse-grained and contain foraminifera and other fossils. The lower limestones, on microscopic examination, are finer grained and less fossiliferous (1791, p. 73).

Thickness. --- The following are some reported thicknesses of the Grayson:

Paleontology. --- Exogyra arietina F. Roemer and Gryphaea mucronata Gabb are currently stated to characterize the Grayson (Del Rio). E. arietina is present in much of the Main Street, and especially in the transition zone at its top, in association with Kingena wacoensis (Roemer); it is abundant in the lower part of the Grayson, where G. mucronata is rare; and rare in the upper Grayson where G. mucronata is abundant. G. mucronata also occurs in the basal Buda. The Grayson in the southern sections contains abundant Haplostiche texana, mostly in siliceous flagstones. There is a notable oyster zone in the Grayson. It comprises Exogyra cartledgei Bose, Exogyra n. sp. aff. texana Roemer, at least two new species of large Exogyra (one resembling E. olisiponensis, and the other smooth), and Exogyra drakei Cragin.

The Grayson contains the highest of five principal Washita zones of pyritic micromorphs. Its pyritic ammonites are somewhat similar to those in the Pawpaw, but show greater advancement, especially in the Mantelliceratidae. Grayson genera are: Scaphites, Hamites, Wintonia, Baculites, Submantelliceras (Cottreauites), Adkinsia, and several undescribed forms.

The foraminiferal fauna is distinctive, and includes the following species, which Mrs. Helen Jeanne Plummer has kindly listed:

The arenaceous foraminifer, "Nodosaria" texana Conrad, has been variously referred to "a new genus somewhat like Cribrogenerina" by the present writer (12, p. 47), to Haplostiche by Plummer (1238, p. 124), and to a new genus Cribratina by Sample (1362a, p. 319). It or closely similar species occurs in Trinity, Fredericksburg, and Washita formations, and is most abundant in the Weno in north Texas and in the Grayson in west Texas.

The following are the most characteristic Grayson ostracoda:
   Cythereis burlesonensis Alexander
   Cythereis roanokensis Alexander
   Cytherelloidea obliquirugata (Jones)
   Macrocypris graysonensis Alexander (lower Grayson only)

⁴³ "A new genus, to be described in a forthcoming paper.