|
Washita Group Nomenclature. Roemer described Georgetown and Grayson fossils from Hill and Comal counties, but gave the group no name. B. F. Shumard (1463, pp. 583, 586-587) described the "Washita limestone," mentioning as localities Austin and Grayson, Fannin, and Red River counties; he says that "according to Dr. G. G. Shumard, it is finely developed at Fort Washita" [northwestern Marshall County, Oklahoma]. He mentions in it various Georgetown fossils from Hamites fremonti to Turrilites brazoensis, making it evident that his formation includes substantially all the section from basal Duck Creek to upper Main Street, i.e., the Georgetown limestone. He placed it correctly under the Grayson (indurated blue marl with Exogyra arietina), and incorrectly over the Eagle Ford blue marl, which he states had not been recognized south of Grayson County. Marcou (1042, pp. 86-97) placed it beneath the Grayson, and above the Caprotina limestone (Glen Rose?). Hill early elevated the Washita to the upper group of his Comanche series (731, p. 298) ,coordinate with the lower (Fredericksburg) group. The type locality, as redefined, is nominally at Fort Washita, but the group, except for its upper formation (Buda), is best developed in the Red River and Trinity River valleys. If judged by priority alone, the name Washita was not available in 1887 (C. A. White, 1741, p. 40) for a division [group] name, because it had been correctly used by Shumard in 1861 for what is now called "Georgetown" limestone. Therefore if priority and not usage should prevail, the name Washita would replace Georgetown, and the "Washita group" would require another name. The Washita group has also been called "Indian Territory division" by Hill(780, p. 88). Stratigraphy and contacts. The basal contact of Washita over Fredericksburg is unconformable south of McLennan County (16, p. 40) . Near New Braunfels, Professor F. L. Whitney has reported that the Edwards is directly and unconformably overlain by Fort Worth or higher Washita formation. 39 It will be recalled that the Goodland-Kiamichi contact seems conformable, but that the Kiamichi-Duck Creek contact shows quartz pebbles and other evidences of unconformity (Scott, 1398a ). These facts lend support to the assignment of the Kiamichi to the Fredericksburg. The Washita group forms the top of the Comanche series in Texas, and its top seems everywhere unconformable. The basal Gulf formation as far south as the Brazos is the Woodbine; farther south, it is a black shale (Pepper) . North of the Brazos, on the outcrop, the Grayson is the uppermost formation. In east Texas, underground, the Washita and Fredericksburg were uplifted and locally planed oft' to differing amounts in different places, but details of this unconformity have not been published. South of the Brazos, at the outcrop, the highest Comanchean formation is the Buda limestone. 39 Whitney, F. L., Report to joint meeting of the San Antonio Geological Society and the Southwestern Geological Society, Austin, October, 1931. Generally its contact with the Gulf series is at least disconformable. Unless the Woodbine is equivalent to existing deposits south of the Brazos, which now seems improbable but awaits further work on donation for a final solution, there is an unconformity south of the Brazos between the Comanche and the Gulf series. Dumble (506, pp. 19-20) says: "West of the Pecos, in certain areas, the beds of the Comanchean were elevated a sufficient length of time for the complete erosion of the entire Washita series and the channeling of the Edwards limestone into deep canyons. This surface was again submerged during the Eagle Ford and these channels filled with its shales." This was probably a result of solution and slump. Facies. (1) The marginal sandy facies occurs at certain levels (Pawpaw, Weno) in southern Oklahoma and in Grayson and Cooke counties, Texas; on the southwest side of the Sabine uplift (Shelby County) ; at the Main Street red-brown sandstone level at El Paso; and to a less extent at other localities. (2) The normal neritic facies of limestone, marl, clay and shale occurs at most places. Presumably a shore line existed in eastern New Mexico, and some of the sands and conglomerates in that area may be of Washita age. (Fig. 15). (3) Reef limestone occupies the entire Washita on the Rio Grande in the Big Bend and in the lower Pecos Valley, and extends northwards at certain levels, interfingering into the neritic facies (Fort Stockton). Areal Geology. The outcrop enters Texas in Grayson, Cooke, and eastern Montague counties, and passes southwards in a strip of narrowing width to the Brazos west of Waco; thence to the turning point of the Comanchean outcrop near New Braunfels. Over the strip from Williamson County southwards, no Washita remains west of the Balcones fault; and from there westward to the Uvalde region, none remains north of the Balcones fault. There are no Washita rocks on the Callahan divide. A large area in the western part of the Edwards Plateau, comprising northern Crockett, southern Upton, Reagan, Irion, Schleicher, western Menard, Sutton, and northern Edwards counties, is capped by Washita. Scattered areas in Lynn, Lubbock, Lamb, and Bailey counties in the Panhandle are composed of Washita rocks. West of the Pecos, Washita covers much of eastern Pecos, Terrell, and Brewster counties, forming a continuation of the Edwards . Plateau. Other exposures of Washita are found in irregular areas, many of them on structural highs, in Brewster and Presidio counties; in several areas emerging from beneath or else covered by the Davis-Presidio-Chihuahua lava flows; in faulted and folded areas in the Van Horn, Tierra Vieja, Eagle, and Quitman mountains, and in the surrounding flats; in a part of the Diablo Plateau in Hudspeth County (Sierra Prieta, Cornudas) ; and in a small area at Cerro de Muleros (Monument Mountain) in New Mexico, and in Chihuahua, opposite El Paso. Uppermost Washita appears in thin outcrops in the Palestine salt dome; elsewhere its outcrops are unknown on the Gulf Coastal Plain. Complete sections occur at many places : from Denison to the edge of the Red River valley; near Belton; at Georgetown; near Austin; near Rock Springs; in the Mesa de Anguila and the Solitario, Brewster County; in the Kent-San Martine area; and near El Paso. In central Texas the Washita group underlies the eastern part of the Grand Prairie, and lies west of, and stratigraphically beneath, the Eastern (Woodbine) Cross Timbers. In north-central Texas the alternate marl and soft limestone formations are thicker and more individualized, and form wider and more distinct belts of outcrop than in south-central Texas. In semi-arid Trans-Pecos Texas, the topographic effects of hard and soft rocks, in this group as in others, is more striking than farther east. The Washita portion of the Edwards Plateau is of the same rudistid facies and the same topography as the rest of the plateau. Thickness and dip. The group has a large thickness on the flanks of the Sabine structure. In Grayson County, at the outcrop, it aggregates about 383 feet; in Tarrant County, about 344 feet; in Dallas County, about 390 feet; in McLennan County, about 312 feet; in Bell County, about 210 feet; in Williamson County, about 200 feet; in Travis County, 205 feet; in Medina County, 160 feet. Westwards the formations thicken, going into the Rio Grande embayment, but the thickness for the group depends on how much Georgetown is present. In Val Verde County, the Washita is likely at least 800 feet thick. In the Terlingua area the Washita is 600 to 900 feet thick, depending on the limits of the Georgetown. At El Paso, its equivalents are given as a maximum of about 850 feet. In the Kent and Fort Stockton areas they would aggregate much less, about 275 feet. Correlation and age. The Washita has been correlated with beds in northern and central Mexico by using zonal fossils; its basal part is represented in the Purgatoire in northeastern New Mexico, Colorado, and the Panhandle of Oklahoma. The Pawpaw is apparently about the dividing line between Upper Albian, represented by Duck Creek to Pawpaw inclusive, and Lower Cenomanian, represented by Grayson and Buda. Further work is necessary, however, in order to clarify the exact details of ranges of Mantelliceras, Submantelliceras (Cottreauites), Stoliczkaia, and other markers. Economic products. Some main Washita commercial products are limestone, used mostly for lime, materials for Portland cement, road metal, clay for bricks and earthenware, quicksilver, manganese, and minor occurrences of artesian water ;at a few places the Washita is an oil reservoir. Paleontology. The following zones, provisional and in part overlapping, serve to partition the Washita strata in Texas:
The following are among the most diagnostic ostracoda in
the Washita: Other Washita ostracoda and foraminifera are listed in Vanderpool, 1681. Ammonites vespertinus Morton, the genotype by original designation of Mortoniceras Meek, is a lower Washita "Pervinquieria" and therefore the name Mortoniceras must be used for the Washita ammonites formerly called Pervinquieria. The Upper Cretaceous ammonites generally called Mortoniceras therefore require a new name, and have been called Texanites by Spath in Part IXof his "Monograph of the Ammonoidea of the Gault" (1511b, p. 379). In this paper he treats several subgenera of Mortoniceras which are common in the Washita group, including the following:
The holotype of Hemiaster elegans Shumard is an echinoid somewhat like H. comanchei Clark, with a distinct peripetalous fasciole, and is apparently referable to Hemiaster, not to Macraster. Lithology.40 The limestone members of the Georgetown are generally light gray, compact, somewhat nodular, and crystalline, with a considerable intermixture of shell fragments and fossils. The matrix is finely crystalline and generally somewhat argillaceous. The strata are thin-bedded, and alternate with more marly strata. Some levels, both in the marl and the limestone, contain aggregations of innumerable Gryphaea shells. Microscopically, Georgetown limestone generally shows the following features: (1) Shelly material is in abundance; it is less detrital than the average central Texas Buda and much more detrital than the Edwards. (2) Globigerina washitensis is often present; it is absent in the Austin chalk and in the Upper Cretaceous generally. (3) Inoceramus prisms are scarce; they are generally abundant in the Austin chalk. (4) Echinoid spines are frequent. (5) Miliolids are generally absent; they are abundant inmany Edwards sections. (6) Spherical bodies may be abundant; they are absent iv the Edwards and absent in most samples in the Austin chalk, or, if present are generally sparse. (7) The matrix is fine grained and generally more abundant than the shelly material. In the Buda, the shelly material is more abundant, in the Edwards and in some Austin chalk samples, shelly material is sparse. Details are given in Winton (1791, pp. 65-74). Spherical bodies are abundant in most Duck Creek, many Fort Worth, and some Weno and Main Street samples (figured in 647a, pi. 3, fig. 2). They have been recently discussed by Helen Jeanne Plummer (1238, pp. 112-118), Brown (1238, p. 115), Twenhofel (1238, p. 115), and H. D. Thomas (1599a, pp. 100-101), and have been figured by Winton (1791, pi. 23). Similar bodies have long been known in the English Upper Cretaceous chalks, where they were described by Blake, Jukes-Brown and Hill, and Tarr, and in the Upper Cretaceous of Poland and elsewhere. Vaughan suggested that oolitic grains might have formed around gas bubbles in calcareous ooze. Similar structures apparently occur in rocks of diverse ages, from which their inorganic origin has been inferred. They are of course not zonal markers in Texas, but their abundance definitely suggests the Washita age of the containing rock. 40 Literature Alexander, 30; Hanna, 647a; Plummer, 1238; Smiser, 1493; Udden, 1656; Winton, 1791. Chemical analyses. Analyses of Washita and other
Cretaceous rocks have been published by Schoch (1378), Hill (803), Winton
and Scott (1790) and others. They show that most of the Washita marly
limestones in north-central Texas are high in lime (85%-91% CaCO3) ;that
the Pawpaw and some Denton and Weno shales have as low as 13%-18% CaC0 3;
but that most Kiamichi, Duck Creek, Weno and Grayson marl samples run high
in lime (31%-83% CaCO3). |
||||||||||||||||||||||||||||||||||||||||||||||||||||