North Texas Fossils - Austin formation

Sediments of this group were deposited in Texas in a transgressing sea, whose margin moved northward during Trinity time; the limit of its advance is shown roughly in Fig. 16. Practically everywhere its base consists of sand or conglomerate, which of necessity is of different ages at different places along the line of advance. It follows that this basal lithologic unit is not a formation, if by formation is meant a rock body of a single restricted contemporaneous age. It is the combined marginal facies of various Trinity levels. The middle of the Trinity group generally though not everywhere consists of a limestone, usually somewhat sandy. The uppermost Trinity is likewise a sand shorewards, a limestone seawards. As in the Fredericksburg group, formations, even though useful in small areas, may not be of the same age over greater distances. Rocks of the same facies are of different ages at different places, and rocks of the same age laterally change rapidly in facies. Therefore the Trinity group willbe treated as a whole, and the various, in part overlapping, formations which have been used in different parts of Texas will be mentioned in their appropriate stratigraphic connection.

Formations of Trinity Group in Texas
Northern and central Texas	Marathon basin	Sierra Blanca-Quitman Mtns.	Stages
Paluxy Glen Rose Travis Peak (absent)	Maxon Glen Rose "Basement sands" (absent)	Cox Glen Rose-Cuchillo Mountain-Las Vigas Torcer	Lower Albian Upper Aptian Valanginian

²⁰ Literature --- Arkansas: Dane, 392; Hill, 753. Oklahoma: Hill, 803; Billiard, 174, 175; Taff, Atoka, Tishomingo folios; Honess, 839, 840. Louisiana: Spooner, W. C, Interior salt domes of Louisiana, Bull. Am. Assoc. Geol., 10: 217-292, 1926, and, Geol. Salt Dome Oil Fields, 269-344, 1926; Veatch, A. C, Salines of north Louisiana, La. Geol. Surv., Spec. Rept., 2, 1902; Shearer, H. X., and Hutson, ,E. 8., Dixie oil pool, Caddo Parish, Louisiana, Bull. Am. Assoc. Petr. Geol., 14: 743-764, 1930; Ross, J. S., Deep sand development in Cotton Valley field, Webster Parish, Louisiana, Bull. Am. Assoc. Petr. Geol., 14: 983-996, 1930; Crider, A. F., Pine Island deep sands, Caddo Parish, Louisiana, Str. Typ. Amer. Oil Fields, II:168-182, 1929; Fletcher, Corbin D., Structure of Caddo field, Caddo Parish, Louisiana, Str. Typ. Amer. Oil Fields, II:183-195, 1929; Spooner, W. C, Homer Oil Field, Claiborne Parish, Louisiana, Str. Typ. Amer. Oil Fields, II:196-228; Teas, L. P., Bellevue oil field, Bossier Parish, Louisiana, Str. Typ. Amer. Oil Fields, II:229-253, 1929. North-central Texas: Bullard, 176, 177; Winton, 1789, 1790, 1791; Shuler, 1454; Adkins, 11, 1(5; Hill, 742, 746, 803; Scott, 1394; Miser, 1113a; Vanderpool, 1679. Central Texas: Hill, 803, 795, 808; Jones, 888, 891; Sellards, 1402; Liddle, 992. Trans-Pecos Texas: King, 936; Baker, 46; Adkins, 12; Udden, 1623, 1625, 1626. Edwards Plateau: Liddle, 991; Beede, 87, 92; Henderson, 704; Hill, 803. Paleontology: Hill, 762, 767, 783; Rauff, 1286; Burckhardt, 180a; Vanderpool, 1678, 1679. Paleobotany: Groves, 633; Fontaine, 545; Torrey, 1608; Sellards, 1416; Wieland, 1758 a, 1759, 1759a; Adkins, 12, 13. Transgressions and regressions: Grabau, 629b; Hill, 803; Scott, 1394.

Hill (731, p. 298) at first placed Shumard's "Caprotina limestone" (== Glen Rose) in the Fredericksburg group and separated the "Upper Cross Timbers" (== Travis Peak) as an independent and earlier unit. The term "Trinity division" was first used by Hill in 1889 (735, pp. xiv-xv). Taff's "Bosque division" (1574, p. 280, 1891) is a synonym. The name "Travis Peak sands" was applied by Hill (766, p. 118) in 1890 to the basal marginal fades of the Trinity group in Travis County; the succeeding limestones were called "Glen Rose or Alternating Beds" by Hill in 1891 (772, pp. 504, 507; 780, p. 83), and the upper Trinity sands were called Paluxy in 1892 (780, p. 84). These three formations comprise the standard type section of the Trinity group. The basal marginal facies, where its exact correlation appears doubtful, has been generally referred to in the literature as "Basement Sands." Along a line (Fig. 18) through Brown, Parker, Wise, and Denton counties, the Glen Rose facies interfingers out into sand on going northwards, and the combined Trinity sand here has been called Antlers (788, p. 303). Much of it is Trinity, but its upper part may be of Fredericksburg age. In west Texas the nomenclature of the Trinity has met with similar difficulties, and some authors, through lack of knowledge of the fossil contents of the beds, have introduced much confusion into the nomenclature. Richardson, in describing the section northwest and northeast of Sierra Blanca, called the basal conglomerate "Campagrande" (1304 a, p. 47), the type locality being in the Finlay Mountains; a higher sandstone was called "Cox" (1304, p. 47) from Cox (Tabernacle) Mountain. The overlying Finlay limestone (1304, p. 47) is of Fredericksburg (Comanche Peak-Edwards) age. The basal conglomerate 4 miles west of Sierra Blanca was called "Etholen" by Taff (1573, p. 723). These formations, and the Maxon sandstone of King (936, p. 92), about Paluxy in age, situated in the eastern part of the Marathon basin, all lie in the thinned shoreward northern extension of the Comanche series in Trans-Pecos Texas. Just south of Sierra Blanca, Taff partitioned the Trinity section into Etholen, Yucca (1573, p. 725), and Bluff (1573, p. 727) beds; farther southwest and down the overlap near Quitman Gap he partitioned the Trinity equivalents into Mountain (1573, p. 730) and Quitman (1573, p. 728) beds. In the Shafter district Udden established the Presidio beds for the basal sands and conglomerates of the Trinity (1623, pp. 25-30), and the Shafter beds (1623, p. 30) for the overlying limestones. In the Conchos River region in Chihuahua, west of Presidio, Burrows has established several formations, which are discussed in the following section. At Malone (Torcer) ,in the Conchos River valley west of Presidio, and probably near Hot Springs south of Sierra Blanca, the earliest Cretaceous in Texas, here included in the emended Trinity group, occurs. It will be discussed first. The Trinity elsewhere in Texas may be conveniently divided into thicker, off-shore, sections, and thinner inshore, in part marginal, sections.

In consequence of the landward (northward) overlap of Trinity sediments onto the old basement rocks across Texas, the Trinity group forms a wedge-shaped mass of rocks, the thin edge pointing inland, and the thicker, buried, seaward extension pointing gulfwards. The Trinity rock sheet reaches inland to an irregular line (Fig. 16) running approximately through El Paso, Fort Stockton, Big Spring, and thence across the central area, denuded of Comanchean rocks, to southern Oklahoma; south of this line there are restricted areas in which Trinity rocks are absent and Fredericksburg rests directly on the old land.

In facies, all Trinity rocks at the outcrop in Texas are epicontinental, and consist of two classes: (1) marginal sediments, conglomerate, sandstone, sandy shales in the typical Travis Peak formation, some sandy phases of the Glen Rose, and the type Paluxy; and (2) an offshore, neritic facies, consisting of limestone and marl, as in the type Glen Rose. The marginal facies is characterized by clastic and detrital material, coarse sands, gravel, and conglomerate, red color, rarity of typical marine invertebrates, presence of fossil wood, cycads, dinosaurs, fish remains, and cross-bedding. The other facies contains limestone (shell breccias, coquina, or organically precipitated limestone), some of it of shallow water origin and containing ripple marks, rain marks, mud cracks, dinosaur tracks, and coral reefs, some black shale, and anhydrite, sulphur, celestite, strontianite and mineral salts. In Louisiana deep wells there is a third, red bed facies, which may be of continental origin.

The formations of the Trinity group have only local validity, because, if viewed regionally, they are facies of one continuous and laterally changing mass of sediments. For convenience they are here treated as local formations, with the reservation that on being traced into the marginal area the offshore units become lithologically similar. The Gillespie near Fredericksburg is a marginal, cycad bearing facies of the upper Glen Rose. The Paluxy is a sandy facies of the uppermost Glen Rose. The Antlers in southern Oklahoma is the combined marginal equivalent of the entire Trinity group, and perhaps includes basal Fredericksburg strata. The limy and marly Trinity is marked by an upper Aptian and lower Albian assemblage of pelecypods and gastropods, corals (resembling those from the Tehuacan, San Juan Raya, and Puebla districts in southern Mexico), and ammonites (Dufrenoya, Parahoplites, Douvilleiceras). The marginal facies is marked by cycads (Cycadeoidea boesiana, C. bart-johnsoni) , wood, conifers, no angiosperms, brackish-water mollusca, and dinosaurs.

Texas outcrop.—The Trinity group thickens on passing from the Red River valley southwards into the Mineral Wells-Fort Worth-East Texas geosyncline, and on passing gulfwards from the boundary between the offshore and marginal facies. This boundary may be taken at the northern and western margin of the outfingerings of Glen Rose limestone into the Antlers sand (1394, 1679, 1574). The margin follows a line running from near Texarkana westward (underground) across northern Denton County, across Wise, Parker, northern Comanche, and eastern Brown counties (outcrop), thence in a large reentrant (removed by erosion) down the Colorado River valley, across Gillespie County (outcrop), westward across the southern part of the Edwards Plateau (underground), through northern Edwards and Val Verde counties to the Pecos south of Pandale. West of the Pecos the thinned margin of the Glen Rose occurs in the northeastern quadrant of the Marathon basin (Purington ranch, and near Gap Tank) on the outcrop, sporadically near Fort Stockton off the high structures (underground), south of Kent (where absent on outcrop), between Black Mountain (where absent) and Sierra Blanca (where present), westwards to near El Paso (where unreported). South and east of this line, the Glen Rose limestone with its normal marine fauna is present; north of the line, the marginal sandy equivalents are present over a strip of variable width, 100 miles or less generally, but formerly extended farther inland for an unknown distance. The presumed northern limit of Trinity deposition is shown on Fig. 16. A section (Fig. 17) from Cooke County southward through Denton to Fort Worth shows (1) that the Glen Rose limestone fingers out to the north into the Antlers sand facies; and (2) that the Trinity group maintains a nearly uniform thickness in these two facies, and, therefore, that much of the Antlers is of Trinity age. In the more northern wells, one thin lentil is logged as lime (in Hampton well, 11 feet) ; farther south three (Denton City, Lewis Atkins) or four (Hughes-Morton, Fritz-Mathis) thicker limes appear. In Tarrant County these consolidate to 347-418 feet of Glen Rose.

In the Red River counties the typical Comanchean well log shows the Woodbine distinct (400-536 feet), the combined Washita and Goodland limestone 425-650 feet thick, and the Antlers 456-1185 feet thick. This variation corresponds to irregularities in the basement, and to local overlap of basal beds.

Facies changes in the Trinity are shown on the outcrop in Parker and Wise counties (Cooperative maps Parker & Wise Cos.; Scott, 1394). About 4 miles west of Decatur a few thin limestone ledges with typical Glen Rose fossils occur. On the Weatherford-Millsap road the Glen Rose, in two limestone seams, is about 50 feet thick. Eastward and southward the Glen Rose thickens, partly by the addition of new limy members above and below. Along the Brazos valley at the Parker-Hood county line, the Glen Rose is about 200 feet thick. In central Travis County the Glen Rose is 355M118 feet thick. In Dallas it has a recorded thickness of 522 to 595 feet (1454, pi. XX). In Parker County, the basement sands and conglomerates, ]50 feet thick or less, consist of quartz sand and pebbles up to 3 inches in diameter, well rounded in comparison with the underlying angular and heterogeneous Pennsylvanian conglomerates, and not derived from them. In both the basement sands and the Paluxy there are deep purple "red beds." The basement sands contain bone fragments, carbonized wood and plant remains, and much silicified logs and wood. The Paluxy is a packsand, bearing silicified wood.

An east-west section, from Erath, Comanche, and Brown counties to beyond Big Spring, shows the Glen Rose rapidly thinning westwards across Comanche County and disappearing near Brownwood, west of which only Paluxy represents the Trinity group (Fig. 16). At Round Mountain, northwestern Erath County, the Glen Rose is represented only by a 5-foot limy ledge. In the Santa Anna, Callahan Divide, and San Angelo area, the Paluxy consists of 150 feet or less of derived material, which rests on an irregularly eroded Permian basement. In Irion County, thin Trinity lime is logged. Near Big Spring and in the Llano Estacado the substratum is Triassic.

In the Colorado River valley, the lateral variation of the Trinity group has been studied by Taff, in a series of sections from Hickory Creek to Mount Barker near Austin (1574). These studies demonstrated that the clastic and conglomeratic features of the Trinity become more refined towards the southeast, and that the Glen Rose limestone thickens and becomes more calcareous and less sandy in that direction. If Taff's section were projected graphically westward, the limit of the Glen Rose limestone would lie in eastern Llano County.

In the Hays-Bianco county section, the Glen Rose thins westwards and disappears in eastern Gillespie County.

In the Kerrville-Fredericksburg section (Fig. 16) the same thinning is shown on outcrop and in subsurface. The marginal facies of the upper Glen Rose near Fredericksburg has taken on a great quantity of red color from the underlying decomposed Cambrian formations. This facies is called the Gillespie formation. It bears cycads and fossil wood.

In the southern part of the Edwards Plateau, numerous sections reveal the offshore thickening of the Trinity group, and the fact that several areas were islands in Trinity times. This area has been investigated by Mr.L.D. Cartwright (201b). The inner margin of the Trinity across the southern part of the Edwards Plateau, and some projecting irregularities in the old land floor (presumably islands) both within the limits of the Glen Rose limestone wedge and in the area of marginal sands are shown on Fig. 16.

Liddle and Prettyman have published numerous sections of the Trinity in the Pecos valley (991). The basement rocks are irregular in this district, and locally no Trinity was deposited (Fig. 16).

In the Marathon-Fort Stockton section, the Glen Rose outcrop occurs as far north as Gap Tank. To the west, in the northern Glass Mountains, the entire Trinity group is locally absent by nondeposition, and the Fredericksburg rests on the Paleozoic. In the Fort Stockton area, the Trinity limestone is of irregular distribution, generally absent on the highs. In several of the wells, sands, with Chora fruits are present, and are probably Trinity. Farther southwest, in the Hovey district, wells show several hundred feet of Glen Rose limestone. Kent is north of the margin of the Trinity limestone, and only thin Trinity sand is present at the outcrop.

In the Sierra Blanca area north of the Texas and Pacific Railway, beneath the Georgetown limestone is 100 feet or less of sandy beds with Kiamichi fossils. Under these is the Finlay limestone, which is of Edwards and Comanche Peak age. Beneath this is the Cox sandstone, probably of Maxon-Paluxy age at its top, of Glen Rose marginal equivalents basally. The Campagrande and Etholen have not been precisely dated by fossils, but are likely of Trinity age. For Fredericksburg equivalents at Sierra Blanca, see page 352. From the Etholen, Baker records Porocystis pruniformis; in the sandstones near the base of the Etholen at the type locality, there are some poorly preserved oysters. Taff (1573, p. 724) records oysters and an Exogyra? in the Etholen conglomerate; Richardson does not mention any specifically Trinity fossils in the Cox or the Campagrande.