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Abstract: Type Locality Descriptions San Vicente Member of Boquillas Formation (Upper Cretaceous)
Ross A Maxwell (1), Edward Marks (2)
LOCATION: Planetable section, 2.4 road mi south of intersection of Panther Junction to Rio Grande Village Park Road 1 and San Vicente Village to Rio Grande Village River Road, and 0.25 mi south of U.S. Geol. Surv. benchmark elevation 1881 ft, starting at the River Road, extending 2240 ft due east, and about 2 mi northeast of the old village of San Vicenti, Big Bend National Park, Brewster Co., Texas.
NAMED BY: R. A. Maxwell, J. T. Lonsdale, R. T. Hazzard, and J. A. Wilson, 1967, Geology of Big Bend National Park, Brewster Co., Texas: Univ. Texas Bull. n. 6711, p. 64-71.
AGE: Upper Cretaceous, Coniacian-Santonian.
CORRELATED WITH: Lower Austin Group: Atco, Vinson, Jonah units (Durham, 1956).
ORIGINAL DESCRIPTION: (Maxwell, et al., p. 66-68) The Member is mostly 350 to 400 ft thick in Big Bend National Park but locally thins to 130 ft. It is gray, thin-to medium-bedded, chalky and argillaceous limestone flags interbedded with gray or yellowish-gray platy marl, or soft gray marl. Some limestone layers are as much as 12 to 18 in thick, but most beds are only 2 to 6 in thick. The more calcareous rocks weather grayish white, whereas the more marly beds weather bluish gray or yellowish gray. The San Vicente Member contains more and thicker marl intervals and more chalk than the Ernst Member. The basal 20 ft of the San Vicente is commonly silty or sandy flagstone and locally the basal 6 in is finely conglomeratic. According to Adkins, the zone of Peroniceras sp. is the lowest Austin faunal zone ... The amount of flagstone decreases upward, and the top of the member is alternating soft gray marl and chalk. The chalk forms ledges resistant to erosion that frequently stand in low cuestas. A chalky ledge (usually a low cuesta) 15 to 20 ft below the top of the San Vicente contains abundant Inoceramus undulatoplicatus ... Above the Inoceramus undulatoplicatus beds as much as 22 ft of gray marl, slightly indurated, with some flagstone, contain numerous large Inoceramus sp. In most localities the San Vicente grades upward into the Pen Formation, but in places the contact is abrupt ... At the type locality of the San Vicente Member is the ... measured section ... It is 331 ft thick. The basal 6 in is conglomerate that rests upon the Coilopoceras beds of the Ernst Member ... above the conglomerate are about 20 ft of gray, buff, and pinkish siltstone, sandstone, argillaceous limestone, and some bentonitic clay with lignitic concretions. These are overlain by chalk beds 12 to 18 in thick, chalky and argillaceous marl, and soft gray marl. The Peroniceras beds are not recognized here.
The Inoceramus undulatoplicatus beds form a 13-foot ledge whose top is 22 ft below the top of the member. They are nodular, gray chalk interbedded with platy, gray marl. The overlying beds, up to the base of the Pen Formation, are mottled, gray and yellowish brown, finely crystalline limestone interbedded with gray marl which yielded several Inoceramus sp. that are partly covered with Ostrea congesta.
COLUMN: (Maxwell, et al, 1967, p. 68) See Fig. 1
Tabulated data; refer to PDF file.
Tabulated data; refer to PDF file.
Durham, C. O., Jr., 1956, The Austin-Taylor relationship in Central Texas, in Resumenes de los trabajos presentados: XX Cong. Geol. Inter., Mexico City, abs., p. 330.
FIG. 1. TYPE LOCALITY COLUMNAR SECTION, SAN VICENTE MEMBER, BOQUILLAS FORMATION, BIG BEND NATIONAL PARK, BREWSTER COUNTY, TEXAS.
FIGURE 2. Location maps, type locality, San Vicente Member, Boquillas Formation, Big Bend National Park, Brewster County, Texas.
End_of_Record - Last_Page 464-------
ACKNOWLEDGMENTS AND ASSOCIATED FOOTNOTES
(1) Texas Bureau of Economic Geology, retired.
(2) Union Oil Company of California.
Austin, Texas 78703
Houston, Texas 77025
Authors are responsible only for supplying the information on type sections.
Copyright © 1999 by The Gulf Coast Association of Geological Societies