Coniacian to lowermost Campanian stratigraphy of the Austin Chalk, northeast Texas.

摘要

The lithostratigraphy, biostratigraphy, and chronostratigraphy of the lower Coniacian to lowermost Campanian (Upper Cretaceous) strata of the Austin Chalk and its equivalents in northeast Texas were studied. The Austin Chalk in Ellis, Dallas, Collin, and southern Grayson County is represented by the Atco, Bruceville, and the Waxahachie members. The informal names "Hutchins member" and "upper chalk" are formally included in the "Waxahachie Member" (new member) herein. The outcrop of the Waxahachie Member at Lake Waxahachie Spillway is designated the type section.; East of Grayson, Fannin, Lamar, and Red River counties, the Austin Chalk equivalents are represented by the Ector Member, the Bonham Mudstone Member, and the Gober Member. A neostratotype is designated for the Bonham Mudstone Member of the Austin Chalk at the Hestand Creek locality herein. Moreover, the definition of the Gober Member at its type locality at Gober (Lamar County) is emended to include the silty chalk and silty chalky mudstones that overly the Bonham Mudstone Member of the Austin Chalk and underlie the Brownstown Member of the Taylor Formation in northeast Texas.; The "Roxton Limestone" of the older literature is redefined. The type locality of the Roxton strata consists of reddish to buff weathered (yellowish to light brown when fresh) calcareous sandstone composed mostly of quartz grains and sparse inoceramids prisms. Hence, the name "Roxton Limestone" is both misleading and inappropriate. Curiously, the Texas Bureau of Economic Geology incorrectly included the "Roxton Limestone" as a member of the Gober Member of the Austin Chalk. Both the Gober Member of the Austin Chalk and the "Roxton Sandstone" are completely different lithologically; they should be separated as different lithostratigraphic units on future geological maps. It is likely that the Roxton Sandstone is genetically related to the clastics of the Tokio Formation of southwestern Arkansas. In northeast Texas the present study demonstrates that the Texas Bureau of Economic Geology incorrectly mapped the Bruceville Member as the Gober Member of the Austin Chalk. Whereas the Gober consists of interbedded silty to sandy chalk and mudstone, the Bruceville consists of indurated chalk and chalky mudstone lacking silt and sand.; The present study indicates that: (1) The Ector Member is the chronostratigraphic equivalent of the lower Coniacian part of the Atco Member; (2) The Bonham Mudstone and Bruceville members are both assignable to the Santonian; and (3) The Gober and the Waxahachie members of the Austin Chalk are assignable to the lowermost Campanian.; New biostratigraphic data have resulted in the revision of the planktonic foraminiferal zonation for Upper Cretaceous (lower Coniacian to lower Campanian) strata of the western Gulf Coastal Plain. Three superzones and four zones belonging to three chronostratigraphic units are established including: (1) Superzone UK4, Zone 4B (lower Coniacian) and Zone 4A (upper Coniacian); (2) Superzone UK3 (Santonian); and (3) Superzone UK2, Zone 2F (lowermost Campanian). (Abstract shortened by UMI.)