CYCLE- AND CHEMOSTRATIGRAPHY OF UPPER ALBIAN LOWER CRETACEOUS COMANCHE SHELF MARGIN, SOUTHWEST TEXAS

Detailed outcrop and petrographic studies document 3rd- and 4th-/5th-order depositional cycles in the Upper Albian Lower Cretaceous carbonate strata in two outcrops in Val Verde County, southwest Texas. On the broad, shallow Comanche carbonate shelf, the depositional environment of the Fort Lancaster Formation in the northern Pandale section is interpreted as shelf interior with two rudist packstone-grainstone biostromes (i.e., “Middle” and “Upper” Caprocks). At the base of the Fort Lancaster in the nodular, recessive Burt Ranch Member, a 4.5m-thick skeletal wackestone exhibits a positive δ¹³C excursion and increasing-upward TOC trend. Such signals possibly point to the environmental stress associated with the early Late Albian Oceanic Anoxic Event 1c.

The Devils River Formation in the southern Pecos River Hwy 90 bridge section is interpreted as a sand shoal surrounding the deep-water intrashelf Maverick Basin. It differs from the time-equivalent Fort Lancaster Formation by its higher energy well sorted, cross-bedded, rudist-peloidal grainstone facies. By tracing maker beds and comparing facies types, the previously defined 3rd-order High-Frequency Sequences (HFS) along the Pecos River Canyon (i.e., Alb 18, Alb 19, Alb 20, Alb 21, A/C 1, A/C 2) are identified in the outcrop sections. In the Pandale section, Alb 18 and Alb 19 are represented by the Burt Ranch Member - lower Middle Caprock and by the upper Middle Caprock. Alb 20 and A/C 1 are represented by two shoaling-upward intervals beginning with oyster beds at the base. A/C 2 is represented by the Upper Caprock. In the Pecos River Hwy 90 bridge section, the upper part of Alb 21 is represented by a swash-laminated rudist grainstone interval, and the overlying part of the section is correlated with A/C 1. Absence of A/C 2 may be related to the erosional contact at the top of the Devils River Formation.

Such correlation reinforces the implication that platform-wide relative sea-level events are possibly responsible for these 3rd-order depositional cycles. The 4th-/5th-order shoaling-upward depositional cycles are defined in both outcrop sections by upward increase of fossil abundance and high-energy facies, and the capping of subaerial exposure surfaces. Localized processes, such as varying sediment accumulation rate, may form these cycles.