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TOLSON, PATRICK M., Maxus Exploration Company, Amarillo, TX

ABSTRACT: Depositional Environments of the Red Fork Sandstone in Custer and Roger Mills Counties, Southwestern Oklahoma

The Desmoinesian Red Fork formation is a prolific, overpressured gas-producing sequence of interbedded sandstones and shales. Total thickness ranges from less than 100 ft (northeast) to more than 1100 ft (south). Isopach maps suggest that syndepositional faulting controlled major depositional trends.

The lower Red Fork, whose base is defined by a persistent, hot, black shale (sequence boundary?), is mainly deep-marine shale and siltstone. Two major shallowing-upward deltaic

sequences separated by a marine transgression are evident in the middle (50-400-ft thick) and upper (30-250-ft thick) Red Fork. The middle Red Fork is marine dominated and was deposited into a relatively deep basin on a steep, unstable delta-front slope. In contrast, the upper Red Fork deltaic sequence is more fluvial dominated and was deposited in shallower water.

The upper Red Fork is overlain by the Pink lime interval, which appears to be shallow-marine/lagoonal black shale. The Pink lime contains fish scales, coffee-ground to branch-size lignitic plant debris, and brackish to shallow-marine ostracodes, linguloid brachiopods, Tasmanites algae, and gastropods.

Most of the Red Fork has an easterly, possibly Ouachita Mountain area source. The prolific Southwest Leedey field has a different mineral assemblage and diagenetic sequence and may have a northern source.

AAPG Search and Discovery Article #90991©1993 AAPG Mid-Continent Section Meeting, Amarillo, Texas, October 10-12, 1993.