Lithostratigraphy of the Lower Permian (Leonardian) Clear Fork Formation of north-central Texas

Rocks known as Clear Fork (Leonardian Series, Lower Permian) are a well-known source of vertebrate fossils in north-central Texas and a celebrated producer of oil and gas in West Texas. In the former region, the Clear Fork is a continental redbed sequence nearly 400 m thick that accumulated on the landward side of the Eastern Shelf of the Midland Basin, where the name Clear Fork is applied to a thick marine interval. Approximately 200 km south of these redbed outcrops, Clear Fork exposures composed of alternating and intertonguing shallow-marine carbonates and paralic to terrestrial mudrocks were divided by early workers into the Arroyo (oldest), Vale, Bullwagon Dolomite, and Choza formations; these rocks grade into the entirely terrestrial sequence of north-central Texas. Although vertebrate paleontologists historically have used the marine-based formation divisions in the redbeds, such terms cannot be applied formally, and the Clear Fork is herein classified as a single formation in the northern terrestrial region. Following the recent discovery of prolific fossil plant deposits in the Clear Fork of north-central Texas, extensive field work and subsurface studies were initiated to better understand the depositional origin of the formation. The Clear Fork is dominated by mudstone (80-85%) but also contains several fluvial sandstone units. The largest is located in the lower part of the Clear Fork and represents a braided bed-load system that is in part correlative to a regionally persistent shale that petroleum geologists have named Big Red or Vale Shale in the subsurface; this unit marks the most widespread influx of terrigenous clastic material on the Eastern Shelf during the Leonardian. Other fluvial systems that originated as high-sinuosity, suspended-load rivers contain abundant fossils in abandoned or restricted channel segments. An absence of fossil occurrences upwards in the Clear Fork corresponds to diminished fluvial sedimentation and the development of coastal sabkhas characterized by red mudstone, thin dolomites, and abundant gypsum.