Middle Permian (Guadalupian) Fusulinids from Subsurface Midland Basin, West Texas, USA, Including Paleoenvironmental Microspheric Parafusulina

Parafusulina fusulinids are preserved in partly dolomitized outer ramp facies in a Middle Permian (Guadalupian) subsurface core from the Midland Basin of West Texas. The fusulinid-bearing carbonate unit was deposited within the basin during a sea level lowstand of the Guadalupian G10 high-frequency sequence and is assigned to the Grayburg Formation. The lowstand carbonate unit consists of dolomitized progradational shallow-water carbonate ramp facies. The fusulinids are preserved only in outermost ramp fusulinid-skeletal packstone facies that overlies dark-gray quartz siltstone slope facies. The assemblage of Parafusulina species indicates that the carbonate unit is of late Roadian-early Wordian age based on the conodont chronostratigraphy. The lowest 1 ft (0.3 m) of the outermost ramp facies interval contains microspheric morphotypes of Parafusulina that could be the product of paleoenvironmental low-light conditions in the lower photic zone of an outermost ramp setting rather than sexual dimorphism.