CONODONTS FROM THE ELM CREEK FORMATION (ARTINSKIAN) OF NORTH-CENTRAL TEXAS U.S.A.: FAUNA FROM A PERMIAN INTERMITTENTLY RESTRICTED SHALLOW SHELF

A limited conodont fauna was collected from the Elm Creek Formation, Wichita-Albany Group, in north-central Texas. The Elm Creek is a regionally extensive platform carbonate and shale sequence deposited on the Eastern Shelf of the Midland Basin. Normal marine conditions apparently did not exist in this part of the Eastern Shelf during Elm Creek carbonate deposition. The carbonates are primarily packstone to wackstone foramol (foraminifera-mollusca) limestones. Current field investigations have found many organisms typical of Permian fauna, such as fusulinids and brachiopods, to generally be absent or extremely limited. The shelf area is interpreted to have experienced deposition during conditions of highly fluctuating salinity. Conodonts were only found in the intervals with slightly more marine character, i.e. those with bryozoans and echinoids that were scattered throughout the 22 m carbonate section. Most samples containing conodonts yielded fewer than five elements/fragments per kilogram. Only two 20 cm beds in the upper portion of the limestone at one locality contained more abundant conodonts, fifty per kilogram. A significant percentage of all recoveries were juveniles and as such were not easily assignable to specific genera.

The majority of conodonts were Pa elements of Sweetognathus,followed in number by Rabeignathus,with a few possibly assignable to Diplognathodus. Other elements were found allowing nearly complete apparatus reconstruction for this group. Some authors now regard Rabeignathusas a Sweetognathuswith accessory nodes on the Pa element. Numerous elements in this collection appear to be transitional. Biostratigraphic correlation based on Rabeignathuswould position the Elm Creek as Artinskian in age using chronostratigraphic scheme adopted by the ICS subcommission in 1996. Components of the Sweetinaapparatus, excluding the P elements, were also obtained. The limited conodont recovery of primarily shallow-water genera further bolsters the interpretation of Elm Creek deposition under primarily non-normal marine conditions.