CONODONT BIOSTRATIGRAPHY OF THE DESMOINESIAN (MIDDLE PENNSYLVANIAN) BRANNON BRIDGE LIMESTONE (GRINDSTONE CREEK FORMATION, STRAWN GROUP), NORTH-CENTRAL TEXAS

The Brannon Bridge Limestone, the lowermost member of the Grindstone Creek Formation, consists of two or three lenses of shallow water limestone interlayered with distal deltaic siliciclastic deposits within the lower to middle Strawn Group of north-central Texas. In Parker County, TX, a quarry (now backfilled) exposed two limestone beds, each ~3 meters thick, separated by a ~6 meter thick shale and sandstone unit. The limestone units show well-preserved Komia in packstone, which may represent Komia “meadows” that flourished between deltaic flooding events. Previous research has used fusulinids to correlate the Brannon Bridge to the middle Desmoinesian (late Cherokee) Beedeina novamexicana-Wedekindellina euthysepta zone. Conodonts are moderately abundant in the limestone units with many ramiform (S) elements preserved in addition to more diagnostic carminate (P1) and angulate (P2) elements.

Our research shows that the Brannon Bridge samples contain a typical shallow water carbonate facies conodont fauna with species of Adetognathus, Hindeodus, Diplognathodus, Neognathodus, and Idiognathodus. The Neognathodus morphotypes are closely comparable to N. roundyi. The first occurrence of N. roundyi is reported from the uppermost Cherokee Verdigris cyclothem, and Idiognathodus specimens in the samples are consistent with the Verdigris, although species from the other genera are not diagnostic. Gondolella species are not present in the samples, whereas they are common in samples from the lowermost Marmaton Group Excello cyclothem. Based on these comparisons, the Brannon Bridge Limestone appears to be equivalent to the Verdigris cyclothem of the upper Midcontinent and latest early Desmoinesian (latest Cherokee) in age.