MOVE OVER BRACHIOPODS AND MAKE ROOM FOR MOLLUSCS: A FAUNAL CHANGE IDENTIFIED IN THE LATE CAPITANIAN REEF TRAIL MEMBER OF THE BELL CANYON FORMATION (DELAWARE BASIN, WEST TEXAS)

A striking feature of the Reef Trail Member (uppermost member of the Bell Canyon Formation) is the occurrence of molluscs, mainly bivalves, gastropods, and scaphopods, not previously observed in the lower members of the formation. Collections of the lower members are dominated by brachiopods. The Reef Trail Member records an ecological change in fauna within the Delaware Basin of West Texas prior to the end-Guadalupian extinction. This transition occurred earlier than the global transition at the Guadalupian–Lopingian boundary documented by Clapham and Bottjer (2007). The Bell Canyon Formation was deposited seaward of an oversteepened reef-rimmed platform by sediment-gravity flows. The Reef Trail Member is late Capitanian in age (Lambert et al., 2002) and records the last normal marine deposition in the basin. An examination of acetate peels of the rocks suggests that the pattern is real and not a preservational bias. We analyzed 10 collections in the Reef Trail Member located on Williams Ranch Road in Guadalupe Mountains National Park. Brachiopods (divided into Rhynchonellata and Strophomenata) and bivalves were counted as articulated individuals, whole single valves, and fragments, if part or the entire hinge is present. Gastropods were counted if greater than 50% of the shell is present. Relative abundance was used to examine the percentage of faunal groups contributing to each collection. The results show that bivalves and gastropods comprise at least 40% to 50% of the Reef Trail assemblages. Bivalves are diverse and include pectens, arcoids, and unidentified infaunal burrowers. Gastropods include both low and high-spire forms. Additionally, in comparison to the lower members, strophomenid brachiopods (e.g., Derbyia) become more abundant than athyridids (e.g., Hustedia). The faunal change may reflect different environmental conditions in the basin, favoring fauna that are tolerant of stressful conditions. An increase in silt occurs in the Lamar and Reef Trail Members associated with a ~60 m drop in sea level and progradation of the reef into the basin.