BRYOZOAN SPECIES COMPOSITION OF THE UNUSUAL SILICICLASTIC FENESTRATE-RICH MUD-MOUND AT LANGSTON GAP (LATEST MISSISSIPPIAN, NORTHEASTERN ALABAMA)

Most fenestrate-bryozoan mud-mounds developed in carbonate paleoenvironments early in Mississippian time, but our purpose is to examine an unusual siliciclastic/terrigenous mud-mound apparently formed by fenestrates during the Late Mississippian.

The mound examined is the lower 10 ft (3 m) of the mountain roadcut up in Langston Gap, 0.6 mi (1 km) southeast of Langston village (SW¼ NE¼ sec. 12, T6S, R5E, Langston 7.5' quad., Jackson Co.). Stratigraphically, it lies at the top of the Pennington Shale (Uppermost Mississippian; highest Chesterian or mid-Namurian). Overall features of the mound are in Gibson ('86 Compass 64:23-29).

The brown, silty, siliciclastic mudstone of the Langston Gap mound contains numerous bryozoans, over half the fossils seen, predominantly delicate-fenestrate fronds and fragments. The bryozoans are well-preserved molds/impressions displaying morphological detail of both frontal and reverse surfaces, so that identification was feasible.

We found 18 bryozoan species among the 160 specimens available for examination. Two species are abundant: Rectifenestella tenax and more robust Polypora gracilis. Three are common (Fenestella ivanovi; Rectifenestella multispinosa, tenuissima), while three more are uncommon (Fenestella cumingsi, frutex, matheri). Ten are rare: Fenestella hemispherica, paradisensis, parallela; Archimedes confertus, fosteri, intermedius, macfarlani, negligens; and the tiny rhabdomesids Streblotrypella major and Klaucena (Spira) altinodata.

This bryofauna has a decidedly Late Mississippian appearance. Most of its species have been recorded from Upper Mississippian strata, largely in North America, but some from Ireland as well (not surprising, due to their paleogeographic proximity). A few of the species herein had also participated in earlier Mississippian carbonate-mud mounds, and one (Rectifenestella tenax) continued on into Permian micrite mounds.

Most of the bryozoans here are fallen, lying horizontally, but several still stand upright in place. Such orientation, coupled with their abundance, confirms the sediment-trapping/baffling role previously inferred for the fenestrates in this mound, as also does the fact that the sediment trapped was siliciclastic (hence loose) rather than carbonate (which could have become cemented early independent of the presence of the bryozoans).