NEW MICROBIAL BIOTA IN A SMALL CHESTERIAN (UPPER MISSISSIPPIAN) CARBONATE MOUND IN NORTHCENTRAL ALABAMA

Chesterian (Upper Mississippian) strata in north-central Alabama are composed of a mixture of shallow marine platform carbonate and siliciclastic units. At Woodville, east of Huntsville Alabama, strata equivalent to the Hartselle Sandstone consist primarily of shale with thin skeletal limestone and calcareous sandstone interbeds. In the western area of the state, the Hartselle is typically composed of quartzarenite that was deposited as a series of barrier islands and bars atop a shallow marine platform. The Woodville site is noteworthy as it contains a small columnar mound (1.2 m high) that nucleated atop an oolitic grainstone deposited during a pause in shale deposition within the Hartselle-equivalent interval. Biogenic buildups such as this are rare within Mississippian strata in north Alabama. The Woodville mound contains a new problematic microencruster we name Aphralysia anfracta, sp. nov., along with encrusting bryozoans (Fistulipora), nonskeletal microbes, and other microencrusters, including Aphralysia capriorae Mamet and Roux 1975, in a carbonate mud matrix. Other biotic constituents of the mound include sponges (including Pileospongia), gastropods, a tabulate coral (Micheliniidae, Michelina?), and coenobytes, including coccoid calcimicrobes. The mound biota, especially the microencrusters, are dramatically different from those in the few other Mississippian carbonate mounds that have been described in Alabama. Those were made by various consortia of rugose corals, fenestrate bryozoa, crinoids, sponges, and nonskeletal microbes. Marine cements, borings and synsedimentary sediment infiltration within voids indicate that the Woodville mound was indurated while on the sea floor. Renewed mud and silt deposition buried the grainstone and the mound, and terminated microbial growth. Shallow marine carbonate sedimentation (Bangor Limestone) resumed domination across the platform in northern Alabama sometime after deposition of the Hartselle-equivalent shale unit, but to the best of our knowledge, Woodville-type mounds did not redevelop anywhere else in the area.