PETROFACIES AND DIAGENESIS OF THE LATE PALEOCENE SALT MOUNTAIN LIMESTONE, SOUTHWESTERN ALABAMA

Salt Mountain Limestone is exposed in one isolated location in southwestern Alabama along the up-thrown side of a normal fault possibly associated with an underlying salt dome (Louann Salt). The 40 m thick section consists primarily of white to cream-colored muddy limestone, but is also known to contain important reefal buildups, particularly in the subsurface beneath southeastern Alabama. Fossiliferous floatstone and bindstone interbeds at the outcrop contain a diverse assembly of fauna including corals (some in situ up to 1 m in diameter), coralline algae, molluscs, sponges and sponge networks, benthic foraminifera and numerous other less abundant invertebrates. Unfortunately, precise identification of much of the biota is difficult due to diagenetic overprinting and recrystallization. Petrographic analysis suggests that Salt Mountain Limestone at the study site can be subdivided into three overlapping mud-dominated petrofacies: 1) skeletal wackestone/packstone, 2) skeletal floatstone and 3) coralgal-sponge bindstone. Minor amounts of grainstone and rudstone also occur as thin interbeds or lenses.

The amount of cementation and porosity in the Salt Mountain outcrop is variable. Some parts of the exposure are friable and recessive due to limited cementation or extensive secondary dissolution. Other parts of the exposure are well indurated, resulting in laterally continuous scarps and blocks. Microkarst features are common across the top of the outcrop and isolated travertine-filled fractures and surface coatings suggest partial cementation of dissolution voids. To date, we have identified several meteoric phases of calcite cement. Most are intergranular drusy non-ferroan spars. Later phases of pore-filling calcite that grew within naturally porous skeletal allochems (e.g., corals) may be zoned and/or more ferroan. This suggests that pore-water chemistry changed subtly and repeatedly during a prolonged episode of calcite precipitation.