The Salt Mountain Limestone of Alabama

Abstract

The Salt Mountain Limestone of southwestern Alabama is a Late Paleocene (Thanetian) tropical-subtropical, photiczone reef constructed primarily of sponges, coralline red algae, and lime mud, but with common scleractinian corals (12 species) and abundant bryozoa. Accessory fauna includes larger and smaller benthic foraminfera, echinoderms, endolithic bivalves, and brachiopods.  The reef developed on an inner neritic siliciclastic shelf, probably over a local diapiric shoal, at an approximate paleolatitude of 31°-32° North.  The final demise of the reef was evidently caused by siliciclastic influx from the prograding Holly Springs Delta System of the Mississippi Embayment.  This massive carbonate underwent several episodes of diagenetic alteration, including early marine micritic cementation.  Smaller benthic foraminifera assemblages suggest depositional water depths of 15-30 meters for forereef sediments, to less than 5 meters for the reef core. The dominant alga is Archaeolithothamnium, which occurs as both thick and thin crusts, and rhodoliths, suggesting water depths between 5-20 meters. At least 15 distinct sponge morphotypes are consistently identified, ranging from small, 2 -10 cm oblong buds to large, vase- or barrel-shaped forms that may have reached 1 meter in length.  Echinoderms are extremely abundant, as indicated by ossicles and plates, but only six species of regular echinoids have been identified.  Conspicuously absent are any remains of irregular echinoids, which may not have occupied reef niches at that time as they clearly did by the Late Eocene and Early Oligocene.  The constructional dominance of coralline algae and sponges in the Salt Mountain could be an indication that scleractinians had not yet fully established their post-Cretaceous role as primary reef framestone builders in the Gulf of Mexico Basin.  The abundance of hexactinellid sponges, however, may be especially significant.  The siliceous hexactinellids are exclusive K-strategists, requiring stable environments, but are bathymetrically tolerant.  They reached their greatest diversity in the Late Cretaceous and were common reef/mound constructors in the Jurassic.  Their presence in the Salt Mountain suggests a temporary role as early post-Cretaceous photic-zone reef constructors prior to the full recovery of scleractinian-dominated reefs in the later Paleogene.