EXCEPTIONAL FORAMINIFERAL AND FISH OTOLITH PRESERVATION REVEALS ENVIRONMENTAL FLUCTUATIONS IN THE OLIGOCENE BYRAM FORMATION (BIG BLACK RIVER LOCALITY, MISSISSIPPI)

Exceptionally preserved assemblages of foraminifers and fish otoliths have provided the basis for a detailed evaluation of environmental fluctuations in the Oligocene Byram Formation (Rupelian) at the Big Black River Locality near Edwards, Mississippi. The site is considered by many as the most complete, best preserved section of the Byram Formation in the Gulf Coast and one of the best marine Oligocene sites in the U.S. The importance of the site is compounded by the presence of the only two Orelian-age land mammals ever discovered in the Gulf Coastal Plain. The present study included independent analyses of over 2,700 foraminifers from 9 beds and over 1,200 otoliths from 8 beds. The foraminifers represented 25 families and 105 species. Fish otoliths from nearshore marine environments often exhibit signs of abrasion and poor preservation. However, the excellent preservation of the fish otoliths at Big Black River Locality allowed for the identification of at least 25 taxa of bony fishes making this site one of the most diverse Oligocene assemblages based on otoliths in the U.S. Gulf Coast.

Several techniques were employed in the foraminiferal paleoenvironmental determinations. These included the percentage abundance of the dominant genus, the alpha index (diversity), and the percentage occurrence of suborders (calcareous benthic, arenaceous, and porcelaneous). Paleoenvironmental determinations based on otoliths were accomplished primarily by comparing fossil otoliths to the environmental parameters of extant species whenever possible or to closely related species when not possible. In addition, the percentages of sciaenids and the codlet Bregmaceros were used as indicators of paleoecology. While both foraminiferal and otolith analyses indicated shallow marine conditions, there were indications of bays and lagoons and hyposaline and hypersaline influences. Although foraminifers have been considered as the most sensitive indicators of paleoenvironmental parameters, the fish otoliths also attest to subtle environmental changes in the beds of the Byram Formation. The outstanding preservation of the fish otoliths and foraminifers and the detailed collecting allow for determination of fluctuations in the paleoecology during this part of the Oligocene, which was a pivotal point in Tertiary climate.