THE UPPER CRETACEOUS TOMBIGBEE SAND AT THE BENTONITE MINES NEAR ABERDEEN, MISSISSIPPI

Historically, the best natural exposures of the fossil-rich Upper Cretaceous Tombigbee Sand in Mississippi have been scenic bluffs along the Tombigbee River. This includes the type locality at Plymouth Bluff near Columbus. However, completion of the Tennessee-Tombigbee Waterway in the 1980s has permanently transformed these historied and magnificent outcrops making them inaccessible to study. In the last eighty years, the industrial need for bentonite, an adsorbent clay with many useful commercial applications, has resulted in deep excavations into the Tombigbee Sand of Itawamba and Monroe counties. Mined the world over, bentonite in the Cretaceous deposits of Mississippi is of the calcium type and is used to clarify vegetable oil. South of Aberdeen, Mississippi, commercial quantities of bentonite lie at a depth of ~90 feet below the stratigraphic top of the Tombigbee Sand, or ~120 feet below the local surface, and consist of one to two clay seams, the thickest averaging seven feet thick. Over the years, substantial, albeit short-lived, sections of the Tombigbee Sand have been repeatedly exposed by several mining companies, including AMCOL (American Colloid). Originally called Panther Creek Mine, the AMCOL site is known today as Fowlkes Bentonite Mine and is operated by BASF Corporation.

The fossils of the massive-bedded fine sands are essentially the same as those preserved elsewhere throughout the outcrop belt of the Tombigbee Sand, and include bones, teeth, coprolites, low-Mg calcite shells, mollusk steinkerns, tubiculous polychaetes, and cassiduloid echinoids. The bentonite beds preserve a different type of fauna. Fossils can be locally common in the soft, unaltered bentonite clay, but fragile and exceedingly difficult to recover intact. However, certain macroinvertebrates and occasional wood fragments are well preserved in calcareous concretionary zones within the bentonite. Concretions are spheroidal, average ~10 inches in diameter, and occasionally contain echinoids and moldic mollusks. The univalves and bivalves are small—the latter dominated by the veneroid clam Cymbophora. The ammonites include elements of the Late Santonian Boehmoceras fauna. The echinoids consist of two burrowing heart urchins, Plesiaster and Mecaster, and the species represented are new biogeographic occurrences.