VERTEBRATE-RICH PHOSPHATIC CONGLOMERATES FROM THE SHALLOW TRANS-SAHARAN SEAWAY, NORTHEASTERN MALI

Voluminous phosphorite sequences were deposited during the Late Cretaceous through Middle Eocene in the Taoudeni and Iullemmeden Basins of northeastern Mali. Repeated sequences of marl, limestone and paper shales are punctuated by condensed sections of phosphatic conglomerate, and represent shallow-water deposition along the margins of the epeiric Trans-Saharan Seaway. The 0.5 to 2 m thick conglomeratic units (pebble to cobble-sized clasts) are of particular interest for their abundant preservation of vertebrate remains. Systematic grid-sampling of a conglomerate near Tamaguilel reveals that at least half of the clasts are derived from vertebrate sources (bones=30%, coprolites=20%, undetermined clasts=50%).

Bones include various skeletal elements of fish (e.g., pycnodonts, eagle rays and amiids) and reptiles (e.g., dyrosaurs and snakes). Coprolites are generally cigar-shaped and spiraled along the long-axis, suggesting that the producers were probably fish with primitive spiraled guts. Adding further evidence of a marine setting, some coprolites and bones are mechanically bored by pholad bivalves. Early phosphate diagenesis strongly favoured the preservation and lithification of phosphate-rich materials (i.e., bone and coprolites), and was likely facilitated by microbes.

Previous interpretations of the conglomerate (and other regional phosphorites) suggested deposition in lagoon, lacustrine or continental settings. Our study demonstrates a strong marine influence on the deposit, though brackish-water conditions cannot be ruled out. In addition to being a valuable economic phosphate resource, the Malian phosphorites preserve a significant volume of vertebrate fossils ideal for systematic and paleoecologic study.