Phosphate taphonomy of bone and coprolite conglomerates: a case study from the Eocene of Mali, NW Africa

Abstract

Repeated sequences of carbonate and shale are punctuated by condensed sections of phosphatic conglomerate in the epeiric deposits of the Trans-Saharan Seaway in northeastern Mali. To characterize the taphonomic and depositional setting of these phosphates, a thick Eocene conglomerate from the area of Tamaguélelt was targeted for quantitative analysis. Systematic grid sampling demonstrates that nearly all of the clasts are derived from vertebrate sources (bones = 27%, coprolites = 20%, probable coprolites = 53%), and invertebrate body fossils are nearly absent. Bony and cartilaginous fish dominate the bone assemblage, which also includes minor reptilian elements from sea turtles, sea snakes, and dyrosaurid crocodilians. Coprolites are of five distinct varieties, including three spiral forms probably produced by separate fish taxa. Repeated episodes of abrasion and minor bioerosion with modest levels of sorting characterize the taphonomy of the phosphate conglomerate and are consistent with a shallow-marine-to-brackish-water depositional environment between fair-weather and storm-wave base. Early phosphogenesis strongly favored the preservation and lithification of phosphate-rich bones and coprolites, probably during periods of marine transgression and sediment starvation. Combined with evidence from sedimentology, these vertebrate-dominated fossil assemblages appear extensively reworked and highly time averaged as a result of amalgamation and concentration by storm activity during periods of marine transgression.