Facies analysis and environmental interpretation of a late Pleistocene coastal lagoon sedimentary sequence containing a Coprinisphaera-bearing terrestrial episode (Kisimayo, southern Somalia)

For the first time, we here report trace fossils of dung beetle brood balls (Coprinisphaera ichnogenus) in late Pleistocene sediments from Somalia. The sediments containing these traces represent a lagoonal sedimentary sequence, cropping out along some quarry walls in the outskirts of the Kisimayo town. The terrestrial ichnofossils are accompanied by some shells of the giant land snail Achatina, which confirms the subaerial exposure. Indeed, the rock, generally strongly weathered, shows a texture of a foraminiferal packstone-grainstone, rich in bivalve and gastropod molds, of marine origin. The related facies stack is indicative of a period of environmental changes, which took place in a sheltered shallow water coastal area of a tropical carbonate shelf. For a short time, due to a slight lowering in sea-level, this lagoonal area experienced subaerial conditions with accumulation of wave- and wind-driven marine sand in a backshore zone, so allowing the development of environmental conditions suitable for the appearance of terrestrial trace fossils and Achatina. The discovery of Coprinisphaera represents one of the rare African records of the ichnogenus; it is also a unique example for the Indian Ocean's African coast of a terrestrial episode being found within a lagoonal sequence.