NW AFRICA’S OLDEST KNOWN SKELETAL FOSSILS: A NEW CAMBRIAN SMALL SHELLY FOSSIL FAUNA FROM SOUTH MOROCCO

The Anti-Atlas area of South Morocco provides an excellent and well-studied geological archive covering large parts of the Proterozoic and Paleozoic. Its uppermost Ediacaran to lower Cambrian shallow water carbonate record (Adoudou and Lie-de-Vin formations) ranks among the best studied of the world. Whereas high-resolution δ¹³C data and several precise U/Pb zircon ages of ash layers were available previously, the fossil record has hitherto been poor. Apart from calcified algae and trace fossils, skeletal fossils have only been reported from the overlying Igoudine Formation (archaeocyathids and trilobites of Adtabanian age). Hence, placing of the Precambrian/Cambrian boundary (542 Ma) in the lower part of the Adoudou Formation is so far primarily based on the correlation of a -6 per mill δ¹³C excursion with a U/Pb zircon dated section in Oman.

As part of a regional geologic project, the very base of the Adoudou carbonate succession has been studied in the eastern Anti-Atlas. There, shallow-water carbonates (containing abundant stromatolites) cover a thick pile of regionally extensive terrestrial clastic and volcanic rocks (Ouarzazate Supergroup) of mostly Ediacaran age. Non-stromatolitic, detritus-rich, massive carbonate interbeds in the first few meters of the Adoudou succession contain sub-millimeter sized shelly fossils of variable shapes. Due to their dolomitic preservation, individual shells could not be isolated from their matrix and their inspection has yet been restricted to thin sections. The fossils display highly variable cross sections with some resembling broken tubes and other sections through Aldanella-like shells. Overall, the fossils seem to constitute a hitherto unknown small shelly fossil fauna.

Time constraints on this first reported small shelly fossil fauna from NW Africa are provided by both δ¹³C profiles from the carbonate host rocks and high-precision CA-ID-TIMS dating of detrital and volcanic zircons. Together, these data sets point towards an early to middle Nemakit-Daldynian (ca. 530 to 540 Ma) age. It is hence suggested that, at least in the eastern Anti-Atlas, the Precambrian/Cambrian boundary (542 Ma) is perhaps not recorded in the carbonate succession of the Adoudou Formation.