SEA LILIES IN THE DESERT

Slabs with beautifully preserved scyphocrinitids from the Sahara region regularly appear at mineral and fossil fairs; most of these fossils are labelled, ‘Erfoud’. Very little work has been undertaken on the detailed stratigraphy of the scyphocrinitid beds of Morocco; hence, the information provided in this chapter is limited. One would hope that future work will increase our knowledge of these remarkable occurrences.

The western Sahara (Fig. 108) underwent several marine transgressions during the Palaeozoic. The extended Silurian transgression started during mid-Llandovery times with the deposition of graptolitic shales. Subsidence increased, so that the thickest part of the succession accumulated during Ludlow times. At the southern edge of the Anti-Atlas in the Dra Plain, in the Tafilalt area near Erfoud and in the Ougarta chains of western Algeria, the Silurian is composed largely of black, sometimes bituminous shales with graptolites. These shales weather on the surface to brighter colours. In the Pridolian part of the series (uppermost Silurian, about 410 million years before present) they contain bands of blue, fossiliferous limestones with Scyphocrinites, orthocone nautiloids (‘Orthoceras’), bivalves (Cardiola) and gastropods. Upper Silurian to Lower Devonian argillaceous shales or laminated clays with limestone intercalations or lenses containing scyphocrinitids and orthocone nautiloids are in fact found in a wide area, from the coast near Casablanca and Rabat, across the High Atlas, to the Dra Plain, the Tafilalt, and the Ougarta chains of western Algeria. The limestones commonly occur as nodular concretions in large cushion-like masses that are interbedded in the shales.