EARLY PALEOZOIC PALEOGEOGRAPHY OF THE NORTHERN GONDWANA MARGIN - NEW EVIDENCE FOR ORDOVICIAN SILURIAN GLACIATION

During the Early Paleozoic, transgressions and the distribution of sed imentary facies on the northern Gondwana margin were controlled by a r egional NNW-SSE to almost north-south striking structural relief. In E arly Silurian times, a eustatic highstand enabled the sea to reach its maximum southward extent. The counterclockwise rotation of Gondwana d uring the Cambrian and Early Ordovician caused the northern Gondwana m argin to shift from intertropical to southern polar latitudes in Ordov ician times. Glacial and periglacial deposits are reported from many l ocalities in Morocco, Algeria, Niger, Libya, Chad, Sudan, Jordan and S audi Arabia. The Late Ordovician glaciation phase was followed by a pe riod of a major glacioeustatic sea-level rise in the Early Silurian du e to the retreat of the ice-cap. As a consequence of the decreasing wa ter circulation in the basin centers (Central Arabia, Murzuk- and Ghad ames basins), highly bituminous euxinic shales were deposited. These s hales are considered to be the main source rock of Paleozoic oil and g as deposits in parts of Saudi Arabia, Libya and Algeria. The following regression in the southern parts of the Early Silurian sea was probab ly caused by a second glacial advance, which was mainly restricted to areas in Chad, Sudan and Niger. Evidence for glacial activity and fluv ioglacial sedimentation is available from rocks overlying the basal Si lurian shale in north-east Chad and north-west Sudan. The Early Siluri an ice advance is considered to be responsible for the termination of euxinic shale deposition in the basin centers.