Black shale, grey shale fossils and glaciers : significance of the Late Ordovician-Early Silurian succession of the Tazzeka Massif, eastern Morocco
Résumé
A 400 m thick clastic succession of Late Ordovician through Silurian age crops out in the Tazzeka Massif, eastern Morocco. Biostratigraphic data (chitinozoa, acritarchs) constrain these rocks to the Late Katian through Hirnantian (coeval with glaciation in Gondwana), with a ~8 Myr hiatus at the OrdovicianSilurian boundary. Sedimentological analysis reveals six facies associations, including interbedded black and grey shale couplets (poorly oxygenated shelf sediments), a bioturbated shale with wave rippled sandstone (inner shelf deposits), several occurrences of diamictite (of probable glaciogenic origin), interbedded sandstone and mudstone (storm and fair-weather wave agitated shoreface), rippled sandstone (storm return flow deposits) and amalgamated sandstone deposits (delta-front debris flows). Direct evidence for glaciation in the Hirnantian deposits is poor. Third order sequence stratigraphic analysis reveals two transgressive systems tracts (Late Katian and intra-Hirnantian), two highstand systems tracts (both intra-Hirnantian), lowstand wedges probably corresponding to two principal glacial lowstands, two maximum flooding surfaces (Late Katian and intra-Hirnantian), and a major ravinement surface (HirnantianSilurian contact). Remote from the centre of glaciation, the Tazzeka succession is suggested to be an excellent reference section with which to understand glacially-moderated sea-level changes in the Late Ordovician. Silurian (Late Llandovery or younger) shale geochemistry indicates organic enrichment (TOC N4.5%). This organic enrichment greatly increases the known extent of the upper of two Silurian organic shales across North Africa (potential hydrocarbon source rocks) across the region.