Evolution of Jurassic Carbonate Platforms in an Active Transtensional Rift: High Atlas of Morocco
P. D. Crevello, J. E. Warme, M. Septfontaine, R. B. Burke
Carbonate platforms developed during the Early and Middle Jurassic on the margins of two seaways that are now exposed in the Middle and High Atlas, Morocco. The seaways occupied transtensional rifts, which, by Early Jurassic, had formed the segmented terrain on which the carbonate platforms established.
Two episodes of platform development occurred in the High Atlas, both recording the effects of tectonic, eustatic, and sedimentologic controls on platform trends, morphologies, and internal stratigraphy: Lias (Sinemurian to Domerian) and Lias-Dogger (Aalenian to Bajocian). Along the borders of Mesetas, platform margins became established over basin-bounding faults, whereas platforms within the seaway developed on pre-existing or active structural highs.
Fault-scarp margins, olistolith and mega-breccia emplacement, and drowned or foundered margins record the tectonic contribution to shaping the margins. Platform interior stratigraphy contains third- and fourth-order depositional sequences that reflect multiple sea level fluctuations during a gradual sea level rise. Structural highs in the interior of the seaways, some remaining active during the Lias, were capped or fringed by platform carbonates or, in deeper water, were covered by sponge-algal buildups. Basin lows filled with turbidites and limestone-marl cycles. Carbonate alluvial fans and neptunian dikes are associated with syntectonic wrenching within the seaway. The Liassic platform was inundated by a Toarcian drowning event, with shale deposition occurring across the southern platform, within the basin, and, locally, condensed sequences forming on intra-basin highs. Lias-Dogger platforms ceased with the transition to continental sedimentation, marking the end of the high Atlas seaway.
AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.