Control of Lower Jurassic microbial reef communities on carbonate platform geometry (Djebel Bou Dahar, High Atlas, Morocco)

The Djebel Bou Dahar (DBD) carbonate platform (Early Jurassic, High Atlas, Morocco) was deposited on the footwall high of an active marine rift. It contains six depositional sequences bounded by footwall unconformities and correlative flooding surfaces along the adjacent hanging wall. The DBD evolved from a low-relief ramp profile (Sequences I-III) to a high-relief steep-fronted platform with slopes up to 30° and 600 m relief (Sequences IV-VI). The architectural evolution was controlled by fault-block rotation, regional subsidence in an extensional tectonic setting, third order eustatic sea level and sediment production and dispersal rates, while the margin geometry was, at least in part, controlled by changing reef communities on the slope and margin. Sequence III consisted of siliceous sponge microbial mud mounds associated with coated grain skeletal packstone and grainstone in middle and outer ramp regions. This deep-water carbonate factory did not build into wave-agitated shallow-water and lacked the capability to construct a high-relief margin geometry. During sequences IV (retrogradational) and V (progradational) the growth of a highly productive coral stromatoporoid microbial reef at the platform margin and on the slope (10-60 m depth), adjacent to deeper water siliceous sponge microbial lenses (60-140 m), promoted the accretion of a high-relief and steep slope and the development of a productive shallow-water platform top. Sequence VI record increased accommodation space creation and retrogradational patterns prior to final platform drowning. Coral stromatoporoid microbial boundstone similar to Sequence V slope lithofacies extend on the outermost platform, 200-500 m inward of the platform break. The DBD Lower Jurassic carbonate platform demonstrates the influence of various carbonate factories and microbialites in building and stabilizing a high-relief geometry. It also shows, in contrast with the generally accepted belief that Lower Jurassic reefs are dominated by platform bivalve bioherms, that similar age reef systems can have substantial contributions by microbialite components.