Drowning of a lower Jurassic carbonate platform: Jbel Bou Dahar, High Atlas, Morocco

The high-plateau of the Jbel Bou Dahar, situated in the Central and Eastern High Atlas of Morocco, represents a Lower Jurassic carbonate platform that drowned at the beginning of the Toarcian. Three phases of platform evoluti on can be distinguished: During the pre-drowning phase (upper Sinemurian - upper Pliensbachian) the platform interior facies reflects a restricted-marine lagoonal environment, protected by scattered buildups and cemented debris at the platform margin . Upper and mid-slope are dominated by coarse-grained, poorly sorted limest ones, deposited through debris flows during sea-level lowstands. Sea-level highstand deposits occur at the toe of slope and are formed by an alternati on of fine-grained litho- and bioclastic pack- to grainstones (turbidites), marls and mud- to wackestones (hemipelagic oozes). A condensed section, reflecting an abrupt and fundamental environmental cha nge along the entire platform, characterises the drowning phase (upper Plie nsbachian lower Toarcian). Within the platform interior densely packed bios parites represent the switch to high-energy environments, causing erosion o f the former pre-drowning lagoonal sediments. These erosional products were redeposited on the platform slope, leading to the formation of coarse-grai ned non-skeletal sparites and micrites. Both platform interior and slope su ccessions show a series of cyclic variations in sediment composition that c ould have been triggered by small-scale sea-level fluctuations. In contrast to the abrupt facies change at the pre-drowning - drowning boun dary, the transition to the post-drowning phase (lower Toarcian - Aalenian) is gradual. During this phase, biopelmicrites and pure micrites were depos ited in all platform sections, followed by the deposition of calcisiltites. The facies point to quiet-water conditions below storm-wave base and displ ay a uniform deep-marine sedimentation. This analysis shows that the drowning of the Jbel Bou Dahar carbonate platf orm was caused by abrupt and fundamental changes in the shallow-water realm . After exposure of the platform, these changes prevented the carbonate fac tory from re-establishing itself and made it impossible for the platform to keep up with the subsequent rise in sea level. These local changes were pr obably triggered by high-frequency sealevel variations in combination with regional or even worldwide changes in ocean circulation patterns.