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A Reference Example for Oolitic Ramps : a 37 km Continuous Outcrop (Jurassic Cliffs of Amellago, High Atlas, Morocco)


Pierre, Aurelien Olivier1, Christophe Durlet2, Philippe Razin3 (1) CHEVRON ETC, San Ramon, CA (2) Universite de Bourgogne (FRANCE), Dijon, France (3) Bordeaux University, Bordeaux, France


Located in the High Atlas Mountains, the cliffs of Amellago offer superb exposures of the Lias-Dogger transition on the northern margin of the Saharan craton. The exceptional dimensions of the continuous outcrop (37 km long and 1000 m high) allow the observation of a ramp stack dip profile at the seismic scale. Three 4th order sequences are observed in detail and placed in a high-resolution sequence-Previous HitstratigraphicNext Hit and biostratigraphic framework, from the proximal to distal environments. The facies changes and their extensions are highlighted by numerous serial logs. The sedimentary features can be observed from the regional scale to the micro-facies scale and allow to reconstruct the facies relationships. Spatial and temporal relationships between facies are the result of physical tracking of banks on the field.


The slope of this ramp, the paleo-bathymetries and the extension of the facies belts, are estimated directly from the Previous HitinterpretationNext Hit of this photo-mosaic profile. Correlation of serial sedimentological sections shows that the ramp profile evolves though time within each 4th order cycle. The main ramp characteristics are: (1) inhibition of oolitic production during early TST at 2nd and 4th order cycle, (2) development of very flat muddy ramp system during early TST at 2nd and 4th order cycle, (3) a carbonate production located in euphotic zone, (4) transport processes dominated by dynamics of tide, wave and storm, depending on proximal/distal and Previous HitstratigraphicTop positions within each 4th order cycle.


These results have implications for construction of a reference ramp model at several scales in oolitic setting.


AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California