Spatial and temporal distribution of ooids along a Jurassic carbonate ramp: Amellago outcrop transect, High-Atlas, Morocco.
Résumé
Carbonate ramp systems are widespread throughout the geological record, but very few areas have seismic-scale, continuous and structurally undeformed outcrops that allow reliable interpretation of facies distributions and stacking patterns. The Amellago outcrop shows the detailed depositional and stratigraphic relationships of an ooid-dominated ramp system that is almost completely exposed along a dip profile (37 km long and 1000 m thick) in the Lower to Middle Jurassic of the southern High Atlas, Morocco. Ammonite and brachiopod fauna provide excellent biostratigraphic control on small scale stacking patterns.
At Amellago, the evolution of depositional environments is evident at different scales of space and time during this period of tectonic quiescence dominated by thermal subsidence. An important observation is that the Amellago ramp system contains micrite-rich, ooid-free intervals that alternate with ooid-rich intervals. The ooid-rich intervals are mainly in the late transgressive and highstand system tracts, whereas the ooid-free intervals occur in the early transgressive phase. More than 25 such alternations were recorded in high frequency cycles and at the scale of one large cycle at the Aalenian/Bajocian transition. These compositional changes and the associated different ramp geometries are interpreted to result from the combined effects of eustatic sea level and climatic changes.