A Reference
Example for Oolitic Ramps :
a 37 km Continuous Outcrop (Jurassic Cliffs of Amellago,
High
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-stratigraphic
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
interpretation 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
stratigraphic 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