Abstract: El Biod Triassic (Algeria). A tectonic and climatic controlled sedimentation

HAMEL, A.

El Biod area constitutes the west border of Ghadames basin, where one of the most important hydrocarbon reservoirs are the Triassic deposits, which on palynological evidence, are dated Carnian-Norian. Overlying unconformably various Paleozoic units, they are overlain by a chronostratigraphic dolomitic horizon (D2), dated Hettangian. Triassic terrigenous clastic rocks of El Biod area were deposited in a west fault-bounded basin which is similar to an half-graben, relative to tectonic setting. Two depositionnal cycles are defined within these clastic rocks. The first one is characterized by three major depositional systems:

1- small alluvial fans were deposited on the northwest side of the basin, adjacent of the major faults. These fans consist of proximal and distal facies that were dominated by streamflood depositional processes.

2- The alluvial fans interfinger with fluvial sediments deposited in the axial part of the basin which represents a tectonically erosional valley oriented SW-NE. This axial depositional system lying on the downthrown side of the faults was dominated by braided fluvial environments which grade vertically and laterally towards the northeast into meandering, then floodplain, depositional environments.

3- Above these two depositional systems and extending towards the northeast a sabkha depositional environment was developed.

The second one is characterized by two major depositional systems:

1- The cycle one sabkha sedimentation was interrupted by tectonism as evidenced by extrusive volcanic and uplift of the westernmost side of El Biod ridge. Again, fluvial environments took place, starting by the braided in the south part of the basin evolving to the meandering and floodplain towards the north.

2- As in cycle one a sabkha sedimentation took place, covering a larger area. Complete submergence by hypersaline marine waters ensued, depositing anhydrite and salt in the north while argillaceous sedimentation continued in the south.

So, it appears clearly that depositional evolution of the basin was primarily controlled by tectonic processes, while climatic, provenance, and drainage basin area were important secondary controls on this evolution.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria