Triassic Sediments of the Oued el-Mya Basin

A schematic map of the typical paleoenvironments of the clastic Triassic sediments (Lower Series, T, and T ) is presented in Fig. 2.12. 

The Lower Series was deposited on a flooded erosion surface over a Paleozoic substrate, a marine surface on which were developed vast lagoons, volcanic flows, tidal flats and barriers. Truely marine conditions prevailed only over the northern part of the region. The sandstones of the Lower Series are not as uniformly continuous as those of the Tj and T. They are generally lenticular or sometimes form wedges interrupted by the volcanic rocks. A great potential for the accumulation of hydrocarbons in lithostratigraphic traps may develop under these conditions like in the sandstones in the upper part of the Lower Series in TKT-i, BKZ-i and BKZ-2. 

In certain parts the volcanic activity and the sedimentation of the Lower Series (especially in its upper portion) took place concurrently as, e.g., between Takhoukht and northwest Guellala. As a consequence the flows mentioned influence the sedimentation of the Lower Series and thus the thinning and thickening of the latter. The discordant surface on the Hercynian folds appears to possess a less pronounced relief at the onset of sedimentation of the Lower Series. The source of supply of the material of the Lower Series must have been smaller hills as throughout the series there is no individual sandstone member which is thicker or of greater extent. In certain areas there is only an argillaceous succession directly above the unconformity surface. 

The maximum thickness of the volcanic rocks was observed in the Guellala area (GLA-i). This volcanic ridge appears to have restricted the deposition of the sandstones of T-i in the northern regions, leading also to a thinning of this unit from El-Hojira (EH-i) in the north towards Guellala in the northeast and Takhoukht (TKT-2) farther to the south. 

Tectonism exerted a strong influence on the sedimentation of the Triassic rocks and controlled the distribution of the sedimentary environments. Along the edges of the basin structmral disruptions led to the development of elongate horsts and grabens. 

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