--> Abstract: Sequence Stratigraphy and Depositional Environments on a Palaeozoic Clastic Ramp Margin, Ahnet-Timimoun Basin, Algeria, by K. J. Myers, J. P. P. Hirst, A. Belmecheri Arezki, and R. Sentouh; #90956 (1995).

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Abstract: Sequence Stratigraphy and Depositional Environments on a Palaeozoic Clastic Ramp Margin, Ahnet-Timimoun Basin, Algeria

K. J. Myers, J. P. P. Hirst, A. Belmecheri Arezki, R. Sentouh

A wide, ramp margin was developed during the Devonian/Carboniferous in the Ahnet-Timimoun Basin, Algerian Sahara. Variations in relative sea level resulted in rapid, long distance (>500km) lateral translations of the clastic facies belts; this was the main influence on the locations of sand depocentres. The geometry and distribution of Gedinnian and Emsian shallow marine sandstones is complex. Understanding the influence of relative sea level, shelf processes and local tectonics is essential to predicting the distribution of potential reservoir units.

The Silurian to Carboniferous succession preserved in the Ahnet-Timimoun Basin can be divided into two major Transgressive-Regressive cycles, each of approximately 45 million years duration (Ashgill to Siegenian; Siegenian to Tournaisian). The T-R cycles comprise several sequences of approximately 10 million years duration. Major source rocks in the basin were deposited in the Early Silurian (Llandovery) and Late Devonian (Frasnian) around the transgressive maximum of the T-R cycles.

In the Ahnet-Timimoun Basin, marine sedimentation prevailed across much of the ramp margin, During Gedinnian times (early Devonian), progradational events associated with each sequence deposited a succession of extensive, shallow marine, coarsening-up sandstone. The sequence boundary marking the regressive maximum of the first T-R cycle (Siegenian) resulted in a rapid transition from an inner shelf environment to braided rivers which deposited a regional high N/G sandstone. Sequence boundaries, although marked by rapid basinward shifts in facies belts, are without significant fluvial incision.

The transgressive sequence set in the overlying T/R cycle, is marked initially by rapid southwards directed trangression and an extensive ravinement surface of early Emsian age. The overall trangression was punctuated by a minor tectonically enhanced sequence boundary which resulted in deposition of a progradational, coarsening-up inner shelf to shoreface sandstone unit The variations in sandstone thickness were influenced by local tectonics/subsidence and transgressive erosion/reworking.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France