Glacioeustatic and Tectonic Event Stratigraphy of the Upper Ordovician - Lower Silurian of Western Libya

By

Ahmed S. El Hawat¹, A.M. Bezan², Adel Obeidi³, Hassan El-Bargathi⁴

(1) Garyounis University, Benghazi, Libya (2) Exploration Dept. National Oil Corporation, Tripoli, Libya (3) Schlumberger, GeoQuest, Tripoli, Libya (4) TotalFinaElf, E&P Libya, Tripoli, Libya

 The Upper Ordovician- Lower Silurian succession of western Libya consists of two depositional sequences. The lower sequence starts with TST of basal sandstone followed by laminated marine shale containing dropstones of reworked pebbles derived from older rocks. It is followed by regionally extended condensed section of sandy coquina hardground that changes southward (Murzuq basin) to Fe-oxide horizon, and northward in the subsurface of Ghadamis basin, into cold-water carbonates. The HST consists of interbedded sandstone and shale grading upward into fine-grained, shallow marine and nearshore sandstone association. These clastics exhibit evidence of pervasive, multiscale soft sediment deformation and turbidite sedimentation, attributed to tectonic seismic activity that overprinted HST sedimentation. In northern Ghadamis basin, it grades laterally into carbonate bryozoan bank build-up. The second sequence starts with a LST of deeply incised channel systems, attributed to a major glacio-eustatic sea-level fall, and followed by coarse-grained fluvial infill of the second deglaciation event. These fluvial channels followed the NW-SE trend of the inherited structural troughs of the Mid Palaeozoic rift system, and form the main reservoir facies in western Libya. The following TST deposits consist of basal nearshore sandstone, grading into graptolitic shale, which terminates by a condensed section of organic matter-rich, radioactive maker throughout Murzuq and Ghadamis basins and beyond. Similarly, two glacioeustatic depositional sequences constitute the same succession in Morocco and Arabia. Whereas sedimentation in western Libya was dominated by periglacial marine and fluvial sedimentation. Deposition in the other areas was relatively proximal to glaciation centres.