Журнал геологии и геофизики
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ISSN: 2381-8719
ISSN: 2381-8719
Wanas HA* and Soliman HE
Facies analysis of the studied Permo-Triassic Qiseib Formation indicates the presence of five lithofacies. These lithofacies are: matrix-supported conglomerates facies (facies A), coarse-to medium-grained sandstone facies (facies B), heterolithic facies (interbedded sandstone and siltstone) (facies C), interbedded siltstone and claystone facies (facies D), and mixed clastic-carbonate facies (facies E). These facies were deposited in two fluvial environments: low sinuosity braided and high sinuosity meandering. The braided stream deposits dominate the southern portion of the studied area, whereas the meandering stream deposits prevailed in the northern portion. The fine-grained meandering stream deposits grade laterally to the northern part of the study area into tidally-influenced fine-grained meandering strata (estuarine mud).
In terms of its sequence stratigraphy, the Qiseib Formation represents a fluvial depositional sequence bounded by two sequence boundaries. This fluvial sequence comprises lowstand-, transgressive- and highstand systems tracts. The Lowstand Systems Tract (LST) is predominantly characterized by braided coarse-grained sandstone deposits (Braided stream facies) with limited fine-grained clastics (floodplain facies). TST includes the coeval tidally-influenced fine-grained meandering strata that occur in the extreme north of the study area. The highstand systems tract (HST) is assigned to the sand bodies of the crevasse splay that encased within the floodplain mudstones, where base level rise slows down. Architectures of deposition of the studied Qisieb Formation were mainly controlled by tectonics and climate change. Influence of sea-level control could be developed during deposition in the northern part of the study area, where the tidally-influenced downstream deposits (estuarine mudsone and siltstone) dominated. Also, the relative proportion of fluvial channel sand bodies and fine over bank deposits of the Qisieb Formation were a result of changes in the balance between accommodation space (A) and sediment supply(S).