Paleogeographic Reconstruction of the Upper Miocene Sequences of the Wadi Yunis Member of the Al Khums Formation, Sirt Basin, Central Libya

摘要

The main objective of this work is to reconstruct the paleogeographic distribution of the depositional facies of the Upper Miocene carbonate sequences of Wadi Yunis Member, Al Khums Formation, Sirt Basin. The sequence stratigraphic correlation of 38 field measured high-resolution sedimentological sections indicates that the Upper Miocene Wadi Yunis Member consists of two successive shallowing-up sequences. Each sequence consists of six vertically stacked depositional facies (oolitic grainstone facies, oo-skeletal grainstone-packstone facies, algal pelle-skeletal facies, bioclastic wackestone facies, algal stromatolite facies, and coarsely crystalline selenite gypsum facies). These carbonate depositional facies formed in restricted shelf lagoonal-tidal inlet and beach-bar shoalsubtidal channel complex environments along a NW-SE trending, 150 km long, wave-dominated carbonate ramp. Mapping analysis of these two depositional sequences resulted in the reconstruction of the paleogeographic distribution of the Wadi Yunis Member depositional facies. During the transgressive systems tract (TST) of the lower shallowing upward sequence, the oolitic grainstone facies belt was deposited above the marly bioclastic wackestone facies. Then a younger channeled oolitic grainstone belt was created above a structural high seaward of the older oolitic grainstone belt, and between these two oolitic belts lagoonal bioclastic wackestone facies were deposited. During the highstand systems tract (HST), channel inlets partially closed and the bioclastic wackestone facies with dwarfed fossils, stromatolites, algal pelletal facie and some gypsum restricted lagoonal facies were deposited. During the lowstand systems tract (LST), the lagoons were completely restricted and selenitic gypsum facies deposited in an eye-ball shaped geometry and the lower shal-lowing-up sequence completed. The upper shallowing-upward sequence preserves the TST. It is made up of channeled lagoonal bioclastic wackestone facies that were later overlain from the seaward side by a new oolitic grainstone belt. The paleogeographic distribution of the depositional facies of the Upper Miocene carbonate sequences of the Wadi Yunis Member, Sirt Basin, indicates a structurally controlled compartmentalized ramp setting. The elongated carbonated ramp complex body is striking along the NW-SE direction and relatively narrow along the depositional dip section. The low energy/restricted lagoonal deposits are encompassed between two high energy parallel oolitic grainstone belts along 150 km.