Sedimentology and Sequence Stratigraphy of the Upper Miocene Carbonate-Evaporite Sequence of the Wadi Yunis Member, Al-Khums Formation, Sirt Basin, Libya

摘要

The investigated Upper Miocene carbonate succession of the Wadi Yunis Member of the Al Khums Formation is exposed in the central Sirt Basin, Libya, along a narrow belt running parallel to the present-day coastline between the villages of Al Aqaylah and Ben Jawwad. It rests unconformably on the Middle Miocene section of the Marada Formation. The study area is located on the northern extension of the northwest-southeast trending Zaltan-Jahama structural platform. It is bound by the Ajdabiya Trough to the east and to Marada Trough to the west. Thirty-eight high-resolution sedimentological sections, which were measured and laboratory analysis of samples, indicate that the Wadi Yunis Member is divided into two successive shallowing-up sequences consisting of six vertically stacked depositional facies. These are: (1) oolitic grainstone facies, (2) oo-skeletal grainstone-packstone facies, (3) algal pelle-skeletal facies, (4) bioclastic wackestone facies, (5) algal stromatolite facies, and (6) coarsely crystalline selenite gypsum facies. Collectively, these facies comprise a carbonate ramp system consisting of high-energy, wave-dominated oolitic shoals, channels and low-energy lagoonal facies. Oolitic shoal paleocurrent measurements suggest a southwest (206?) vector mean transport direction, a result of a northeasterly dominated Mediterranean wind. Facies ratio, isopach, and paleogeographic maps of the study area demonstrate that the oolitic shoals carbonate factory was located to the northwest, whereas the shelf lagoon was influenced by siliciclastic input from the southeast. At times, during eustatic lowstands, the shelf lagoon was subjected to restricted circulation and evaporate precipitation. Due to environmental compartmentalization and rapid lateral facies changes, a multi-datum technique was used in this study. This carbonate ramp body with its elongated narrow northwest-southeast morphology and the southwest-oriented unimodal paleocurrent mode (206? azimuth) is the end product of wave-dominated sedimentological processes that influence the paleocurrents flow directions and facies distribution. However, at some stage in time, the basin suffered from restriction and complete isolation. The gypsum ratio map shows a bulls-eye pattern with evaporites in the basin center surrounded entirely by carbonates, typical of enclosed basins. Detailed laboratory work for petrographic analysis and diagenesis of the limestone facies has been conducted on 53 representative sample thin sections. The dolomitization and gypsification ratio maps indicated that the diagenetic patterns are facies controlled. The dolomitization process is extensive but largely confined to the permeable high-energy deposits of sand size, while the gypsification process is largely restricted to the impermeable low-energy deposits that are muds.