FACIES CHARACTERISTICS, DEPOSITIONAL STYLES AND
EVOLUTION OF THE SYN-RIFT MIOCENE SEQUENCES IN
NUKHUL-FEIRAN AREA, SINAI SIDE OF THE GULF OF
SUEZ RIFT BASIN, EGYPT.
MOUNIR H. EL-AZABI
Geology Department, Faculty of Science, Cairo University, Egypt.
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Highly variable, syn-rift Miocene clastic/carbonate sequences are well exhibited in Nukhul-Feiran area. They experience complexity of facies and diversity of environments. Complex facies patterns were influenced by basin palaeotopographic irregularity, rift tectonic events, irregular subsidence/uplift, climatic changes, variable sediment input and relative sea level fluctuations. This caused the initiation of diversified synchronous depositional realms and facies, which include alluvial fan/fan delta conglomerate and sand, estuary mouth/beach shoreface sand, offshore shale, mudstone and limestone, intertidal/ shallow subtidal reefal limestone and algal stromatolite and supratidal sabkha. The lateral shift of the depositional realms led to vertical stacking of the various facies associations. The studied succession is punctuated by five unconformities that separate the enclosing sediments into six distinctive unconformity-bounded depositional sequences, each marked by spectacular litho-/biofacies aspects and has been formed in different environmental settings. Unconformities and facies deviations maintain the repeated phases of tectonic unrest in the Gulf of Suez rift basin. The initial early syn-rift sequence of Nukhul sediments has been deposited in a lower intertidal/shallow subtidal regime, partly quiet-water deep subtidal. It is succeeded by richly fossiliferous cyclic shale, mudstone and limestone of Lower Rudeis Formation (Seq. II). Microfauna signifies deposition in an open deep marine site below effective wave-base up to 200m water depth. This contrasts markedly with the overlying shallow marine clastic/carbonate deposits of Sarbut EI-Gamal Formation. The latter holds seven fan sequences with four depositional sequences at Gabal Abu Alaqa, which merge into three depositional sequences in Wadi Feiran and Wadi Baba. Sedimentary facies aspects of Sarbut EI-Gamal clastics assert deposition in a constricted subaerial coastal setting as prograding alluvial fans (Fan seq. I-III) and in a shallow marine site as subaqueous fan-delta complex (Fan seq. IV-VII) due to ephemeral intense fluvial discharges. Shallow marine reefal facies prevailed during periods of ceased clastic input. The formation displays an ideal avenue of facies variation along Wadi Sidri, where the clastic/carbonate sediments adjacent to EI-Gamal fault zone change radically downstream into alternating shale/mudstone, evaporite, sandstone and limestone of Upper Rudeis, Kareem and Belayim formations (Seq. III-VI). Indeed, the tectonic impact has played the greatest role in controlling the Miocene depositional sequences, their facies patterns and environmental distributions. Global eustatic sea level change was of minor important in sedimentary facies development. |
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