Coralline red algae from the Early Pliocene Shagra formation of Wadi Wizer, Red Sea coast, Egypt, and their implications in biostratigraphy and paleoecology

摘要

Coralline red algae are highly abundant and well diversified in the well exposed carbonate deposits of the Early Pliocene Shagra Formation at Wadi Wizer, Red Sea coast, Egypt. Lithostratigraphically, the Shagra Formation unconformably overlies the Late Miocene Marsa Alam Formation and underlies the Quaternary deposits. This formation included two members, from base to top: 1) Dashet El Dabaa Member; 2) Sharm El Arab members. The coralline red algae and foraminifera are important constituents of thesedeposits and highly abundant. This carbonate facies is dominated by different assemblage of coralline red algae in the form of in situ crusts, rhodoliths, fragments and corals, bivalve shell fragments, bryozoans, benthonic and palnktonic foraminifera. The systematic study and the taxonomic investigations carried out on the coralline red algae, twenty one coralline algal species were recognized and described for the first time (most of which belonged to nongeniculated coralline algae, they are represented by: seven nongeniculated coralline algal genera in the form of Lithothamnion, Mesophyllum, Spongites, Lithophyllum, Neogoniolithon, Sporolithon, and Lithoporelld). The geniculated coralline algae is described and represented by Corallina sp. One species of green algae (Chlorophyaceae) depicted in the genus Halimeda sp. On the basis of the stratigraphic vertical distribution of these coralline red algae, the studied Early Pliocene sequence could be subdivided into two local coralline algal assemblagezones from base to top: Neogoniolithon sp. Mesophyllum lemoinaea Assemblage Zone and Lithothamnion saipanense Lithophyllum prelichenoid.es Assemblage Zone. Detailed microfacies analysis of the Early Pliocene Shagra Formation led to the recognition of eight microfacies types (six of carbonate facies and two of siliciclastic facies) on the basis of Lithological characters, faunal content and sedimentary strutcures, indicating that the Early Pliocene sequence was deposited in transgressive - regressive cycles ranged from near shore, warm shallow inner to middle neritic marine water environments with development of reefal facies. Paleoecologically, it is noted that both of Lithophullum, Neogoniolithon and Spongites are dominated in the reefal carbonate facies. However, the Mesophyllum, Lithothamnion and Sporolithon are main components of the relatively more shallower carbonate facies.