Diversity, growth forms and rhodolith distribution: factors controlling the fabric of coralline algae of the early Miocene deposits, Cairo – Suez road, Egypt

Abstract

The Lower Miocene deposits in Gabal Gharra, Cairo – Suez District, unconformably overlies the Oligocene and Eocene deposits. They consist predominantly of regressive and transgressive mixed siliciclastic / carbonate sediments of shallow marine environments. The Lower Miocene deposits are represented lithostratigraphically by the Gharra Formation that comprises four members, from base to top: Agrud, Sadat, Hommath and Genefe members. Two measured sections were described and investigated: Gabal Gharra section and Gabal Agrud section. The coralline red algae are highly abundant in Sadat and Genefe members as fragments, crusts and rhodoliths. Larger foraminifera, corals, echinoids, bivalves, gastropods, bryozoans, benthic and planktonic foraminifers, dasyclades are also present. These two members consist of dolomitized bioclastic packstone, grainstone to floastones / rudstone with rhodoliths (up to 3cm in size). The systematic study led to the recognition of 14 coralline algal species. Lithophyllum sp. dominates the coralline associations at the generic level, followed by Mesophyllum spp, Lithothamnion spp and Spongites spp. The vertical distribution of the coralline red algae in the two concerned sections allowed to subdivide the Lower Miocene sequence into two local coralline algal zones, from base to top: Lithophyllum ghorabi Zone (including the Sadat Member) and Mesophyllum iraqense Zone (covering the Genefe Member). The rhodoliths populations in the two sections were analyzed in terms of their size, shape, massiveness, symmetry, algal growth forms, algal species diversity, epibionts and borings. These parameters reflect environmental differences between the two studied sections. A comparison of the two the analyzed rhodoliths populations show common features, but also significant differences are recorded. The rhodoliths populations of Gabal Gharra were formed under low energy conditions characterized by porous rhodoliths with higher primary porosity, less frequent epibionts, more boring and thick coralline algal thalli as well as greater asymmetry. On the contrary, in Gabal Agrud the analyzed rhodoliths population was formed in high energy conditions of shallower water marine environments and characterized by more mobile, symmetrical rhodoliths, more frequent epibiont, thin coralline thalli.