Macrofauna associated with the Pleistocene limestones of Zabargad Island (Red Sea): paleoenvironmental and stratigraphic significance.

In 1979 an Italian geological mission visited Zabargad island (St. John’s: 23°37’ N, 36°12’ E). The island, located about 50 Km W of its axis, is considered key area to understanding the kinematics of the Red Sea rift. Giorgio Marinelli, who was in the party carrying out the first field reconnaissance, inspired this trip, followed by two other expedition sin 1980 and 1983. Thought to be an uplifted block of upper mantle and crustal rocks, Zabargad exposes a unique suite of rocks that are: (1) Fresh peridotites; (2) Metamorphic rocks (gneiss and amphibolites); (3) Doleritic-basaltic dikes; (4) Sandstone, limestone, and black shale of the Zabargad Formation (Early Cretaceous); (5) Evaporites (Miocene); (6) Old Reef Limestones; (7) Old conglomerates and breccias; (8) Young Reef Limestones (MIS5e and Holocene). The Old Reef Limestones (ORL) is an informal stratigraphic unit represented by diachronous fossiliferous biocalcarenites and reefal limestones. Although very fossiliferous, this unit has been affected by considerable diagenetic processes and most of its fossil content is now represented by moulds of corals, molluscs, echinoids, barnacles and coralline algae. ORL is tentatively assigned to the Pleistocene, although the concomitant absence of planktonic microfossils and the lack of radiometrically-datable macrofossils hamper at present a more precise age assessment. Preserved shells or latex and gypsum casts at times permit a more refined taxonomic identification and with no exception the fossils are consistently approachable to taxa inhabiting the present Red Sea-Indian Ocean region and at times known from Pleistocene outcrops in the Red Sea region. This similarity permits to drive some regarding the marine setting of Zabargad during the pre-MIS5 Pleistocene suggesting that ORL represents coral reefal and shallow lagoonal environments. The presence around Zabargad of shallow coral reef environments is unquestionably documented by various massive, branching and encrusting zooxanthellate scleractinian corals such as: Porites (massive and columnar growth forms), Stylophora, Acropora, Cyphastrea, Favites, Favia, Pavona, Goniopora, Caulastrea (phaceloid colony) and the solitary coral Fungia. The occurrence of the infaunal bivalve Periglypta and epifaunal pectinids is consistent with reefal habitats. The presence of shallow marine lagoons (backreef?) entering depressed areas pf the island’s interior is documented by infaunal bivalves (e.g. Diplodonta) and gastropods (e.g. Strombus). ORL represents the first unequivocal proof that the Zabargad peridotitic block was already an island at the time of its deposition in the pre-MIS5 Pleistocene (> 125ky BP), thus setting a minimum age for its alleged tectonic uplift.