Depositional styles of Early Ediacaran terrestrial volcanosedimentary succession in Gebel El Urf area, North Eastern Desert, Egypt

Abstract

Abstract Located 100 km northwest of Hurghada, the volcanosedimentary successions of Gebel El Urf is exposed between latitude 27° 45′ 30″ and 27° 51′ 00″ N and longitude 32° 49′ 00″ and 32° 59′ 00″ E. The volcanosedimentary successions of Gebel El Urf crop out in an area dominated by different Late Proterozoic plutonic complexes. Both, erosional and intrusive contacts with different granitoid units have been identified. Two SHRIMP ages have been obtained from crystal-rich and lithic-poor ignimbrites yielding 615 ± 4 and 616.0 ± 5.4 Ma placing the evolution of the inter-montane basins, described here, into the Early Ediacaran. In the Gebel El Urf area, a southeastward dipping succession (Gebel El Urf Succession, GUS) of ca. 2000 m thickness rests on coarse-grained granite with an erosional unconformity. Another succession present in the area (Wadi Kefri Succession, WKS) represents volcanogenic sediments which exhibit degrees of metamorphic overprint. In places, it is presumed to be older than GUS. For the GUS, 14 lithofacies types have been differentiated and grouped to seven lithofacies associations. Subdivided into four depositional phases, GUS starts with a thick, massive and clast-supported conglomerate of alluvial fan facies (well-rounded clasts up to 100 cm). GUS continues with pelitic to sandy-turbiditic lacustrine and sandy braided river deposits with occasional volcanic glassy fragments (now illite) (Phase 2). The upper half of the GUS is dominated by volcanic deposits, starting with a 50 m thick package of alternating ignimbrites and synvolcanic sedimentary mass flow deposits, the latter related to phreatomagmatic vents (Phase 3). A thick succession of welded to non-welded ignimbrites follows, with one 20 m intercalation of coarse well-rounded conglomerates (Phase 4). Numerous SiO 2 -rich and – poor dykes and sills emplaced into GUS. The GUS development displays a cycle from high to low energy sedimentation and back, under humid climatic conditions. Phase 2 was characterized by extension and down faulting of basin shoulders. Tectonic activity presumably also led to damming of the river creating a sweat water lake. Volcanism commenced with small vents during Phase 2 and terminated with voluminous eruptions in Phase 4.