Evidence for Early and Mid-Cryogenian glaciation in the Northern Arabian–Nubian Shield (Egypt, Sudan, and western Arabia)

Evidence of Earlyto Mid-Cryogenian (c. 780 Ma and c. 740 Ma) glacial activity is summarized for the northern Arabian–Nubian Shield (ANS), including structural framework, stratigraphy, lithological descriptions and relationships with younger and older units, banded iron formation chemostratigraphy, other characteristics, geochronological constraints, and discussion. The ANS is a broad tract of juvenile continental crust, formed from accreted arc-backarc basin terranes developed around the margins of the Mozambique Ocean. As a result, these successions formed in marine environments at some distance from continental margins. Deposits include banded iron formation (BIF) and possibly glacial diamictite scattered over broad regions of the Central Eastern Desert of Egypt, NW Arabia and possible correlative units in NE Sudan. The older (c. 780 Ma) examples (Meritri group, NE Sudan; basal Mahd group, Arabia) occur in the central ANS, on the southern flank of an important lithospheric boundary, an ophiolite-decorated suture zone. Mahd group diamictite is thin (1–5 m thick) and rests above the earliest (Cryogenian) ANS unconformity. The Meritri group interval near Port Sudan is much thicker and part of a deformed passive margin. Both Mahd and Meritri group deposits need further study before they are accepted as glaciogenic; confirmation of this interpretation would indicate that Neoproterozoic glacial activity began at least as early as 780 Ma ago. The younger (c. 740 Ma) glacial deposits include diamictite and BIF: the Atud diamictite and BIFs of the Central Eastern Desert of Egypt and the correlative Nuwaybah diamictite and BIF of NW Arabia. Northern ANS-BIF is a well-layered chemical sediment of interlaminated hematite-magnetite and jasper. A glacial origin for the Atud-Nuwaybah diamictites is inferred because large clasts and matrix zircons have ages (Palaeoproterozoic and Neoarchean) and compositions (especially quartzite, arkose, and microdiamictite) that require transport from outside the ANS Cryogenian basin. Northern ANS-BIF may also reveal glacial influence, having been deposited in response to reoxygenation of a suboxic ocean. The 740 Ma diamictite and/or BIF may correlate with Tambien Group diamictites in Ethiopia (Miller et al. 2011). Northern ANS diamictite and BIF were deposited in an oceanic basin of unknown size, as indicated by association with abundant ophiolites; they are strongly deformed, obscuring many primary features. There is no strong evidence for or against Ediacaran glaciation in the ANS, largely because the region was uplifted at this time. The c. 600 Ma ANS peneplain may have been partly cut by Ediacaran glaciation. Some of the post-accretionary basins of Arabia could preserve glaciogenic deposits of Ediacaran age, but assessing this possibility requires further investigation. The Arabian–Nubian Shield (ANS) consists of mostly Neoproterozoic outcrops around the Red Sea in NE Africa and West Arabia, exposed by Oligocene and younger uplift and erosion. The ANS is one of the largest tracts of juvenile continental crust of Neoproterozoic age on Earth; its evolution accompanied a supercontinent cycle that defined Neoproterozoic tectonics, beginning with the break-up of the end-Mesoproterozoic supercontinent Rodinia in early Neoproterozoic time (Stern 2008). ANS juvenile crust (intra-oceanic arcs and oceanic plateaux) was generated around and within the Mozambique Ocean and coalesced as this ocean closed (Stern 1994). The tectonic cycle culminated in a protracted collision beginning c. 630 Ma, forming the East African Orogen (EAO), an important weld in the end-Neoproterozoic supercontinent of ‘Greater Gondwana’ (Stern 1994) or ‘Pannotia’ (Dalziel 1997). In reconstructed Gondwana, the EAO extends from the Mediterranean (Tethys) southward along the eastern margin of Africa and across East Antarctica. Sedimentary evidence of early Cryogenian glacial episodes is likely to be preserved in ANS sequences, because these episodes occurred when ANS crustal components were mostly below sea level (Stern et al. 2006). In contrast, evidence of late Cryogenian to Ediacaran glacial episodes may be absent because this was a time of collision and uplift in the ANS. Some evidence for Ediacaran climate may be preserved in post-accretionary basins of Ediacaran age in Arabia (Johnson 2003). In this chapter we describe possible glaciogenic units from both flanks of the Red Sea in the northern ANS; the interested reader should see the chapter by Miller et al. (2011) for an overview of possible glacial deposits from the southern ANS as well as Allen et al. (2011a, b) for chapters covering successions in Oman, to the east of the ANS. The units summarized here include (i) Meritri group metaconglomerate (E. Sudan) and basal Mahd group diamictite (central Arabian Shield); (ii) Atud diamictite (E. Egypt) and Za’am group Nuwaybah diamictite (NW Arabian Shield); and (iii) BIF from E. Egypt, NW Saudi Arabia (Sawawin deposit) and NE Sudan (Fodikwan). Locations of these units are shown in Figure 22.1 and co-ordinates are listed in Table 22.1. We know less about the Meritri group and basal Mahd group and are less confident that they are glaciogenic than we would like to be, but if confirmed, they would be the oldest known Neoproterozoic glaciogenic deposits. These are discussed here to encourage further detailed studies. The deposits of E. Sudan and the central Arabian Shield have not been the subject of focused sedimentological study. The Meritri group conglomerate of Sudan was first identified and briefly described by Abdelsalam & Stern (1993). The basal Mahd group diamictite was first mentioned by Johnson et al. (2003) and then discussed in somewhat greater detail by Stern et al. (2006). The diamictite and banded iron formations to the north in Egypt and NW Arabia (Fig. 22.1) have been studied for some time. The Atud Formation was first described as conglomerate (El-Essawy 1964). Since its first recognition, the Atud Formation has only been recognized in eastern Egypt between 258N and 268N. At the type locality near Gebel Atud, it includes schist, metamudstone, and metagreywacke in addition to diamictite, all of which From: Arnaud, E., Halverson, G. P. & Shields-Zhou, G. (eds) The Geological Record of Neoproterozoic Glaciations. Geological Society, London, Memoirs, 36, 277–284. 0435-4052/11/$15.00 # The Geological Society of London 2011. DOI: 10.1144/M36.22 suffered greenschist–facies metamorphism (El-Essawy 1964). The term ‘conglomerate’ is not appropriate for the poorly sorted, matrix-supported deposits of the Atud Formation. These better fit the description of diamictite by Flint et al. (1960) as poorly sorted, heterolithic, and very coarse terrigenous sediments. Accordingly, in this chapter we refer to the Atud diamictite (Stern et al. 2006). Structural framework The Meritri group conglomerate (Sudan) is preserved within the B’ir Umq-Nakasib Suture Zone, whereas the Mahd group diamictite (Saudi Arabia) is preserved just south of this suture zone. This is one of the longest and best-defined ophiolite-decorated suture zones in the ANS (Johnson et al. 2003) and extends (with the Red Sea closed) ENE–WSW over 600 km from the Nile in Sudan into the central Arabian Shield. The suture zone itself consists of rocks that originated in a variety of juvenile oceanic environments and include strongly deformed ophiolite nappes, and metavolcanic, metasedimentary, and intrusive rocks. Dating of the ophiolites, volcanic rocks, and preand syntectonic plutons indicates that oceanic magmatism in the region was active c. 870–830 Ma, whereas suturing occurred c. 780–760 Ma (Hargrove III et al. 2006). Structural complexities (folding, faulting, shearing, etc.) are especially severe for the Meritri group, which is part of a southwards-directed nappe stack (Fig. 22.2a), discussed in the following section (Abdelsalam & Stern 1993). The Mahd group diamictite is less folded and faulted because it lies south of the suture zone. The Atud and Nuwaybah diamictites and associated BIF in Egypt and NW Saudi Arabia are also folded and faulted. BIF is strongly deformed although much of this deformation may have occurred as a result of slumping of dense, weak sediments in a tectonically unstable basin. BIF and surrounding sediments are metamorphosed to greenschist facies. Tectonic deformation began as a result of collision between arc terranes prior to c. 680 Ma (Ries et al. 1983). Other deformation resulted from pervasive left-lateral strike-slip shearing along the Najd fault system, which was active during Ediacaran time (Sultan et al. 1988). Najd deformation was a far-field manifestation of collision between fragments of east and west Gondwana (Abdelsalam & Stern 1997). Najd faulting imparted a penetrative shear fabric to most Cryogenian supracrustal units in Egypt and NW Arabia.