Evidence for Early and Mid-Cryogenian glaciation in the Northern Arabian–Nubian Shield (Egypt, Sudan, and western Arabia)
Robert J. Stern, Peter R. Johnson, Kamal A. Ali, Sumit K. Mukherjee, 2011. "Evidence for Early and Mid-Cryogenian glaciation in the Northern Arabian–Nubian Shield (Egypt, Sudan, and western Arabia)", The Geological Record of Neoproterozoic Glaciations, Emmanuelle Arnaud, Galen P. Halverson, Graham Shields-Zhou
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Evidence of Early- to 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.
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The Geological Record of Neoproterozoic Glaciations
In recent years, interest in Neoproterozoic glaciations has grown as their pivotal role in Earth system evolution has become increasingly clear. One of the main goals of the IGCP Project No. 512 was to produce a synthesis of newly available information on Neoproterozoic successions worldwide similar in format to Hambrey & Harland’s (1981) Earth’s pre-Pleistocene Glacial Record. This Memoir therefore consists of a series of overview chapters followed by site-specific chapters. The overview chapters cover key topics including the history of research on Neoproterozoic glaciations, identification of glacial deposits, chemostratigraphic techniques and datasets, palaeomagnetism, biostratigraphy, geochronology and climate modelling. The site specific chapters for 60 successions worldwide include reviews of the history of research on these rocks and up-to-date syntheses of the structural framework, tectonic setting, palaeomagnetic and geochronological constraints, physical, biological, and chemical stratigraphy, and descriptions of the glaciogenic and associated strata, including economic deposits.