Evolution of the Arabian Nubian Shield and Snowball Earth

Abstract

Neoproterozoic evolution of the Arabian-Nubian Shield (ANS) and East African Orogen (EAO), 870–541 Ma, spanned revolutionary changes in Earth Systems, including a supercontinent cycle (Rodinia break-up, opening/closing of the Mozambique Ocean, Gondwana assembly), extreme climate fluctuations between long-lived glacial episodes as postulated by the Snowball Earth Hypothesis (1992–2002), marked increases in oceanic and atmospheric oxygen levels, and expansion of the biosphere from simple microbial life to the inclusion of larger and more diverse multicellular organisms. Understanding of these Earth System transitions has advanced tremendously over the past two decades through the integration of global studies of Neoproterozoic sedimentary successions with refined geochronologic techniques. This approach applied to Neoproterozoic Snowball Earth (NSE) localities now indicates that Neoproterozoic glaciations included two global-scale (panglacial) episodes, the ~717–659 Ma Sturtian and ~650–640 to 635 Ma Marinoan glaciations—which together comprise the Cryogenian Period, and more regional episodes during the Ediacaran Period and possibly, albeit controversially, during the earlier Tonian Period. Recent geochronologic, geochemical, and sedimentologic studies of low metamorphic grade ANS successions substantially contribute to the global Neoproterozoic dataset and, along with recently revised age constraints for the Cryogenian Period, facilitate an updated assessment of how Neoproterozoic glaciations may have influenced the sedimentary record of the ANS during its development. Tonian and Sturtian glaciations would have occurred following Rodinian break-up and major phases of juvenile crust formation in arc/island arc settings of the Mozambique Ocean (870–690 Ma), while latent terrane accretion and magmatism were still active. Paleogeographic reconstructions for these intervals generally place the ANS at tropical latitudes, where chemical weathering rates of juvenile crust terranes would have been high. Evidence supporting Tonian glaciation in the ANS is unresolved, with banded iron formation (BIF) and possible glacial diamictite scattered over the Central Eastern Desert (CED) of Egypt, NW Arabia, and possible correlative units in NE Sudan, as strongest candidates based on available age control (~780–740 Ma) and lithologic compatibility with NSE episodes. New age constraints for some of these localities (i.e., Atud diamictite and Um Nar, El-Hadid, Um Ghamis, and Wadi Kareim BIF localities in the CED) now demonstrate that deposition coincided with the Sturtian panglacial interval. Strong evidence of Sturtian glaciation in the ANS also occurs at the top of the Tonian-early Cryogenian Tambien Group in Northern Ethiopia, where polymict diamictite (<719.7 ± 0.5 Ma) bearing clasts consistent with glacial transport transitionally overlies limestone with pre-Sturtian ⁸⁷Sr/⁸⁶Sr values of 0.7066. Diamictite clast compositions similar to lower Tambien Group units suggest derivation from Tambien Group source terranes within the ANS, such as may have developed during early structural emergence of the EAO and/or associated eustatic sea-level fall. Carbonate units preserving negative carbon isotope excursions correlated to the ~800 Ma Bitter Springs anomaly, ~737 Ma Islay anomaly, and ~720 Ma pre-Sturtian transition, demonstrate that the Tambien Group is an important archive for studying the Tonian transition to extreme climates of the Cryogenian. The Marinoan (~645–635 Ma) glaciation overlapped with incipient development of the EAO, resulting from convergence, uplift, and structural deformation of earlier formed arc and accreted arc terranes, as the Mozambique Ocean closed between cratonic fragments of West and East Gondwana. Because most of the ANS was likely elevated above sea level, the ANS/EAO had few depocenters capable of preserving Marinoan sedimentation. Some peripheral margin basins, such as Murdama and Furayh basins in Arabia, overlapped with the Marinoan glaciation, but their sedimentary records have not been systematically studied for glacigenic characteristics. The onset of sedimentation in some post-amalgamation basins of the northern ANS (e.g., Jibalah Group of NW Arabia and possible equivalents in Jordan and Israel) may have overlapped with the Marinoan glaciation or Marinoan sediments could have been subsequently reworked in alluvial systems and redeposited during early basin formation. Following continental collision (~630–600 Ma), Ediacaran glaciations would have coincided with continued shortening and orogenic uplift (~600–540 Ma), when vast alluvial fan systems transported sediments away from EAO highlands. Ediacaran paleogeographic reconstructions generally place the ANS at higher tropical or temperate latitudes in the S. Hemisphere that may have supported regional scale glaciation. Within post-amalgamation basins of the northern Arabian Shield, the Jibalah Group sedimentary record includes polymict conglomerate, matrix-supported diamictite, and occasional dropstones that could be glacigenic. Regional deposition, constrained between underlying shield rocks (likely ≤605 ± 5 Ma) and the overlying Lower Cambrian basal unconformity (~540–520 Ma) and confirmed by U–Pb zircon dating of volcanic intervals within several basins, would have spanned the ~580 Ma Gaskiers glaciation, the ~575–567 Ma Shuram negative carbon isotope excursion, and younger Ediacaran glaciations. Although highly variable, sedimentary fill in many basins begins as polymict conglomerate with increasing limestone abundance in the higher succession, possibly consistent with a marine transgression. Carbonate δ¹³C values in combination with detrital zircon ages indicate that the basal conglomerate units pre-date the Shuram anomaly and could correlate with the ~580 Ma Gaskiers glaciation. Post-glacial supersequences may have been similarly deposited throughout northern Gondwana (Israel, Jordan, Saudi Arabia, Oman). Metazoan trace and probable body fossils are documented in Dhaiqa and Jifn basins above conglomeratic strata (Mataar Fm and Jifn Polymictic Conglomerate) that have been prospectively correlated with the Gaskiers glaciation. The lowest fossil horizons are no younger than 577 ± 5 Ma (Jifn) and 569 ± 3 Ma (Dhaiqa), similar to the Newfoundland record, where Ediacaran fauna appear ~9.5 myr after the ~580 Ma Gaskiers glaciation (Pu et al. in Geology 44:955–958, 2016). Follow-up studies are needed to assess the timing and glacigenic affinity of conglomerate and diamictite units, and to establish if and when marine deposition occurred within the Jibalah Group. A conspicuous stratigraphic feature of the northern Gondwanan margin is the widespread occurrence of an erosional unconformity throughout North Africa and Arabia separating Neoproterozoic basement from Cambro-Ordovician age sandstone that was principally sourced from erosion of the EAO. Although EAO erosion would have initiated as soon as regional uplift began, the exceptional power of Marinoan and Ediacaran ice sheets acting on a Himalaya-scale orogen may have contributed to ~650–540 Ma beveling of the Afro-Arabian Peneplain.