Significance of the Tambien Group (Tigrai, N. Ethiopia) for Snowball Earth events in the Arabian–Nubian Shield

Abstract

Juvenile continental crust of the Arabian-Nubian Shield (ANS) formed within a Neoproterozoic supercontinent cycle. Subsequent late Neoproterozoic deposition overlapped a series of dramatic climatic events that are unparalleled in subsequent Phanerozoic time, as proposed by the "Snowball Earth" hypothesis. In particular, extreme negative δ 13 C excursions coincident with glacial diamictite and cap carbonate sequences imply that profound carbon flux changes accompanied widespread glacial transitions (Snowball Events). Such a succession appears to be partially preserved in metasediments of the diamictite-bearing Tambien Group of northern Ethiopia (Negash syncline). Here, a pronounced negative carbon excursion from values of +7 to −2 per mil accompanies the transition to diamictite deposition. New zircon evaporation dates from granites intruding the base of the sequence but post-dating early deformation of the entire sequence suggest that the Tambien Group is older than 613 Ma, and therefore diamictite genesis is incompatible with younger Marinoan, Varanger, or "Ediacaran" glacial intervals. Syn-tectonic granites elsewhere within the region date between 750 and 800 Ma and provide an older age constraint for the Tambien Group. The limestone sequence underlying the diamictite has an average 87 Sr/ 86 Sr composition of 0.7066, which supports either a middle (ca. 750-740 Ma) or late (ca. 725-720 Ma) Sturtian age. Combined Sr and C isotopic compositions of the limestones and the upper age constraint from the granite ages provide a compelling argument that the Tambien Group sediments are ∼720-750 Ma or a little older and strongly support the prospect that the Negash diamictite is the product of Sturtian glaciation. If so, these are the first Sturtian Snowball Earth sequences identified in the ANS.

Geomorphology of the Adwa District

“…Avigad et al (2007) dated this granitoid body at *612.3 ± 57 Ma, correlating it with another post-tectonic intrusive body in a nearby area next to the southern Eritrean border, the Mareb granite (Tadesse et al , 2000Avigad et al 2003;Miller et al 2003;Asrat et al 2004).…”

Stratigraphy and geochronology of the Tambien Group, Ethiopia: Evidence for globally synchronous carbon isotope change in the Neoproterozoic

“…1) with U-Pb dates of ca. 610 Ma (Miller et al, 2003;Avigad et al, 2007) accompanied orogenesis; these plutons provide a minimum age constraint on the timing of Tambien Group deposition (Fig. 1).…”

Neoproterozoic Glaciated Basins: A Critical Review of the Snowball Earth Hypothesis by Comparison with Phanerozoic Glaciations

“…However, the exact nature of the relationship between 13 C depleted carbonates and glaciation is a subject of debate. Negative δ 13 C anomalies occur in preglacial stratigraphic units in Svalbard (Halverson et al, 2004), Namibia , Ethiopia (Miller et al, 2003), Canada , Australia (McKirdy et al, 2001) and Scotland . Schrag et al (2002) proposed that the low-latitude position of continental landmasses during this time would have led to more efficient burial of organic carbon and the development of large methane reservoirs, the slow release of which resulted in the negative δ 13 C anomalies observed in preglacial carbonates.…”