THE TAMBIEN GROUP, ETHIOPIA: AN EARLY CRYOGENIAN (CA. 800-735 MA) NEOPROTEROZOIC SEQUENCE IN THE ARABIAN-NUBIAN SHIELD

Abstract

The Tambien Group in Tigrai, Ethiopia, comprises a number of inliers each containing 2-3 km thickness of interbedded carbonate and clastic sediments, capped in one inlier by a glaciogenic diamictite. A range of geochemical indices suggest near-pristine C- and Sr-isotope values are preserved and these, together with lithological variations, allow local correlation between these inliers and correlation with the global Neoproterozoic isotope stratigraphy. A composite section of the Tambien Group shows δ13C in carbonate of ~+6‰ at its base, decreasing upwards to twin lows of ~-4‰ separated by a brief excursion back to positive values, then rises again to a plateau of ~+6‰ before finally decreasing sharply to -2‰ beneath the Negash diamictite at its top. No glaciogenic sediments are observed associated with the lower twinned negative anomalies. The 87Sr/86Sr values in carbonates change from ~0.7063 in the lower units to ~0.7067 in the upper units. The δ13C of organic matter changes little through the sequence (-24.2 ± 1.3‰), with the result that the C-isotope fractionation between carbonate and organic matter decreases from +26 to +21‰ up through the second low in carbonate δ13C before increasing to ~+29.5‰ in the rest of the sequence. Together with existing radiometric age constraints, the Sr-isotope data indicate that the Negash diamictite is Sturtian in age, and the lower negative C-isotope anomalies appear to correlate with the non-glaciogenic Bitter Springs Stage recognised in Australia and Svalbard. According to this correlation the Tambien Group was deposited in the interval ca. 800-735 Ma. Our data reinforce the emerging view that not all Neoproterozoic negative C-isotope anomalies are associated with glaciation. The variations in C-isotope fractionations within our sequence contrast with those from Australia and are attributed to differences in local environmental variables, probably temperature. These differences are consistent with the recent proposal that the Bitter Springs Stage anomaly is the result of a pair of Inertial Interchange True Polar Wander events [Halverson, G.P., Maloof, A., Schrag, D., Dudas, F., Hurtgen, M., in press. Stratigraphy and geochemistry of a ca 800 Ma negative carbon isotope stage in northeastern Svalbard. Chem. Geol.] [Maloof, A.C., Halverson, G.P., Kirschvink, J.L., Schrag, D.P., Weiss, B.P., Hoffman, P.F., in press. Combined paleomagnetic, isotopic and stratigraphic evidence for true polar wander from the Neoproterozoic Akademikerbreen Group, Svalbard. GSA Bull.], and further high-resolution dating and palaeomagnetic studies of the Tambien Group should allow testing of this hypothesis. © 2006 Elsevier B.V. All rights reserved.