CRYOGENIAN CARBONATE-SLATE SEQUENCES OF THE TAMBIEN GROUP, N. ETHIOPIA: PRE-“STURTIAN” CHEMOSTRATIGRAPHY AND REGIONAL CORRELATIONS

Ethiopia's youngest Neoproterozoic outcrops are “Sturtian” diamictites that cap the Tambien Group (Tigre, N. Ethiopia), a modestly thick (2-3 km) slate-carbonate sequence that records the Cryogenian evolution of the Mozambique Ocean within the southern Arabian-Nubian Shield (ANS). Litho- and chemostratigraphic variations of the Tambien Group, compiled from four areas of Tigre, demonstrate that δ¹³C_carb and ⁸⁷Sr/⁸⁶Sr stratigraphies are effective for regional correlation. The regional record reveals two consecutive positive-to-negative carbon isotope excursions, the first associated with an abrupt transition to carbonates with cap carbonate-like features and the second associated with the transition from relatively organic-rich black limestone to “Sturtian” diamictite (Negash synclinorium). Sr isotope compositions rise impressively from values less than 0.7055 near the base of the carbonate sequence to a plateau near 0.7068 in upper limestones, before possibly declining to 0.7064 (or lower) in the transition to diamictite deposition. Sr contents of limestones increase (10x) systematically above the lower negative δ¹³C_carb interval. Textural and chemical properties of the Assem Limestone (Mai Kenetal locality), in addition to its depositional context, suggest a transgressive cap-carbonate sequence. Although conformably underlain by laminated slate without definitive evidence of glaciation, its lithostratigraphic position as the lowest significant carbonate unit correlates above polymict volcaniclastic agglomerates and greywackes (Negash and Samre localities) previously suspected to have a glacigenic association. Chemical weathering indices (PIA: 92-99) of slates below this regional horizon suggest intense silicate weathering of the Tsaliet arc accretion complex that would have acted to lower atmospheric pCO₂. These findings raise the possibility that the lower Tambien Group records an earlier pre-“Sturtian” cooling event, possibly equivalent to the Kaigas glaciation. If so, the Assem Limestone is the first cap carbonate sequence recognized in the ANS. The intra-oceanic setting of the Tambien Group adds an important new paleogeographic perspective for the early development of Cryogenian Earth systems.