Chapter 21 The Tambien Group, Northern Ethiopia (Tigre)
The Tambien Group of northern Ethiopia (Tigre), with probable correlatives in Eritrea, is a 2–3-km-thick siliciclastic–carbonate succession that was deposited in an intra-oceanic arc platform setting within the southern Arabian–Nubian Shield (ANS) area (southern extension of the Nakfa Terrane) of the Mozambique Ocean. Its deposition occurred prior to ocean closure between converging fragments of East and West Gondwana and concomitant structural emergence of the East African Orogen (EAO). The Tambien Group is well exposed and best studied in the Mai Kenetal and Negash synclinoria, where litho- and chemostratigraphy (including δ13Ccarb, 87Sr/86Sr) provide the basis for a composite reference section. Two glaciogenic intervals have been suggested from exposures within the Didikama and Matheos Formation in the Negash Synclinorium. No reliable palaeomagnetic data exist to constrain the palaeolatitude of Tambien Group deposition and the southern ANS, but palaeogeographic reconstructions and evaporite pseudomorphs in lower carbonate units (Didikama Formation) imply low to intermediate latitudes (<45°). Integration of available geochronological information (regional magmatism and detrital zircon) suggests c. 775–660 Ma as a plausible window constraining deposition of the prospective glacial intervals.
The Tambien Group appears to preserve a coherent chemostratigraphic framework that can be effectively subdivided according to shifts in δ13Ccarb polarity [polarity intervals A (+), B (–), C (+), D (–)]. Slates underlying and interstratified with polarity interval A carbonate preserve evidence of extreme chemical weathering that lessened prior to deposition of polarity interval B carbonate. Tambien Group carbonate units have sedimentological characteristics consistent with both shallow and deeper marine depositional settings. The lower prospective glacial interval lacks diagnostic sedimentological evidence of synglacial deposition, but is overlain by negative δ13C carbonate (polarity interval B) with sedimentological characteristics consistent with well-documented cap-carbonate successions. The upper prospective glacial interval in the Negash Synclinorium (Matheos Diamictite) best exhibits characteristics consistent with glaciogenic deposition (matrix-supported polymictic clasts, possible dropstones, possible bullet-nosed and striated clasts). In contrast to pericratonic rift margin settings that are common for Cryogenian glaciogenic deposits, palaeogeographic reconstructions for the 775–660 Ma timeframe place northern Ethiopia within an intra-oceanic setting that was likely far removed from cratonic hinterlands. More work on Tambien Group sedimentology, geochronology and palaeogeography is required to better evaluate the extent and timing of glacial conditions associated with the prospective glaciogenic intervals.