Chapter 21 The Tambien Group, Northern Ethiopia (Tigre) NATHAN R. MILLER 1 *, DOV AVIGAD 2 , ROBERT J. STERN 3 & MICHAEL BEYTH 4 1 Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, TX 78712-0254, USA 2 Institute of Earth Sciences, Hebrew University of Jerusalem, Jerusalem 91904, Israel 3 Geosciences Department, University of Texas at Dallas, Richardson, TX 75083-0688, USA 4 Geological Survey of Israel, Jerusalem, Jerusalem 95501, Israel *Corresponding author (e-mail: nrmiller@mail.utexas.edu) Abstract: 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 con- verging 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 d 13 C carb , 87 Sr/ 86 Sr) 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 palaeo- latitude 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 (,458). 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 d 13 C carb polarity [polarity intervals A (þ), B ( – ), C (þ), 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 set- tings. The lower prospective glacial interval lacks diagnostic sedimentological evidence of synglacial deposition, but is overlain by nega- tive d 13 C 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 con- sistent 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. Supplementary material: Supplementary Table 21.1 of Tambien Group geochronological age constraints is available at http://www. geolsoc.org.uk/SUP18462. The Tambien Group is exposed (Fig. 21.1) throughout portions of northern Ethiopia (Tigre Province) and Eritrea (NNE extensions from Figure 21.1; Bizen domain and Adobha Abi terrane of Beyth et al. (2003) and De Souza Filho & Drury (1998)), in greenschist-grade terranes comprising the southern portion of the Arabian – Nubian Shield (ANS). It may be equivalent to similar carbonate-rich units in NE Sudan (Bailateb Group; Stern et al. 1994), SW Saudi Arabia (Hali Group; Greenwood et al. 1976), and possibly western Yemen (inferred from its Red Sea conjugate position). Its deposition marks mainly marine siliciclastic and car- bonate sedimentation within the Mozambique Ocean, an extinct Neoproterozoic ocean basin destroyed during the late Neoprotero- zoic consolidation of Greater Gondwana with the emergence of the East African Orogen (EAO) (Stern 1994). The Tambien Group is best studied in northern Ethiopia from the Mai Kenetal (western flank near 13855 0 N, 38850 0 E) and Negash (southern extent near 13850 0 N, 39837 0 E) synclinoria, where much of the Tambien Group is continuously exposed, and these localities provide the substantial basis for regional litho- and chemostratigraphy. Two possible glaciogenic intervals have been suggested within the Tambien Group (Beyth et al. 2003; Miller et al. 2003, 2009). The lower interval occurs as a greywacke-conglomerate interval within slate of the lower Didikama Formation in the Negash Synclinorium (Fig. 21.2a, column D), and the top of this interval may correlate with the base of the Assem Limestone in the Mai Kenetal Synclinorium (see discussion). The upper prospective glacial interval tops the Tambien Group as diamictite of the Matheos Formation in the core of the Negash Synclinorium (Fig. 21.2a), and this interval may be equivalent to arkosic sand- stone and conglomerate of the Dugub Formation that similarly tops the Tambien Group in the western Shiraro area (Fig. 21.1, Avigad et al. 2007). The upper diamictite unit at Negash was orig- inally defined as the ‘Pebbly slate’ (Beyth 1972). It was subsequently assigned within the Matheos Formation (Garland 1980) and informally described as ‘Pebbly Slate (diamictite)’ in Miller et al. (2003), ‘Slate/Pebbly slate (Diamictite Slate)’ in Alene et al. (2006) and ‘Negash Diamictite’ in Avigad et al. (2007). Miller et al. (2009) subdivide the Matheos Formation into three members, the youngest corresponding to the diamictite facies. In consideration of a possible lower glaciogenic interval in the Negash Synclinorium, the informal term ‘Matheos Diamic- tite’ is suggested for the upper prospective glacial interval. In addition to these northern Ethiopian localities, Eritrea hosts poss- ible glaciogenic units that have not been systematically studied. The southern ANS is still very much a frontier region in need of systematic sedimentological, geochemical, and geochronological studies of Neoproterozoic units. Much of what is known about the Cryogenian Period for the region has been learned in only the past decade. The earliest suggestion of a Late Proterozoic glaciation was by Bibolini (1920), who described faceted clasts (facce piane) within an unnamed pebbly mudstone-conglomerate unit (conglom- erati poligenici) in northern Eritrea. The basic Neoproterozoic stratigraphic framework for northern Ethiopia was established in regional mapping by Beyth (1972), including description of the From:Arnaud, E., Halverson, G. P. & Shields-Zhou, G. (eds) The Geological Record of Neoproterozoic Glaciations. Geological Society, London, Memoirs, 36, 263–276. 0435-4052/11/$15.00 # The Geological Society of London 2011. DOI: 10.1144/M36.21