Chapter 21

The Tambien Group, Northern Ethiopia (Tigre)

NATHAN R. MILLER1*, DOV AVIGAD2, ROBERT J. STERN3 & MICHAEL BEYTH4 1Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, TX 78712-0254, USA 2Institute of Earth Sciences, Hebrew University of Jerusalem, Jerusalem 91904, Israel 3Geosciences Department, University of Texas at Dallas, Richardson, TX 75083-0688, USA 4Geological Survey of Israel, Jerusalem, Jerusalem 95501, Israel *Corresponding author (e-mail: [email protected])

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 13 87 86 d Ccarb, Sr/ 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 13 shifts in d Ccarb 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 d13C 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 Matheos Formation in the core of the Negash Synclinorium northern Ethiopia (Tigre Province) and Eritrea (NNE extensions (Fig. 21.2a), and this interval may be equivalent to arkosic sand- from Figure 21.1; Bizen domain and Adobha Abi terrane of stone and conglomerate of the Dugub Formation that similarly Beyth et al. (2003) and De Souza Filho & Drury (1998)), in tops the Tambien Group in the western Shiraro area (Fig. 21.1, greenschist-grade terranes comprising the southern portion of the Avigad et al. 2007). The upper diamictite unit at Negash was orig- Arabian–Nubian Shield (ANS). It may be equivalent to similar inally defined as the ‘Pebbly slate’ (Beyth 1972). It was carbonate-rich units in NE Sudan (Bailateb Group; Stern et al. subsequently assigned within the Matheos Formation (Garland 1994), SW Saudi Arabia (Hali Group; Greenwood et al. 1976), 1980) and informally described as ‘Pebbly Slate (diamictite)’ in and possibly western Yemen (inferred from its Red Sea conjugate Miller et al. (2003), ‘Slate/Pebbly slate (Diamictite Slate)’ in position). Its deposition marks mainly marine siliciclastic and car- Alene et al. (2006) and ‘Negash Diamictite’ in Avigad bonate sedimentation within the Mozambique Ocean, an extinct et al. (2007). Miller et al. (2009) subdivide the Matheos Formation Neoproterozoic ocean basin destroyed during the late Neoprotero- into three members, the youngest corresponding to the diamictite zoic consolidation of Greater Gondwana with the emergence of the facies. In consideration of a possible lower glaciogenic interval East African Orogen (EAO) (Stern 1994). The Tambien Group is in the Negash Synclinorium, the informal term ‘Matheos Diamic- best studied in northern Ethiopia from the Mai Kenetal (western tite’ is suggested for the upper prospective glacial interval. In flank near 138550N, 388500E) and Negash (southern extent near addition to these northern Ethiopian localities, Eritrea hosts poss- 138500N, 398370E) synclinoria, where much of the Tambien ible glaciogenic units that have not been systematically studied. Group is continuously exposed, and these localities provide the The southern ANS is still very much a frontier region in need of substantial basis for regional litho- and chemostratigraphy. systematic sedimentological, geochemical, and geochronological Two possible glaciogenic intervals have been suggested within studies of Neoproterozoic units. Much of what is known about the the Tambien Group (Beyth et al. 2003; Miller et al. 2003, 2009). Cryogenian Period for the region has been learned in only the past The lower interval occurs as a greywacke-conglomerate interval decade. The earliest suggestion of a Late Proterozoic glaciation within slate of the lower Didikama Formation in the Negash was by Bibolini (1920), who described faceted clasts (facce piane) Synclinorium (Fig. 21.2a, column D), and the top of this interval within an unnamed pebbly mudstone-conglomerate unit (conglom- may correlate with the base of the Assem Limestone in the Mai erati poligenici) in northern Eritrea. The basic Neoproterozoic Kenetal Synclinorium (see discussion). The upper prospective stratigraphic framework for northern Ethiopia was established in glacial interval tops the Tambien Group as diamictite of the 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/M36.21 264 N. R. MILLER ET AL.

Fig. 21.1. Location of key Tambien Group exposures (unshaded units) (A, Shiraro area; B, Mai Kenetal Synclinorium; C, Negash Synclinorium; D, Samre area) within northern Ethiopia (Tigre Province) modified from Miller et al. (2009). Metavolcano–sedimentary block boundaries (Tadesse et al. 1999) occur only within the Tsaliet Group, and are inferred to represent accreted arc terranes or slivers in a supra-subduction zone setting. Stars show geochronological localities for syn-tectonic intrusives (white), post-orogenic intrusives (black) and detrital zircons (white stars with black dots) discussed in the text and shown in Figure 21.5 (also in Supplementary Table 21.1). Prospectively equivalent upper Tambien Group strata of the Gulgula Group occur in western Eritrea (arrow) just west of the Shiraro Area, as well as in the Adobha Abi terrane and Bizen Domain (arrows) to the north in Eritrea. Dashed box east of Mai Kenetal outlines the Werii study area of Sifeta et al. (2005). M. Alem marks the western limb locality (Madahne Alem) within the Negash Synclinorium bearing 774.7 + 4.8 Ma zircons in slate c. 16 m below lowest Tambien Group carbonate beds (Avigad et al. 2007). The Negash Synclinorium is structurally bounded to the east by the Atsbi Horst (AH). Inset map (below) shows the location of an unnamed pebbly mudstone unit in northeastern Eritrea (Cecioni 1981) that could correlate to the Tambien Group. units now posited to have glacial associations. Motivated by the ANS consists of a patchwork of Neoproterozoic tectonostrati- Snowball Earth hypothesis, a number of reconnaissance studies graphic terranes, now bisected by the Oligocene and younger have since explored the depositional context of the Tambien Red Sea rift. ANS terranes include significant volumes of Group (Beyth et al. 2003; Miller et al. 2003; Alene et al. 2006; see juvenile Neoproterozoic crust generated within spreading centers, also Stern et al. 2006). More comprehensive regional investigations, arc and back-arc settings of the Mozambique Ocean (Stern 1994). involving U–Pb zircon geochronology, chemical weathering Sutures between terranes, many with ophiolites and dated meta- indices, and higher resolution sampling for integrated C- and morphic assemblages, document collisional deformation and help Sr-isotope stratigraphy, have further refined the age and range of constrain the timing of terrane amalgamation. Many sutures have litho- and chemostratigraphic variations in the Tambien Group appreciable strike–slip offsets, and these may broadly relate to (Sifeta et al. 2005; Avigad et al. 2007; Miller et al. 2009). c. 600 Ma escape tectonics (Burke & Sengo¨r 1986), during which ANS terranes were progressively sandwiched by, and offset between, obliquely converging Gondwana cratonic blocks (de Structural framework Souza Filho & Drury 1998, and references therein). Tambien Group exposures in Tigre occur within the presumed southern (See additional region-specific structural information in the extension of the Nakfa terrane of Eritrea. The Nakfa terrane is ‘Glaciogenic deposits and associated strata’ section.) The greater one of several suture-bounded low-grade volcano-sedimentary THE TAMBIEN GROUP, NORTHERN ETHIOPIA 265

(a) Relation A. B. C. D. to syn- Shiraro Plains Mai Kenetal Negash Negash & post- Region Synclinorium Synclinorium Synclinorium tectonic Tadesse Beyth 1972 Miller et al. 2009 Beyth 1972 Beyth 1972 intrusives 1999 (Tadesse 1999) (Garland 1980) Pz & Mz Enticho Ss Enticho Ss Seds Adigrat Ss Adigrat Ss Enticho Ss Algal* Ls Mai Kenetal Ls (200m) Pebbly slate (200m) Diamictite Mbr (Tselim Imni Ls) ? ? Black detrital ls Transitional Mbr Shiraro Cgl Tsedia Slate (500m) (300m) ? (subdivided into

Matheos Fm Black Ls Mbr Arkosic ss upper Bilato and Mica dolomite & Enda Zebi Fm

Shiraro Group lower Logmiti Slates) slate (700m) Mentebtab Ss Dugub Fm - Mentebtab Assem LS (300m) Slate & dolomite Dolomite > Slate Bedded ls (Filafil Ls) (200m) Mica dolomite Slate > Dolomite Purple slate w/grn Purple slate w/grn Werii Slate (1000m) reduction spots; GCI ? Post-orogenic granitoids

reduction spots (Segali Slate) greywacke (300m) Didikama Fm Lower Slate ---- Tambien Group ------Tambien Didikama Fm ? (Atsbi Horst) Undiff. Tsaliet Tsaliet Tsaliet Syn- Tsaliet Group Metavolc. Metavolcanics Metavolcanics Metavolcanics (b) d13 C carb Polarity

Mai Kenetal Synform Negash Synform Afro-Arabian Peneplain ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Mai Kenetal LS C (++) Matheos FmD (-) Diamictite Mbr ??? Transition Mbr 3 2 Tsedia Slate C (+) C (++) Black LS Mbr ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~unconformity? Assem LS B (-) Didikama FmB (-) Upper (dol>slate) ~~~~~~~~~~~~~~~ ???paraconformity? GCI ??? 1 Werii Slate A (+?) A (+) Lower (slate >dol) Lower Slate (Atsbi Horst)

Tsaliet Metavolcanics Tsaliet Metavolcanics

Fig. 21.2. (a) Lithostratigraphic subdivision of the Tambien Group in previous work. Prospective glacial intervals in columns A, C and D are superimposed from Beyth et al. (2003) and Miller et al. (2009). The upper Algal Limestone unit in the Shiraro plains region (column A), originally termed the ‘Algal, stromatoporoidea (?) limestone’ by Beyth (1972) was not mapped in the Axum Sheet (Tadesse 1999) and may occur in the Badme region (north of Shiraro, Fig. 21.1). (b) Chemo- and lithostratigraphic correlations between the Mai Kenetal and Negash synclinoria used to create a composite Tambien Group stratigraphic section (see discussion). 13 Stratigraphic intervals in each section are alphabetized (A–D) according to the succession of associated d Ccarb polarity (þ or –) intervals. See Figure 21.3 for associated 13 87 86 chemostratigraphic characteristics (circled data sets: d CTOC, Sr/ Sr, [Sr] and chemical weathering index) that support the correlations. Numbers 1–3 correspond to the different segments used in constructing the composite stratigraphy of Figure 21.6. Abbreviations (A&B): Fm, formation; GCI, Greywacke–Conglomerate Interval, the lower prospective glacial interval in the Negash Synclinorium; grn, green; Ls, limestone; Mbr, member; Metavolc, metavolcanics; Mz, Mesozoic; Pz, Palaeozoic; Ss, sandstone; Undiff, undifferentiated. (Modified from Miller et al. 2009.) terranes comprising the greater Nubian Tokar superterrane et al. 2005). An unconformable basal contact also was postulated (Kro¨ner et al. 1991). from considerable lateral variations observed among basal The Nakfa terrane supracrustal assemblage records a lower Tambien metasediments (Arkin et al. 1971; Beyth 1972; Tadesse sequence of igneous rocks associated with arc magmatism in and 1999). Tambien Group deposition, including the two prospective around the southern ANS sector of the Mozambique Ocean, as glacial intervals, is thought to have occurred in an intra-oceanic exemplified by calc-alkaline plutons and associated metavolcanic platform setting (above a substratum of consolidated arc terranes) rocks of the Tsaliet Group. In western Tigre, the Tsaliet Group may following the main phase of Tsaliet arc magmatism in the region be preserved as a series of accreted volcanic arc terranes within a (c. 780 + 30 Ma; Avigad et al. 2007). The extent to which suprasubduction setting (Fig. 21.1; volcano-sedimentary blocks of Tsaliet arc subterranes were fully accreted prior to Tambien Tadesse et al. 2000). Cessation of Tsaliet arc volcanism was fol- Group deposition is uncertain, and some syndepositional relief lowed by deposition of weathered arc detritus (Sifeta et al. 2005) differentiation related to ongoing shortening and/or extension is and in turn increasing proportions of carbonate sediments, as possible (Miller et al. 2009). exemplified by the Tambien Group. The contact between the The entire Nakfa basement complex was deformed after depo- Tsaliet and Tambien Group is poorly understood, ranging from sition of the Tambien Group in conjunction with closure of the seemingly conformable and gradational to fault-bounded (Sifeta Mozambique Ocean between fragments of east and west Gondwana, 266 N. R. MILLER ET AL. and the concomitant structural emergence of the EAO, beginning area, splitting Beyth’s informal Tambien Group facies into the c. 630 Ma. As a consequence, NNE-trending upright/overturned basal Didikama Formation and overlying Shiraro Group folds and faults dominate the regional structural grain, with Tambien (Fig. 21.2a, column A). Designated within the latter were three Group exposures best preserved in synclinoria and grabens. Inter- metasiliciclastic formations (Enda Zebi, Dugub and Mentebtab nal minor or secondary folds and faults complicate measured sec- formations), each with respective lateral facies variations. tions in the Mai Kenetal and Negash synclinoria. Restoring In the eastern limb of the Negash Synclinorium, Beyth (1972) shortening (200%) associated with these collisional structures mapped the base of the Tambien Group as an interval of arkosic greatly extends the minimal aerial extent of the Tambien basin or sandstone and conglomerate, above a possible tectonic contact basins. The deformed Tsaliet-through-Tambien Group succession with the Astbi Horst (Fig. 21.1, Fig. 21.2a column C). This was subsequently intruded by c. 610 Ma (‘Mareb’) granitoids coarse metasiliciclastic interval corresponds to the lower prospec- associated with crustal thickening and differentiation of the matur- tive glacial interval, the Greywacke-Conglomerate interval (GCI) ing EAO. Whether or not, and for how long, Tambien Group depo- of the Didikama Formation. The basal contact of the GCI has not sition continued prior to emplacement of the ‘Mareb’ post-orogenic been studied in detail, but the underlying depositional sequence granitoids is unknown. Despite these post-depositional orogenic involves a minimum of several hundred metres of conformably processes, temperatures and pressures did not exceed low- bedded fine-grained marine metasedimentary units (mainly slate greenschist grade metamorphism and well-preserved primary sedi- with episodic thin carbonate interbeds up to a few centimetres mentary structures are common in many Tambien Group carbonate thick). This lower slate succession contrasts from coarse volcanic successions (Alene et al. 2006; Miller et al. 2009). agglomerate and conglomerate of the Tsaliet Group exposed at the Extensive erosion of the EAO resulted in cutting of a widespread top of the Atsbi Horst (Fig. 21.1), but the contact between these regional unconformity prior to Cambro-Ordovician time (Avigad units has not been systematically mapped or studied. Miller et al. 2005), herein termed the Afro-Arabian Peneplain (AAP). et al. (2003, 2009) include this lower slate interval (below the Uppermost Tambien Group exposures in the core of the Negash GCI) within the Tambien Group, as the informal Lower Slate Synclinorium (Matheos Diamictite) and possibly equivalent silici- member of the Didikama Formation (Fig. 21.2a, column D). clastic deposits of the Dugub Formation in the western Shiraro Above the GCI, the Didikama Formation transitions into varie- area (Fig. 21.1) are thought to be northern Ethiopia’s youngest gated slate that includes the first prominent dolomite beds (infor- preserved Neoproterozoic metasediments below the AAP. mal Slate . Dolomite member) and thicker bedded dolomite at the top of the formation (informal Dolomite . Slate member). The GCI and overlying purple slate (with green reduction spots; Stratigraphy the ‘Purple Egg Slate’ of Beyth 1972), with estimated combined thickness of 300 m, forms a distinctive dark (grey-purple) Beyth (1972) documented Tambien Group outcrops in the Shiraro marker unit that can be traced laterally throughout much of the area, in several en echelon synclinoria to the east (i.e. Mai Kenetal, c. 30-km-long exposed eastern synclinorial limb. Tsedia, Chemit and Negash), and the Escarpment area east of The Didikama Formation is overlain sharply, and very likely Adigrat (Fig. 21.1, study areas A–C). Apparent regional facies unconformably (Beyth 1972, p. 74; Garland 1980, p. 14), by dis- variations within the internal stratigraphy of the Tambien Group tinctive black limestone of the Matheos Formation. Miller et al. were described, and a preliminary correlation was proposed for (2009) subdivide the Matheos Formation into three members. three areas: Shiraro (west), Mai Kenetal (centre) and Negash The basal Black Limestone Member is well laminated and partly (east). Subsequent workers have modified the stratigraphy in detrital (intraclasts), and transitions upward into non-calcareous various ways as described below and shown in Figure 21.2a. slate (Transitional Member) and eventually pebbly slate (Diamic- Figure 21.2b shows a recent correlation scheme proposed for the tite Member). Tambien Group based on integrated litho- and chemostratigraphy of the Mai Kenetal and Negash synclinoria (Miller et al. 2009), which is further elaborated in the discussion section. Prospective glaciogenic deposits and associated strata Above the Tsaliet Metavolcanics, Beyth (1972) assigned four formational designations within the Tambien Group based on the Lower prospective glaciogenic unit Mai Kenetal type section (in stratigraphic order): Werii Slate, Assem Limestone, Tsedia Slate and Mai Kenetal Limestone. In Greywacke–Conglomerate Interval, lower Didikama Formation (Lower the Negash Synclinorium, Beyth defined informal facies (in strati- Tambien Group). Beyth (1972) described the GCI within the graphic order: greywacke and purple slate with green reduction eastern flank of the Negash Synclinorium as poorly sorted, light- spots, slate and dolomite, mica dolomite and slate, black detrital grey to black greywacke, containing mostly medium grained to limestone,andpebbly slate), which he suggested might correlate pebble-sized, subrounded quartz, in a mud (slate) supported with the Mai Kenetal section (Fig 21.2a, columns C v. B). matrix (Fig. 21.2a, column C). Thin conglomeratic layers and Dolomite-slate successions similar to those of the lower Tambien ripple marks (exposed in bedding planes) also occur within the Group in the Negash Synclinorium were subsequently documented unit. The unit fines upward into purple slate with distinctive in the Escarpment east of Adigrat, as well as in synclines in the ovate greenish-yellow patches, interpreted as reduction spots. Shiraro area. Garland (1980) formalized Beyth’s Tsaliet Metavolca- This lower interval (c. 300 m thick, Beyth 1972) is overlain by var- nics as the Tsaliet Group, and subdivided the Negash Tambien iegated slate (orange and green), which in turn grades upward Group succession (Fig. 21.2a, column D) into the lower Didikama (with poor outcrop) into the lowest exposed dolomite beds of the Formation (after similar dolomitic rocks in the Shiraro Area) and Didikama Formation. Similar purple spotted slate is reported overlying Matheos Formation (equivalent to Beyth’s informal below the Didikama Formation in the Shiraro area (Beyth 1972; black detrital limestone and pebbly slate facies). In mapping of Fig. 21.2a, column A). The GCI is underlain by well-layered the Mai Kenetal synclinorium, Tadesse (1999) used local names slate (with occasional sub-decimetre-thick carbonate interbeds to address the possibility that some of Beyth’s original type areas and minor foliation) that crops out continuously down-section (i.e. Werii Slate, Tsedia Slate) might reside in different volcano- (eastward) for at least several hundred metres in the hanging sedimentary arc terranes (Fig. 21.1). The main difference between wall flank of the Atsbi Horst (Fig. 21.1). the two lithostratigraphic schemes is differentiation of Beyth’s Tsedia Slate into a lower Logmiti Slate and overlying Bilato Lime- Upper Werri Slate (Lower Tambien Group). In the NW flank of the stone and Slate (Fig. 21.2a, column B). Tadesse (1999) also formal- Mai Kenetal Synclinorium, the Assem Limestone lies sharply ized a new Neoproterozoic stratigraphic scheme for the Shiraro and conformably above the Werii Slate and shares THE TAMBIEN GROUP, NORTHERN ETHIOPIA 267 sedimentological and isotopic characteristics of well-studied cap- rocks overlie carbonate units assigned to the Didikama For- carbonate sequences (i.e. abrupt basal lithological transition to mation (Tadesse 1999) and crop out within the Shiraro Block high-energy carbonate facies, basal dolomite bed (c. 1 m thick) (Fig. 21.1), a graben-like depression that is fault-bounded to transitioning upward into limestone, consistently negative the east against the Adi Hageray Block and extends westward 13 d Ccarb, primary and/or early diagenetic (pre-compaction) pris- beyond the studied area into Eritrea. The areally most extensive matic and radial fibrous cements (Miller et al. 2009, figs S2D– unit is the Dugub Formation, which we summarize following G; Hoffman et al. 2007)). The observation that many Cryogenian Tadesse (1999) and our own studies 4.5 km ENE of the town carbonate successions with negative d13C compositions are of Shiraro (Fig. 21.1). affiliated with glaciogenic intervals (e.g. Yoshioka et al. 2003; The Dugub Formation consists of weakly metamorphosed con- Halverson et al. 2005; Corsetti et al. 2007 and other arguments, glomerate, arkosic sandstone and siltstone (Tadesse 1999), which see discussion) raises the possibility that the pre-Assem Limestone are regionally intercalated at different scales. Well-preserved depositional sequence could have a glacial affiliation. The under- primary sedimentary structures are common as graded bedding, lying Werri Slate is mainly finely laminated, well-foliated, non- cross-lamination, ripple marks, and flame and slump structures. calcareous slate that does not exhibit obvious glaciogenic sedimen- Elliptical (rounded to subrounded) and well-sorted metaconglome- tological characteristics. However, the nature of the upper Werri rate clasts (8 cm) comprise up to 40% of the rock volume. Clast Slate and its contact with the Assem Limestone has not been compositions include low-grade (Tsaliet-like) volcanic rocks studied regionally and poorly sorted metasiliciclastic intervals (chlorite schist, tuffaceous metasediments), phyllite, granite, are documented, at least locally, in the Werri Slate. For example, quartz pegmatite, quartzite and chert. Fine- to medium-grained Beyth (1972) reported occurrences of greywacke within the chlorite, muscovite, feldspar and quartz are notable components Werii Slate regionally, and encountered poorly sorted rhyolitic of the metaconglomerate groundmass. Carbonate is notably agglomerate with well-rounded metre-scale quartzitic fragments absent, but is reported (as scarce marble layers a few to tens of at the southern end of Mai Kenetal syncline (around 138470N metres thick) in the underlying Enda Zebi Formation (Tadesse and 388510E). Greywacke is also observed near its gradational 1999). Dugub metasandstone contains quartz pebbles (up to a contact with Tsaliet Group (Alene et al. 2006). few millimetres across) and grades laterally and vertically into non-pebbly sandstone and siltstone. Cross bedding exhibits set heights ranging between 0.1 and 0.5 m; coset heights range up to Upper prospective glaciogenic unit 1.5 m. Orientations of asymmetrical ripple marks and slump structures suggest west-southwestward transport and palaeoslope Diamictite Member, Matheos Formation (Upper Tambien Group). The directions (Tadesse 1999; Avigad et al. 2007). Matheos Formation, comprising the interior of the Negash Syncli- norium, records a conformable transition from black limestone to diamictite. Ranging from faintly layered (massive) to well-bedded, Prospective glaciogenic deposits in Eritrea with near vertical dips, the c. 250-m-thick basal Black Limestone Member (Fig. 21.2a, column D) forms a distinctive ridge that The high-grade Arag terrane in NE Eritrea (c. 400 km NNW of rims the interior of the Negash Synclinorium. Individual beds Negash) has a widespread unnamed pebbly mudstone unit, consist- range in thickness from decimetre- to metre-scale and are typically ing of silicic clasts within an argillaceous-siliceous matrix (Verri finely laminated in thin section. Intraclastic grainstone intervals are 1909; Bibolini 1920, 1921, 1922). The unit is reported as widely common. Towards the synclinorial axis, limestone beds thin exposed between 178Nand178450N along its north-northwesterly (becoming sub-decimetre thick) and interstratify with increasing trend (c. 90 km) and penetrated by post-orogenic granitoids (acid proportions of light to dark grey phyllitic slate. The overlying c. aplitic intrusions, Cecioni 1981), which have not been dated. 100 m Transitional Member includes non-calcareous phyllitic Cecioni (1981) described the pebbly mudstone matrix as a hard, slate that passes upward into pebbly slate, without interbedded violet, clay-quartz cement, and the clasts as pebbles and boulders carbonate. The primary depositional fabric involves mainly fine of quartzite and silicified schist. Some of the small pebbles (,1 to 3 cm scale) horizontal beds. Initial disparate sedimentary (,1 cm) are pitted and knotted; others are angular. Bibolini clasts are sub-centimetre scale (compositions have not been (1920, 1921) noted the occurrence of faceted clasts and postulated studied). The overlying c. 200-m-thick Diamictite Member a glaciogenic origin. To our knowledge this is the earliest suggestion occupies the tightly folded core of the Negash Synclinorium and of a possible Late Proterozoic (‘algonkiano’) glaciation within the is distinguished by an overall upward increase in clast abundance ANS. Cecioni (1981) preferred a gravity flow origin for this unit, and size (typically ,10 cm, but up to 20 cm in diameter). Clasts but recognized a possible glaciogenic association due to the are matrix-supported and the finer matrix retains horizontal layer- occurrence of faceted pebbles. This unit trends more or less along ing, with some lateral pinching and swelling, throughout the strike with Tambien Group exposures in northern Ethiopia and member. Syn-orogenic compression of the synclinal core has prospective Bizen Domain equivalents in southern Eritrea, but has deformed the diamictite to varying extents. More pelitic parts of apparently not been further studied in relation to a glacial the diamictite display foliation and associated clasts are often some- association. what elongated with pressure shadows. Despite these superimposed The Neoproterozoic supracrustal succession of western Eritrea features, clasts that appear to preferentially deform underlying (just west of the Shiraro area in northern Ethiopia, Fig. 21.1) con- matrix laminae are relatively common. Diamictite clasts have sub- sists of volcano-sedimentary assemblages (Augaro Group) uncon- rounded, elongate and angular shapes (including some bullet-nosed formably overlain by basin-fill metasediments of the Gulgula clasts), and some planar clast surfaces may be striated (Miller et al. Group (Teklay et al. 2003; Teklay 2006). The Gulgula succession 2003). Clast lithologies are polymictic, including felsic volcanic involves greenschist-grade polymictic conglomerate, shale, phyllite rocks, fine-grained black limestone and dolomite (including well- with interstratified marble lenses, in addition to carbonaceous sand- rounded clasts that retain primary sedimentary structures; e.g. stone, quartz arenite and allodapic carbonate, which, like the oolite), low-grade semipelitic sediments, and rare volcanic con- Tambien Group, are deformed as upright, recumbent, and isoclinal glomerate consistent with the upper Tsaliet Group. folds. Teklay (2006) describes the polymictic conglomerates as clasts, both matrix- and clast-supported, ranging from a few centi- Dugub Formation, Shiraro Group (Upper Tambien Group). Conglo- metres to a half-metre of granite, slate, phyllite and epidotite. The merate and arkosic metasediments comprise youngest Tambien Gulgula Group merges southwards into the Shiraro block in north- Group exposures in the lowland Shiraro area of western Tigre ern Ethiopia, suggesting a composite Gulgula-Shiraro depositional (Shiraro Group of Tadesse 1999; Fig. 21.2a, column A). These basin and a lithostratigraphic affiliation with the Shiraro Group 268 N. R. MILLER ET AL.

(upper Tambien Group). The origin of Gulgula Group polymict data comprising most (c. 80%) analyses (Fig. 21.3). This compi- conglomerates, in addition to those described above in the lation reveals significant chemostratigraphic trends, from which Shiraro Group (Dugub Formation) require further study to clarify regional correlations are proposed (Fig. 21.2b, discussion). whether or not glacial processes were involved. Carbonate d13C compositions for the Tambien Group range between –8 and þ8‰ and can be characterized as fluctuating repeatedly upsection between positive and negative polarity Boundary relations with overlying and underlying (Fig. 21.3). Low-grade Bizen Domain stromatolitic carbonates non-glacial units in SE Eritrea, considered to be Tambien Group equivalents 13 (Beyth et al. 2003), are among the most negative d Ccarb values Lower prospective glaciogenic unit reported (range, –7.2 to –4.8‰; 3 cal, 5 dol) but sample preser- 13 vation was not critically assessed. Highest d Ccarb compositions, Greywacke-Conglomerate Interval (GCI) of the Didikama Formation, with maxima near þ7‰, derive from the Mai Kenetal Limestone Negash Synclinorium. Where examined in the eastern limb of the and Matheos Formation Black Limestone Member. The negative 13 Negash synclinorium, the GCI occurs within the Didikama For- d Ccarb compositions of the Assem Limestone (c. –1 to –4‰) 13 mation (Fig. 21.2a, columns C and D) above thick slate (Lower are similar to those comprising the lower negative d Ccarb interval Slate Member) and below variegated (purple/green/orange) in the Negash Synclinorium (c. –1 to –3‰) (despite their contrast- 13 slate that, in turn, grades upward into lowest dolomite beds of ing mineralogies), as well as negative d Ccarb intervals mapped as the Didikama Formation (Slate . Dol Member). The continuity Assem Limestone in the Chemit (–4.4 to –3.0‰) and Tsedia of the transition from the Lower Slate member to the GCI is uncer- (–4.5 to –0.7‰) synclinoria (Alene et al. 1999, 2006; Miller tain due to minor folding and possible faulting associated with the et al. 2009). Bizen Domain stromatolitic dolomites with negative 13 eastward structural transition to the Atsbi Horst (Fig. 21.1), d Ccarb (Beyth et al. 2003) may correlate with the negative 13 whereas the transition above the GCI appears to be continuous d Ccarb interval of the Didikama Formation. The Negash and conformable. carbonate sequence is so far unique in recording two negative 13 d Ccarb excursions. 18 Upper Werri Slate, Mai Kenetal Synclinorium. Miller et al. (2009) Carbonate d O compositions range between 0 and –16‰, with make chemostratigraphic arguments that the equivalent of the dolomite typically enriched (by 2–5‰) relative to stratigraphi- GCI may occur in the Mai Kenetal Synclinorium below the cally proximal (or prospectively equivalent) limestone. Although Assem Limestone, either within the upper Werii Slate or substantial scatter is apparent, median compositions are mainly between the Werii Slate and Assem Limestone as a paraconfor- between –4 and –10‰, similar to other Cryogenian datasets mity. The abrupt Werii Slate-Assem Limestone contact can be (Fig. 21.3; Jacobsen & Kaufman 1999; Robb et al. 2004; Halver- followed laterally in the field as well as traced (.13 km) in son et al. 2005, supplementary information). Values more negative satellite imagery. than –11‰ may be below the modal range of Cryogenian samples (Kaufman et al. 1993), and could be particularly altered. The Mai Kenetal Limestone and Matheos Formation Black Limestone 13 87 86 Upper prospective glaciogenic unit (units considered best preserved for d Ccarb and Sr/ Sr, see discussion) have median compositions between –7.6 and Matheos Formation Diamictite Member, Negash Synclinorium. The –3.5‰, with Matheos Black Limestones underlying the transition Matheos Formation Diamictite Member is mapped only within to diamictite deposition most enriched. 13 the folded core of the Negash Synclinorium. Within the Matheos Carbon-isotopic compositions of organic matter (d CTOC)in Formation, it overlies a distinctive but gradational transition the Tambien Group, principally from the Mai Kenetal and from black limestone (Black Limestone Member) to non- Negash synclinoria, are mainly in the range of –20 to –30‰, calcareous slate (Transitional Member). Although a direct with extremely light values (e.g. , –40‰) suggesting contri- contact has not been mapped, the oldest overlying sediments are butions from methanogenic biomass (Fig. 21.3). Both Mai Ordovician Enticho Sandstone, the base of which marks the AAP. Kenetal and Negash successions show similar stratigraphic enrich- 13 ment trends in d CTOC with pre-diamictite values in uppermost Dugub Formation, Shiraro Area. The Dugub Formation in the Matheos Formation limestones about 3‰ heavier than uppermost western Shiraro Area (Fig. 21.1) occupies a grossly similar Mai Kenetal Limestone exposures. stratigraphic position within the Tambien Group that could be The Tambien Group has a large compilation of 87Sr/86Sr com- equivalent to the Matheos Diamictite, but boundary relations for positions in studies by Miller et al. (2003, n ¼ 9; 2009, n ¼ 71) this low-lying unit are poorly known. and Alene et al. (2006, n ¼ 5). Results obtained from the same units are complementary among the studies despite contrasts in sample processing approaches. Least-altered 87Sr/86Sr compo- Chemostratigraphy sitions (normalized to SRM 987 ¼ 0.710240) are mainly between 0.7055 and 0.7068 (Fig. 21.3). The Mai Kenetal succes- Chemostratigraphic data for Tambien Group exposures in Ethiopia sion shows a stratigraphic enrichment trend that compares with a (Alene et al. 1999, 2006; Miller et al. 2003; Sifeta et al. 2005) and more extensive enrichment trend in the Negash succession. 13 87 86 likely equivalents in Eritrea (Beyth et al. 2003) have become The lower negative d Ccarb interval at Negash has Sr/ Sr available in only the last decade (Fig. 21.3). For the most part, che- compositions (avg: 0.706178 + 40, n ¼ 2) within error of those mostratigraphic surveys have been limited in terms of types of ana- for Assem Limestone (avg: 0.706175 + 14, n ¼ 14) 13 lyses performed and stratigraphic sampling frequency within a (Miller et al. 2009). Negative d Ccarb compositions in the given lithological unit, with fewer than 30 sample intervals Assem Limestone may initiate with 87Sr/86Sr compositions near considered in any one study (Fig. 21.3b). Miller et al. (2009) sub- 0.70597 (Miller et al. 2009). stantially expanded the regional chemostratigraphic database, Tambien Group carbonate units have Sr compositions ranging assessing 100 carbonate and 30 slate intervals from the Shiraro, up to about 4000 ppm, with pronounced stratigraphic enrichment Samre, Mai Kenetal and Negash regions. The integrated Tambien evident in both the Mai Kenetal and Negash synclinoria. Sr com- Group record now constitutes a significant chemostratigraphic data positions less than 1000 ppm typify limestone and dolostone in set for evaluating Cryogenian marine secular variations (including the Assem Limestone and Didikama Formation (also Didikama 71 stratigraphic intervals with 87Sr/86Sr measurements on Formation in Shiraro and Samre areas), whereas much higher con- least-altered units), with the Mai Kenetal and Negash synclinoria centrations averaging 2000–3000 ppm occur in the Mai Kenetal (a) H ABE RU,NRHR TIPA269 ETHIOPIA NORTHERN GROUP, TAMBIEN THE

(b)

13 18 13 87 86 Fig. 21.3. Summary of chemostratigraphic data sets for the Tambien Group. (a) Statistical summaries of chemostratigraphic (d Ccarb , d Ocarb , d CTOC , Sr/ Sr, Sr concentration and chemical weathering index) data 13 for Tambien Group lithostratigraphic units by locality and in stratigraphic order. Letters A–D mark stratigraphic units of alternating d Ccarb polarity; corresponding circled data sets indicate possible correlations between the Mai Kenetal and Negash synclinoria (black vertical bars) and other Tambien Group localities (see discussion); line types of circled data sets clarify vertical correlations. Black triangles indicate prospective glaciogenic intervals. Data compiled from Miller et al. (2009) and Alene et al. (2006, vein calcite samples excluded) are restricted to samples considered least altered for 87Sr/86Sr. Chemical Index of Weathering (CIA or PIA) data exclude samples containing lithic fragments or carbonate minerals. (b) Distribution of chemostratigraphic analyses for each study used to compile the upper graph statistical summaries. The histogram (top-to-bottom) bar order corresponds to analysis type, as listed from left to right under the ‘Total Analyses/Least Altered Analyses’ compilation. 270 N. R. MILLER ET AL.

Limestone and Matheos Formation Black Limestone Member. derivations from comparable juvenile crustal sources. Associated Intermediate Sr compositions (avg: 1198 + 91 ppm, n ¼ 2, chemical weathering indices (Fig. 21.3), which proxy the degree Miller et al. 2009) occur in the Tsedia Slate in the Mai Kenetal Syn- of weathering from fresh rock (50) to full clay conversion (100), clinorium. Similar transitional compositions are missing in the are generally between 70 and 90 for the Chemical Index of Altera- Negash Synclinorium and instead there is a sudden stratigraphic tion (CIA, Nesbitt & Young 1982) and .75 for the Plagioclase jump in Sr concentrations between the Didikama Formation dolo- Index of Alteration (PIA, Fedo et al. 1995). mite and directly overlying Matheos Formation Black Limestone. A range of Sr concentrations have been measured for matrix car- bonate (980 + 712 ppm, n ¼ 11; Miller et al. 2003) and carbonate Palaeolatitude and palaeogeography clasts in the Matheos Formation Diamictite Member (512 ppm, oolite cobble; Miller et al. 2009). There are no reliable palaeomagnetic constraints for Tambien The petrological and chemical transition from largely metavol- Group deposition within the southern ANS. Strike–slip displace- canic and metasiliciclastic (slate) units in the Tsaliet Group and ment and structural shortening associated with closure of the lower Tambien Group (Werri Slate) to predominant carbonate Mozambique Ocean and formation of the EAO constitute deposition in the higher Tambien Group (Assem Limestone and significant challenges for early Cryogenian palaeogeographic Didikama Formation) was investigated by Sifeta et al. (2005) in reconstructions within the ANS. the Werii area east of the Mai Kenetal Synclinorium (Fig. 21.1). Palaeogeographic reconstructions spanning the 775–660 Ma Metavolcanic rocks are sub-alkaline, with chemical fingerprints interval (Fig. 21.4a–c) generally place the ANS (as inferred that are compatible with island arc (primitive or evolved) and/ oceanic arcs) within the greater Mozambique Ocean between sub- or MORB settings. Lower Werri Slate compositions indicate sequently flanking Gondwana fragments (e.g. between India and

(a) (b)(c)

Au - Ma Au - Ma Continents with In c. 750 Ma Pacific Pacific palaeomagnetic Ka In Ka Au SC data In S Mozambique M EA Az Mozambique L Az Mozambique Co Adamaster Adamaster Ocean SF RP RP B La EAO Co Ka La

Adola SF Co RP Sah Am Braziliano SF Brasiliano Braziliano Ocean WA Postulated Am Schematic Am WA subduction WA continental zone collision 800 Ma 750 Ma 630 Ma After Meert 2003; Meert & Torsvik 2003 Collins & Pisarevsky 2005 (d) West Gondwana Tambien Group ( ) Mz-Cz Orogen ? Pz-Mz Orogen c. 550 Ma + + +

Pz Orogen +

+ Saharan + + AAP Neoproterozoic Orogen West Meta- ANS African + + craton Cration + + AAP + East Gondwana + + + + + O + + + Congo A

+ E Indian + + + + SF Craton +

~ + + Shield + + Amazonian Mad Australia c + + i t +

Craton + + + + + c SL

+ r + + + + + + +

W. + a + t

Kalahari n + A East + + Colour RP Craton + N

+ ~ + +

Antarctic + S online= +

Shield + + colour + Subduction Zone hardcopy Southern limit of Early Palaeozoic sandstone marking African-Arabian peneplain (AAP)

1000 km Modified: Meert & Lieberman 2008

Fig. 21.4. Global palaeogeographic reconstructions for (a) 800 Ma, (b) 750 Ma, (c) 630 Ma and (d) 550 Ma, showing the inferred palaeogeographic location and structural-tectonic setting of the ANS/Tambien Group (black star) during the Cryogenian Period. Prospective glaciogenic intervals in the Tambien Group were likely deposited during the earlier Cryogenian (.630 Ma) prior to the emergence of the East African Orogen (EAO). (a, b) Rifting and break-up of Rodinia (c. 900–750 Ma) was associated with sea-floor spreading, arc and back-arc basin formation, and terrane accretion in the Mozambique Ocean. (c) Accommodation space in the southern ANS basin likely inverted by 630 Ma in response to closure of the Mozambique Ocean between converging elements of West and East Gondwana and emergence of the EAO. Occurrence of undeformed post-orogenic intrusives of this age or older that puncture the deformed Neoproterozoic supracrustal sequence in Ethiopia and Eritrea, suggests that the Tambien Group at 630 Ma was likely deformed and uplifted within the EAO. (d) Location of the Tambien Group at c. 550 Ma within the context of Gondwana amalgamation and emergence of the Antarctic-EAO. The modern southern limit of Early Palaeozoic sandstone (superimposed from Avigad et al. 2005) documents the minimal extent of the Afro-Arabian Peneplain (AAP) and extent of regional uplift and Cambro-Ordovician erosion associated with the emergent EAO (Avigad et al. 2005). Reconstructions modified from (a) Meert 2003; Meert & Torskvik 2003; (b, c) Collins & Pisarevsky 2005; (d) Meert & Lieberman 2008; Grey et al. 2008. Abbreviations for cratons and continents: Am, Amazonia; AuMa, Australia/Mawson Block; Az, Azania; B, Baltica; Co, Congo/Tanzania/Bangweulu Block; EA, East Antarctica; Ka, Kalahari Block; La/L, Laurentia; In, India; Mad, Madigascar; RP, Rio de la Plata; Sah, Saharan Metacraton; SC, South China; SF, Sao Francisco; S, Siberia; WA, West Africa; Adola, Adamastor, Braziliano, Mozambique, Pacific, oceanic basins; Mz, Mesozoic, Cz, Cenozoic; Pz, Palaeozoic. THE TAMBIEN GROUP, NORTHERN ETHIOPIA 271

500 Enticho Ss (Ord) C detrital zircon 550 2 2 Fig. 21.5. Radiometric age constraints age distribution 2 Mai Post-orogenic plutons Mareb R. Kenetal bearing on the age of the Tambien Group Eritrea Sibta Shire 6 7 600 1 1 6 within the magmatic evolution of the 8 Ediacaran 9 Hawzien Negash southern ANS and EAO. Tsaliet arc 650 E. Sudan Youngest detrital zircons in upper magmatic products and ‘Mareb’ prospective glacial intervals Matheos Tambien post-orogenic pluton ages arranged 700 Dugub Fm Diamictite Group st according to their relative west-to-east (Shiraro) Zircons below 1 dolomite beds (Negash) Carbonate Age (Ma) 1 positions in Tigre. Magmatic equivalents 750 2 2 1 1 Deposition for localities in Eritrea and Sudan (open

Cryogenian 2 1 800 Eritrea Azeho Deset 2 Madahne BSS squares) are shown to the left. The Enticho NEOPROTEROZOIC5 4 PHZ Hawzien Alem Sandstone detrital zircon age spectrum 3 850 9 10 Chila Rama (Negash) (Avigad et al. 2007) shown at the left of the E. Sudan Tsaliet Group syn-tectonic granites and metavolcanics figure derives from the Enticho area as 900 Ton. shown in Figure 21.1. The shaded Relative W-to-E position (Tigre) horizontal bar shows the age range (827+6 to 770 + 7 Ma) for the Bitter Springs stage 13 STUDY DATE TYPE/METHOD (BSS) negative d Ccarb excursion of Australia (Halverson et al. 2005, 2007; and 1. Avigad et al. (2007) 6. Miller et al. (2003) Pb-Pb zircon U-Pb zircon references therein). Note that this range 2. Tadesse et al. (2000) 7. Asrat et al. (2004) Rb-Sr Zircon evaporation 3. Tadesse et al. (1999) 8. Teklay et al. (2001) overlaps with the main phase of Tsaliet Sm-Nd Youngest detrital zircon 4. Teklay (1997) 9. Kröner et al. (1991) syn-tectonic arc magmatism; see text for CHIME zircon (SHRIMP U-Pb) 5. Teklay et al. (2002) 10. Teklay et al. (2003) discussion.

Congo cratons). For example, Collins & Pisarevsky (2005) place Geochronological constraints ophiolite-bearing strata of the Adola Belt (southern Ethiopia) out- board of the Congo Craton (Fig. 21.4b). As the Adola Belt is gener- The age of Tambien Group deposition is bracketed by magmatism ally considered to mark thesouthern extent of the EAO, itis reasonable associated with formation of the underlying Nakfa basement to infer a similar outboard location for the Tambien Group. complex (Tsaliet Group) and orogenic thickening and differen- The earliest reliable regional palaeomagnetic data derive from tiation related to maturation of the EAO (‘Mareb’ granitoids). the time of Gondwana amalgamation (Fig. 21.4d); late Cryogenian Affiliated magmatism is widely dated throughout the ANS (e.g. (593 + 15 Ma) Dokhan volcanics of Egypt (Davies et al. 1980; Meert 2003; Johnson & Kattan 2007, and references therein). Wilde & Youssef 2000), interpreted to have a subtropical palaeo- Our geochronological review is restricted to southern ANS latitude (P-lat: 20.6 + 5.08, A95: 10.08, Trindade & Macouin studies in eastern Sudan (Kro¨ner et al. 1991), Eritrea (Teklay 2007; but see Nairn et al. 1987). The Dokhan palaeopole has 1997; Teklay et al. 2001, 2003; Andersson et al. 2006), northern been widely used in subsequent palaeogeographic reconstructions Ethiopia (Tadesse et al. 1997, 2000; Miller et al. 2003; Asrat (e.g. Meert 2003; Meert & Torsvik 2003; Macouin et al. 2004; et al. 2004; Avigad et al. 2007) and western Ethiopia (Ayalew Trindade & Macouin 2007). et al. 1990) closest to Tambien Group exposures and its prospec- Low subtropical palaeolatitudes (9–138) were also reported for tive equivalents (Figs 21.1, 21.5, Supplementary Table 21.1). Huqf Supergroup units in Oman thought to represent deposition Detrital zircon age spectra from mature Ordovician sand (Enticho during and after an upper Cryogenian (Fiq Formation) glaciation Sandstone) capping the AAP in Tigre are interpreted by Avigad et al. before 544 Ma (Kempf et al. 2000; Kilner et al. 2005), when (2007) to reflect the magmatic history of the regional Neoprotero- Oman was likely amalgamating with the ANS (detrital zircon zoic–Ordovician basement complex, as it was uplifted and eroded data in Rieu et al. 2007). Subsequent work (Rieu et al. 2006; within the EAO. The Enticho Sandstone zircon age distribution Allen 2007, and references therein) demonstrates that Mirbat (Fig. 21.5) reinforces that Neoproterozoic magmatism occurred in Group localities in both palaeomagnetic studies correspond to an two main episodes: (i) arc-related calc-alkaline magmatism older Cryogenian (Ghubrah-Ayn Formations) glaciation, con- c. 850–740 Ma (c. 780 Ma peak), largely related to petrogenesis strained radiometrically to be ,722 Ma and ongoing at of the Tsaliet Group, and (ii) post-orogenic magmatism c. 660– 711.8 + 1.6 Ma. The fact that palaeolatitudes for Oman units 580 Ma (c. 630 Ma peak) after Tambien Group deposition. associated with both glacial intervals (between c. 722 and Additional geochronological constraints elaborated below and 544 Ma) are similarly low, and also close to published c. 550 Ma shown in Figure 21.5 further suggest that the lower and upper palaeopoles from Gondwana, has yet to be fully reconciled with prospective glacial intervals within the Tambien Group were depos- the available geologic data (Allen 2007). However, even if low ited during the lull between these two magmatic episodes, with 775 palaeolatitudes are confirmed, Oman had yet to accrete with the to .660 Ma as a plausible depositional window (see Discussion). ANS at the time of the older Cryogenian (Ghubrah-Ayn For- mations) glaciation, and therefore these constraints could not be precisely extended to the Tambien Group. Pre-Tambien Group magmatism To the extent that the ANS occupied a gross latitudinal range similar to the Congo-Sa`o Francisco and/or East-Sahara cratons Deformed (syn-tectonic) plutons and metavolcanic rocks of the during the early Cryogenian (c. 750 Ma), as implied by various Tsaliet Group (lower calc-alkaline magmatic components of the palaeogeographic reconstructions (e.g. Fig. 21.4a,b; Trindade & Nakfa terrane) have been dated by a number of techniques (e.g. Macouin 2007, fig. 3a), the Tambien Group may have occupied Pb/Pb zircon, Rb–Sr, Sm–Nd, CHIME zircon, U–Pb zircon, low to intermediate latitudes (,458). Occurrence of evaporite zircon evaporation) with varying associated uncertainties (Sup- pseudomorphs in lower Didikama Formation dolomite (interpreted plementary Table 21.1 – Geochronological Data). These ages as ,774.7 + 4.8 Ma, Miller et al. 2009) may support a subtropical provide an indirect lower age limit for Tambien Group carbonate palaeolatitude, as palaeomagnetically constrained evaporite basins deposition. The oldest rocks in this region include an as old as 2.25 Ga have statistically significant volume-weighted 862 + 6 Ma granite clast within the Gulgula Group (Fig. 21.1, concentrations in the palaeo-subtropics (Evans 2006). Teklay et al. 2003), 854 + 3 Ma deformed volcanic rocks and 272 N. R. MILLER ET AL.

811 + 11 Ma granite from Eritrea (Teklay 1997). In the Axum continuity of the slate-to-carbonate transition, leading to negative 13 area (western Tigre), syn-tectonic pluton ages range between d Ccarb compositions above the lower prospective glacial interval 13 806 + 21 Ma and 756 + 33 Ma (Tadesse et al. 2000). A (GCI) in the eastern limb and positive d Ccarb compositions 784 + 14 Ma syn-tectonic granitoid occurs SSW of Hauzien above variegated slate in the western limb (see Miller et al. 2009 (Fig. 21.1; Avigad et al. 2007). A conformable felsic interval for details). The available regional data suggest that Tambien 13 shortly (c. 17 m) below initial dolomite beds of the Didikama For- Group carbonate deposition began with positive d Ccarb compo- mation, in the western limb of the Negash Synclinorium (Madahne sitions (interval A). For Negash this interval (c. 250 m thick) in Alem), produced a zircon U–Pb date of 774.7 + 4.8 Ma (Avigad the western limb may crop out within an east-verging thrust et al. 2007; Figs 21.4 and 21.5). block, whereas the interval in the eastern limb may be faulted out or yet unrecognized below the GCI. The nature and continuity of the transition from Tsaliet Group agglomerates to lower Didi- Post-Tambien Group magmatism kama Formation slate and dolomite remains poorly understood and a systematic stratigraphic transition from lower positive (inter- 13 Post-orogenic ‘Mareb’ granitoid plutons (Fig. 21.1) penetrate the val A) to higher negative (interval B) d Ccarb compositions has yet older deformed Neoproterozoic complex (including the Tambien to be documented in a conformable sequence. Based on similar Group) throughout northern Ethiopia and Eritrea. In Tigre, ages lithostratigraphic position and character (slate with negligible car- for these commonly rounded and undeformed intrusives range bonate content and advanced chemical weathering indices; from 613.4 + 0.9 Ma (Mai Kenetal Granite, Avigad et al. Fig. 21.3), the informal lower slate member of the Didikama 2007) to 545 + 24 Ma (Mareb Granite, Tadesse 1997). The Formation could be equivalent to the Werii Slate and these units Negash Pluton, just west of the Negash syncline, is c. 606 Ma are tentatively included in polarity interval A. (606.0 + 0.9 Ma, Miller et al. 2003; 607 + 7 Ma, Asrat et al. The lower prospective glacial interval in both sequences 13 2004). A small granitoid body puncturing the western Negash underlies a lower carbonate interval with negative d Ccarb (interval limb (lower Didikama Formation) is interpreted to have a compar- B) that is in turn overlain by a distinctive black limestone succession 13 able age, but did not render suitable zircons for geochronology with the most enriched d Ccarb compositions of the entire Tambien (Miller et al. 2009). Ages of post-orogenic intrusives are somewhat Group (interval C; Fig. 21.2b). Polarity interval B and C units in et al. 13 87 86 older in Eritrea (628 + 4 Ma, 622 + 1 Ma; Teklay 2001) and both localities have highly complementary d CTOC, Sr/ Sr and Sudan (SE of Tokar: 652 + 14 Ma, Kro¨ner et al. 1991). The good Sr compositions that support their regional correlation (Fig. 21.3). agreement between individually dated syn-tectonic and post- The lithological and chemostratigraphic transition from interval B orogenic magmatic products with corresponding modal peaks in to C appears to be continuous in Mai Kenetal, whereas this transition the Enticho Sandstone zircon age distribution (Fig. 21.5), and in Negash is abrupt and likely associated with an unconformity the lack of detrital zircons younger than c. 739.2 + 6.3 Ma in (Miller et al. 2009). Above the black limestone interval in the 13 either of the prospective glaciogenic units capping the Tambien Negash Synclinorium is the second ‘upper’ negative d Ccarb excur- Group (Dugub Fm, Matheos Diamictite) are strong evidence that sion (interval D) associated with a conformable transition to the the Tambien Group is older than EAO post-tectonic granitic upper prospective glacial unit (Matheos Diamictite). rocks (Avigad et al. 2007). In the vicinity of Mai Kenetal and Negash, this constraint is c. 610 Ma, but its upper age is likely much older considering that regional deformation and crustal Composite chemostratigraphic reference section thickening must have preceded the ‘Mareb’ intrusives. The oldest dated post-tectonic pluton in the region (652 + 14 Ma; Figure 21.6 presents a composite chemostratigraphic reference Kro¨ner et al. 1991) and Enticho sandstone detrital zircon record section for the Tambien Group based on the correlations between (initiation of the second magmatic phase at c. 660 Ma; Avigad the Mai Kenetal and Negash synclinoria shown in Figure 21.2b et al. 2007) suggest that the Tambien Group may have been depos- and the reasoning above. The Mai Kenetal section above the Werii ited prior to c. 660 Ma (Fig. 21.5). Slate (Assem Limestone, Tsedia Slate and Mai Kenetal Limestone) essentially replaces the Negash sequence above the basal positive 13 d Ccarb interval of the lower Didikama Formation and below Discussion (or within) the lower portion of the Matheos Formation Black Limestone Member. This composite effectively replaces dolomite Regional chemostratigraphic context of the prospective lithologies below a probable unconformity (Negash) with a continu- glacial intervals ous and conformable succession of limestone lithologies; these seem to show gradational chemostratigraphic trends into highly enriched The observation that many Tambien Group exposures in Tigre values that correlate well with the lower Matheos Formation Black exhibit a similar succession of lithofacies that have comparable che- Limestone Member (Miller et al. 2009). The abrupt transition mostratigraphic characteristics suggests that the Tambien Group pre- from positive (polarity interval A) to negative (polarity interval 13 serves a regionally coherent chemostratigraphic framework (Beyth B) d Ccarb compositions may be a structural contact, but we 13 1972; Alene et al. 2006; Miller et al. 2009). Tambien Group deposi- suggest that the relative stratigraphic order of d Ccarb polarity is tional history, including the depositional context of the prospective maintained in the composite. The 87Sr/86Sr evolution of Cryogen- glaciogenic intervals, can be effectively considered relative to ian seawater (albeit poorly delineated) involves a general trend of 13 87 86 stratigraphic changes in the polarity of associated d Ccarb.These increasing Sr/ Sr (Halverson et al. 2007). That least altered relationships are best demonstrated in the Mai Kenetal and Negash samples from polarity interval A dolomite beds have lowest synclinoria (Figs 21.2b, 21.3) but are consistent with the stratigraphic 87Sr/86Sr compositions is consistent with early Cryogenian depo- succession in other areas (e.g. Shiraro and Samre). sition within the Tambien Group carbonate platform. The evolution The carbonate successions in the Mai Kenetal and Negash syn- of Tambien Group depositional environments is now considered clinoria both initiate above thick marine slate sequences; however, based on the composite reference section (Fig. 21.6). the nature of this transition to carbonate deposition differs in each locality. In Mai Kenetal the transition from Werii Slate to the Assem Limestone is a sharp conformable or paraconformable Palaeoenvironmental changes contact, the Assem Limestone beginning with and maintaining 13 negative d Ccarb compositions. In Negash, faulting in the lower The Tambien Group involves mainly a transition from weathered portion of each synclinorial limb interrupts the stratigraphic Tsaliet arc detritus (lower slate intervals; Sifeta et al. 2005) to THE TAMBIEN GROUP, NORTHERN ETHIOPIA 273

d13 Enticho Ccarb Sr (ppm) TOC Sandstone -8 -40 4 8 02000 4000 0.0 0.1 1.0 2000 (d) oolite matrix cobble IntervalInterval ddevoidevoid ooff bbeddededded carbonatecarbonate

1800 Tr

1600 3

1400 LS

MK (c)

1200 Bilato

1000 Logmiti Black LS Diam. Tsedia SlateTsedia Fm Matheos

800 Tambien Group Tambien

Stratigraphic Height (m) Stratigraphic (b) MTS 600 LS Assem CC ? 2 ? 400 >Dol Slate (a)

200 774.7±4.7 Ma

Slate ExtremeExtreme wweatheringeathering iindicesndices ((PIA:PIA: 992-99)2-99) Lower 1 0 Tsaliet Gp (Meta- 0.705 0.706 0.707 -15 -10 -50 -45 -35 -25 volcanics) 87 86 d18 d13 Sr/ Sr Ocarb CTOC

MTS CC Sheet crack cements Evaporite pseudomorphs Stromatolites Internal slump Molar tooth structures Probable dropstones Cap carbonate-like textures

Fig. 21.6. Composite chemostratigraphic reference sequence for the Tambien Group based on the Mai Kenetal and Negash synclinoria. Segments 1–3 denote the basis for constructing this composite sequence (cf. Fig. 21.2b) and the black triangles denote the prospective glacial intervals. The composite sequence is differentiable by 13 four changes (a–d, separated by horizontal dashed lines) in the polarity of associated d Ccarb and consists of limestone units except for segment 1 (dolomite comprising the lower portion of the Didikama Formation in the western limb of Negash). The lower horizontal shaded interval indicates the stratigraphic range of slates having extreme chemical weathering indices (PIA, plagioclase index of alteration of Fedo et al. 1995). The upper horizontal shaded interval corresponds to the upper portion of 13 the Matheos Formation lacking bedded carbonate; the darker grey d Ccarb field in the Matheos Diamictite shows the range for diamictite matrix carbonate (Miller et al. 2003), whereas the uppermost data point corresponds to an oolite cobble (Miller et al. 2009). Abbreviations: Ls, Limestone; Dol, dolomite; Tr, transitional, Diam-Matheos diamictite; MK, Mai Kenetal. predominant carbonate deposition in a marine arc-accretion plat- associated with polarity interval A are interpreted to have been form setting. The lack of obvious ash beds, volcaniclastic intervals deposited in high alkalinity tidal flat and intertidal settings. Mod- and syn-tectonic intrusive rocks in the main carbonate succession erate energy shallow intertidal or subtidal settings are interpreted (polarity interval B and higher) suggests deposition during a mag- for the Assem Limestone and upper Didikama Formation matically quiet interval (Avigad et al. 2007), and the upward (polarity interval B) on the basis of prominent domal and interdi- waning of slate deposition in favour of carbonate may represent gitate stromatolites, coarse rip-ups, and cross-bedded grainstones a phasing out of arc magmatic activity in the region. (Miller et al. 2009, figs S2, S3). The base of the Assem Lime- Chemical weathering indices of slates in polarity interval A stone occurs abruptly above the Werii Slate as a metre-thick suggest an overall stratigraphic trend of increasing chemical weath- dolomite bed, with sedimentary features similar to those ering of source areas to extreme levels, which lessened somewhat described for well-studied transgressive cap-carbonate sequences. prior to deposition of polarity interval B (Fig. 21.6). The association Lower carbonate units of the Tambien Group (polarity intervals 87 86 13 of extreme weathering indices with increasing Sr/ Sr is consist- A–B) have low TOC contents with highly variable d CTOC, ent with an interval of rapid chemical weathering of arc terranes in including light compositions, which together with the sedimen- the southern ANS portion of the Mozambique Ocean. The fine tary characteristics are consistent with nearshore environments grained nature of slates and absence of shallow water indicators (Miller et al. 2009, figs 7B,C and 9). suggests a moderately deep-water depositional setting. Deeper subtidal environments with lower (but still variable) Above the lower slate intervals, the Tambien Group carbonate energy levels and ubiquitous micrite are interpreted for upper pile is broadly differentiable between lower carbonate units carbonate units of the Tambien Group (polarity interval C). (polarity intervals A–B) suggestive of shallow marine deposi- The transition appears to begin at the base of the Tsedia Slate in tional settings and upper carbonate units (polarity interval C) sug- Mai Kenetal (equivalent to the proposed unconformity between gestive of deeper marine environments. Based on the occurrence the Didikama and Matheos Formations in Negash), and this bound- of evaporite pseudomorphs, sheetcrack cements, microbialami- ary could mark the base of a transgressive system following nates and stromatolites (Miller et al. 2009, fig. S5), carbonates regression. Both the Mai Kenetal Limestone and the Matheos 274 N. R. MILLER ET AL.

Formation Black Limestone are distinctive dark grey to black lime- 827 Ma), are substantially older than ages determined by Rb–Sr stone units characterized by fine horizontal layering and high lateral whole rock methods (c. 770–670 Ma). The younger ages have continuity (Miller et al. 2009, figs S3 and S6). The lack of stroma- been interpreted to reflect isotopic disturbance due to regional tolites and high-energy sedimentary structures (coarsely graded deformation of the arc complex, possibly as it accreted with the beds, cross-bedding) but occurrence of dark intraclastic intervals African continent (Kro¨ner et al. 1991; Saharan Metacraton?). and intraformational slumping may indicate slope deposition Structural deformation of the orogen may thus have begun below storm wave base. These upper carbonate units are relatively before 670 Ma. As this deformation was post-depositional and pre- 13 enriched in TOC that has less variable d CTOC, consistent with ceded the first post-orogenic intrusives (652 + 14 Ma) by tens of more open marine environments (Miller et al. 2009, fig. 7B,C). millions of years, the Tambien Group minimum age, and the age 13 Upper carbonate units are also most enriched in d Ccarb and of the upper prospective glacial interval, could be appreciably 13 d CTOC, particularly in the upper part of the Matheos Formation older than 652 + 14 Ma. Black Limestone Member. This mutual stratigraphic enrichment pattern, together with distinct stratigraphic increases in 87Sr/86Sr and Sr concentrations, is consistent with high rates of organic Evidence for glacial influence on sedimentation matter burial and related fractionation of the upper (photic) 13 marine d CDIC pool in the lead up to Matheos Diamictite depo- The lower prospective glacial interval of the Tambien Group, 18 sition. Although d Ocarb is the marine proxy most likely to as the GCI in Negash and a possibly equivalent interval or paracon- undergo post-depositional alteration, it is notable that the intervals formity at the top of the Werri Slate is most controversial. with most enriched compositions (consistent with cryogenic Direct sedimentologic evidence of syn-glacial deposition (i.e. sequestering of d16O) occur in association with the two prospective definitive dropstones, striated clasts) is so far lacking, with the glacial intervals (Fig. 21.6). The possibility that d18O records fluc- possible exception of matrix supported horizons in the GCI tuations in ice volume is more likely for the upper prospective (Beyth 1972). On the other hand, both of these intervals are over- 13 glacial interval because the lower prospective glacial interval is lain by negative d Ccarb carbonates with similar chemostrati- underlain by dolomite, which is typically enriched by 2–4‰ graphic characteristics. Where the base of this carbonate interval over coeval limestone (Jaffre´s et al. 2007, and references therein). is well exposed (Assem Limestone, Mai Kenetal) it exhibits fea- tures consistent with well-documented Cryogenian post-glacial cap carbonates. Timing of the prospective glacial intervals Alene et al. (2006) discounted a glacial association for the Assem Limestone, but did not document or sample the base of Integration of available geochronological information suggest c. this unit above its abrupt contact with Werri Slate. They suggested 775–660 Ma as a plausible window for Tambien Group deposition the Assem Limestone may correspond to a non-glacial negative 13 bracketing the two prospective glacial intervals. This depositional d Ccarb interval possibly equivalent to the Bitter Springs stage window is not definitive, but corresponds to the lull between (BSS) of Australia (as suggested by Halverson et al. (2005, 13 Tsaliet arc magmatism and later magmatism associated with matu- 2007) for similar negative d Ccarb intervals within early Cryogen- ration of the EAO, on the basis of regional stratigraphy and cross- ian sections from NW Canada (Little Dal Group) and Svalbard cutting relationships. Direct dates on magmatic products from each (Akademikerbreen Group)). The age for the BSS in Australia magmatic episode vary locally within Tigre but concur with the requires interbasinal correlations that are difficult to confirm, and detrital zircon age distribution from the immediately overlying the age range has a corresponding large degree of uncertainty Ordovician Enticho Sandstone (Fig. 21.5). Somewhat older arc between 827 + 6 Ma (Gairdner Dyke Swarm) and 777 + 7Ma magmatism is indicated in Eritrea and eastern Sudan. (Boucat Volcanics) among various studies (Halverson et al. The 774.7 + 4.8 Ma date obtained from zircons recovered from 2005, 2007). Although a correlation with the BSS is possible, the variegated (tuffaceous?) slate, c. 17 m below the first positive c. 827–770 Ma age range would correspond to a time of active 13 d Ccarb (polarity interval A) dolomite beds of the Didikama For- arc magmatism in the southern ANS (Fig. 21.5). The Assem Lime- mation, is a possible maximum age constraint for significant stone and higher Tambien Group succession lacks evidence of (bedded) carbonate deposition in the Tambien Group (Fig. 21.5). active magmatism (ash beds/slate intervals/syn-tectonic intru- The interval was originally interpreted as either a volcaniclastic sives) suggesting it was deposited after the c. 780 Ma acme of interval or sill injected into a volcaniclastic interval. Subsequent arc magmatism in the region. The lower Negash negative 13 petrographic analysis of this interval revealed probable sedimen- d Ccarb interval occurs several hundred metres above the varie- tary textures, including rounded quartz grains and a large (3 cm) gated slate interval dated at 774.7 + 4.8 Ma (zircon). This 13 rounded quartzose clast. If this date holds, the lower prospective suggests that the lower negative d Ccarb interval could be appreci- glacial interval occurring at least c. 250 m higher is likely to be ably younger than c. 775 Ma. As there are definitively younger appreciably younger. The upper prospective glacial interval is post-glacial cap carbonates (e.g. above the c. 755 Ma Kaigas For- older than deformation and later magmatism associated with the mation, southern Namibia, Gariep Belt, Hoffmann et al. 2006) EAO. The oldest indication of EAO magmatic activity in the we suggest that a lower prospective glacial association within southern ANS is from a 652 + 14 Ma post-orogenic pluton in the Tambien Group remains a viable interpretation. It is conceiva- eastern Sudan (southern Red Sea Hills, c. 500 km NNW of ble that the glaciogenic association could be indirect. For example, 13 Negash; Kro¨ner et al. 1991), whereas locally dated ‘Mareb’ gran- the cap-carbonate features and negative d Ccarb excursion could itoids in Tigre are c. 610 Ma (avg: 609.8 + 3.8 Ma, Mai Kenetal, be products of enhanced upwelling associated with global Negash, Hauzien plutons). cooling and/or vigorous circulation associated with post-glacial The dated Red Sea Hills locality in eastern Sudan occurs climate change in a setting that did not earlier accumulate glacially c. 50 km NW of the area of prospective glacial deposits (pebbly transported clastic sediments. Both of these intervals in the Mai mudstones with flattened clasts) in northern Eritrea (Fig. 21.1) Kenetal and Negash synclinoria warrant additional study to (Cecioni 1981). If the pebbly mudstones are glaciogenic and evaluate glacial associations. correlative with the upper prospective glacial interval of the Of the proposed upper glaciogenic intervals for the Tambien Tambien Group, the Red Sea Hills age offers an important Group, the Matheos Formation Diamictite Member best exhibits minimum age constraint for the Tambien Group. The most sedimentological characteristics consistent with glacially influ- precise age estimates on older (pre-pebbly mudstone) associated enced deposition. In addition to matrix supported polymict arc metavolcanic and plutonic rocks in the Red Sea Hills area, clasts, clasts that appear to preferentially deform underlying obtained by single zircon 207Pb/206Pb methods (c. 870– bedding are interpreted as ice-rafted debris. Some bullet-nosed THE TAMBIEN GROUP, NORTHERN ETHIOPIA 275 and possibly striated clasts are consistent with mechanical abrasion Pan-African orogens: implications for global environment. Earth during glacial entrainment (Kuhn et al. 1993). This interval and Planetary Science Letters, 240, 818–826. is further significant because of its gradational contact with the Avigad, D., Stern, R. J., Beyth, M., Miller,N.&McWilliams,M. underlying Matheos Black Limestone Member, which records a 2007. Detrital zircon U–Pb geochronology of Cryogenian diamictites 13 87 86 and Lower Palaeozoic sandstone in Ethiopia (Tigrai): age constraints decline in d Ccarb and possibly also Sr/ Sr, prior to diamictite deposition. If the latter is not the product of diagenetic alteration, on Neoproterozoic glaciation and crustal evolution of the southern Arabian–Nubian Shield. Precambrian Research, 154, 88–106. the declining Sr-isotope compositions could signal a strong Ayalew Bell Moore Parrish decrease in continental weathering input relative to oceanic , T., , K., ,J.M.& , R. R. 1990. U-Pb and hydrothermal input, as consistent with increasing ice cover. Rb-Sr geochronology of the Western Ethiopian Shields. Geological Society of America Bulletin, 102, 1309–1316. The possibly equivalent Dugub Formation in the Shiraro area Beyth, M. 1972. The geology of central western Tigre, Ethiopia. PhD occupies a gross stratigraphic position similar to the Matheos thesis, Bonn, University of Bonn. Diamictite as well as similar detrital zircon age distributions Beyth, M., Avigad, D., Wetzel, H. U., Matthews,A.&Berhe,S.M. (Avigad et al. 2007), but the underlying stratigraphy is poorly 2003. Crustal exhumation and indications for snowball Earth in the known. Palaeocurrent proxies suggest a north-northeastern East African Orogen: North Ethiopia and East Eritrea. 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