Depositional environments during the Late Palaeozoic ice age (LPIA) in northern Ethiopia, NE Africa

Accepted Manuscript Depositional environments during the Late Palaeozoic ice age (LPIA) in northern Ethiopia, NE Africa Robert Bussert PII: DOI: Reference:

S1464-343X(14)00096-X http://dx.doi.org/10.1016/j.jafrearsci.2014.04.005 AES 2017

To appear in:

African Earth Sciences

Received Date: Revised Date: Accepted Date:

12 August 2013 2 April 2014 7 April 2014

Please cite this article as: Bussert, R., Depositional environments during the Late Palaeozoic ice age (LPIA) in northern Ethiopia, NE Africa, African Earth Sciences (2014), doi: http://dx.doi.org/10.1016/j.jafrearsci.2014.04.005

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Depositional environments during the Late Palaeozoic ice age (LPIA) in northern

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Ethiopia, NE Africa

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Robert Bussert1 1

FG Explorationsgeologie, Technische Universität Berlin, Sekr. ACK 1-1, Ackerstraße 76, D-

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13355 Berlin, Germany, [email protected]

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ABSTRACT

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The Late Palaeozoic sediments in northern Ethiopia record a series of depositional

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environments during and after the Late Paleozoic ice age (LPIA). These sediments are up to

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200 m thick and exceptionally heterogeneous in lithofacies composition. A differentiation of

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numerous types of lithofacies associations forms the basis for the interpretation of a large range

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of depositional processes. Major glacigenic lithofacies associations include: 1) sheets of

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diamictite, either overlying glacially eroded basement surfaces or intercalated into the sediment

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successions, and representing subglacial tillites, 2) thick massive to weakly stratified muddy

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clast-poor diamictites to lonestone-bearing laminated mudstones originating from a

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combination of suspension settling of fines and iceberg rainout, 3) lensoidal or thin-bedded

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diamictites deposited from debris flows, 5) wedges of traction and gravity transported coarse-

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grained sediments deposited in outwash fans, 6) irregular wedges or sheets of mudstones

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deformed primarily by extension and incorporating deformed beds or rafts of other lithofacies

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formed by slumping, and, 7) irregular bodies of sandstone, conglomerate and diamictite

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deformed by glacial pushing.

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The dominance of laminated or massive clast-bearing mudstones in most successions indicates

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ice-contact water bodies as the major depositional environment. Into this environment, coarse-

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grained sediments were transported by various gravity driven transport processes, including

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dropstone activity of ice-bergs, slumping, cohesive debris flow, hyperconcentrated to

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concentrated flow, hyperpycnal flow, and by turbidity flow. Close to glacier termini, wedge-

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shaped bodies of conglomerate, sandstone, diamictite and mudstone were deposited primarily

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in subaqueous outwash-fans. Soft-sediment deformation of these sediments either records ice

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push during glacier advance or re-sedimentation by slumping. Apart from an initial glacier

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advance when thick ice of temperate or polythermal glaciers covered the whole basin, many

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sections document at least a second major phase of ice advance and retreat, and some sections

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additional minor advance-retreat cycles.

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Whether most of the LPIA sediments in northern Ethiopia were deposited in lakes or in fjords

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is not yet clear. Although univocal evidence of marine conditions is missing, the presence of

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carbonate-rich beds and the trace fossil assemblage are compatible with a restricted marine

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environment such as broad palaeofjords affected by strong freshwater discharge during

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deglaciation. A restricted marine environment for most of the sediments in northern Ethiopia

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could challenge models of the LPIA sediments in Arabia as primarily glaciolacustrine and

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glaciofluviatile deposits.

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Key words: Late Palaeozoic ice age (LPIA), Gondwana glaciation, northern Ethiopia,

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subglacial tillites, subaqueous outwash fans, push moraines

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1 Introduction

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During the Late Palaeozoic ice age (LPIA), referred to as “Earth’s best known pre-Quaternary

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glaciation” (Soreghan et al. 2011), glaciers successively covered vast areas of Gondwana while

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the supercontinent migrated across the South Pole (DuToit, 1921; Fielding et al., 2008a; Henry

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et al., 2012). In many parts of Gondwana, extent and dynamic of the glaciation are

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documented by erosional palaeolandforms and glacigenic sediments. Additionally, a number of

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sedimentary successions record the termination of the ice age and the transition to greenhouse

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conditions (e.g. Visser, 1997; Scheffler et al., 2006). Although glaciers apparently affected

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large parts of Gondwana, indicators of terrestrial glaciation such as glacially eroded basement

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surfaces and terrestrial tillites are relatively rare (Eyles et al., 2006), with noticeable exceptions

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in Oman (Martin et al., 2012) and Australia (Huuse et al., 2012). In contrast, the LPIA

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sedimentary record is dominated by subaquatic, primarily glaciomarine sediments, very likely

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because of their higher preservation potential when compared to terrestrial glacigenic deposits

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(e.g. Eyles, 1993; Eyles and Lazorek, 2007). Nonetheless, Late Palaeozoic glaciofluviatile and

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glaciolacustrine deposits occur in South America (e.g. Tomazelli and Junior, 1997; Netto et

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al., 2009), Australia (e.g. Jones and Fielding, 2004; Fielding et al., 2008b), Antarctica (e.g.

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Miller, 1989; Woolfe, 1994), East Africa (e.g. Wopfner and Kreuser, 1986), India (e.g.

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Casshyap and Tewari, 1982; Sen and Banerji, 1991) and Arabia (e.g. Melvin et al., 2010;

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Martin et al., 2012).

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On the Arabian Peninsula, LPIA glaciers appeared first during the middle Carboniferous (Al-

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Husseini, 2004), considerably before the LPIA peaked and glaciers reached maximum extent

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across Gondwana, during the Late Pennsylvanian to Early Permian (Isbell et al., 2003; Fielding

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et al., 2008a). Also in Arabia, ice remained present until the Early Permian (Levell et al., 1988;

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Melvin and Sprague, 2006; Martin et al., 2008, 2012; Keller et al., 2011). In this region, LPIA

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glaciers left behind predominantly glaciolacustrine sediments (Braakman et al., 1982; Kruck

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and Thiele, 1983; Melvin et al., 2010; Martin et al., 2012) but additionally outwash fan and

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sandur deposits; the latter form productive hydrocarbon reservoirs in Oman and Saudi Arabia

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(Levell et al., 1988; Melvin and Sprague, 2006; Le Heron et al., 2009; Martin et al., 2012).

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LPIA deposits occur also in northern Ethiopia, yet these are, in comparison to their

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counterparts in Arabia, relatively poorly studied – despite their potential for an improved

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reconstruction of the extent and nature of the LPIA in Africa (e.g. Visser, 1997; Wopfner,

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2002). As a first step forward, this study tries to 1) interpret the depositional conditions of the

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glacigenic sediments in northern Ethiopia, 2) analyse the palaeotopography of the glaciated

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basin, and 3) compare the LPIA record in northern Ethiopia with sedimentation models

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recently proposed for LPIA sediments on the Arabian Peninsula (Melvin and Sprague, 2006;

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Melvin et al., 2010; Keller et al., 2011; Martin et al., 2012). It is based on the investigation of

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sections in two major outcrops areas in the Tigray Province of northern Ethiopia, the Mekelle

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Basin, and the Adigrat-Adua Ridge (Fig. 1). To the north, outcrops of LPIA sediments reach

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near to the Ethiopian-Eritrean border. They possibly extend into Eritrea, yet all glacigenic

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deposits studied by Kumpulainen et al. (2006) and Kumpulainen (2007) in Eritrea are

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interpreted as being Upper Ordovician in age. The question of the extension of LPIA sediments

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and ultimately of LPIA glaciers in NE Africa might be solved by cross-border field work

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linking both outcrop areas. Unfortunately during the last decade, border dispute between

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Ethiopia and Eritrea has prevented such studies.

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Lithofacies codes used in this study follow, with minor modifications, the classification

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schemes of Eyles et al. (1983) and Benn and Evans (1998). The petrography of the sediments

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has been investigated by standard XRD and thin section analyses.

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2 Geological setting

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The LPIA deposits in northern Ethiopia belong to an approximately 2000 m thick succession

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of Phanerozoic sediments that overlay Neoproterozoic basement, primarily Pan-african

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metavolcanics and metasediments intruded by granitoid rocks (Fig. 1) (Beyth, 1972a; Beyth et

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al., 2003; Miller et al., 2003; Alene et al., 2006). Following a period of extensive peneplanation

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in the latest Neoproterozoic to earliest Palaeozoic, the region alternated for most of the

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Palaeozoic and Mesozoic between periods of slow subsidence and sedimentation and periods

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dominated by erosion or non-sedimentation. Post-Oligocene uplift and subsequent erosion of

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the Ethiopian dome has resulted in modern-day exposure of the complete Phanerozoic section.

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In northern Ethiopia, LPIA sediments are exposed in two major regions, along the margin of

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the Mekelle Basin and at the foot of the Adigrat-Adua Ridge. The Palaeozoic sediments in

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northern Ethiopia were first differentiated by Dow et al. (1971) and Beyth (1972a,b) into the

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sandstone-dominated Enticho Sandstone and the mudstone-rich Edaga Arbi Glacials,

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interpreted by these authors as correlative glacigenic deposits. In a first attempt to date the

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sediments using palynomorphs, Beyth (1972a) concluded an age “not older than Devonian”.

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Based on trace fossil evidence, Saxena and Assefa (1983), Kumpulainen et al. (2006) and

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Kumpulainen (2007) came to a different result and correlated the sediments with Late

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Ordovician glacigenic deposits in North Africa and on the Arabian Peninsula. When Late

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Palaeozoic (latest Carboniferous to Early Permian) palynomorphs were discovered in

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mudstones of the Edaga Arbi Glacials (Bussert and Schrank, 2007) and Early Palaeozoic trace

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fossils (e.g. Arthrophycus alleghaniensis) in sandstones of the Enticho Sandstone that overlay

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older, most likely Upper Ordovician glacigenic sediments (Bussert and Dawit, 2009), it became

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apparent that deposits of two Palaeozoic ice ages occur in northern Ethiopia (Fig. 1). The

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Early Palaeozoic glacigenic successions are sandstone-dominated, whereas the LPIA sediments

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are more heterogeneous and dominated by mudstones. In order to differentiate between Early

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Palaeozoic and LPIA sandstones that were formerly included into the Enticho Sandstone,

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Bussert and Schrank (2007) introduced the names “Lower Enticho Sandstone” and “Upper

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Enticho Sandstone”. Accordingly, the Upper Enticho Sandstone and the Edaga Arbi Glacials

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form lateral facies equivalents of Late Palaeozoic (latest Carboniferous to Early Permian) age

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(Bussert and Schrank, 2007). They attain a combined maximum thickness of about 200 m

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(Bussert and Dawit, 2009). Palynomorphs indicative of a latest Carboniferous to Early Permian

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age were encountered in mudstones in sections Agve, Megab, Edaga Arbi Village and Enticho

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(for location, see Fig. 1). The presence of Granulatisporites/Microbaculispora (Bussert and

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Schrank, 2007) that first appear in the palynological zone OSPZ 2 (Asselian-Sakmarian,

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lowermost Permian) in Oman and Saudi Arabia (Stephenson et al., 2003) suggests an earliest

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Permian age for the sediments. For outcrops of glacigenic sandstone that neither contain

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characteristic Early Palaeozoic trace fossils nor yielded any Late Palaeozoic palynomorphs,

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differentiation of the glacigenic sediments is partly problematic. In many locations, the LPIA

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sediments are unconformably overlain by Triassic (or Upper Permian) to Middle Jurassic

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siliciclastics of the Adigrat Sandstone (Beyth, 1972a,b; Dawit, 2010).

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3 Evidence of glaciation

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Two types of evidence demonstrate that LPIA glaciers affected northern Ethiopia, on the one

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hand palaeolandforms of glacial erosion and on the other hand glacigenic sediments.

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Palaeolandforms of glacial erosion are almost omnipresent on the contact surface of the LPIA

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sediments to basement rocks. They include microscale features such as polished rock surfaces,

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glacial striae, chatter marks and muschelbrüche as well as mesoscale landforms, for example

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roche moutonnées, whalebacks, rock drumlins and troughs (Bussert, 2010)(Plate 1, Fig. a). In

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some places, the basal erosion surface is overlain by “exotic” boulders up to 6 m in diameter

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(Plate 1, Fig. b,c). Besides, scattered outsized clasts, sometimes striated, faceted and bullet-

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shaped (Plate 1, Fig. d), occur also in fine-grained and partly rhythmically laminated sediments.

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Clasts within such fine-grained successions are usually associated with soft-sediment

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deformation structures in underlying beds and with on-lap or bending of overlying strata (Plate

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1, Fig. e,f), structures which are typical of dropstone activity (e.g. Thomas and Connell 1985).

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Some diamictites contain boulder beds, mostly horizontal single-layer boulder accumulations

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with flattened, in part striated upper boulder surfaces (Plate 1, Fig. g,h). Most likely, these

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boulder beds represent subglacial boulder pavements (e.g. Visser and Hall, 1985).

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4 Lithofacies associations

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The studied sections in northern Ethiopia consist of 16 major lithofacies types (Table 1). Based

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on lithofacies composition and geometry, nine major lithofacies associations (LFA) have been

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identified.

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4.1 LFA1: Coarse-grained lithofacies association (subaerial outwash fan deposits)

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Description

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Lithofacies association 1 (LFA1) is dominated by conglomerates (lithofacies Gx, Gcm) that

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contain polymict striated clasts up to 0.3 m in diameter, and by sandstones (lithofacies Sh, Slw,

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Sx). Less common are diamictites (lithofacies Dmm). Both conglomerates and sandstones form

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sharp-based lenses and sheets, but they differ in size. Whereas conglomerates primarily

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constitute lenses that reach up to 3 m in thickness and 20 m in width, most of the sheets and

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lenses of sandstone are only a few decimetres to 1 m thick (Plate 2, Fig. a,b). Lenses of cross-

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bedded conglomerate or sandstone commonly consist of single cross-beds. Beds show erosive

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basal contacts and are sub-horizontally oriented or inclined, with dip angles of up to 25 degrees

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(Plate 2, Fig. c). Intercalated lenses of diamictite are sometimes associated with soft-sediment

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deformation of underlying strata. Also sporadically present are downward tapering wedges of

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crudely banded or massive sand. Overall, LFA1 forms wedges or sheets up to 8 m thick which

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extend in the dominant flow direction for tens to few hundreds of metres. The lateral extent of

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the lithofacies association is also considerably restricted perpendicular to the major flow

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direction, because in this direction it usually cannot be traced to outcrops some tens to few

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hundred metres away.

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Interpretation

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The dominance of coarse-grained sediments that contain polymict striated clasts suggests

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deposition close to a glacier termini. Sharp vertical changes in lithofacies indicate sudden

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variations in transport conditions, possibly related to a pulsed meltwater run-off and to

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irregular gravity-driven sediment flows. Lenses of conglomerate and sandstone that consist of

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single cross-beds were deposited as subaquatic dunes in simple and most probably short-lived

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channels. Intercalated lenses of diamictite either represent flow tills or debris flow deposits.

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Inclined conglomerate and sandstone beds represent fan or delta forests. Vertically downward

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oriented sand wedges conceivably formed as ice-wedge pseudomorphs.

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Similar coarse-grained lithofacies associations can form in proximal glaciofluvial environments,

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for example in subaerial outwash fans (e.g. Maizels, 1993; Zieliński and van Loon, 1999) and

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in eskers or kames (e.g. Brennand, 1994; Warren and Ashley, 1994; Mäkinen, 2003; Bennett et

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al., 2007), but also in subaqueous settings, such as in glaciomarine grounding-line fans (e.g.

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Lønne, 1995; Plink-Björklund and Ronnert, 1999; Lønne et al., 2001; Henry et al., 2012) and

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in glaciolacustrine deltas and fans (e.g. Martini, 1990; Russell and Arnott, 2003; Johnsen and

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Brennand, 2006; Winsemann et al., 2007). In the case of LFA1, rare ice-wedge pseudomorphs

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suggest a subaerial setting, while the inclined bedding, the restricted lateral extent and the

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dominance of traction-transported sediment indicate a deposition in outwash fans (e.g.,

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Dobracki and Krzyskowski, 1997; Zieliński and van Loon, 1999; Pisarska-Jamroży, 2008;

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Koch and Isbell, 2013). Nevertheless, subhorizontal oriented to gently inclined examples of

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LFA1 that overlay sediments of LFA3 (interpreted as distal subaqueous outwash sediments)

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might represent proximal parts of shallow-water fans or deltas (e.g. Nemec, 1990; Postma,

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1990).

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4.2 LFA2: Heterogeneous coarse-grained lithofacies association (proximal subaqueous

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outwash fan deposits)

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Description

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The lithofacies association is dominated by conglomerates (lithofacies Gcm, Gmm, Gx) that are

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interbedded with sandstones (lithofacies Smc, Sh, Slw, Sx), diamictites (lithofacies Dmm) and

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mudstones (lithofacies Fm). The conglomerates often build up lenses that cut steep into

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underlying sediments (Plate 2, Fig. d) and contain basement as well as mudstone clasts up to 1

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m in diameter (Plate 2, Fig. e). Some conglomerates and sandstones however form large-scale

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foresets (Plate 2, Fig. f) with dip angles up to ~200 that rapidly thin out down-dip and pass

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within short distance into alignments of clasts. Interbedded mudstones contain scattered

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outsized clasts and are partly soft-sediment deformed. The lithofacies association is subdivided

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by erosional unconformities into units with differing dip angle and orientation. Overall, LFA2 is

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crudely fining-upward and forms sediment bodies up to 28 m thick that are laterally traceable

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for some tens to a few hundreds of metres. Laterally the lithofacies association usually

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interfingers with mudstones of LFA4. It is mainly because of this facies relationship, the

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presence of mudstone interbeds and the abundance of mudstone clasts, that LFA 2 differs from

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LFA1.

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Interpretation

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In LFA2, the dominance of massive coarse-grained sediments that contain clasts up to boulder

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size suggests deposition by hyperconcentrated, concentrated and debris flows (lithofacies Gcm,

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Gmm, Dms) as well as by high energy traction-dominated turbulent flows (lithofacies Gx) very

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near to a sediment source. Mudstones indicate suspension settling of fines. Lenses of

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conglomerate with steep borders were deposited in deeply incised channels whereas

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conglomerate foresets represent lobe-like large-scale clinoforms, reflecting a considerable

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angle of slope during deposition. The high flow strength necessary to transport gravel sheets

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with boulders up to 1 m in diameter suggests violent meltwater outbursts possibly of similar

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magnitude as jökulhlaups (e.g. Maizels, 1997; Russell, 2007; Marren et al. 2009). The rapid

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downslope thinning of conglomerates implies a sudden drop of transport energy, presumably in

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front of a meltwater outlet (e.g. Powell, 1990). Upper plane bed conditions and high sediment

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concentrations are also suggested for the deposition of horizontal or low-amplitude wavy

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bedded sandstones. Unconformity bounded sediment units that dip in variable directions likely

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record fluctuations in sediment delivery such as shifts of the entrance point and the avulsion of

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distributary channels, related to successive meltwater outbursts or to oscillations of the ice

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terminus. Soft-sediment deformation resulted either from downslope slumping of

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unconsolidated sediment on large-scale foreset surfaces or from rapid loading of water-

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saturated deposits.

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The combined presence of clinoforms, coarse-grained channel-fills and slump deposits points

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to a deposition of LFA2 in fans or deltas. As Winsemann et al. (2007) have outlined, in

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particular deposits of coarse-grained, debris flow-dominated subaqueous fans are difficult to

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distinguish from their subaerial counterparts, because of an almost identical lithofacies

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composition. In the case of LFA2, a subaquatic deposition is suggested primarily by three

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indications, 1) by mudstones containing scattered clasts that are interpreted as dropstones and

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2) by the lateral interfingering of LFA2 with sand-, silt- and mudstones of LFA3 and

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mudstones of LFA4, and 3) the abundance of mudstone clasts representing reworked

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subaqueously deposited fines. Although large-scale foreset beds (or clinoforms) can form both

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in delta and fan settings, the lack of distinct topsets differs from typical delta deposits (e.g.

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Reading and Collinson, 1996) and supports a subaqueous outwash fan origin, either as

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grounding-line or tunnel-mouth fans (e.g. Mäkinen, 2003; Russell et al., 2007, Bennett et al.,

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2007). Subaqueous fan sediments of similar geometry and lithofacies composition occur both

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in proglacial or subglacial lacustrine settings (e.g. McCabe and Ò Cofaigh, 1994; Benn, 1996;

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Johnsen and Brennand, 2006; Winsemann et al., 2004, 2007; Livingstone et al., 2012) and in

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marine environments (e.g. Lønne, 1995; Plink-Björklund and Ronnert, 1999).

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4.3 LFA3: Interbedded sand-, silt- and mudstones (medial to distal subaqueous outwash

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fan or delta deposits)

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Description

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Lithofacies association 3 is dominated by sheets of fine- to medium-grained, massive, ripple

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cross-bedded or small-scale trough cross-bedded sandstone (lithofacies Sm, Sr) that are

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frequently interbedded, in part showing a crude rhythmicity, with layers, drapes or lenses of

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silt- or mudstone (lithofacies Fm, Fl)(Plate 2, Fig. g).In the sandstones, symmetrical,

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asymmetrical, starved and climbing ripples occur (Plate 2, Fig. h). Irregularly intercalated are

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sharp based lenses of massive or ripple cross-bedded sandstone up to 0.4 m thick and 1.5 m

a 1 275

wide. The lithofacies association forms gentle wedges to sheets at maximum 4 m thick and

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laterally traceable for more than 100 m.

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Interpretation

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The interbedding of sand-, silt- and mudstones demonstrates that periods of turbulent aquatic

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sediment transport and periods of largely stagnant water dominated by suspension settling of

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fines alternated. The large variability in ripple cross-bedding indicates a changing current and

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wave influence, thus a relatively shallow water setting, and substantial fluctuations in sediment

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discharge. In contrast, sharp-based sheets and lenses of sandstone are interpreted as lobe and

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channel-fill deposits primarily of turbidity flows (e.g. Jopling and Walker, 1968; Gustavson et

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al., 1975; Shaw, 1975). Rhythmically bedded successions probably originated from recurring

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meltwater-fed sand deposition by hyperpycnal flows (e.g. Zavala et al., 2011; Girard et al.,

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2012) during summer and suspension settling of fines during winter (e.g. Gustavson et al.,

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1975; Ashley, 2002). LFA3 is therefore interpreted as bottomsets in glacigenic shallow water

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fans or deltas. It might represent a coarse-grained variation of the ‘delta varves’ described by

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Gustavson et al. (1975) or the ‘proglacial deltaic bottomsets’ of Brodzikowski and van Loon

291

(1991).

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4.4 LFA4: Fine-grained lithofacies association (subaqueous basin floor sediments)

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Description

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LFA4 consists primarily of laminated, bedded or massive dark grey mudstones (lithofacies Fl,

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Fm). The mudstones frequently contain beds of calcareous fine-grained sandstone or coarse-

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grained siltstone (lithofacies Sm, Smc, Sr) (Plate 3, Fig. a) and clay-rich, relatively clast-poor

a 1 300

diamictites (lithofacies Dmm, Dms) as well as beds or lenses of lithofacies Smc or Gcm. The

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lamination or bedding in the mudstones is partly rhythmical although often deformed, either by

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outsized clasts, by soft-sediment deformation structures such as folds or faults (Plate 3, Fig. b),

303

and occasionally by wedge-shaped structures (Plate 3, Fig. c). Some successions, however,

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completely lack outsized clasts. Most intercalated sandstone beds are thin, only a few mm to

305

cm thick. They display sharp basal contacts and are in part normal graded. Sandstone lenses

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reach up to 1 m in thickness. Diamictites occur in two types, either as massive beds with

307

gradational contacts to mudstones that can reach several metres in thickness (lithofacies

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Dmm), or as lenses that show sharp basal contacts (lithofacies Dms)). Arthropod trackways

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locally occur on bedding planes of fine-grained sandstone. Moreover, in several sections the

310

lithofacies association contains terrestrial palynomorphs (e.g. Bussert & Schrank 2007).

311

Successions of LFA4 are up to 60 m thick (e.g. in section Edaga Arbi North, Fig. 1).

312 313

Interpretation

314

The dominance of mudstone in LFA4 indicates deposition mainly by suspensions settling in

315

stagnant water, interrupted by low-density turbidity flows that intercalated sharp-based

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sandstone beds. Sandstone lenses most probably represent channelized turbidity flows whereas

317

conglomerate lenses formed from hyperconcentrated flows. Thin sandstone laminae in

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rhythmically laminated mudstone-sandstone successions might represent periodic underflows

319

or hyperpycnal flows. Massive mudstones stem from the rapid settling of suspended clay,

320

possibly derived from turbidity currents, with the settling probably accelerated either by

321

flocculation (e.g. O’Brien and Pietraszek-Mattner, 1998; Hodder and Gilbert, 2007) or flow

322

ponding (e.g. Sinclair and Tomasso, 2002). Frequent ice-contact is suggested because of

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scattered outsized clasts, interpreted as dropstones. Gradually intercalated clay-rich diamictites

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(lithofacies Dmm) most likely formed by increased rainout of basal glacial debris from icebergs

a 1 325

or under-surfaces of floating ice sheets ("waterlain tills"; e.g. Menzies and Shilts 2002)

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whereas diamictites with sharp basal contacts (lithofacies Dms(d)) are interpreted as debris

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flow deposits. The abundance of slump structures in LFA4 indicates unstable subaquatic slopes

328

prone to gravity-driven sediment-mobilisation. The lamination in the mudstones, which is

329

largely undisturbed by the activity of infaunal organisms, suggests generally hostile bottom

330

water living conditions, presumably because of dysaerobic or anoxic conditions, considering

331

also the dark sediment colour. Occasional arthropod trackways on sandstone bedding planes

332

show a temporary invasion of the environment by vagile epibenthos, possibly enabled by the

333

inflow of oxygen-rich water in the course of turbidity flows. LFA4 thus represents a relatively

334

deep and source distal subaqueous setting that was frequently affected by gravity-driven

335

sediment transport, mainly by turbidity, hyperpycnal and debris flows, by slumping, and

336

occasionally by ice-berg rainout.

337 338 339

4.5 LFA5: Sandy sheet-like diamictite association (subglacial tillite complexes)

340 341

Description

342

LFA5 consists predominantly of massive, chaotically or crudely bedded silty to sandy

343

diamictites (lithofacies Dmm, Dms)(Plate 3, Fig. d,e) that contain faceted and striated clasts of

344

both local and “exotic” provenance. At the base, it partly includes thin layers of stratified or

345

foliated siltstone and crudely stratified diamictites with subhorizontally orientated clasts. In its

346

middle and upper part, mildly deformed lenses of massive or horizontally stratified sandstone

347

(lithofacies Sm, Sh) are locally present. Boulder beds sporadically occur at the top and within

348

diamictite beds. Locally the upper surface shows a rounded low relief hummocky morphology

349

(Plate 3, Fig. f). The lithofacies association usually forms sheets 1.5 to 2.5 m thick that either

a 1 350

overlay glacial erosion surfaces composed of Neoproterozoic basement rocks or Lower

351

Enticho Sandstone or are intercalated with sharp subhorizontal basal contact into other

352

glacigenic sediments.

353 354

Interpretation

355

A subglacial origin of most sheet-like sandy diamictite associations is suggested because of

356

their common presence directly on top of glacially eroded bedrock surfaces, wide distribution,

357

relatively constant and limited thickness as well as because of the abundance of polished and

358

striated clasts of both local and "exotic" provenance. Thin foliated basal siltstones at the base

359

likely stem from locally intensified crushing and abrasion of debris during subglacial shearing,

360

and subhorizontal clast fabrics from shearing during lodgement. Chaotic deformation structures

361

could represent glacitectonic deformation features, whereas lenses and layers of sandstone

362

intercalated in the middle and upper part of diamictite associations resemble intratill meltwater

363

sediments deposited in englacial cavities (e.g. Lawson, 1981; Haldorsen and Shaw, 1982). The

364

mild deformation of lenses and layers as well as the preservation of internal sedimentary

365

structures is rather attributable to later differential compaction then to subglacial deformation

366

during lodgement, because the latter process usually strongly modifies or even completely

367

destroys primary sedimentary structures (e.g., Piotrowski et al., 2001). Boulder beds at the top

368

of these diamictites probably represent boulder pavements that resulted from the winnowing of

369

diamictites by meltwater erosion during periods of glacier retreat, followed by ice overriding

370

and subglacial abrasion during a later ice re-advance forming striated upper boulder surfaces

371

(e.g. Eyles 1988). Hummocky shaped upper surfaces either formed by meltwater erosion or by

372

glacier erosion during glacier re-advances. Intra-diamictite boulder pavements might stem from

373

the selective subglacial lodgement of boulders (e.g. Visser and Hall, 1985).

a 1 374

The sheet-like diamictite associations are therefore interpreted as fossil “subglacial tills” or

375

“subglacial till complexes” (Brodzikowski and Van Loon 1991; Evans and Hiemstra, 2005;

376

Evans et al., 2006) and likely originated from a combination of subglacial lodgement, sediment

377

deformation and melt-out. Relatively thin tills are typical of terrestrial environments (Eyles and

378

Eyles, 1992; Benn and Evans, 2010).

379 380 381

4.6 LFA6: Lenticular diamictite association (debris flow deposits)

382 383

Description

384

The lithofacies association consists of lenses, thin sheets or irregular bodies of clayey to sandy

385

diamictites that occur as intercalations in various parts of the section. Lenses are at maximum 3

386

m thick and reach laterally for up to 20 m, while sheets are few dm thick and laterally

387

extensive. With sharp convex-down to irregular basal contact it overlays various other

388

lithofacies associations, most commonly mudstones of LFA4, or occurs as stacked units (Plate

389

3, Fig. g). Internally the diamictites are massive, crudely bedded, contain deformed (e.g.

390

sheared) soft-sediment clasts or show various soft-sediment deformation structures (Plate 3,

391

Fig. h).

392 393

Interpretation

394

Different from the sandy sheet-like diamictite association (LFA5), the lenticular diamictite

395

association is interpreted as debris flow sediments, based on their sharp basal contacts, laterally

396

restricted extent and lenticular geometry, as well as on the occurrence of soft-sediment

397

deformation structures. In most cases it probably formed by the gravitational reworking and

398

mixing of glacigenic sediments, e.g. tills, outwash fan sediments and glaciolacustrine

a 1 399

mudstones. It however cannot be ruled out that it partly represents "flow tills" deposited more

400

or less directly by the ice.

401 402 403

4.7 LFA7: Soft-sediment deformed fine-grained lithofacies association (slump deposits)

404 405

Description

406

LFA7 consists principally of massive, clast-bearing mudstones (lithofacies Fm) that contain

407

soft-deformed lenses or beds of sandstone (primarily lithofacies Sr, Sm) and large-scale

408

sediment blocks (“rafts”; McCarroll and Rijsdijk, 2003). Rafts are variable in shape, lenticular,

409

ellipsoidal or blocky, and consist of conglomerates (lithofacies Gcm) or sandstones (lithofacies

410

Sm, Sr, Sh) (Plate 4, Fig. a,b). Normally rafts are tilted, in some outcrops with similar tilt

411

orientation. Soft-sediment deformation structures comprise normal faults (Plate 4, Fig. c,d),

412

open folds and basal subhorizontal shear faults. Steeply dipping or deformed sheets of massive

413

sandstone occasionally transect the lithofacies association (Plate 4, Fig. b). These sheets are

414

planar to concave-up in cross-section, widen downward and reach up to 20 cm in width and

415

~15 m in length. The presence of soft-sediment deformation structures, sediment rafts and

416

scattered clasts results in an irregular to chaotic appearance of LFA7. The lithofacies

417

association reaches a thickness of up to 30 m and occurs normally within largely undeformed

418

mudstones of LFA4, though in section Megab West it is truncated by a diamictite association

419

(LFA5).

420 421

Interpretation

422

The dominance of mudstones indicates subaqueous deposition primarily by suspension settling

423

and the presence of scattered outsized clasts dropstone activity, thus ice-contact. Rafts of

a 1 424

poorly sorted conglomerate or sandstone likely represent detached and transported outwash

425

fan sediments. Their transport into place by loading-induced vertical sinking is unlikely, on the

426

one hand because such processes usually produce rounded blocks and vertical clast fabrics

427

(e.g. Eissmann, 1981; Rijsdijk, 2001; Aber and Ber, 2007) and on the other hand because

428

similar sediments are missing in directly overlying beds. Ice transport and subsequent melt-out

429

of rafts (e.g. Menzies, 1990; Hoffmann and Piotrowski, 2001) is also improbable, at least for

430

outcrops where rafts show a similar tilt orientation – a feature that can hardly result from an

431

irregular process such as subglacial melt-out. Whereas a similar orientation of blocks in

432

individual outcrops could have resulted from domino-style block rotation in slumps, their

433

lenticular or ellipsoidal shape might originate from slump internal block rotation and shearing

434

(e.g. Hölzel et al., 2006). Thus, the rafts most likely represent blocks of slumped outwash fan

435

deposits. Deformation structures such as open folds can form by bending over non-planar

436

failure surfaces within advancing slumps (e.g. Færseth and Sætersmoen, 2008) or in the

437

downslope contractional zone of slumps (e.g. Woodcock, 1976).The steeply dipping sandstone

438

sheets are interpreted as clastic dykes (e.g. van der Meer et al., 2009). Their simple geometry

439

and massive texture contrast with complex dykes commonly reported in subglacial settings

440

(e.g. Rijsdijk et al., 1999; Le Heron and Etienne, 2005; Denis et al., 2009; van der Meer et al.,

441

2009). LFA7 also lacks the increasing distinctness of strain signatures upward that is

442

characteristic of subglacial deformation (Benn and Evans, 1996). This suggests gravity-driven

443

slumping of the sediments in a proglacial setting (McCarroll and Rijsdijk, 2003), with the basal

444

shear faults representing glide planes. Most features of LFA7 can be explained in a slump

445

context. Whereas a similar orientation of blocks in individual outcrops could have resulted

446

from domino-style block rotation in slumps, their lenticular or ellipsoidal shape might originate

447

from slump internal block rotation and shearing (e.g. Hölzel et al., 2006). Deformation

448

structures such as open folds can form by bending over non-planar failure surfaces within

a 1 449

advancing slumps (e.g. Færseth and Sætersmoen, 2008) or in the downslope contractional

450

zone of slumps (e.g. Woodcock, 1976). The high variability of soft-deformation structures

451

present in LFA7 is also typical of slumps (e.g. Martinsen, 1994, 2003). Sandstone dykes

452

probably were triggered by the rapid loading of water-saturated sediments, causing pore-fluid

453

overpressure and the opening of fractures, followed by liquefaction, fluidization and upward-

454

injection of underlying sediment. In glacial subaqueous environments, various processes can

455

trigger slumps, e.g. rapid loading by sediment or ice, abrupt water-level fluctuations, meltwater

456

surges, emplacement of debris flows, or seismic events (e.g. Chapron et al., 2004; Virtasalo et

457

al., 2007). Although most examples of LFA7 are interpreted as slumps, in section Megab West

458

it is probably of a hybrid origin and resulted from an initial slumping of blocks in a subaqueous

459

proglacial setting and subsequent overriding of the slump deposits by an advancing glacier.

460 461 462

4.8 LFA8 Soft-sediment deformed coarse-grained lithofacies association (push moraines)

463 464

Description

465

LFA8 consists of sandstones (lithofacies Sr, Sm, Sx), conglomerates (lithofacies Gcm, Gx) and

466

diamictites (lithofacies Dms) as well as minor mudstones that are large-scale soft-sediment

467

deformed, mainly by thrust and reverse faults but also by flexures and folds. Large sediment

468

rafts are missing, different to LFA7. The base of the lithofacies association is bounded by soft-

469

sediment low-angle faults whereas the upper contact is erosive to diamictites or proximal

470

outwash fan deposits. Under- and overlying sediments are largely undeformed. The lithofacies

471

association occurs in isolated outcrops. In section Enticho, soft-sediment deformed sandstones

472

and conglomerates attain a thickness of approximately 10 m and extend laterally for about 50

473

m. With soft-sediment fault contact, the deformed sediments overlay clast-poor diamictites;

a 2 474

they are truncated by diamictitic conglomerates. In section Edaga Hamus North, a sequence of

475

thrusted and moderately inclined fine- to coarse-grained sandstones rests on a diamictite unit

476

and on approximately horizontally oriented conglomerates and sandstones (lithofacies Sm, Sx)

477

and is overlain with irregular contact by subhorizontally oriented to gently inclined

478

conglomerates and sandstones (Plate 4, Fig. e). Further examples of LFA8 occur in and around

479

sections Megab West (Plate 4, Fig. e) and Dugum.

480 481

Interpretation

482

Syndepositional deformation of LFA8 primarily by horizontal compression is indicated by the

483

dominance of soft-sediment thrust and reverse faults. The abundance of coarse-grained

484

outwash fan sediments and diamictites both in the association and in under- and overlying

485

strata suggests an ice-proximal setting and supports the interpretation of the lithofacies

486

association as push or thrust moraines (e.g. van der Wateren, 1986; Bennett, 2001) that

487

formed at the front of an advancing ice margin or glacier snout. Considering the widespread

488

occurrence but limited size of the lithofacies association, most examples probably formed

489

during advance-retreat cycles of local glacier lobes.

490 491 492

4.9 LFA 9: Multi-coloured lithofacies association (post-glacial sediments)

493 494

Description

495

LFA9 consists mainly of multi-coloured, often red, brown or grey, mostly fine-grained

496

sandstones (lithofacies Sr, Sh, Sm, Sx), mudstones (lithofacies Fm) and minor conglomerates

497

(lithofacies Gcm). The lithofacies partly forms fining-upward sequences, but also occur as thick

498

units lacking any apparent grain-size trend. In some sections (e.g. Edaga Arbi West), it shows

a 2 499

a slight overall coarsening-upward trend. Lenses and sheets of sandstone or conglomerate are

500

irregularly intercalated. Ripple structures are of both symmetric and asymmetric geometry and

501

show variable ripple indexes (Plate 4, Fig. g). The mudstones contain scattered well-rounded

502

sand grains and occasionally bioturbations, root traces and desiccation cracks (Plate 4, Fig. g).

503

Body fossils are missing. According to XRD analysis, red and brown coloured sediments

504

contain hematite, and light grey coloured sediments partly calcite. The lithofacies association

505

is up to 30 m thick and extends laterally for many hundred of meters.

506 507

Interpretation

508

The presence of fine-grained and partly clay-rich lithofacies, as well as the occurrence of

509

symmetrical ripples with low ripple indexes (which is typical of subaqueous wave ripples, e.g.

510

Pye and Tsoar, 1990) and of asymmetric ripples that likely formed in weak currents implies

511

deposition in both slowly flowing and stagnant water. The complete lack of outsize clasts

512

points to a non-ice contact environment. Common hematite staining suggests cementation

513

under oxidising conditions, most probably during groundwater-controlled early continental

514

diagenesis (e.g. Walker, 1976; Turner, 1980). Massive mudstones and fine-grained sandstones

515

that show bioturbations, sediment-filled cracks and root traces were either deposited in

516

floodplain ponds or playa lakes episodically exposed to desiccation, plant colonisation and

517

initial pedogenesis, or in sabkhas and lagoons of a shallow marine coastal environment. Well-

518

rounded sand grains in mudstones might represent wind transport of dune sand into playa or

519

sabkha lakes, akin to examples in the Mesozoic of eastern North America (Hubert and Hyde,

520

1982; Smoot and Olsen, 1988) and in the Upper Rotliegend of the southern North Sea

521

(George and Berry, 1993). However, the well-rounded grains might also have formed as beach

522

sand. Sandstone-dominated fining-upward sequences either represent fluviatile or tidal

523

channels, and thin sandstone lenses intercalated into mudstones small-scale floodplain or tidal

a 2 524

creeks. Lenses and sheets of coarse-grained lithofacies either record episodic high-energy

525

sediment transport in channels and in sheetfloods (e.g. Turnbridge, 1981; Hubert and Hyde,

526

1982). As pedogenesis was apparently limited to root penetration and did not progress to well-

527

defined soil horizons, plant growth likely was retarded by unfavourable growth conditions,

528

probably water stress or salinity. The presence of calcite that likely represents calcrete-like

529

cementation indicates an arid to sub-humid palaeoclimate during deposition (e.g. Goudie,

530

1983).

531 532 533

5 Sedimentary sections

534 535

Sections of Late Palaeozoic glacigenic sediments were logged in two major outcrop areas in

536

the Tigray Province of northern Ethiopia, along the southern and northern margin of the

537

Mekelle Basin, and at the northern and southern foot of the Adigrat-Adua Ridge, close to the

538

border of Ethiopia to Eritrea.

539 540 541

5.1 Sections at the southern margin of the Mekelle Basin

542 543

Description

544

Sections at the southern margin of the Mekelle Basin are extremely variable both in thickness

545

and in lithofacies composition. All sections start with basal diamictites (LFA5) that overlay

546

erosional surfaces composed of Neoproterozoic metasediments and displaying palaeolandforms

547

of glacial erosion, most prominently whalebacks and roche moutonnées. Close to section

a 2 548

Samre East (base 13011.4’N/39013.3’E; see Fig. 1 for location), large exotic boulders, mainly

549

granites, are exposed on the exhumed basal erosion surface (Plate 1, Fig. b).

550

Section Samre West (Fig. 2; base 13010.5’N/39011.5’E) consists predominantly of clast-

551

bearing mudstones with interbedded sheets and lenses of siltstone, sandstone and diamictite

552

(LFA4) (Plate 5, Fig. a,b). Thick diamictites, in part containing boulder beds (LFA5), in part

553

sediment rafts and sandstone dykes (LFA7), are intercalated in the middle and upper part of

554

the section. These sediments are overlain by conglomerates and sandstones (LFA1) (Plate 5,

555

Fig. c). The uppermost part of section consists primarily of multi-coloured sand-, silt- and

556

mudstones (LFA9).

557

Section Samre North (Fig. 2; base 13012.1’N/390 13.1’E); is dominated by coarse-grained

558

sediments, mainly conglomerates and sandstones (LFA2) (Plate 5, Fig. d). These sediments

559

contain scattered basement and mudstone boulders up to 1.2 m in diameter (Plate 5, Fig. e).

560

Beds dip in various directions with angles of up to 10 degrees and interfinger laterally with

561

mudstones of LFA4. Most conglomerates form steep sided lenses that are at maximum 2 m

562

thick and 10 m wide, whereas the sandstones predominantly constitute sheets up to few

563

decimetres thick. The uppermost part of the section is built up of stacked beds of fine-grained

564

cross-bedded or cross-laminated sandstones and siltstones (LFA9).

565 566

Interpretation

567

The diamictites at the base of sections in the Samre region likely represent basal tillites,

568

because of their position directly above glacially eroded basement surfaces, their wide

569

geographical spread and relatively constant thickness. In section Samre East, the basal

570

diamictite seems to have experienced some winnowing by meltwater erosion during glacier

571

retreat. A marked heterogeneity in lithofacies and thickness of the sections in the Samre region

a 2 572

most likely relates on the one hand to a pronounced palaeotopography, on the other hand on

573

differences in distance to a glacier terminus as the major sediment source.

574

In section Samre West, the dominance of LFA4 indicates deposition in a relatively source (ice)

575

distal subaqueous basin. Sandstone and diamictite interbeds mirror frequent subaqueous

576

gravitational mass-transport by low-density or high-density turbidity flows as well as debris

577

flows into the basin, with thick diamictites representing debris flows and waterlain tills. A

578

boulder pavement indicates the winnowing of a diamictite, likely a subglacial till, by meltwater

579

erosion and subsequent overriding by a re-advancing glacier (e.g. Eyles, 1988; Boyce and

580

Eyles, 2000). In the upper part of the section, conglomerates were deposited in outwash fan

581

channels that possibly served as meltwater conduits during final melting and retreat of the ice.

582

The predominantly coarse-grained sediments in section Samre North were deposited as

583

channel-fills and sheet-flows in a large outwash fan complex, with a subaqueous setting being

584

indicated primarily by their lateral interfingering with thick mudstones. In the section, an

585

overall slight fining-upward reflects a subtle decline in transport energy, possibly caused by fan

586

retrogradation during ice retreat. Overlying sand- and siltstones likely originated in a medial to

587

distal outwash fan environment.

588 589 590

5.2 Sections at the northern margin of the Mekelle Basin

591 592

Description

593

Sections at the northern margin of the Mekelle Basin are on average more homogeneous and

594

fine-grained when compared to sections at the southern margin.

595

In section Dugum (Fig. 3; base 130 51.0’N/39029.2’E), a basal diamictite association (LFA5)

596

overlays a glacial erosion surface composed of Neoproterozoic metamorphic rocks. In the

a 2 597

neighbourhood, exhumed granite boulders up to 6 m in diameter occur on this surface.

598

Following upward in the section are sandstones and minor conglomerates (LFA2). Most of the

599

section however is dominated by clast-bearing mudstones and muddy diamictites that include

600

minor thin sandstone beds and lenses (LFA 4), but coarse-grained lithofacies associations are

601

intercalated in different levels. In its middle part, lenses of mud-rich diamictite (LFA6) are

602

intercalated, and a thick unit of foreset beds of conglomerates to diamictites, sandstones and

603

mudstones (LFA2)(Plate 5, Fig. f9 in the upper part. At the top, the section is truncated by a

604

fault and therefore lacks a regular contact to the Adigrat Sandstone.

605

With fault contact to metamorphic basement, section Megab West (Fig. 3; base

606

13056.2’N/39021.5’E); starts with interbedded sand- and mudstones (LFA3). These sediments

607

are erosively overlain by a crudely fining-upward sequence of conglomerates and sandstones

608

(LFA1). The overlying section is dominated by clast-bearing mudstones and clast-poor muddy

609

diamictites (LFA4), in which a unit of diamictic mudstones with soft-deformation structures,

610

sediment rafts and sandstones dikes (LFA7) is intercalated. The soft-deformed sediments are

611

truncated by a complex diamictite association (LA5)(Plate 5, Fig. g,h) that contains sheet-like

612

diamictites and a boulder pavement and is overlain by mudstone-rich diamictites. Lower in the

613

section another diamictite unit which also includes a boulder pavement is intercalated. The

614

upper part of the section consists of clast-bearing dark grey mudstones that transform

615

gradually upward into clast-free red and brown coloured mudstones and sandstones.

616

Section Megab Southwest (base 13055.4’N/39021.0’E) consists, except of a basal sandy

617

diamictite complex, primarily of mudstones with a changing content of lonestones and thin

618

diamictite, conglomerate and sandstone interbeds (Plate 6, Fig. a,b).

619 620

Interpretation

a 2 621

Apart from a basal tillite, section Dugum represents primarily proglacial basin floor sediments

622

that embed several subaqueous outwash fan complexes. Although no other indicators for

623

subglacial deposition except of the basal tillite occur, the very coarse-grained nature of the

624

uppermost outwash fan association suggests deposition directly in front of a glacier terminus,

625

thus indirectly of a second glacier advance. Diamictites gradually intercalated into mudstones

626

formed by the rainout of clasts from ice-bergs, and sharply intercalated lenses of diamictite as

627

debris flow sediments.

628

In section Megab West, the presence of two sheet-like diamictite associations that contain

629

boulder pavements suggests that at least two ice-front advances reached the location. The

630

complex upper diamictite association likely formed during a polyepisodic ice advance that

631

included an intervening retreat phase. The first of these ice advances probably caused slumping

632

and deformation of proglacial sediments in a water body, with the degree of deformation

633

possibly intensified during subsequent overriding of the slump by the ice. Thick clast-poor

634

massive diamictites gradually intercalated into mudstones of LFA4 originated as waterlain

635

(rainout) tills. At the top of the section, a gradual change from grey clast-bearing to red or

636

brown coloured clast-free mudstones likely reflects a relatively slow transformation from

637

glacial to post-glacial conditions.

638

The dominance of basinal mudstones in section Megab Southwest in contrast to the nearby

639

diamictite-rich section Megab West primarily reflects a small-scale heterogeneity in deposition

640

and a laterally restricted impact of ice advances.

641 642 643

5.3 Sections at the southern foot of the Adigrat-Adua Ridge

644 645

Description

a 2 646

Sections at the southern foot of the Adigrat-Adua Ridge are dominated by mudstones (LFA4)

647

and contain only minor diamictites and conglomerates. These sections are generally more fine-

648

grained when compared to those at the margin of the Mekelle Basin.

649

Section Edaga Arbi Village (Fig. 4; base 14002.4’N/39004.4’E) consists predominantly of grey

650

mudstones with interbedded horizontal lamina or thin beds of calcareous, horizontally or

651

ripple-laminated siltstone to fine-grained sandstone (LFA4) (Plate 6, Fig. c). Outsized clasts

652

occur only in the lowermost part of the section. In the upper part, the grey toned mudstones

653

change gradually into red or brown coloured, otherwise light grey toned, fine-grained

654

sandstones and siltstones (LFA9).

655

Similar to section Edaga Arbi Village, section Edaga Arbi West (Fig. 4; base

656

14002.3’N/38059.3’E) is dominated by mudstones of LFA4. However, diamictites and soft-

657

sediment deformation structures such as thrust faults as well as frequent sandstone beds occur

658

in the basal part of the section (Plate 6, Fig. d). The upper basal part of the succession is free

659

of outsized clasts and contains abundant laminae or thin beds of carbonaceous siltstone or fine-

660

grained sandstone. Scattered outsized clasts as well as diamictites again are present in the

661

middle upper part of the section, and are overlain by clast-free, initially grey but changing

662

upward into red to brown coloured siltstones and fine-grained sandstones.

663

In section Edaga Arbi North, which is probably correlative to the lowermost part of section

664

Edaga Arbi Village, a thick succession of clast-bearing massive mudstone contains soft-

665

sediment normal faulted lenses of massive to ripple cross-bedded sandstone (Plate 6, Fig. e).

666 667

Interpretation

668

Section Edaga Arbi Village represents a relatively ice distal setting, for which ice-contact is

669

only indicated by outsized clasts in mudstones during early sedimentation. During most of the

670

deposition, suspension settling of fines was dominating, occasionally interrupted by low-

a 2 671

density turbidity flows. In the upper part of the section, sedimentation conditions changed

672

gradually into a warmer environment.

673

In Section Edaga Arbi West, the abundance of outsized clasts in the succession points to an

674

ice-contact setting during long periods of deposition. The combined occurrence of a massive

675

diamictite and soft-sediment deformation in the lowermost part of the section suggests

676

subglacial lodgement and deformation during ice re-advances, thus an initial ice-marginal

677

setting with several advance-retreat cycles. After a period without obvious signs of ice-contact,

678

a second ice advance reached the region and resulted in the deposition of dropstones and

679

waterlain tillites. In the following, glacial ice-conditions changed to a non-ice contact and

680

finally to a post-glacial environment.

681

In section Edaga Arbi North, the normal faulted sandstone lenses most likely formed by the

682

slumping of subaqueous channel fills in an ice-contact setting.

683 684 685

5.4 Sections at the northern foot of the Adigrat-Adua Ridge

686 687

Description

688

Sections in this region, e.g. sections Bizet, Adigrat Northwest and Adigrat North (see Fig. 1),

689

consist primarily of mudstones (LFA4) that contain, apart from scattered clasts, units of

690

mudstone which include rafts or large bodies of soft-sediment deformed sandstone (LFA7) )

691

(Plate 4, Fig. c). Massive and bedded sandy diamictites with large basement clasts and sporadic

692

boulder beds (LFA5) are intercalated at or near to the base and in the upper part of sections

693

(Plate 6, Fig. g). They occur together with large-scale soft-sediment deformed and primarily

694

thrusted sandstones, conglomerates and mudstones (LFA8) (Plate 6, Fig. h). In section

695

Enticho, within a succession that consists of diamictites (LFA5), clast-bearing mudstones

a 2 696

(LFA4) as well as of sandstone and conglomerate (LFA2), a horizon of dolomitic concretions

697

is intercalated.

698 699

Interpretation

700

In these sections, the dominance of mudstone suggests that most sediments were deposited in a

701

relatively ice-distal subaquatic environment. Widespread soft-sediment deformation, the

702

presence of large rafts and soft-deformation of sandstone bodies implies unstable subaquatic

703

slopes prone to the slumping of water-saturated unconsolidated sediments. The occurrence of

704

thrusted conglomerates and sandstones associated with sandy diamictites at the base and in the

705

upper part of sections is taken as evidence of ice push during at least two glacier advances.

706

The horizon with dolomitic concretions intercalated within the glacigenic sediments in section

707

Enticho might indicate that at least in one interglacial period the region was transgressed by

708

the sea.

709 710 711

6 Discussion

712 713

6.1

Depositional environments and sedimentation pattern

714 715

The high lithofacies heterogeneity and strong lateral changes in thickness of the LPIA

716

sediments in northern Ethiopia were controlled by two major factors, firstly by a pronounced

717

basin floor palaeotopography and secondly by distance to glaciers as the main sediment source.

718

The dominance of mudstones in most regions of the basin suggests that the vast majority of

719

sediments were deposited in low-energy subaqueous environments. Subaerial deposition was

720

restricted to the lateral basin margins, and within the basin to successions at the base and top of

a 3 721

sections. In almost all outcrop regions except the lateral basin margins, basal tillites directly

722

overly glacially eroded basement surfaces (Fig. 5a), suggesting that during initial glacier

723

advance thick ice of a large ice stream covered the complete basin. Soft-sediment deformation

724

of the basal tillites and internal striations indicate minor glacier advance-retreat cycles during

725

this advance. The absence of basal tillites in sections at the basin margins either reflects non-

726

deposition due to the lack of a considerable ice cover, or later erosion by meltwater. During ice

727

retreat, outwash fans formed at the ice terminus, subaerial fans along the lateral basin margins

728

and subaqueous fans within the basin. In the following, sedimentation in the basin was

729

dominated by the subaqueous suspension settling of fines, interrupted by periodic hyperpycnal

730

sediment flows and by irregular event-type turbidity and debris flows, and occasionally by the

731

dumping of ice-rafted debris. After a period dominated by subaqueous deposition, glacier re-

732

advances in several regions of the basin resulted in renewed deposition of subglacial tillites and

733

in the deformation of older glacigenic sediments in form of push moraines. Nevertheless,

734

different from the initial ice advance documented basin-wide at the base of sections, the re-

735

advances were laterally restricted and did not affect the whole basin. The laterally limited

736

occurrence of subglacial tillites and push moraines during a suggested second major ice

737

advance indicate deposition of glacigenic sediments in palaeovalleys or palaeofjords separated

738

by topographic highs (Fig. 5b).

739

In the studied LPIA sediments in northern Ethiopia no body fossils were discovered, probably

740

because in regions close to glacier margins, body fossils show a low preservation potential

741

(Aitken, 1990). Although the degree of bioturbation of the sediments is low, a low diversity

742

trace fossil assemblage composed of small sized arthropod trackways and resting traces

743

assignable to Cruziana cf. problematica and Isopodichnus or Rusophycus occurs on sandstone

744

bedding planes. The ichnotaxa Cruziana cf. problematica and Isopodichnus or Rusophycus are

745

facies crossing types which lived in both continental and marginal-marine environments (Schatz

a 3 746

et al., 2011). Low diversity and small size of the traces suggests a highly stressed environment,

747

probably due to salinity fluctuations, high rates of sedimentation, oxygen-depleted conditions,

748

high water turbidity, and variable degree of substrate consolidation (Buatois and Mángano,

749

2011), typical of fjord environments. Very similar trace fossil assemblages and lithofacies

750

associations as in northern Ethiopia are reported from many Late Palaeozoic fjords and deep

751

coastal lakes influenced by strong discharges of meltwater (e.g. Buatois et al., 2010; Schatz et

752

al. 2011). The slight increase of carbonate-rich silt- and sandstone beds towards the north

753

might also indicate an enhanced marine influence in this direction. Similar LPIA glacigenic

754

successions dominated by mudstones and containing diamictites, outwash fan sandstones and

755

mass transported sediments primarily occur in glaciomarine settings, such as in the Canning

756

Basin in Western Australia (Mory et al., 2008) and in basins in western Argentinia (e.g. Henry

757

et al, 2008).

758

Concerning the diamictites in the LPIA successions in northern Ethiopia, most of them did not

759

form subglacially but either represent waterlain (rainout) tillites or debris flow deposits. The

760

majority of outwash fans extended approximately perpendicular to the direction of ice advance

761

and likely formed grounding line fans, although some outwash fans are aligned approximately

762

in direction of ice advance and might have represented “interlobate moraines” (e.g. Russell and

763

Arnott, 2003; Mäkinen, 2003).

764

Sections in the southern part of the basin, e.g. in the vicinity of the villages of Samre and

765

Megab, are generally more heterogeneous in lithofacies and contain higher amounts of coarse-

766

grained sediments, predominantly outwash fan deposits, when compared to sections close to

767

the village of Edaga Arbi in the north. This very likely reflects a more proximal position of the

768

former sections in respect to the ice spreading centre. Sections around Edaga Arbi were

769

deposited in relatively ice distal and deep subaqueous environments dominated by suspension

770

fall-out. These sections contrast primarily in the abundance of indicators of ice contact, such as

a 3 771

dropstones and push moraines. Large differences in nearby sections support a depositional

772

model of topographically largely separated valleys or fjords.

773 774 775

6.2 Palaeotopography

776 777

The areal distribution of LPIA sediments in northern Ethiopia and their lateral changes in

778

thickness and facies indicate deposition in a NNE trending glaciated trough that was at least 60

779

km wide, 140 km long and at maximum about 200 m deep. The abundance of glacial

780

palaeolandforms at the bottom of the trough points to an active role of glacial erosion in its

781

shaping. Ice movement was towards the north, as glacial erosional palaeolandforms (striations,

782

whalebacks, roche moutonnée and others) demonstrate (Bussert, 2010). They represent Late

783

Palaeozoic palaeolandforms and not Early Palaeozoic relics, because such features were

784

nowhere observed at the contact of the Early Palaeozoic Lower Enticho Sandstone to

785

Precambrian basement.

786

Estimating the depth of glacial incision in glaciated basins is difficult, as the stratigraphic hiatus

787

in such basins is not only controlled by (i) the depth of glacial erosion, but also by (ii)

788

preglacial erosion related to uplifts and (iii) preglacial differential subsidence (Ghienne et al.,

789

2012). In large parts of the Arabian Peninsula, middle Carboniferous Hercynian tectonism (Al-

790

Husseini, 1992; Sharland et al., 2004) resulted in NS-trending fault blocks, sags and swells (Al-

791

Husseini, 2004). Given the pre-Red Sea rifting proximity of Arabia and northern Ethiopia, the

792

Late Palaeozoic trough in northern Ethiopia might have formed during this tectonic phase.

793

Because Hercynian tectonism probably was soon preceded by glacial erosion, the individual

794

share of these two processes in the formation of the basement trough is difficult to

795

differentiate. Assuming a uniform thickness of Early Palaeozoic sediments in northern Ethiopia

a 3 796

in the range of the preserved maximum thickness and considering their local survival beneath

797

Late Palaeozoic glacigenic deposits, the depth of glacial erosion in northern Ethiopia is

798

estimated to be generally less than 200 m. For the most parts of the basin, the relative relief

799

between the base of local palaeodepressions and palaeohighs formed by glacial erosion was

800

very likely below 100 m.

801 802 803

6.3 Glacier dynamics

804 805

The extensive erosion surface at the base of the LPIA sediments that features a large variety of

806

glacial erosional palaeolandforms suggests intense glacial erosion during initial glacier advance

807

by a thick and extensive ice stream or ice sheet. Glacial erosional palaeolandforms, including

808

whalebacks and roche moutonnée occur both in palaeovalleys and on palaeoridges, showing

809

that at least partly the shape of palaeovalleys resulted from glacier erosion. The occasional

810

presence of glaciofluvial sediments at the base of palaeovalleys nevertheless implies that

811

meltwater erosion also contributed to their shaping. Intense subglacial erosion is typical of

812

temperate (wet-based) and polythermal ice sheets (e.g. Sugden and John, 1976) whereas cold-

813

based ice is believed to have little potential to erode (e.g. Kleman et al., 2008). This

814

assumption was questioned by field studies in Antarctica (Atkins et al., 2002; Lloyd Davies et

815

al., 2009) which have proven the ability of cold-based glaciers to erode. The presence of cold-

816

based glaciers during the LPIA in the study area however is unlikely, on the one hand because

817

erosional palaeolandforms such as roche moutonnée rely on meltwater in their formation and

818

on the other hand because basal successions of coarse-grained lithofacies are atypical of cold-

819

based glaciers, since such glaciers carry little rock debris when compared to their temperate

820

and polythermal counterparts (e.g. Sugden and John, 1976; Kleman et al., 2006). Although the

a 3 821

abundance of glaciofluvial outwash sediments favours temperate ice conditions, abundant

822

diamictites and rarity of supraglacial deposits fit better to a polythermal ice regime (e.g.

823

Hambrey and Glasser, 2012).

824

In many sections in northern Ethiopia, indicators of the direct glacier impact occur in form of

825

subglacial tillites and push moraines. The abundance of push moraines points to relatively

826

mobile and frequently surging glaciers. In contrast to the basin-wide presence of signs of

827

glacier activity at the base of the sections, indicators of glacial activity higher in the section are

828

regionally restricted, probably reflecting deposition in discrete palaeovalleys. This suggests that

829

the second major ice advance and associated minor ice advance-retreat cycles were largely

830

limited to individual ice tongues that entered separate palaeovalleys or fjords. However, the

831

relief of these depressions was relatively moderate.

832 833 834

6.4 Regional correlation

835 836

LPIA glaciers were active in further parts of Ethiopia, as sediments and palaeolandforms of

837

glacial erosion in the Blue Nile gorge and in the Chercher Mountains (Lebling and Nowack,

838

1939; Jepsen and Athearn, 1964), in the Negelle area (Tadesse and Melaku, 1998) and in the

839

Ogaden Basin (Hunegaw et al., 1998) suggest. The impact of LPIA glaciers is also

840

documented in other regions of Africa north of the present day Equator, in the Central African

841

Republic (Censier and Lang, 1992), in Niger (Lang et al., 1991) and possibly also in northern

842

Eritrea (Merla et al., 1979)(Fig. 6). Diamictites and varved lake sediments in the border region

843

of northwestern Sudan to southwestern Egypt are attributed to a local mountain glaciation by

844

Klitzsch (1983).

a 3 845

The presence of LPIA sediments in the border region of Sudan and Egypt and in Niger, far

846

from other LPIA relics, suggests that local ice centres or mountain glaciations existed in

847

Africa. Palaeo-ice flow direction in Ethiopia was towards the North, compatible with either a

848

source area in the central African part of a massive ice sheet covering much of southern

849

Gondwana, or a local ice-spreading centre positioned in the Horn of Africa. The existence of

850

several small ice centers in Gondwana during the LPIA has been emphasised by Isbell et al.

851

(2003, 2012).

852

Local ice centres were recently also suggested for LPIA deposits in Oman (Martin et al., 2012)

853

In Arabia, LPIA sediments and palaeolandforms are very widespread. In Yemen, a continental

854

glaciation is attested by polished and striated glacial pavements (Wopfner and Li, 2009) that

855

are overlain by up to 130 m of fine-grained glaciolacustrine sediments and diamictites of three

856

glacier advances (Kruck and Thiele, 1983). Similar in Oman, three phases of glaciation were

857

recognised by Alsharhan et al. (1993), but two phases of ice advance by Martin et al. (2012).

858

The glacigenic sediments reach only a thickness of few tens of metres in outcrops (Al Belushi

859

et al., 1996) yet several hundred metres in the subsurface (Alsharhan et al., 1993). Similar

860

LPIA successions also occur in central eastern and in south-western Saudi Arabia (Melvin and

861

Sprague, 2006; Melvin et al., 2010; Keller et al., 2010). Both on sedimentological and

862

palaeontological arguments, e.g. the complete lack of marine fossils, these authors concluded

863

glacial lakes and glaciofluvial outwash plains as the major depositional environments. They also

864

notized an abundance of ice-distal mudrocks, debris flow and gravity flow deposits as well as

865

the presence of small-scale push moraines, and indicative of minor glacial readvances (Melvin

866

et al., 2010).

867

The areal extent and thickness of fine-grained subaquatic glacigenic sediments in both Arabia

868

and northern Ethiopia implies that large proglacial water bodies, either large lakes or fjords,

869

existed in both regions. Glacier retreat successions in both regions include abundant

a 3 870

dropstones, gravity-flow and slump deposits. The common presence of push moraines further

871

advocates that both regions were affected by mobile warm-based or polythermal glaciers. In

872

northern Ethiopia as well as in Arabia, the basal erosion surfaces are marked by diverse

873

glacigenic erosional palaeolandforms, implying that both regions were affected by warm-based

874

or polythermal continental glaciers. Parallels in the LPIA successions in both northern Ethiopia

875

and Arabia likely reflect primarily pre-Red Sea rifting palaeogeographic proximity (e.g.

876

Torsvik and Cook, 2004). Considering the comparable ice-flow directions towards the N or

877

NE and high similarity in lithofacies architecture, both regions were probably overridden by ice

878

lobes of the same ice sheet or ice cap, favouring a joint ice centre in the south as the source.

879

However, a local ice centre seems to have existed at least temporarily in the Huqf High in

880

Oman (Martin et al., 2012).

881

The depth of glacial erosion in both regions differed, reaching down to the Precambrian

882

basement in northern Ethiopia but only to Early Palaeozoic strata in Arabia. The glacigenic

883

sediments that overlay the basal erosion surfaces are also differing, with diamictites dominating

884

in northern Ethiopia and glaciofluvial deposits in Arabia (Melvin and Sprague, 2006). A

885

supposed greater depth of glacial erosion in northern Ethiopia might indicate a higher ice

886

thickness and erosive potential of the glaciers, when compared to those in Arabia. The relative

887

paucity of glaciofluvial sediments overlying the basal erosion surface in northern Ethiopia

888

could point to their cannibalisation by re-advancing grounded glaciers, probably caused by a

889

reduced accommodation space in an intracratonal setting such as in northern Ethiopia. The

890

contrasting structural setting, rate of subsidence and amount of available accommodation space

891

might account also for differences in the thickness of the glacigenic sediments and in the

892

number of preserved glacial advance-retreat cycles in northern Ethiopia when compared to

893

basin centres on the Arabian Peninsula. A relatively stable intracratonal setting of northern

894

Ethiopia, in a rather interior part of Gondwana, likely reduced the potential for the deposition

a 3 895

of glacigenic sediments and for the preservation of complete glacial advance-retreat

896

successions, whereas a more marginal cratonal position of basins in Arabia allowed the

897

preservation of thicker successions of glacigenic sediments and of further glacial advance-

898

retreat cycles.

899 900 901

7

Conclusions

902 903

A detailed analysis of lithofacies associations of LPIA sediments in northern Ethiopia resulted

904

in the identification of a wide range of glacigenic depositional processes and environments. The

905

sediments document two major and probably additional minor glacier advance-retreat cycles.

906

Intense glacial erosion during ice advance, evidence of subglacial deformation and the presence

907

of numerous push moraines indicate the action of grounded temperate or polythermal glaciers.

908

Most sediments were deposited during deglaciation in subaquatic environments, either in ice-

909

contact lakes or in fjords, by turbidity, hyperpycnal, hyperconcentrated and debris flows,

910

slumping and rainout from floating ice. Subaqueous outwash-fans formed close to glacier

911

termini. Mass transport and re-sedimentation of glacigenic sediments in slumps and debris

912

flows was very common. Differences in the thickness and the facies of the glacigenic sediments

913

and in the number of preserved glacial advance-retreat cycles in northern Ethiopia compared to

914

the Arabian Peninsula likely reflect contrasting structural settings and related rates of

915

subsidence and generation of accommodation space. Furthermore, the study raises the question

916

of a possible fjord-like depositional environment for part of the sediments.

917 918 919

Acknowlegements

a 3 920 921

Dr. Enkurie Dawit (now of Mekelle University, Ethiopia) is thanked for his tremendous help

922

during field work, for many fruitful discussions and friendly company. I also like to thank

923

Mathias Hinderer (TU Darmstadt, Germany) and an anonymous reviewer for very valuable

924

suggestions that significantly improved the manuscript.

925 926 927

8

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Torsvik, T.H., Cocks, L.R.M., 2004. Earth geography from 400 to 250 Ma. A

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palaeomagnetic, faunal and facies review. Journal of the Geological Society of London 161,

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555–572.

1442 1443

Turnbridge, I.P., 1981. Sandy high-energy flood sediments – some criteria for their

1444

recognition, with an example from the Devonian of S.W. England. Sedimentary Geology 28,

1445

79–95.

1446 1447

Turner, P., 1980. Continental Red Beds. Developments in Sedimentology 29, 562 pp.

1448 1449

Van der Meer, J.J.M., Kjær, K.H., Krüger, J., Rabassa, J., Kilfeather, A.A., 2009. Under

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pressure: clastic dykes in glacial settings. Quaternary Science Reviews 28, 708–720.

1451 1452

van der Wateren, D., 1986. Structural geology and sedimentology of the Dammer Berge push

1453

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1454

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1455 1456

Virtasalo, J.J., Kotilainen, A.T., Räsänen, M.E., Ojala, A.E.K., 2007. Late-glacial deposition in

1457

a large, low relief, epicontinental basin: the northern Baltic Sea. Sedimentology 54, 1323–

1458

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1459

a 6 1460

Visser, J.N.J., Hall, K.J., 1985. Boulder beds in the glaciogenic Permo-Carboniferous Dwyka

1461

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1462 1463

Visser, J.N.J., 1997. A review of the Permo-Carboniferous glaciation in Africa. in: Martini, I.P.

1464

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1465

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1466 1467

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1468

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1469

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1470 1471

Warren, W.P., Ashley, G.M., 1994. Origins of the ice-contact stratified ridges (eskers) of

1472

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1473 1474

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1475

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1476

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1477 1478

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1479

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1480

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1481 1482

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1483

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1484

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1486

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1487 1488

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1489

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1490 1491

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1492

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1493 1494

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1505 1506

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characteristics and depositional model. Geologie en Mijnbow 79, 93–107.

1508 1509

a 6 1510

Figures

1511 1512

Fig.1. Geological sketch map of northern Ethiopia (based mainly on Arkin et al., 1971; Garland

1513

et al., 1978; Alemu et al., 1999; Hailu, 2000) and corresponding stratigraphic column (based

1514

mainly on Beyth, 1972a,b; Bussert and Schrank, 2007; Dawit, 2010). The location of

1515

investigated sections is indicated by asterisks.

1516 1517

Fig. 2. Two logged sections (sections Samre West and Samre North) in the southern Mekelle

1518

Basin. For locations, see Fig. 1; for codes of lithofacies and lithofacies associations, see Tab. 1

1519

and Tab. 2.

1520 1521

Fig. 3. Two logged sections (sections Megab West and Dugum) in the northern Mekelle Basin.

1522

For locations, see Fig. 1; for codes of lithofacies and lithofacies associations, see Tab. 1 and

1523

Tab. 2. For legend, see Fig. 2.

1524 1525

Fig. 4. Two logged sections (sections Edaga Arbi West and Edaga Arbi Town) at the southern

1526

foot of the Adigrat-Adua Ridge. Recognize different scale of logs. For locations, see Fig. 1; for

1527

codes of lithofacies and lithofacies associations, see Tab. 1 and Tab. 2. For legend, see Fig. 2.

1528 1529

Fig. 5a. Schematic WNW-ESE cross-section from Megab to Wukro (for location, see Fig. 1).

1530

Illustrated are major glacial and non-glacial erosion surfaces, the internal architecture of the

1531

glacigenic succession and overlying post-glacial sediments. Note that the cross-section only

1532

covers an inner part of the palaeovalley shown in Fig. 5b.

1533

a 6 1534

Fig. 6. Distribution of LPIA glaciated and non-glaciated basins in Gondwana. Dashed line

1535

outlines “limit of glaciation” in Africa according to Wopfner and Jin (2009). Numbered

1536

glaciated basins: 1) Northern Ethiopia Basin, 2) Blue Nile Basin, 3) Ogaden Basin, 4) NW

1537

Yemen and SW Saudi Arabia basins, 5) South Oman Basin, 6) Eastern and Central Saudi

1538

Arabia Basin, 7) SW Central African Republic Basin, 8) Niger Basin, 9) SW Egypt/NW Sudan

1539

Basin. Modified after Wopfner and Jin (2009).

1540 1541

a 6 1542

Plates

1543 1544 1545 1546 1547 1548 1549 1550

Plate 1. Indicators of LPIA glaciation in northern Ethiopia a) Polished and striated basal erosion surface overlain by outwash fan sandstones. Close to section Samre East. b) Exhumed basal erosion surface composed of Neoproterozoic schists overlain by scattered granite boulders. Close to section Samre East. c) Exhumed granite boulder in the basal sediment succession, halfway between sections Dugum and Megab Southeast.

1551

d) Polished and striated metavolcanic clast. Section Megab West.

1552

e,f) Dropstones in laminated to thin-bedded basin floor mud- and sandstones. Section

1553 1554 1555

Megab Southwest. g,h) Boulder pavements associated with sandy diamictites. Sections Megab West (g) and Samre West (h).

1556 1557 1558 1559 1560 1561 1562 1563 1564 1565

Plate 2. Lithofacies associations LFA1 – LFA3 a) LFA1. Stacked channel-fills consisting of cross-bedded and massive conglomerates and sandstones. Lower part of section Megab West. b) LFA1. Cross-bedded to massive conglomerates interbedded with sheets and lenses of horizontally laminated sandstones. Lower part of section Megab West. c) LFA1. Conglomeratic foreset beds downlapping on horizontally bedded sandstones. Section Abi Adi. d) LFA2. Conglomeratic channel-fill deeply incised into sandstones and mudstones. Middle part of section Samre North.

a 6 1566 1567 1568 1569 1570 1571 1572 1573

e) LFA2. Large mudstone boulder (m) within a conglomeratic channel-fill. Lower part of section Samre North. f) LFA2. Outwash fan foreset beds composed of conglomerates, sandstones and mudstones. Upper part of section Dugum. g) LFA3. Symmetric and asymmetric ripple marks in sandstone-mudstone succession. Lower part of section Megab West. h) LFA3. Climbing ripple cross-lamination in fine-grained sandstones. Lower part of section Megab West.

1574 1575 1576 1577 1578

Plate 3. Lithofacies associations LFA4 – LFA6 a) LFA4. Laminated mudstones, siltstones and fine-grained sandstones. Section Megab Southwest. b) LFA4. Laminated to thin-bedded mudstones and sandstones. Soft-sediment

1579

deformation caused by syn-depositional slumping. Section Megab Southwest.

1580

c) LFA4. Wedge-shaped soft-sediment deformation structure, possibly caused by a

1581

grounding ice floe or small iceberg. Section Megab Southwest.

1582

d) LFA5. Basal diamictite overlying metamorphic schists. Section Dugum.

1583

e) LFA5. Sandy basal diamictite in section Edaga Arbi West.

1584

f) LFA5. Basal diamictite with smoothly rounded upper surface overlain by dropstone-

1585 1586 1587 1588 1589 1590

bearing diamictite and sandstone beds. Section Megab Southwest. g) LFA6. Stacked diamictites that show soft-sediment deformation structures and clast lags. Section Dugum. h) LFA6. Debris flow diamictite in mudstones. Middle part of section Samre West.

a 6 1591 1592 1593 1594 1595 1596 1597 1598 1599 1600 1601 1602

Plate 4: Lithofacies associations LFA7 – LFA9 a) LFA7. Rafts of conglomerate and sandstone (r) in clast-bearing massive mudstone. Middle part of section Megab West. b) LFA7. Clastic dykes (d) and sandstone rafts (r) in clast-bearing massive mudstone. Middle part of section Megab West. c,d) LFA7. Soft-sediment normal faulted ripple cross-bedded sandstones in massive to laminated mudstone. Section Bizet. e) LFA8. Tilted and soft-sediment thrust-faulted sandstones truncated by coarse-grained outwash fan sediments. Section Edaga Hamus. f) LFA8. Soft-sediment thrust-faulted outwash fan sediments. Lower part of section Megab West. g) LFA9. Massive to ripple cross-laminated sandstones with symmetrical ripple marks

1603

interbedded with mudstones. Upper part of section Megab West.

1604

h) LFA9. Root casts in post-glacial sediments in section Samre West.

1605 1606

Plate 5: Sections in the Mekelle Basin

1607

a) Dropstones in a laminated mudstone-siltstone succession. Section Samre West.

1608

b) Beds of hyperconcentrated flow sandstones containing floating clasts, in dropstone-

1609

rich mudstone-siltstone succession. Lower part of section Samre West.

1610

c) Conglomeratic outwash fan channel-fill. Upper part of section Megab West.

1611

d) Conglomeratic and sandy outwash fan foreset beds. Section Samre North.

1612

e) Stacked conglomeratic outwash fan channel-fills. Middle part of section Samre North.

1613

f) Outwash fan foreset beds. Upper part of section Dugum.

1614

g, h) Subglacial sheet-shape tillite with boulder pavement overlain by waterlain tillites and

1615

debris flow sediments. Section Megab West.

a 6 1616 1617 1618 1619 1620

Plate 6: Sections in the Mekelle Basin and in the Adigrat-Adua Ridge a) Thick succession of deglaciation to post-glacial mudstones truncated by channel-fill sandstone. Section Megab Southwest. b) Channelized hyperconcentrated flow sandstones with floating clasts in dropstone-rich

1621

mudstone-siltstone succession. Section Megab Southwest.

1622

c) Mudstone-dominated middle part of section Edaga Arbi Town.

1623

d) Thrust-faulted sandy basal diamictite in section Edaga Arbi West.

1624

e) Soft-sediment normal faulted sandstones in clast-bearing mudstone. Section Edaga

1625

Arbi North.

1626

f) Thrust-faulted sandstone lens in mudstone. Section Bizet.

1627

g) Massive and bedded sandy diamictites overlying soft-sediment folded and injected

1628 1629 1630 1631 1632 1633

sandstones. Section Adigrat North. h) Soft-sediment sheared mudstones and sandstones in section Adigrat North.

a 6 1634

Tables

1635

Tab. 1: Lithofacies types in LPIA sediments in northern Ethiopia.

1636

Tab. 2: Lithofacies associations in LPIA sediments in northern Ethiopia.

1637 1638

Figure 1 BW Review

Figure 1 Colour Review

Figure 2 Black and White

Figure 2 Color

Figure 3 BW Review

Figure 3 Colour Review

Figure 4 Black and White

Figure 4 Color

Figure 5 BW Review

Figure 5 Colour Review

Figure 6 Black and White

Figure 6 Color

Plate 1 Black and White

Plate 1 Color

Plate 2 Black and White

Plate 2 Color

Plate 3 Back and White

Plate 3 Color

Plate 4 Black and White

Plate 4 Color

Plate 5 Black and White

Plate 5 Color

Plate 6 Black and White

Plate 6 Color

Table 1

Lithofacies

Description

Interpretation

Fm

Massive or crudely laminated mudstones. Dark grey to green variety often contains scattered sand- to bouldersized clasts. Multi-coloured, often red to brown variety shows bioturbations and root traces but lacks outsized clasts. Beds are up to 4 m thick.

Rapid settling of suspended fines in standing water. Outsized clasts represent dropstones. Multi-coloured variety formed in post-glacial alluvial or shallow marine environments.

Fl

Partly rhythmically laminated mudstones. Rhythmic couplets are commonly few mm to 3 cm thick, composed of basal silt to fine-grained sand and of overlying mud. Basal contacts are mostly sharp; contacts between coarseand fine-grained laminae are either sharp or gradational. Lamination often deformed by outsized clasts.

Suspension-settling of fines and surge-type low-density turbidity flows as well as hyperpycnal underflows. Outsized clasts (dropstones) signal ice-contact during deposition.

Sr

Ripple cross-stratified or climbing-ripple stratified, fineto medium-grained, well sorted sandstones. Ripple structures are predominantly asymmetric; climbing ripples are often supercritical.

Current and wave ripples; climbing ripples document rapid deposition from sediment-laden turbulent currents.

Sh

Horizontally laminated, bedded or low-angle stratified, fine- to coarse-grained, moderately to well sorted sandstones. Individual beds are ~1 to 3 cm thick, cosets are up to 0.7 m in thickness.

Low-amplitude bed waves. Formed at the transition from dune to upper flow regime plane bed conditions.

Slw

Low-angle wavy to low-angle convex-up stratified, medium-grained to pebbly sandstones, in part with scattered outsized clasts. Solitary bedforms are a few decimetres in height.

Upper flow regime bedforms, probably in part antidunes.

Sx

Trough or tabular-planar cross-bedded, fine- to coarsegrained, moderately to well sorted sandstones. Form lenses with concave-up erosive basal contacts. Cosets are up to 8 m thick.

Sinuous to straight crested subaqueous sand dunes.

Sm

Massive, partly normal graded coarse-grained siltstones to sandstones; in part calcareous, partly ripple crosslaminated. Basal contacts are sharp and flat, except of sporadic sole marks. Upper contacts are either abrupt or gradational. Forms sheet-like beds or lenses typically 1 to 10 cm thick. Arthropod tracks are occasionally present on bedding planes.

Low to high density turbidity flows. Carbonate content might stem from reworked Neoproterozoic carbonates.

Smc

Massive, moderately to poorly sorted fine- to coarsegrained sandstones with floating granules or gravels. Basal contacts are sharp to erosive, upper contacts are commonly sharp. Forms sheets, lenses or trough-shaped beds up to 40 cm thick.

Hyperconcentrated flows.

Gx

Tabular to trough cross-bedded, clast-supported, moderately to poorly sorted conglomerates. Clasts are subrounded to rounded. Basal contacts are erosive and concave-up. Forms trough-shaped to lenticular bodies up to 1.2 m thick.

Subaqueous gravel dunes or simple gravel bars. Deposited by high-energetic turbulent flows in gravel-bed channels.

Gcm

Clast-supported, massive to inverse graded, sandy conglomerates. Contain basement and mudstone clasts up to 1 m in diameter. Basal contacts are erosive and concave-upward or flat. Tops are sharp and often irregular, with protruding outsized clasts. Forms lenses, sheets or gentle to moderate dipping wedges up to 1.5 m thick.

Grainflows or hyperconcentrated flows. Lenses represent channelised flows, whereas inclined wedges formed as fan lobes.

density

Gmm

Matrix-supported, massive to crudely stratified conglomerates. Clasts float in a sandy to muddy matrix. Basal contacts are concave-upward and erosive. Forms lenticular to trough-shaped bodies at the maximum 1.8 m thick.

Channelised debris flows.

Dms

Matrix supported, crudely stratified to massive, partly soft-sediment deformed muddy to sandy diamictites with variable amounts of clasts up to boulder size. Contains rafts or deformed lenses of other lithofacies. Overlays sharp basal contacts.

Debris flow sediments or flow tillites.

Dmm(m)

Massive, primarily mudstone-rich diamictites that contain variable amounts of scattered, facetted and/or striated clasts up to 1.2 m in diameter. Clasts are in part vertically oriented. Basal contacts are gradational. Commonly forms beds several metres thick.

Rain-out (waterlain) tillites.

Dmm(s)

Commonly clast-rich sandy, massive to crudely stratified and in part foliated diamictites. Overlay sharp basal contacts and occasionally contain deformed lenses of well sorted sediment as well as boulder pavements. Forms thin sheets up to 2.5 m thick.

Subglacial basal tillites.

(d) = with dropstones

Table 2

Lithofacies association

Major lithofacies

Minor lithofacies

Geometry

Interpretation

LFA1

Gx, Gcm

Sh, Slw, Sx, Dmm

wedges (to sheets)

subaerial outwash fan or delta deposits

LFA2

Gcm, Gmm, Gx, Smc, Sh, Slw, Sx

Dmm, Fm

wedges

proximal subaqueous outwash fan deposits

LFA3

Sm, Sr

Fm, Fl

wedges to sheets

medial to distal subaqueous outwash fan or delta deposits

LFA4

Fl, Fm,

Sm, Smc, Sr, Dmm, Dms

sheets

basin floor sediments

Sm, Sh

sheets

subglacial tillite complexes

lenses

debris flow deposits

irregular sheets or wedges

slump deposits

irregular

push moraines

sheets

post-glacial alluvial plain or coastal plain sediments

Smc, Gcm LFA5

Dmm, Dms

LFA6

Dmm, Dms

LFA7

Fm

LFA8

Sr, Sm, Sx, Gcm, Gx, Dms

LFA9

Sr, Sh, Sm, Sx, Fm

Gcm, Sr, Sm, Sh,

Gcm