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Chapter 3. the Early Palaeozoic Glacial Deposits of Gondwana

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Chapter 3. the Early Palaeozoic Glacial Deposits of Gondwana CHAPTER THE EARLY PALAEOZOIC GLACIAL DEPOSITS OF GONDWANA: OVERVIEW, CHRONOLOGY, AND 3 CONTROVERSIES D.P. Le Heron1, S. Tofaif1 and J. Melvin2 1Royal Holloway, University of London, Surrey, United Kingdom, 2Saudi Aramco, Dhahran, Eastern Province, Saudi Arabia 3.1 INTRODUCTION 3.1.1 OVERVIEW There is a widespread record of deposits of Early Palaeozoic age that provide evidence for glacia- tion (Fig. 3.1). As this chapter will show, much of the record of the Early Palaeozoic is restricted to the Ordovician, although there is some evidence for glaciations of early Silurian age in some regions. Much of the sedimentary record is archived in present-day North Africa and Arabia, pro- viding an interesting contrast between these modern-day hyper-arid settings and an ancient, 443- million-year-old (Hirnantian) glacial record. As a result of the outcrop distribution in these modern desert settings, rock exposure is typically excellent, and superb examples of ancient glacial land- forms and successions can be documented. In this chapter, we provide illustrated examples through- out from the North African and Arabian record in particular, paying particular attention to exploring what makes this glacial sedimentary record unique and important. Compared to other Precambrian and Phanerozoic glacial records, that of the Hirnantian is unusually sandy. This simple observation demands an explanation, not least because the very sand-prone nature of the deposits accounts for their major resource importance as regional oil and gas reservoirs (Huuse et al., 2012). 3.1.2 PALAEOGEOGRAPHIC CONTEXT AND ORIGINS OF THE GLACIATION The palaeogeographic context at 443 Ma, during the Hirnantian, explains the present-day distribution of glaciogenic deposits of this age, including how extensive outcrop belts occur in the modern-day Sahara and Arabian deserts. At 443 Ma, parts of the Gondwana supercontinent became glaciated. This supercontinent, which comprised South America, Africa, Madagascar, Arabia, India, East Antarctica, and Australia (Fig. 3.2)(Allen, 2007; Torsvik and Cocks, 2009), extended from the South Pole to the tropics during the Early Palaeozoic (Torsvik and Cocks, 2009), and hence it records the full spectrum of warm to polar climate indicators in its sedimentary archive (Scotese et al., 1999). Past Glacial Environments. DOI: http://dx.doi.org/10.1016/B978-0-08-100524-8.00002-6 © 2018 Elsevier Ltd. All rights reserved. 47 48 CHAPTER 3 THE EARLY PALAEOZOIC GLACIAL DEPOSITS OF GONDWANA FIGURE 3.1 Outline of present-day countries which contain Early Palaeozoic glacial deposits highlighted in yellow, with the black arrows indicating the direction of palaeo-ice flow. From Hambrey, M., 1985. The Late OrdovicianEarly Silurian glacial period. Palaeogeogr. Palaeoclimatol. Palaeoecol. 51, 273289; Hambrey, M.J., Harland, W.B., 1981. Earth’s pre-Pleistocene glacial record. Cambridge University Press, London, 1022 pp.; Ghavidel-syooki, M., A´lvaro, J.J., Popov, L., Pour, M.G., Ehsani, M.H., Suyarkova, A., 2011. Stratigraphic evidence for the hirnantian (latest ordovician) glaciation in the zagros mountains, Iran. Palaeogeogr. Palaeoclimatol. Palaeoecol. 307, 1À16. doi:10.1016/j. palaeo.2011.04.011; Grahn, Y., Caputo, M.V., 1992. Early Silurian glaciations in Brazil. Palaeogeogr. Palaeoclimatol. Palaeoecol. 99, 9À15. doi:10.1016/0031-0182(92)90003-N; Le Heron, D.P., Craig, J., 2008. First-order reconstructions of a Late Ordovician Saharan ice sheet. J. Geol. Soc. Lond. 165, 19À29. doi:10.1144/0016-76492007-002; Le Heron, D.P., Craig, J., Etienne, J.L., 2009. Ancient glaciations and hydrocarbon accumulations in North Africa and the Middle East. Earth-Sci. Rev. 93, 47À76; Monod, O., Kozlu, H., Ghienne, J.F., Dean, W.T., Gu¨nay, Y., Le Heriss´ e,´ A., et al., 2003. Late Ordovician glaciation in southern Turkey. Terra Nov. 15, 249À257. doi:10.1046/j.1365-3121.2003.00495.x; Robardet, M., Dore,´ F., 1988. The Late Ordovician Diamictic Formations from SW Europe: N Gondwana Glaciomarine Deposits. Palaeogeogr. Palaeoclimatol. Palaeoecol. 66, 19À31; Storch,ˇ P., 1990. Upper Ordovician—lower Silurian sequences of the Bohemian Massif, central Europe. Geol. Mag. 127, 225À239. doi:10.1017/S0016756800014503; Vaslet, D., 1990. Upper Ordovician glacial deposits in Saudi Arabia. Episodes 13, 147À161. 3.1 INTRODUCTION 49 FIGURE 3.2 Reconstruction of Gondwana during Early Ordovician and Mid-Silurian from Torsvik and Cocks (2009). The formation of Gondwana resulted in widespread amalgamation of terranes, intracratonic moun- tain building, and consequently extensive weathering (Berry and Finney, 2001; Smith, 1997). The decrease of atmospheric CO2 (by increased organic activity and continental weathering (Brenchley et al., 1994; Young et al., 2004) is considered to be one of the key drivers for the Hirnantian glaciation. The increase in weathering activity could be responsible for the decrease in atmospheric CO2 and in turn induced the start of the Hirnantian glaciation (Kump et al., 1999; Berry and Finney, 2001). In addition, the formation of high grounds could have accelerated the formation of mountain glaciers which in turn helped increase the weathering rate (Young et al., 2004; Raymo and Ruddiman, 1992). The opening and closure of oceanic gateways is presumed to be an important contributor to cooling and was possibly a primary cause of the Hirnantian glaciation (Smith and Pickering, 2003). The closure of an isthmus in central America may have isolated Gondwana from the warming effects of circumpo- lar currents, promoting insulation and thus refrigeration (Smith and Pickering, 2003). 50 CHAPTER 3 THE EARLY PALAEOZOIC GLACIAL DEPOSITS OF GONDWANA 3.2 EXTENT OF GLACIATION AND CHRONOLOGY 3.2.1 OVERVIEW Two end-member scenarios were proposed by Ghienne (2003) with regard to ice sheet extent in the Late Ordovician. These were (1) a large, pan-African ice sheet and (2) disconnected, though likely synchronous, ice sheets (Fig. 3.3). These ideas were tested by Le Heron and Dowdeswell (2009) who calculated the likely volume of water stored in the ice sheets in both end-member cases, concluding that the smaller scenario was probably the most realistic based on available data. In a similar way, it is important to establish whether the ice caps around GondwanaÀparticularly those that extended into South AmericaÀwere part of a single, ‘short snap’ Hirnantian event, or whether they evolved, and persisted, during a longer cold phase (Delabroye and Vecoli, 2010). The debate concerning whether the South American deposits are latest Ordovician or early Silurian is consid- ered below, where we explain ‘the essentials’ of regional subdivisions in the Late Ordovician record, in order to communicate our understanding of the glacial sedimentary system and to explain the controversies surrounding timing and extent of ice masses on Gondwana. We will briefly explain, with reference to Fig. 3.4, the glacial stratigraphy of four key areas: (1) North Africa, (2) Arabia, (3) southwest Europe, and (4) South America. 3.2.2 NORTH AFRICA: MOROCCO, ALGERIA, LIBYA In North Africa, Late Ordovician glacial deposits sit unconformably above a paralic succession of variable age (typically mid-Ordovician to preglacial Hirnantian (Loi et al., 2010; Moreau, 2011), exhibit name changes across regional borders (Fig. 3.4), and are characterized by their internal stratal complexity. Not all authors recognize or even use the official formation terminology, owing to the dramatic facies changes and inferred depositional setting over several kilometres. Nevertheless, in Libya, the Mamuniyat Formation is widely recognized. This is correlated with the Tamadjert Formation in Algeria and with the Upper Second Bani Formation in southern Morocco (Fig. 3.4). Strata in northern Morocco are assigned to a multitude of different formations owing to their location within disparate basement inliers (see Le Heron et al., 2007). Study has focused particularly on the Libyan successions around the borders of Al Kufrah Basin in the east (Le Heron et al., 2010, 2014; Le Heron and Howard, 2010), and the Murzuq Basin in the west (Le Heron et al., 2004, 2005, 2013; Ghienne et al., 2003, 2007, 2010a, 2013; Girard et al., 2013a,b, 2015). The westernmost flank of the Murzuq Basin is part of the extensive Tassili N’Ajjer outcrop belt, where the classic studies of Beuf et al. (1971) have motivated highly detailed, recent efforts to unravel the complexities of the glacial sedimentology and stratigraphy (Deschamps et al., 2013). In this latter paper, the recognition of two main types of glacial erosion surface associated with glacial palaeovalleys: (1) a sharp, irregular contact recording predominantly meltwater processes, and (2) a ‘smoother’ type of surface associated with soft-sediment striations (see Box 3.1) and soft-sediment deformation, primarily recording subglacial shearing. These ideas have simultaneously been developed on the equivalent succession in the Anti-Atlas of Morocco (Clerc et al., 2013; Ravier et al., 2015). 3.2 EXTENT OF GLACIATION AND CHRONOLOGY 51 FIGURE 3.3 Reconstruction of Gondwana during the Hirnantian with the location and size of posited ice sheets superimposed. Distribution of ice sheets initially from Ghienne (2003). Figure from Le Heron, D.P., Craig, J., Etienne, J.L., 2009. Ancient glaciations and hydrocarbon accumulations in North Africa and the Middle East. Earth-Sci. Rev. 93, 47À76. 52 CHAPTER 3 THE EARLY PALAEOZOIC GLACIAL DEPOSITS OF GONDWANA FIGURE 3.4 Regional Early Palaeozoic stratigraphy of North Africa and part of the Middle East, with particular focus on glaciogenic strata of Hirnantian age. Note the complex regional variance in stratigraphic names. Modified after Le Heron, D.P., Craig, J., Etienne, J.L., 2009. Ancient glaciations and hydrocarbon accumulations in North Africa and the Middle East. Earth-Sci. Rev. 93, 47À76 and updated to reflect new subdivisions of Melvin, J., 2015. Lithostratigraphy and depositional history of Upper Ordovician and lowermost Silurian sediments recovered from the Qusaiba-1 shallow core hole, Qasim region, central Saudi Arabia. Rev. Palaeobot. Palynol. 212, 3À21. doi:10.1016/j.revpalbo.2014.08.014 in Saudi Arabia.
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