Hirnantian glacial and deglacial record in SW Djado Basin (NE Niger).

Abstract

Pluridisciplinary fieldwork highlights features generated by an extended ice-sheet in the Djado Basin during the Hirnantian. Two glacial palaeovalley systems associated with glacial pavements and separated by thin glaciomarine interstadial series are revealed. Rigid glacial pavements characterised by abrasion erosion are differentiated from soft glacial pavements characterised by soft-bed deformation. Glacial pavements are associated with subglacial bedforms such as megaflutes, flutes and meltwater channels. They are also associated with clastic dykes and glaciotectonic structures such as deformed flutes, subglacial folds and duplex structures. This record demonstrates that ice was warm-based and flowed rapidly on the highfluid-pressure soft substrate, as for ice streams. The erosional glacial landscape is typical of areal scouring, and the depositional sediment-landform assemblage corresponds to subglacial processes. These data afford a reconstruction of glacial events which is consistent with the two polyphased low-frequency glacial cycles inferred in previous studies. During interstadial and postglacial stages, grabens, normal faults, radial extensional microfaults and extensional dihedrons were generated by extensional tectonics during glacio-isostatic rebound. In sectors highly affected by this tectonics, doleritic dykes reflect a basal crust fusion increase induced by adiabatic decompression.

The Late Ordovician Glacial Sedimentary System of the North Gondwana Platform

“…Here, fauna-rich marine shales, originally considered as pre-Hirnantian strata (Legrand, 1993), and now attributed to the extraordinarius Biozone (P. Storch, pers. comm., 2004) were deposited between two major glacial erosion surfaces (Denis et al, 2007). These data imply that the transgression advanced deep into North Africa at least within erosional troughs.…”

“…The inner, middle and outer domains of the glaciated continental shelf were subject to multiple phases of ice-sheet growth and decay. On the inner glaciated shelf, typified by the Niger succession (Denis et al, 2007), two glacial erosion surfaces and associated overlying sequences are recorded, and, are separated by the mid-Hirnantian graptolitic shales (see above). This domain was deglaciated during the mid-Hirnantian, but remained glacierised during phases of ice-front retreat within each of the two first-order glacial cycles.…”

“…Renewed interest arose in the 1990s, when oil companies recognised the association of syn-glacial strata (reservoir rocks) with lower Silurian shales (source rocks) as one of the most significant plays in the North African Lower Palaeozoic succession (Lüning et al, 2000). Recent work has included detailed field and subsurface studies conducted in Mauritania (Ghienne, 1998;Ghienne and Deynoux, 1998;Ghienne, 2003), Morocco (Ouanaimi, 1998;Sutcliffe et al, 2000Sutcliffe et al, , 2001Le Heron et al, 2007), Algeria (Hirst et al, 2002;Eschard et al, 2005), Niger (Denis et al, 2007), Libya (McDougall and Martin, 2000;Smart, 2000;Ghienne et al, 2003;El-ghali, 2005;Moreau, 2005), Jordan (Abed et al, 1993;Powell et al, 1994;Turner et al, 2005), Saudi Arabia (Senalp and Al-Laboun, 2000) and the Horn of Africa (Ethiopia and Eritrea, Kumpulainen, 2005). In addition to these areas, studies have also been undertaken around the northern Gondwana periphery to better provide an overall understanding of the glacial record as a whole, e.g.…”

Effect of the Ordovician paleogeography on the (in)stability of the climate

“…Spjeldnaes (1962), working on floro-faunal data, was the first to postulate a glaciation during the Ordovician. Since then, numerous studies widely documented the Ordovician glacial sedimentary record in North Africa (e.g., Beuf et al, 1971;Denis et al, 2007;Ghienne et al, 2007), South Africa (e.g., Young et al, 2004) and South America (e.g., Díaz-Martínez and Grahn, 2007). Dating these deposits remains highly difficult (e.g., Díaz-Martínez and Grahn, 2007) and geochemical methods bring conflicting results (see for example Trotter et al, 2008 andFinnegan et al, 2011).…”

The Cambrian-Ordovician succession in the Ougarta Range (western Algeria, North Africa) and interference of the Late Ordovician glaciation on the development of the Lower Palaeozoic transgression on northern Gondwana

“…As a consequence, the combination of glacial erosion and post-glacial high rate of sea-level rise forced shorelines to shift drastically to the south. Early Silurian shorelines reach the south of the Koufra Basin (Libya-Chad boundary; Lüning et al 2000), and were located south of the Djado area in Niger (Legrand 2003, Denis et al 2007 or to the SE of the Hodh area in Mauritania , Ghienne 2003. From these southern locations, Silurian shorelines have then migrated throughout the platform to the NW (Bellini & Massa 1980, Lüning et al 2000.…”