The Late Ordovician Glacial Sedimentary System of the North Gondwana Platform

Abstract

The Late Ordovician (Hirnantian) glaciation is examined through the North Gondwana record. This domain extended from southern high palaeo-latitudes (southeastern Mauritania, Niger) to northern lower palaeo-latitudes (Morocco, Turkey) and covered a more than 4000 km-wide section perpendicular to ice-flow lines. A major mid-Hirnantian deglaciation event subdividing the Hirnantian glaciation in two first-order cycles is recognised. As best illustrated by the glacial record in western Libya, each cycle comprises 2-3 glacial phases separated by ice-front retreats several hundreds kilometres to the south. From ice-proximal to ice-distal regions, the number of glacial surfaces differentiates (i) a continental interior with post-glacial reworking of the glacial surfaces), (ii) a glaciated continental shelf that is subdivided into inner (1-2 surfaces), middle (2-5 surfaces) and outer (a single surface related to the glacial maximum) glaciated shelves, and (iii) the non-glaciated shelf. Ice-stream-generated glacial troughs, 50-200 km in width, cross-cut these domains. These troughs are zones of preferential glacial erosion and subsequent sediment accumulation. A glacial depositional sequence, bounded by two glacial erosion surfaces, records one glacial phase. The position either within or outside a glacial trough controls the stratigraphic architecture of a glacial sequence. Glaciomarine outwash diamictites are developed at or near the maximum position of the ice-front. During ice-sheet recession, and in an ice-stream-generated trough, a relatively thin sediment cover blankets the foredeepened erosion surface. An initial rapid ice-sheet withdrawal is inferred. Marine-terminating ice fronts then evolve later into more slowly retreating, land-terminating ice fronts. In adjacent inter-stream areas where a more gradual ice-sheet recession occurred, fluvioglacial deposits prevailed. The progradation of a delta-shelf system, coeval with fluvial aggradation, that may be locally interrupted by a period of isostatic rebound, characterises the late glacial retreat to interglacial conditions. This model should facilitate the sequence stratigraphic interpretation of Late Ordovician glacial deposits and other ancient glacial successions.

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

“…The Late Ordovician Hirnantian is characterized by the rapid growth of large ice sheets (e.g., Ghienne et al, 2007;Loi et al, 2010;Finnegan et al, 2011). Recent geochemical data suggest an abrupt tropical SST cooling at that time (Trotter et al, 2008;Finnegan et al, 2011).…”

“…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).…”

“…Trotter et al (2008) proposed a critical role of the oceanic thermal state during the GOBE, showing that the long-term cooling from a very warm ocean to present-day-like temperatures during the Early to Middle Ordovician may have led to optimal conditions for widespread taxonomic radiations and the appearance of complex communities. Cooler conditions in the Late Ordovician are generally associated with the onset of large ice sheets over the southern Gondwana (e.g., Ghienne et al, 2007). The growth and the reduction of these ice sheets are concomitant with the two pulses of the global extinction (Sheehan, 2001).…”

Climate change and the selective signature of the Late Ordovician mass extinction

“…The primary pulse of extinction near the Katian/Hirnantian stage boundary closely coincided with the rapid growth of south polar ice sheets on Gondwana (1)(2)(3)(4). Expansion of continental ice sheets was accompanied by substantial cooling of the tropical oceans (5, 6), a major perturbation of the global carbon cycle (7)(8)(9) and a large drop in eustatic sea level (2,5,10,11), which drained the vast cratonic seaways that characterized the Late Ordovician world (12). Extinction rates were particularly high around the tropical paleocontinent of Laurentia (13) where retreat of cratonic seas drove a sharp reduction in the area of preserved sedimentary rock between Katian and Hirnantian time (Fig.…”

Thuringian affinity of the Silurian–Lower Devonian succession from the Eastern Taurus, Turkey

“…Figure 9A) This time interval is linked to the Late Ordovician glaciation, whose culmination is during the early-midHirnantian (Page et al, 2007;Ghienne et al 2007Ghienne et al , 2010Loi et al, 2010;Melchin et al, 2013 and references therein). Paleoglacial reconstructions suggest that southern Turkey was situated at high to intermediate paleolatitudes (higher than 45°) at the end of the Ordovician (Ghienne et al, 2007Kozlu and Ghienne, 2012). Under such climatic conditions the cold, heavier, oxygen-rich surface waters sink into the depths and oxygenate the bottom sediments.…”