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The Late Ordovician glacial record

Abstract : An outline of the North Gondwanan, Late Ordovician glacial record is proposed. The related palaeogeographic domain extended from southern high palaeo-latitudes (southeastern Mauritania, Niger) to northern lower palaeo-latitudes (Morocco, Turkey, Sardinia) and covered a more than 4000 km-wide section perpendicular to ice-flow lines. Glacial advances are spatially and stratigraphically evidenced by subglacial structures (tunnel valley, shear zones) delineating glacial erosion surfaces, while deglaciation periods and interglacial intervals are reflected in fluvio-deltaic, glaciomarine or shallow shelf successions. A major mid-Hirnantian deglaciation event subdividing the Hirnantian glaciation in two first-order cycles is recognised. Each cycle comprises 2-3 glacial phases separated by several hundreds kilometres ice-front retreats. 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. Glaciomarine outwash diamictites are developed at or near the maximum position of the ice-front. During ice-sheet recession, a relatively thin sediment cover blankets the erosion surfaces in foredeepened ice-stream areas. Marine-terminating ice fronts then evolve later into more slowly retreating, land-terminating ice fronts. In adjacent areas (inter ice-stream) where a more gradual ice-sheet recession occurred, fluvio-glacial deposits prevailed. The progradation of a delta-shelf system, coeval with fluvial aggradation, characterises the late glacial retreat to interglacial conditions (Ghienne et al., 2007, The Late Ordovician glacial sedimentary system of the North Gondwana platform. Special Publication n°39, International Association of Sedimentologists, Blackwells, pp. 295-319). This model for sequence stratigraphic interpretation of the North Gondwana glacial record allows other proxies of Late Ordovician environmental changes derived from low palaeolatitude settings (e.g. sea level change, isotopic excursions or biodiversity) to be confronted to the reconstructed ice-sheet evolution (timing, extent) throughout the Hirnantian. Of particular interest are the Moroccan successions that comprise in intermediate palaeolatitudes a full glacial record with, in addition, evidences for Katian (pre-Hirnantian) glacially-driven lowstands. The deglaciation of the North Gondwana resulted in a major, latest Ordovician transgression, which was followed during the Lower Silurian by an additional glacio-eustatic sea-level rise reflecting the final recession of the ice-sheet in the continental interior. However, glacier influences maintained throughout the Lower Silurian as shown by the South American glacial record. From an ice-free, pre-glacial status (resp. post-glacial) to full glacial conditions (Hirnantian glacial maximum), the cumulative amplitude of the inferred glacially driven sea-level fall (resp. rise) has probably been in the 150-250 m range. The Hirnantian event may have account for at least 100 m, depending on the growth and extent of pre-Hirnantian glaciers inside the Gondwana landmass.
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Submitted on : Tuesday, April 21, 2009 - 8:57:29 AM
Last modification on : Thursday, April 23, 2020 - 2:26:28 PM

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  • HAL Id : insu-00377126, version 1

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Jean-François Ghienne, Alfredo Loi, Florentin Paris, Jean-Loup Rubino. The Late Ordovician glacial record. Palaeozoic climates, Aug 2008, Lille, France. ⟨insu-00377126⟩

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