Ordovícico

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The Late Ordovician glacial sedimentary system of
the North Gondwana platform
JEAN-FRANCOIS GHIENNE*, DANIEL PAUL LE HERON*†, JULIEN MOREAU*,
MICHAEL DENIS‡ and MAX DEYNOUX*
*Ecole et Observatoire des Sciences de la Terre, Centre de Géochimie de la surface, CNRS-UMR 7517, 1 rue Blessig,
67084 Strasbourg, France (e-mail:ghienne@illite.u-strasbg.fr)
†Institut für Geologie, Leibniz Universität Hannover, Callinstrasse 30, D-30167, Germany
‡UMR 5561, Biogéosciences, Université de Bourgogne, 6 Bd Gabriel, 21 000 Dijon, France

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 lowerpalaeo-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 tothe 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 ofLate Ordovician glacial deposits and other ancient glacial successions.
Keywords Glacial record, Hirnantian, North Africa, ice stream, sequence stratigraphy.

INTRODUCTION

The Late Ordovician glacial record comprises
extensive exposures distributed over the former
North Gondwana cratonic platform. The size of
this domain is >1500 km from south to north
(in present-day coordinates), andextended from
Mauritania to Arabia. Southern regions were

positioned near the ice centres while the northern
part of the platform was located at lower palaeolatitudes (Fig. 1). During the 1960s –1980s, studies
in Morocco (Destombes, 1968a, Hamoumi, 1988),
Algeria (Beuf et al., 1971), Libya (Klitzsch, 1981;
Massa, 1988), Mauritania (Deynoux, 1980; 1985;
Deynoux and Trompette, 1981) and Arabia(McLure,
1978; Vaslet, 1990; McGillivray and Husseini, 1992)

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Fig. 1 The North Gondwana platform during the Hirnantian glacial event. (A) Main names cited in the text (see also

Fig. 8). Names in bold indicates study areas comprising the data base. The black star locates the pre-glacial (Late
Ordovician)...