Jean-François Ghienne

Late Pleistocene and Holocene dune activity and wind regimes in the western Sahara Desert of Mauritania

The western Sahara Desert in Mauritania is dominated by extensive sand seas consisting largely of linear dunes. Analyses of Landsat images, geomorphic and stratigraphic studies, and optically stimulated luminescence dating of dunes in the Azefal, Agneitir, and Akchar sand seas provide evidence that three main generations of dunes were formed during the periods 25–15 ka (centered around the Last Glacial Maximum), 10–13 ka (spanning the Younger Dryas event), and after 5 ka. The wind regimes that occurred during each of these periods were significantly different, leading to the formation of dunes on three distinct superimposed trends—northeast, north-northeast, and north—and the development of the sand seas as composite geomorphic features.

Late Ordovician sedimentary environments, glacial cycles, and post-glacial transgression in the Taoudeni Basin, West Africa

Abstract The sedimentary record and worldwide palaeontological and isotopic data support the existence of short-lived, latest Ordovician (Hirnantian) ice sheets over western Gondwanaland. Located in the cratonic Taoudeni Basin (West Africa), the study areas are distributed across a 500-km-long profile ranging from ice-proximal to ice-distal depositional conditions (Hodh and Adrar areas, respectively; central and northwestern Mauritania). Glaciation-related but mainly non-glacial deposits form the Tichitt Group, which rests upon Cambrian–Ordovician rocks on top of a basin-wide erosional surface. The glacial record consists of depositional successions bound by unconformities of glacial origin in ice-proximal areas, or of sub-aerial origin in ice-distal areas. Facies associations reflect a variety of environments (braided streams, flood-dominated alluvial plains, delta plain to delta slope, tidal or storm-wave influenced shallow-marine settings). The upper bounding surface of the Tichitt Group generally corresponds to a wave–ravinement surface, overlain by uppermost Ordovician to lower Silurian shales. Four, inter-regionally distributed units are vertically superimposed and laterally juxtaposed. A unit is a hundreds of kilometres long sedimentary body, up to 100 m thick, which is laterally discontinuous as it typically infills palaeodepressions or palaeovalleys. In ice-proximal areas, aggrading fluvial deposits are identified. Coarse-grained braided stream deposits, including glacial surfaces related to minor glacial advances, predominate upstream. Downstream, finer-grained flood-dominated fluvial deposits are identified. In ice-distal areas, thick fluvial-dominated delta sediments are deposited. Each of the four units, of climatic significance, and built under high accommodation conditions, records a recession stage of the northern Gondwana ice sheet following a major glacial advance. The overall backstepping of the glacial units characterises the large-scale depositional architecture. The fourth unit, characterised by glaciomarine deposits in ice-proximal areas, and non-glacial, bioturbated storm-dominated deposits in ice-distal areas, records the final retreat of the northern Gondwana ice sheet. The lower bounding surface of this later glacial unit marks a Late Hirnantian major transgressive surface of a Late Ordovician–Silurian relative sea-level rise starting before the glaciation.

Large-scale channel fill structures in Late Ordovician glacial deposits in Mauritania, western Sahara

Abstract Late Ordovician siliciclastic glacial and related deposits on the North Gondwana continental shelf comprise striking incised features referred to as large-scale channel-fill structures. These are described from exposures and aerial photographs in two areas (Adrar and Hodh) in Mauritania. The channel-fill structures are up to several kilometres in length and several hundred metres wide. They are slightly sinuous narrow sandstone bodies deeply incised into the Late Ordovician glacial drift and the Cambro–Ordovician bedrock. Palaeogeographical reconstructions indicate that these structures were located in the ice-marginal zone. They are preferentially oriented parallel to palaeo-ice-flow directions. Sedimentary facies analysis reveals three vertically stacked architectural units infilling a U-shaped erosional basal unconformity. Unit 1 is thin and made up of conglomeratic sandstones of various origins comprising debris-flows, deltaic-like progradational foresets and trough cross-stratified sands. In Adrar, architectural unit 2 shows sheet-like, evenly laminated, fine- to medium-grained, well sorted sandstones characteristic of a high-energy marine environment. In contrast, in the Hodh area, unit 2 displays vertically stacked successions of coarse-grained sandstones that indicate high-discharge sediment-laden flows; these are interpreted as meltwater sediments. In both Adrar and Hodh, architectural unit 3 is made up of gravelly coarse-grained, trough cross-stratified sandstone emplaced in a braided, low sinuosity fluvial environment. The three-stage infilling history and the palaeogeographic location combined with size and shape criteria make it possible to compare the channel-fill structures with Pleistocene tunnel-valleys found in areas of low-relief in cool temperate and low Arctic zones. Such features are incised by high-pressure subglacial meltwater and later infilled by proglacial to postglacial deposits. Based on this comparison, architectural unit 1 represents subglacial or proglacial outwash, high sediment discharge in unit 2 took place in a proglacial environment in the Hodh area while an isostatically downwarped shelf was responsible for a marine incursion in the Adrar channels, and deposits in unit 3 are fluvial postglacial sediments. Identification of preserved channel-fill structures may be useful in reconstructing Late Ordovician ice-sheet dynamics.

A Cenozoic-style scenario for the end-Ordovician glaciation

The end-Ordovician was an enigmatic interval in the Phanerozoic, known for massive glaciation potentially at elevated CO2 levels, biogeochemical cycle disruptions recorded as large isotope anomalies and a devastating extinction event. Ice-sheet volumes claimed to be twice those of the Last Glacial Maximum paradoxically coincided with oceans as warm as today. Here we argue that some of these remarkable claims arise from undersampling of incomplete geological sections that led to apparent temporal correlations within the relatively coarse resolution capability of Palaeozoic biochronostratigraphy. We examine exceptionally complete sedimentary records from two, low and high, palaeolatitude settings. Their correlation framework reveals a Cenozoic-style scenario including three main glacial cycles and higher-order phenomena. This necessitates revision of mechanisms for the end-Ordovician events, as the first extinction is tied to an early phase of melting, not to initial cooling, and the largest δ13C excursion occurs during final deglaciation, not at the glacial apex.

The formation of a geometrical ridge network by the surge-type glacier Kongsvegen, Svalbard

Traditionally, geometrical ridge networks are interpreted as the product of the flow of subglacial sediment into open basal crevasses at the cessation of a glacier surge (‘crevasse-fill’ ridges). They are widely regarded as a characteristic landform of glacier surges. Understanding the range of processes by which these ridge networks form is therefore of importance in the recognition of palaeosurges within the landform record. The geometrical ridge network at the surge-type glacier Kongsvegen in Svalbard, does not form by crevasse filling. The networks consist of transverse and longitudinal ridges that can be seen forming at the current ice margin. The transverse ridges form as a result of the incorporation of basal debris along thrust planes within the ice. The thrusts were apparently formed during a glacier surge in 1948. Longitudinal ridges form through the meltout of elongated pods of debris, which on the glacier surface are subparallel to the ice foliation and pre-date the surge. This work adds to the range of landforms associated with glacier surges.

High palaeolatitude (Hodh, Mauritania) recovery of graptolite faunas after the Hirnantian (end Ordovician) extinction event

Marine shales directly overlying lower Hirnantian (uppermost Ordovician) glacially related sediments in Mauritania (northwest Africa) have produced a rich graptolite fauna spanning the Ordovician–Silurian boundary in an area of high palaeolatitude. The lowermost transgressive sandy shales are barren of graptolites, but overlying shales show a sudden appearance of a diverse fauna indicative of the terminal Ordovician persculptus Zone, suggesting that with improving conditions, colonisation by a relatively cold-tolerant fauna was possible. This fauna is replaced by a low-diversity assemblage dominated by long-ranging taxa, probably representing the basal Silurian acuminatus and atavus Zones. With the extinction of the persculptus Zone fauna, conditions were still hostile to warm water Silurian graptolites, and a Normalograptus fauna was again established. A sudden influx of fairly diverse taxa marks the base of the acinaces Zone and the establishment of a typical Lower Silurian fauna with the establishment of warmer water conditions.

Moraine development at the high-arctic valley glacier Pedersenbreen, Svalbard

ABSTRACTThe formation of a moraine mound (hummocky moraine) complex at the Neoglacial limit of Pedersenbreen, Svalbard is discussed. The moraine mounds are composed for the most part of sheared bas...

Did the Late Ordovician African ice sheet reach Europe

ABSTRACTPaleovalleys and their infi lling successions are described from outcrops and drill cores of the Cantabrian Range (northern Spain). A Hirnantia fauna and associated diamictites with striated lonestones indicate that the paleovalleys are related to the Hirnantian (latest Ordovician) glacial event. Based on overall geometry, depositional facies, and associated deformation structures, the paleovalleys are interpreted as subglacial tunnel valleys. They were most likely related to the North Gondwana ice sheet. The ice sheet therefore reached the Ibero-Armorican domain that was still attached to the Gondwana landmass at least until the latest Ordovician.INTRODUCTION The Hirnantian glacial record documents one or several ice sheets throughout Africa, Arabia, and South America, the extents of which are still controversial (Ghienne et al., 2007; Schonian and Egenhoff, 2007). Although glaciomarine strata are widespread in European peri-Gondwa-nan terranes (Robardet and Dore, 1988; Brenchley et al., 1991), they do not precisely indicate the presence of glaciers at those localities. Because a number of these terranes would have been attached to the Gondwana land-mass during the Late Ordovician (e.g., Robardet, 2003), the Gondwanan ice sheet may have reached some of them. The Hirnantian paleovalleys of the Cantabrian Range (northwest Spain) are the fi rst glacial valleys to be recognized outside the Gondwana landmass. They may be related to a Gondwanan ice sheet lobe or to a peri-Gondwana satellite ice cap.

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

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.

Lower Palaeozoic unconformities in an intracratonic platform setting: glacial erosion versus tectonics in the eastern Murzuq Basin (southern Libya)

The stratigraphic record of the eastern Murzuq Basin has been importantly influenced by deformation resulting in angular and/or deeply erosional unconformities, though the overall context is intracratonic. Major transgressive events and the Ordovician glaciation are nevertheless documented, allowing the delineation of tectonic-, eustasy- or climate-driven unconformities. Lower Palaeozoic key events and related unconformities that characterize the North Gondwana platform have therefore a signature in the eastern Murzuq Basin. The basement/cover unconformity, also known as the infra-Tassilian surface, truncates all the deformed and metamorphosed Lower Cambrian and older rocks. Above is a ?Middle Cambrian to Lower Ordovician megasequence (Murizidié and Hasawnah Fms.), which is in turn truncated by an intra-Ordovician, angular unconformity. This megasequence is unconformably overlain by a Middle Ordovician (Hawaz Fm.) to Silurian (Tanzzuft and Akakus Fms) megasequence, which includes the Upper Ordovician glaciogenic unit (Mamuniyat Fm.), bounded at the base by a polygenic glacial erosion surface showing corrugated glacial lineations, tillites, and glaciotectonic structures. The Middle Ordovician to Silurian megasequence is finally truncated by a base-Devonian, angular unconformity overlain by fluvial sandstones. Regarding the possibility that those fluvial deposits may be as younger as Late Devonian in the eastern Murzuq Basin based on palaeoflora, the so-called Caledonian unconformity might be here a much younger (mid-Eifelian?) surface, and the occurrence of the Lower Devonian “Tadrart Fm.” is questioned. The Upper Ordovician glacial erosion surface, which is sometimes referred to as the Taconic unconformity, usually truncates Middle Ordovician strata in the Murzuq Basin but reaches significantly deeper stratigraphic levels in places that have been previously involved in the intra-Ordovician deformation event. In the Murizidié (southeastern Murzuq Basin), the infra-Tassilian surface, the intra-Ordovician unconformity, and the Upper Ordovician glacial erosion surface amalgamate together. Here, an estimate of the glacial erosion depth cannot be derived from the stratigraphic hiatus beneath the glacial incision, the main part of which relate to the intra-Ordovician tectonic event. The Upper Ordovician climate-related glacial erosion surface is not a valid unconformity for a sequence hierarchy framework of the Lower Palaeozoic, although it presents most of the physical attributes of tectonic-driven unconformities.