Sylvain Clerc PhD thesis

Abstract

The icehouse period during the lower Palaeozoic led to the development of a large continental ice-sheet over the Gondwana. This ice-sheet fluctuated in size and repeatedly recovered the north-Gondwana platform. The resulting glacial record includes major erosional surfaces of regional extent, with subglacial landforms and morphologies, including glacial valleys, and specific glaciogenic sedimentary record. Among these valleys, tunnel valleys refer to Quaternary analogues, associated with the development of ice-sheets over Europe and North-America. Tunnel valley defines elongated, linear to slightly sinuous depressions, measuring few kilometers in width and several kilometres in length. They start and terminate abruptly, are generally a few hundred of meters deep, and display frequent overdeepening along the floor. They are expected to be formed subglacially by pressurized meltwaters.Ordovician tunnel valleys are described from North Africa to the Middle East. Ordovician glacial deposits are considered as a major target for hydrocarbon exploration in these regions, with tunnel valleys forming lithological heterogeneities with excellent reservoir properties. In Europe, Quaternary tunnel valleys are targeted for groundwater resources as they form reliable aquifers. A major interest therefore exists for these valleys, as well as a need for better understanding of the nature and the stratigraphy of the infill, their origin and the parameters controlling their distribution. The recherché project presented in this thesis is based on an extensive fieldwork that focused on three main issues, regarding tunnel valleys: (1) the subglacial environment, (2) the processes and depositional environments associated with their infill and (3) the parameters controlling their distribution.

The subglacial environment is complex, and despite improving investigation techniques, it is hardly accessible for direct observations and remains poorly understood. This environment is generally associated with coarse-grained, poorly sorted facies, and is considered to be subordinate to proglacial environments. The study carried out in Killiney Bay, Ireland, demonstrated that a wide range of facies could be deposited but also preserved in a subglacial environment, because subglacial accommodation space can be provided by the subglacial topography. The different facies display specific characteristics that record the close interaction between the substrate the overflowing ice-sheet, through coupling/uncoupling phases. Subglacial deposits display specific stratigraphic and sedimentological characteristics, as well as typical deformation structures related to fluid overpressures between the ice and the substrate.

Today, the different scenarios for tunnel valley infill are associated with the ice-sheet decay during the deglaciation and the subsequent ice-front retreat. The greater part of sediments is deposited in proglacial environments, either in glaciomarine or glaciofluvial settings. However, based on the diagnostic criterion defined in the quaternary record, subglacial facies were identified in Alnif tunnel valley infill and these facies represent nearly 50 % of the whole valley infill. A subordinate amount of these subglacial facies, restricted to the basal infill, are deposited under a fully grounded ice-sheet. The rest of the sediments is deposited under a lightly grounded ice-sheet, which is locally and temporary grounded on the substrate. By analogy with recent observations in Antarctica, this subglacial environment is at the transition between the fully grounded ice-sheet located above the coupling line, and the proglacial environment, where the ice-sheet is floating in the sea, seaward from the grounding line. Beneath the lightly grounded ice sheet, the sedimentary record will differ according to the amount of accommodation space. Glaciturbidites, associated with expanding flow from a subglacial conduit will be deposited in large accommodation space setting. Conversely, subglacial braided canal network, which develop through lateral migration and overdeepening will characterise low accommodation space setting.

Different examples of tunnel valley are described in the Anti-Atlas, allowing their morphology and the preglacial architecture to be compared in different locations. The results demonstrate the combined influence of lithological and hydrological parameters on tunnel valley shape and distribution. Tunnel valley density is higher where the preglacial substrate is characterised by low diffusivity, where no valley is formed where the preglacial strata are composed of high diffusivity sediments. Low diffusivity sediments have low capacity for groundwater transfer and channels will form at the ice-bed interface to efficiently drain meltwaters towards the margin. These channels will develop to form tunnel valleys. Conversely, above high-diffusivity sediments, meltwaters are fully drained as groundwater. In addition, the preglacial sedimentary architecture has a strong influence on tunnel valley shape. Low-diffusivity units, along the first 200 m of preglacial sediments, form permeability barriers which unable groundwater to flow vertically through the sediment, and therefore stop the tunnel valley overdeepening. These parameters explain the difference in shape, from shallow and narrow valleys to deep and large tunnel valleys. Finally, both the lithological and hydrological parameters are influenced by the regional structural configuration, which controls the evolution of preglacial depositional environments, and thus lithological heterogeneity distribution, but also (2) influences the subglacial drainage configuration.