Interactions between glaciation / deglaciation, deformation and stratigraphic record. Application to Late Ordovician-Silurian series in the Djado Basin (Niger) and north african basins
Defended on the 11th December 2007
The Hirnantian ice sheet (Late Ordovician, ~ 445 Ma) grew over the Gondwana cratonic area, then located in a south polar position, and lasted 1-2 M.y.
Field study in the Djado Basin (northeastern Niger) evidenced two major ice sheet advances, separated by an interstadial major melting event. Interstadial series record that the southermost retreat of the ice sheet extended as far as the southern Djado Basin but north of the Kandi Basin (Benin). Glacial series isopach maps also show the extent of glacial margins during the two glacial cycles. Unlike glacial marginal areas such as Algeria and Libya glacial surfaces are strongly amalgamated in the Djado Basin, the thickness of glacial series is reduced, and the size, as well as the pattern of the subglacial drainage system, suggests poor meltwater drainage efficiency.
Several types of subglacial shear zones are differentiated. The older glacial pavement shows structures typically indicative of strong coupling between the glacier and sandstone bedrock, which resulted into slow basal sliding. Conversely, subsequent glacial shear zones suggest glacier decoupling from ductile subglacial clay and sand. High strain rates, associated with fluid overpressure, were induced by rapid glacier velocity. Pervasive soft-bed deformation, megaflute development and glacial trough incision, provide evidence for ice stream dynamics, following Pleistocene and present-day ice sheet behaviour. The subglacial deformation model established demonstrates that ice streaming initiated through dramatic increase in fluid pressure at the glacier-impermeable bed interface. In Iceland, subglacial deformation also closely depends on flui pressure, even if the type of subglacial shear structure differs from the Djado Basin, accounting for a subglacial lithology contrast. Using bibliographic data, remote-sensed images and the glacial isopach map, ice stream dynamics is shown to be widespread in Saharan basins and depositional thicks at their mouth.
In the Djado Basin, deglaciation is marked by extensional tectonics, and at larger scale by the inversion of crustal deformation formerly induced by glacial overloading. Depocentre heterogeneity, controlled by ice sheet extent during glacial cycles as well as ice stream dynamics, associated with Silurian differential subsidence, documents the Late Ordovician-Lower Silurian petroleum system.
late Ordovician, Hirnantian, glaciation, Niger, Djado Basin, ice streams, subglacial deformation