Late Ordovician subglacial record in the eastern Murzuq basin, Libya

Category Environmetal Geology
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Moreau, Julien۱; Ghienne, Jean Francois۲; Rubino, Jean Loup۳
Holding Date 08 October 2008

During the Late Ordovician, the Gondwana intracratonic shelf was located close to the South Pole. Ice masses had covered this super-continent several times and different forms of glacial activity have been recognised, particularly in North Africa. The Murzuq basin was part of this very large low gradient glaciated shelf and its eastern rim exposes some of the best evidence of Late Ordovician glacial activity, consisting of a 200km by 80 km strip of Cambrian- to Silurian-era rocks. The glacial series shows an complex sedimentary architecture due to cross-cutting of at least five major glacial erosion surfaces. Each glacial surface has composite subglacial landform associations. Some of the subglacial features that have been identified include also meltwater incisions, and glaciotectonic structures. These geomorphological features are in close association and give information about the ice-sheet behaviour on each of the five glacial erosion surface. For example, high elongation ratio landforms, interpreted as mega-scale glacial lineations (MSGL) are recurrent throughout the glacial series and are considered to have been formed by several ice-streams. Different types of glacial lineations on the same surface record different ice-sheet flow rate events, sometimes generating MSGL with elongation ratios reaching 100.
Subglacial sediments are exposed, however, only few metres of glacio-fluvial sediments are preserved on the glacial surfaces. Nevertheless, some glacial surfaces contain melt-water channels subsequently in-filled by subglacial deposits. Despite the variability of the tunnel channel filling sediments, a common architecture can be recognized. At the base the channel erosion surface is moulded by a few meters of conglomeratic glacio-fluvial sediments, above which the main channel in-fill (<50 m) onlaps. Some injection- and glaciotectonic phenomena are associated with the deposition of the main infill of the channels. The basal till is considered to record the normal behaviour of the ice stream, whilst the channel main in-fill records catastrophic meltwater discharge below the ice. This second, channel in-fill phase is considered to be associated with high hydrostatic pressure responsible for the injection- and glaciotectonic structures. This overpressure increases lubrication at the base of the ice resulting in accelerated ice-sheet flow, possibly indicated by the very high elongation ratio MSGL. The repeated sequences of MSGL and meltwater channel in-fill indicate normal fast-flowing ice-streams followed by catastrophic retreats and/or proglacial outbursts at the ice front. There is some evidence for these fluctuations in the large proglacial fan-deltas exposed in the area. Being able to correlate the proglacial systems with the subglacial sedimentary record could potentially be of great importance in understanding the interaction between the evolution of the Late Ordovician ice-sheet, the meltwater discharge and the sediment transport.