Development of the Murzuq Basin, Southwest Libya, during the late Ordovician

The extent to which the Murzuq Basin, southwest Libya was glacierised during the late Ordovician is investigated by field studies, which are complemented by LANDSAT TM and seismic data analyses. A clear four-fold glaciation during the late Ordovician is supported by sediment architectural analysis of upper Ordovician rocks and the interpretation of syn-sedimentary deformation structures contained within them. Each phase of ice sheet advance is characterised by the development of a subglacial erosion surface or a correlative glacioisostatic-Ioading surface in front of it. Evidence for subglacial erosion includes streamlined subglacial bedforms ranging in scale from striations to palaeo-ice streams. At the ice sheet termini, glaciotectonic deformation structures formed with sustained ice sheet advance. Multigenerational, cross cutting tunnel valleys were incised under enhanced hydrostatic pressure and filled during ice sheet recession. The architectural development of both tunnel valleys and their intervalley areas was dependant upon whether ice sheets were grounded in terrestrial or marine settings. During ice sheet recession, sandstone deposition occurred on ice-proximal sandar, intertidal to shallow marine ice contact fans, and laterally extensive, storm and tide dominated open shelf areas. Turbidites were deposited in the lee of continental slope aprons and within tunnel valleys. Mudrock deposition was restricted to low energy ice distal locations and topographic lows. During the final deglaciation of the basin, glacioisostatic rebound occurred and reactivated long lived basement structures, promoting regional uplift and the development of both Gilbert-type delta systems and a "structured basin", consisting of half-graben depocentres and associated forced folds.