Glaciogenic structures at Ordovician tunnel valley margins and their implications for tunnel valley genesis in southern Jordan

Penecontemporaneous deformation and erosional structures are described from the inclined bedrock margins of Late Ordovician (Hirnantian) tunnel valleys in southern Jordan. These include micro-faults, glacial striae and grooves, and newly described micro-slips. Together with the basal glacial diamictites, they indicate contact of the basal ice mass with the underlying Late Ordovician bedrock (the Tubayliyat Sandstone Formation) during erosion of the tunnel valleys. The striae/grooves are oriented parallel and at right angles to the tunnel axes; this latter sense of movement is supported by the associated micro-slips and the throw on the micro-faults. Two types of glacial margin are present: erosional or folded (ductile deformation). Locally preserved basal diamictites (tillites), including far-travelled granitoid and metamorphic clasts derived from the Arabian Shield, support a primary ice–bedrock erosional origin for the tunnel valleys. The presence of abundant erosional/deformational structures oriented towards the tunnel valley axes indicate subsequent movement of the basal ice mass towards the tunnel valley axes, possibly as a result of the collapse of sub-glacial streams during deglaciation. The deformational features are interpreted as the result of high hydrostatic groundwater pressure in the bedrock at the base of the ice sheet. The tunnel valleys were subsequently modified and infilled by two or more phases of fluvio-glacial siliciclastic sedimentation.