THE LOWER LIAS GROUP OF THE HEBRIDES BASIN

The lower and greater part of the Lias Group of the Hebrides Basin com prises shallow-marine to marginal-marine mudstones, siltstones, sandst ones and limestones arranged in unambiguous transgressive-regressive f acies cycles. In the northern region of the basin, the strata were dep osited in a series of half graben bounded on their western margins by major eastward-dipping normal faults developed principally during the Triassic. Interpretation of the palaeogeographic setting of the lower Lias is complicated because exposure of the sequences occurs in the vi cinity of transfer zones that have subsequently acted as foci for Pala eocene igneous activity. The principal exposures have been measured or remeasured in this study, and detailed sedimentary logs are presented that incorporate previously unpublished ammonite determinations. In t he northern part of the Inner Hebrides Trough (Skye-Pabay-Raasay area) , two formations are recognized: the Broadford Formation and the Pabay Shale Formation. The Broadford Formation, as used in this study, is e quivalent only to the 'Lower Broadford Beds' of previous authors. The Pabay Shale incorporates the former 'Upper Broadford Beds' and 'Pabba Shale' of previous usage. Two new predominantly arenaceous members of the Pabay Shale Formation are formally defined for the northern area: the Hallaig Sandstone Member (Lower Sinemurian; Semicostatum-Turneri Z ones) occurs within the lower Pabay Shale and has its type locality at Hallaig Waterfall on Raasay; the Suisnish Sandstone Member (Upper Sin emurian Lower Pliensbachian; Raricostatum Jamesoni zones) occupies the middle part of the Pabay Shale Formation and has its type locality at Rubha Suisnish and in the cliffs along the southern shore of Loch Eis hort, on Skye. In the southern area (Mull-Morvern) use of the terms 'L ower Broadford Beds' or 'Upper Broadford Beds' is also inappropriate. In Ardnamurchan, and to a limited extent in more northerly exposures, the Broadford Formation interdigitates with its more southerly offshor e equivalent, the Blue Lias Formation. The Pabay Shale of the Ardnamur chan-Morvern-Mull area is recognized down to the junction with the Blu e Lias. A sandstone unit is well developed at about the Lower Sinemuri an Upper Sinemurian boundary: it is here named the Torosay Sandstone M ember of the Pabay Shale Formation and is incompletely exposed at seve ral localities on the southeastern coast of Mull. Although no age-diag nostic fossils have been found within the sandstone, it must lie withi n the upper Obtusum Zone to lower Raricostatum Zone interval. Overall, there is a general trend from more proximal environments of depositio n in the north and east, towards more distal environments in the south and west. This regional pattern is modulated by the localized occurre nce of sand-rich, very proximal successions in the vicinity of the sup posed transfer zones, such as the area immediately SE of the northern termination of the Camasunary Fault. Records from the Upper Glen-1 bor ehole, in NW Skye, show that the expanded lower Lias sequence in the c entre of the Sea of the Hebrides Little Minch Basin can be subdivided in a similar manner to that exposed in the Inner Hebrides Trough; for example, it shows the development of more sandy strata at about the Lo wer Sinemurian to Upper Sinemurian boundary. Like the Ardnamurchan sec tion, it also shows an intercalation of the Broadford Formation and Bl ue Lias. The upper half of the Pabay Shale appears to be truncated in the borehole: it is debatable whether this resulted from erosion prior to deposition of the Scalpa Sandstone, from sediment starvation or, a lternatively, intersection of the borehole by a normal fault.