The lower Lias Group of the Hebrides Basin

Abstract

Synopsis The lower and greater part of the Lias Group of the Hebrides Basin comprises shallow-marine to marginal-marine mudstones, siltstones, sandstones and limestones arranged in unambiguous transgressive-regressive facies cycles. In the northern region of the basin, the strata were deposited 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 vicinity of transfer zones that have subsequently acted as foci for Palaeocene 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have been measured or remeasured in this study, and detailed sedimentary logs are presented that incorporate previously unpublished ammonite determinations.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 the 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 equivalent 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 Zones) occurs within the lower Pabay Shale and has its type locality at Hallaig Waterfall on Raasay; the Suisnish Sandstone Member (Upper Sinemurian–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 Eishort, on Skye. In the southern area (Mull–Morvern) use of the terms ‘Lower 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 offshore equivalent, the Blue Lias Formation. The Pabay Shale of the Ardnamurchan–Morvern–Mull area is recognized down to the junction with the Blue Lias. A sandstone unit is well developed at about the Lower Sinemurian–Upper Sinemurian boundary: it is here named the Torosay Sandstone Member of the Pabay Shale Formation and is incompletely exposed at several localities on the southeastern coast of Mull. Although no age-diagnostic fossils have been found within the sandstone, it must lie within the upper Obtusum Zone to lower Raricostatum Zone interval. Overall, there is a general trend from more proximal environments of deposition in the north and east, towards more distal environments in the south and west. This regional pattern is modulated by the localized occurrence of sand-rich, very proximal successions in the vicinity of the supposed transfer zones, such as the area immediately SE of the northern termination of the Camasunary Fault. Records from the Upper Glen-1 borehole, in NW Skye, show that the expanded lower Lias sequence in the centre 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 Lower Sinemurian to Upper Sinemurian boundary. Like the Ardnamurchan section, it also shows an intercalation of the Broadford Formation and Blue 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, alternatively, intersection of the borehole by a normal fault.