Dalradian slides and basin development: a radical interpretation of stratigraphy and structure in the SW and Central Highlands of Scotland

Abstract

Movements on syndepositional listric faults are responsible for many of the lateral variations in Dalradian stratigraphy. Sedimentary facies and thickness variations show that the surface traces of these faults correspond, in many cases, to what have previously been interpreted as tectonic slides, although normal faults, thrusts and sedimentary contacts may also mark their position. Three main episodes of faulting are described in the Islay to Lochaber area. Appin Group sedimentation was controlled by a fault now represented by the Loch Skerrols Thrust-Fort William Slide-Ossian steep belt. In the Islay-Jura area an anomalously thick Islay Subgroup succession accumulated in a half-graben bounded by a listric normal fault to the NW and terminated by transfer faults at each end. The deposition of a thick Easdale Subgroup section was controlled by movements on what is now the Benderloch-Ballachulish Slide. This listric fault terminates southwards in the Scarba Fault and probably extends north into the Ossian steep belt. The thinness of the Appin and lower Argyll Group succession between the Ericht-Laidon and Bridge of Balgie Faults is an original feature. The stratigraphic gap represented by the Boundary Slide here is reinterpreted as a disconformity. However, further east the Boundary Slide may well be a listric fault, movement on which accommodated the thick succession around Schiehallion. The development of the major structure was influenced by the geometry of the sedimentary basins. The Schiehallion-Tummel area is reinterpreted as lying on the lower limb of the Ben Lui fold. This resolves the problem of the emplacement of the Tay nappe which is likely to be a gravity structure derived from above the Ossian steep belt.