The Lower Old Red Sandstone at Balmaha–Aberfoyle and its bearing on the nature of the Highland Boundary and Gualann Faults

Abstract

The final stages of the emplacement of the Dalradian block into Scotland are recorded in the Old Red Sandstone rocks that lie faulted against the main Dalradian belt. In the case of the Balmaha–Queen Elizabeth Forest sequence this involves the Gualann Fault, a splay of the Highland Boundary Fault, which transects c . 6.5 km of Lower Old Red Sandstone conglomerates and sandstones between Balmaha and Aberfoyle. A sequence of coarse conglomerates at Balmaha, near the top of the succession, derived from the SE, was deposited unconformably on the Highland Border Complex. This conglomerate has been studied in detail and the sequence is interpreted as an oblique section through an alluvial fan. Fitting the average dimensions of other alluvial fans to the unfolded cross-section suggests that it once extended some 5 km to the north, over the position of the Dalradian block. The conglomerate–sandstone sequence beneath was also deposited in alluvial fans that accumulated in a strike-slip, extensional basin in which units of conglomerate become progressively younger to the SW. Immediately NW of the Gualann Fault, in the Queen Elizabeth Forest, sub-horizontal red sandstones and breccias rest unconformably on the Highland Border Complex and were probably deposited beyond the reach of the coarse conglomerates. It is suggested that the Gualann Fault was initially a very low-angle or flat thrust, which was rotated in mid-Devonian times during the development of the steep NW limb of the Strathmore Syncline. It brought sub-horizontal breccias and sandstones, originally deposited 10–15 km to the NW, well out of the influence of the conglomerates on the SE side of the fault, into juxtaposition with folded coarse conglomerates south of the Fault. As the Gualann Fault is a splay of the Highland Boundary Fault this implies that the latter may also be a thrust that brought the Dalradian block over the Highland Border rocks. The whole sequence, from Balmaha to the Queen Elizabeth Forest, reflects the unroofing of an existing source of conglomerates to eventually erode their quartzite basement. The Highland Boundary Fault, although a fairly straight and prominent feature to the NE, becomes displaced and difficult to follow in the area to the SW where it is accompanied by many splays trending roughly N–S. The area, in and around the Firth of Clyde, has been regarded as the product of a locked phase of the fault movement that produced an extensional basin in Upper Old Red Sandstone times, but is also likely to have generated the more northern basins seen here. There are a few records of conglomerates and associated rocks cut by the Highland Boundary Fault to the NE, suggesting that Old Red Sandstone basins extended well beyond the present Highland Boundary Fault. These basins may also have relevance to the timing of the incoming of the Dalradian block, absent from Scotland during earlier periods.