Palaeogeographic setting of the Wrae Limestone: an Ordovician submarine-slide deposit in Tweeddale

Synopsis The Wrae Limestone occurs in association with alkaline lava and tuff as a thin horizon within Ordovician greywackes in Tweeddale. The limestone, containing a shallow-water fauna, occurs as clasts ranging in size up to large boulders several metres across, variously set in a muddy grit, clastic breccia, or tuff matrix. The greywackes of Tweeddale were deposited by mass-flow processes in deep water, and the limestone breccia was probably introduced into this environment by debris flow from a volcanic palaeo-high. The intimate association of tuff and limestone suggests that volcanic activity in the source area may have initiated the submarine sliding. Following recent plate tectonic models it is suggested that the original volcanic edifice is either not exposed at the present level of exposure in the Southern Uplands, or was not detached from the descending oceanic plate during subduction. The Wrae Limestone occurs within a distinctive group of shaly mudstones and associated fine-grained and pebbly greywackes, known as the Lowther Shales. This unit is well-developed for several kilometres to the north of the Ordovician/Silurian boundary along a strip of the Northern Belt which extends from the Lowther Hills to the Moorfoot Hills. Sparse fossil evidence suggests that it is in large part of Upper Caradoc age. As with the majority of greywacke units in the Southern Uplands, biostratigraphic control is inadequate for the Lowther Shales association to merit formational status. On the grounds of its lithological heterogeneity, it is suggested that the unit is best renamed the Lowther Beds.