The Lamington Conglomerate: further evidence for an ophiolitic source for the Ordovician Marchburn Formation, Northern Belt, Southern Uplands

Abstract

Formerly regarded as an outlier of Lower Old Red Sandstone conglomerate within the Northern Belt of the Southern Uplands, the newly defined Lamington Conglomerate Member is now proposed to be an integral part of the Marchburn Formation (Tappins Group) of Ordovician (Caradoc) age. Pebbles and cobbles (up to 210 mm in size) are dominated by gabbroic and basaltic lithologies, with subsidiary amounts of tonalite, serpentinite, volcanic breccia and sedimentary lithic clasts. The matrix to the conglomerate is composed of a distinctive pyroxene-bearing sandstone. The member also has unusually high magnetic susceptibility, reminiscent of other units within the Tappins Group further to the SW, and consistently higher than any Old Red Sandstone conglomerate. The clast content supports an ophiolitic provenance for this part of the Marchburn Formation and is consistent with the overall petrography of the Tappins Group.