The Green Beds of the SW Highlands: deposition and origin of a basic igneous-rich sedimentary sequence in the Dalradian Supergroup of Scotland

Abstract

Synopsis The Green Beds are distinctive chlorite- and epidote-rich metasedimentary rocks, which collectively represent a series of important lithostratigraphical marker units in the Southern Highland Group of the dominantly late Precambrian Dalradian Supergroup of Scotland. In a recent re-evaluation of the nature and origin of the Green Beds, integrated mapping, sedimentological, petrological and geochemical studies have been carried out. The Green Beds form relatively thin but laterally extensive units within thick sequences of siliciclastic metasedimentary rocks, and were deposited from high-density turbidity currents and debris flows in a deep-sea slope apron or ramp setting. They comprise a physical mixture of basic and siliciclastic material, representing periodic influxes of basic igneous detritus into the ‘background’ siliciclastic sedimentary system. The mineralogy of the Green Beds is largely the result of metamorphism, although rare relict detrital minerals from both granitic and basic sources are preserved. Geochemical studies show that the Green Beds and other Southern Highland Group metasedimentary rocks have a similar trace element signature, indicating a common siliciclastic source area. Two suites of Dalradian lavas, the Tayvallich and Loch Avich pillowlavas, were also analysed. Although it is unlikely that these particular volcanic units were the source of the Green Bed basic material, it is believed that the mafic component of the Green Beds was derived from episodes of near-contemporaneous volcanic activity.