Torridonian volcanic sediments

Publisher Website
Google Scholar
Abstract
Synopsis: Detailed evidence of volcanic activity within the Torridonian is provided for the first time. A distinctive sedimentary unit within the Stoer Group contains abundant angular volcanic fragments including pumice, shards and devitrified glass (interpreted as palagonite) which comprise from 40 to 80% of the sediment. The unit, which averages about 12 m in thickness, forms a significant marker horizon which can be traced some 50 km from Stoer southwest along the coast to Poolewe. Accretionary lapilli are the best indication of contemporaneity of volcanic activity with deposition of the unit. Their best development is at Coigach where they occur throughout 10 m of the upper portion of the unit which attains its maximum thickness here. This abundance, in combination with thickness, suggests that the eruptive source-vent was within several kilometres of Coigach. Indications of geometry and areal extent are in the main limited to a section along the strike. Throughout most of the outcrop conformable stratigraphic relations exist with the underlying and overlying sediments. The sharp, clear-cut basal contact is an erosional surface, broadly planar with minor relief. At Stoer, however, the unit abuts and laterally intrudes a high (>3 m) bank of sandstones and mudstones, warping and contorting them and carrying away blocks as large as 2.5 m. Irregular, branching, pipe-like segregations of pink authigenic feldspar (which originated during emplacement of the unit and in some cases are slightly post depositional but not unrelated in time) are of late vapour phase origin; some may represent fossil fumaroles. This volcanic sediment horizon was deposited as an ash flow resulting from a highly explosive eruption of phreatic nature. Deposition occurred primarily in the shallow water, near-shore environment where conditions were favourable for local transition to hot lahars.