Volcanic and sedimentary facies of part of the Borrowdale volcanic group, Cumbria

Detailed studies of sequences of volcaniclastic rocks, lavas and intrusions have been undertaken in four areas in the Ordovician Borrowdale Volcanic Group in the English Lake District, which together comprise a vertical section some 4km thick. Fieldwork has been complemented by petrographical study of over 300 samples from these rocks. On the eastern side of Derwentwater, a sequence of blocky lava: flows of andesitic and basaltic andesite composition is inter bedded with hyaloclastites, ashfall deposits, and reworked ashfall material. The deposits of mudflows and lacustrine turbidites have also been recognized here. In the overlying Thirlmere section, a 2500m thick sequence of andesites contains hyaloclastites, local pillows and peperite complexes thought to have been formed by the intrusion of high level andesite sills into wet, unconsolidated volcanic sediments. Ashfall and mudflow deposits are also present. Overlying the andesites is a thick sequence of welded acid ignimbrites, with interbedded ashfall tuffs. A third section at Sour Milk Gill, southern Borrowdale, spans the contact of the andesites and ignimbrites seen at Thirlmere. A thick bedded ashfall sequence shows evidence of lacustrine deposition, and below the overlying pyroclastic flow deposits, thin units of base surge origin are seen. Overlying the thick ignimbrite sequence in Langdale, parallel bedded tuffs with well developed sequences of sedimentary structures, are interpreted as turbidites, and are sharply overlain by ?subaerially reworked coarse tuffs, which are succeeded by blocky andesitic lava flows. It is concluded that most of the sequence accumulated in an area of subdued topography, allowing widespread development of the various volcanic units. Comparisons are drawn between the studied sections and modern volcanic areas such as New Zealand and Mexico