The nature and origin of breccias associated with central complexes and lava fields of the British Tertiary Igneous Province
The British Tertiary Igneous Province (BTIP) contains several large ‘breccia’ outcrops, classically interpreted as ‘vent agglomerates’ (Harker 1904; Bailey et al. 1924; Richey & Thomas 1930). Re-examination of localities on Ardnamurchan, Mull and Skye provides new evidence that many of these represent mass flow deposits, with implications for the environment and topography of the BTIP at the time of deposition. At Carraig Mhor, east of Carsaig Bay on the south coast of Brolass, Isle of Mull a sequence consisting of ignimbrite, grading vertically into siltstones and sandstones, is overlain by thick, coarse breccias The breccias comprise pale igneous clasts (2cm - 1m) with crenulate, angular and rounded shapes, set within a dark, fine-grained, laminated, sedimentary matrix. Complex inter-relationships between clasts and matrix are interpreted as reflecting magma-sediment-water interactions. The breccias are interpreted as peperites and the sequence below provides evidence of local silicic pyroclastic activity during the early stages of formation of the Mull Lava Field. ‘Breccias’ on the Ardnamurchan Peninsula have previously been interpreted as ‘vent agglomerates’. Within them, distinct ‘Ben Hiant’ and ‘Northern’ vents have been identified (Richey & Thomas 1930). However, there is no evidence for primary pyroclastic or explosive activity forming these deposits, although clasts of ignimbrite indicate earlier silicic pyroclastic activity. The ‘breccias’ are typically conglomerates with clasts ranging from rounded to (less commonly) sub-angular, and from 2 cm up to 3m in size. Shattered ‘megablock’ deposits (up to 30m in length) are present locally. Together, these deposits form a stratified sequence containing numerous sedimentary structures. Analysis of clast/matrix relationships and of distribution patterns provides evidence for debris flow/avalanche deposition with increasing clast size, heterogeneity and roundness away from ‘source’.