The architecture of an alluvial suite: rocks between the Townsend Tuff and Pickard Bay Tuff Beds (early Devonian), southwest Wales

Abstract

The Townsend Tuff Bed and the Pickard Bay Tuff Bed are distinctive ash-fall tuffs traceable within a red-bed sequence over an area measuring about 35 km from west to east by 12 km from south to north. The succession between these markers is a mere 15-30 m thick, but contains six other tuffs which, though not themselves distinctive, can also be traced over the area by virtue of their position relative to the two main falls. The mudstone-dominated succession between the two main markers contains eight calcretes (pedogenic limestones), most of which extend over practically the whole of the area, and 16 generally local upward-fining or ‘coarse’ sequences composed of sandstones overlying an erosion surface and passing up into mudstones. The mudstones are massive and seldom laminated, apparently having been organically destratified. The coarser-grained upward-fining sequences involve cross-bedded, parallel-laminated and cross-laminated sandstones, whereas the finer-grained examples normally consist of interbedded mudstones and sharp-based cross-laminated sandstones. Intraformational conglomerates and stringers of mudstone clasts not infrequently accompany the coarser sandstones. Evidence of subaerial exposure is common towards the tops of both the coarser- and finer-grained sorts of upwardfining sequence. The careful tracing of the marker tuffs shows the upward-fining sequences to be of two main kinds. Examples of the least common type occur within localized and relatively deep depressions, probably incised valleys, which they fill to a height less than the depth of the depression. The more frequent variety is also localized but not obviously incised, having apparently grown up in harmony with the accumulation of mud in the area as a whole. The succession between the Townsend Tuff and Pickard Bay Tuff Beds lies in the transition between a clearly marine-influenced facies below and a clearly fluviatile facies above. It appears to record an extensive but comparatively featureless marginal mudflat influenced by both rivers and the sea. The development of calcretes (fossil soils) within the muddy sediments, and the shifting and valley-building activities of mixed tidal and river channels, could have been under the control of relative fluctuations of sea level perhaps on as short a time scale as of the order of 10 4 years. The channellized currents seem typically to have been vigorous and of low to moderate sinuosity.