Arden Sandstone
Late Triassic (Carnian)
PERIOD |
GROUP |
FORMATION |
Triassic |
Mercia Mudstone |
Arden Sandstone |
Lithology
Heterolithic, consisting of grey, green and purple mudstones interbedded with paler grey-green to buff coloured siltstones and fine- to medium-grained, varicoloured green, brown, buff, mauve sandstones; beds of conglomerate occur locally. Laminated and thinly interbedded sediments are commonly extensively bioturbated and show structures indicative of thixotropic deformation. The siltstones and finer sandstones show small-scale ripple drift cross bedding; thicker sandstone beds show trough and planar cross bedding. The proportion of fine to coarse clastics varies laterally within the Formation. The thicker sandstone units, composed of several individual beds, have a lenticular geometry and occupy the inferred former courses of fluvial distributary channels in a deltaic or estuarine environment. Less arenaceous, mudstone and siltstone-dominated successions occupy intervening interdistributary areas. The Arden Sandstone Formation is differentiated from the reddish-brown, blocky weathering mudstones of adjacent formations by its predominantly greenish grey colour, the presence of a significant, though often subordinate, proportion of sandstone, and the predominance of finely laminated lithologies throughout.
Upper Boundary
Placed at the base of the predominantly red mudstones and siltstones of the Branscombe Mudstone Formation where they rest on the interbedded dark grey-green siltstones and pale grey sandstones of the underlying Arden Sandstone Formation; the boundary is abrupt or a rapid, interbedded transition. At outcrop, the boundary is typically marked by a change from the grey, slightly sandy clay soils of the Arden Sandstone Formation to the reddish-brown clayey soils of the Branscombe Mudstone Formation. Where the Arden Sandstone forms a marked cuesta feature, the boundary lies at the down-dip limit of the dip slope.
Lower Boundary
Placed at a rapid upward transition from the red mudstones or siltstones of the Sidmouth Mudstone Formation to the dominantly grey or green, partly or largely arenaceous beds of the Arden Sandstone Formation. Conformable in all areas. At outcrop, the boundary is typically marked by a change from the reddish-brown clayey soils of the Sidmouth Mudstone Formation to the grey, slightly sandy clay soils of the overlying Arden Sandstone Formation. Where the latter forms a marked cuesta feature, the boundary typically lies at a slight concave break in the scarp slope.
Representative Sites
Glebe Barn Quarry
Newnham Cliff
Information on lithology and boundaries taken from the British Geological Survey Lexicon of Named Rock Units |