Branscombe Mudstone

Late Triassic (Norian)

Lithology
Mudstone and siltstone, red-brown with common grey-green reduction patches and spots. The mudstones are mostly structurless, with a blocky weathering habit. Gypsum/anhydrite, locally of economic importance, is common throughout in beds, nodules and veins. Many of these sulphate beds have been named and form distinct markers on geophysical logs. Sporadic thin beds of argillaceous sandstone and silty dolomite occur in the lower part of the formation. Beds of thinly interlaminated, dark grey-green mudstone and dolomitic siltstone occur locally towards the top of the formation.

Upper Boundary
Placed at an abrupt or rapid upward change from the red-brown, silty mudstones of the Branscombe Mudstone Formation to the green or grey-green, dolomitic mudstones of the Blue Anchor Formation. The boundary may be locally unconformable. At outcrop, the boundary is marked by a change in soil colour from the reddish brown clayey soils of the Branscombe Mudstone Formation to grey clayey soils of the overlying Blue Anchor Formation. Typically, this boundary lies near the base of a scarp slope formed by the Blue Anchor Formation and capped by the Penarth Group.

Lower Boundary
Conformable in all areas. Placed at the base of predominantly red, structureless mudstones where they rest on the interbedded dark green-grey siltstones and pale grey fine-grained 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.

Representative Sites
Garden Cliff

Information on lithology and boundaries taken from the British Geological Survey Lexicon of Named Rock Units