Blue Anchor Formation

Late Triassic (Norian-Rhaetian)

Lithology
The formation typically comprises pale green-grey, dolomitic silty mudstones and siltstones with thin arenaceous lenses and a few thin, commonly discontinuous beds of hard, dolomitic, pale yellowish-grey, porcellanous mudstone and silltstone ("Tea Green Marl" of Etheridge, 1865). In southern England and Wales only, the "Tea Green Marl" is overlain by the "Grey Marls" (Richardson, 1906). This unit (equivalent to the upper part of the Rydon Member and the whole of the Williton Member of Mayall, 1981) comprises grey, black, green and, rarely, red-brown dolomitic mudstones with, in the higher beds, yellowish-grey dolostones; also present are laminated siltstone beds with mudcracks, scarce pseudomorphs after halite, and locally abundant gypsum; miospores occur throughout and bivalve fossils and bioturbation become increasingly common upwards.

Upper Boundary
Slightly unconformable or disconformable, marked by an abrupt upward transition from green or grey-green, dolomitic mudstones of the Blue Anchor Formation to black, fossiliferous shales of the Westbury Formation (Penarth Group). The unconformity surface is commonly slightly irregular and small clasts of Blue Anchor Formation lithologies may occur at the base of the Westbury Formation. The Blue Anchor Formation is both burrowed and bored to a depth of several centimetres below the boundary. In parts of southern England and Wales (e.g. on the west Somerset coast) the upper part of the formation (Williton Member of Mayall, 1981) includes lithologies similar to that of the overlying Westbury Formation, giving rise to an apparently gradational and interdigitating upper boundary. At outcrop, the boundary is marked by a change in soil type from the generally paler grey clay of the Blue Anchor Formation, to the darker grey clay of the overlying Westbury Formation of the Penarth Group. Typically, this boundary lies about half way up a scarp slope capped by limestones of the Penarth Group and lowermost Lias Group.

Lower 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. In parts of southern England and Wales the transition occurs over a greater thickness of beds or above a unit of interbedded red and green strata (e.g. on the west Somerset and south Devon coasts). 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 underlying Branscombe Mudstone Formation to the grey clayey soils of the Blue Anchor Formation. Typically, this boundary lies near the base of a scarp slope, which is formed by the Blue Anchor Formation and capped by limestones of the Penarth Group and lowermost Lias Group.

Representative Sites
Garden Cliff

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