Avon Group [Lower Limestone Shale]

Early Carboniferous (Courceyan)

Lithology
Interbedded grey mudstones and thin- to medium-bedded skeletal packstones with one to several thick units of ooidal and skeletal grainstones. Thin units of calcite mudstone and mudstone locally present. Sparse thin ironstones. Represents mid to inner shelf/ramp deposits with coeval barrier, back barrier and coastal plain sediments.

Upper Boundary
Gradational in southern settings, and taken at the incoming of stacked dark grey fine to coarse skeletal packstones with scattered mudstone partings [Barry Harbour Limestone Formation] of the Black Rock Limestone Subgroup above the interbedded mudstones and skeletal packstones of the Avon Group. Boundary taken at the top of the last significant bed of grey mudstone.

Lower Boundary
Conformable in southern settings, where it is taken at the sharp to gradational incoming of grey calcite mudstones, mudstones and limestones of the Group, above the red sandstones, siltstones and mudstones of the Upper Old Red Sandstone [Quartz Conglomerate Group and Tintern Sandstone Formation]. Disconformable along the north crop of the South Wales Coalfield, and Monmouth and Chepstow area where it is taken at the sharp erosive base of the transgressive skeletal and ooidal grainstones above the sandstones of the underlying Quartz Conglomerate Group, Tintern Sandstone Formation, and Brownstones Formation.

Representative Sites
Stowe Green Quarry
Blue Rock Quarry & Cutting

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