Charmouth Mudstone
Early Jurassic (Sinemurian-Pliensbachin)
PERIOD |
GROUP |
FORMATION |
Jurassic |
Lias |
Charmouth Mudstone |
Lithology
Dark grey laminated shales, and dark, pale and bluish grey mudstones; locally concretionary and tabular limestone beds; abundant argillaceous limestone, phosphatic or ironstone (sideritic mudstone) nodules in some areas; organic-rich paper shales at some levels; finely sandy beds in lower part in some areas.
Upper Boundary
At marked or gradational upward change to coarser siliciclastic deposits of the Dyrham Formation.
Lower Boundary
Taken at the top of Blue Lias Formation. The top of the Blue Lias coincides with a marked upward decrease in frequency, thickness and lateral persistence of limestone beds. In many cases, the horizon to be taken is obvious; where there is doubt, the thickness and vertical frequency of limestone beds should be considered, and the following guidelines used: At the top of a more or less persistent, thick (>0.10m) limestone above which the overlying mudstone (beneath any subsequent similar limestone) is at least twice the thickness typical of the underlying limestone-mudstone interbedded unit and above which there is no return, within the Lias Group, to limestone-mudstone spacing on that previous scale.
Representative Sites
Maisemore Cliff
Ham's Gulley Brook
Information on lithology and boundaries taken from the British Geological Survey Lexicon of Named Rock Units |
