Geodiversity
Oxfordshire Rock Types: Clypeus Grit
Clypeus Grit Member (Salperton Limestone Formation)
Some Key Words
brachiopod
a marine animal of the phylum Brachiopoda, having a dorsal and ventral shell with bilateral symmetry; a lamp shell.
calcareous
Containing calcium carbonate.
echinoid
Echinoids are marine animals belonging to the phylum Echinodermata and the class Echinoidea. They have a hard shell (test) covered with small knobs (tubercles) to which spines are attached in living echinoids. The test and spines are the parts normally found as fossils.
peloid
A peloid is a spherical to ellisoidal particle consisting of micrite (lime mud) smaller than 2 mm with no recognisable concentric or radial structure. Small smooth peloids are interpreted as carbonate fecal pellets produced by organisms that consume carbonate mud.
pisoid
A pisolith, also known as a pisoid, is a concentric sedimentary grain, >2 mm in diameter, formed as a concretion. Pisoliths are often found in carbonate rocks.
Parent group:
Inferior Oolite
Period:
Jurassic
Age:
Bajocian/Bathonian
(166.1–170.3 Ma)
Description:
The Clypeus Grit is a pale grey to pinkish brown rock which can be either fine or coarse-grained. It can be ooidal or peloidal, containing bioclastic fragments. The Clypeus grit can therefore be described as either a packstone or a grainstone, with large orange-skinned peloids/pisoids and aggregate grains; common whole shells especially in upper part. Characteristic fauna includes the large echinoid Clypeus ploti, large myacean bivalves and terebratulid brachiopods Stiphrothyris tumida.
Upper Boundary:
Upward change into fine- to medium-grained ooidal grainstone (Chipping Norton Limestone Formation) in northeast. Generally transitional, but locally marked by a hardground
Lower Boundary:
Upward change from shelly coarsely shell-fragmental limestone (Upper Trigonia Grit Member), to limestone as described under Lithology. Commonly a non-sequence, marked by a hardground, though locally appears to be transitional.
Thickness:
10-12 m
Spatial distribution (Oxfordshire):
west of a line from Hook Norton to Tackley where overlapped by Great Oolite Group.
Type reference sections:
All in neighbouring Gloucestershire