Geodiversity

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