Geodiversity
GEOLOGY | CLYPEUS GRIT MEMBER
Inferior Oolite Group
Salperton Limestone Formation
Clypeus Grit Member
Jurassic
Bajocian-Bathonian Age
[166.1 - 170.3 Ma*]
limestone: a rock that is formed chiefly by accumulation of organic remains (such as shells or coral), consists mainly of calcium carbonate.
Image: Clypeus Grit Member (Ⓒ Crown Copyright)
* Ma is an abbreviation for million years
Description:
The Clypeus Grit is a pale yellow to grey and in some places, 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 the upper part.
Fossil Assemblage:
This member of the Salperton Limestone Formation has abundant fauna. Characteristic fauna includes large myacean bivalves and terebratulid brachiopods Stiphrothyris tumida. The eponymous echinoid Clypeus plotii (named after the first keeper of the Ashmolean Musum, Robert Plot) is ubiquitous.
Upper Boundary:
Upward change into fine to medium-grained ooidal grainstone (Chipping Norton Limestone Formation) in north-east. Generally transitional, but locally marked by a hardground.
Lower Boundary:
Upward change from shelly coarsely shell-fragmental limestone (Upper Trigonia Grit Member), to limestone. Commonly a non-sequence, marked by a hardground, though locally this 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.
Key Words
bioclast
A single, often broken, fossil fragment eg shell material.
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 periproct is a tough membranous area surrounding the anus. Aboral surface is the surface opposite to the location of the mouth, whereas, Adoral is towards the mouth.
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) <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.
POUND STONES
Cotswold dairymaids working before standardised weights and measures, often used 'pound stones' to weigh out butter and cream. These pound stones, commonly found in the fields of Oxfordshire, were particularly useful because all were uniform in shape, size and weight.
What were these ubiquitous rocks that aided rural dairy farmers?
They are the fossilised remains of Clypeus plotii. Clypeus is a type of irregular echinoid. Irregular echinoids have a strong bilateral symmetry, unlike the regular echinoids that are round and have a radial symmetry. Clypeus had spines, however, the spines would have been hair-like.
Diagnostic Features of
Clypeus plotii:
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The test is large, circular, or with blunt posterior margin, low domed to subconical in profile, adoral surface more or less flat.
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Apical system central, tetrabasal, posterior ocular plates posteriorly prolonged and extending to periproct.
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The petals are long, broad, open distally, with broad, tapering poriferous zones, the outer pore is slit-like, all ambulacral pores double beneath the petals.
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The periproct is close to the apex, surrounded by the posterior plates of the apical disc; opening into a deep and sharply defined anal sulcus that runs from the apex to the posterior margin.
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The peristome is pentagonal, small and slightly anterior to centre.
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Bourrelets are poorly differentiated although interambulacral zones stand out because ambulacral zones are sunken towards the peristome; there are no real zones of dense tuberculation surrounding peristome.
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The phyllodes are double pored, with pore-pairs arranged in arcs of three forming a broad, but not bowed, band.
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There are no buccal pores.
Images: aboral surface of
Clypeus plotii
Bibliography
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Barron, A J M, 1999. Geology of the Naunton area. British Geological Survey Technical Report WA/98/27.
Cox, B M and Sumbler, M G. 2002. British Middle Jurassic Stratigraphy. Geological Conservation Review Series No.26. (Peterborough: Joint Nature Conservation Committee).
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Lycett, J, 1857. The Cotswold Hills. London.
Parsons, C F, 1976. Ammonite evidence for dating some Inferior Oolite sections in the North Cotswolds. Proceedings of the Geologists' Association, 87, 45-63.
Parsons, C F, 1980. Aspects of the statigraphy and ammonite faunas of the Aaenian-Bajocian stages in Great Britain. Unpublished PhD thesis, University of Keele.
Richardson, L. 1929. The Country around Moreton in Marsh. Memoir of the Geological Survey of Great Britain, Sheet 217 (England and Wales).
Woodward, H B, 1894. The Jurassic Rocks of Britain, Vol.4. The Lower Oolitic Rocks of England (Yorkshire excepted). Memoir of the Geological Survey of the United Kingdom.