Geodiversity
Oxfordshire Rock Types: Hampen
Hampen Formation
Some Key Words
marls
A marlstone is a calcium carbonate or lime-rich mud or mudstone which contains variable amounts of clays and silt. The dominant carbonate mineral in most marls is calcite, but other carbonate minerals such as aragonite, dolomite, and siderite may be present.
packstone
A packstone is a rock with a grain-supported texture with the intergranular voids filled with a finer matrix.
grainstone
Grainstones are grain-supported carbonate sedimentary rocks that contain no micrite. The spaces between grains are filled with sparry cement.
Parent group:
Great Oolite
Age:
Jurassic/Bathonian
(166.1–168.3 Ma)
Description
Previously known as the Marly beds (Woodward, 1894) these are limestones with subordinate interbedded marls.
The Hampen Formation are characteristically grey to brown, thinly bedded, fine to very fine-grained, well-sorted, ooidal grainstone to packstone, commonly slightly sandy or silty, with small-scale cross-bedding.
Thickness of Formation
The vertical thickness of this formation ranges from 4 to 11 metres.
Environment of deposition
The Hampen Formation are interpreted as a crabonate facies deposited in a marine environment with some siliciclastic terrigenous input.
Boundaries
Vertically the Hampen formation has a limestone and marl, resting sharply on the upper ooidal grainstone of the Taynton Limestone Formation. The upper boundary is more diffuse with the White Limestone Formation. Laterally the Hampen Formation passes into a very shallow marine-paralic in the Rutland Formation. Westwards it passes into the upper parts of the Fuller's Earth Formation.
Spatial distribution:
North Cotswold Hills.
Type/reference sections:
The type section and reference sections are all located in neighbouring Gloucestershire.
Landform contribution:
The Hampen formation tends to form hollows between the more resistant, harder units of the Taynton Limestone Formation below and the White Limestone Formation above.