Forest Marble Formation
GEOLOGY | FOREST MARBLE FORMATION
Great Oolite Group
Forest Marble Formation
[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: Forest Marble Formation (false bedding) overlain by the lower half of the Cornbrash Formation at Greenhill Quarry (Ⓒ 1925 Crown Copyright)
* Ma is an abbreviation for million years
A flaggy, sandy, carbonate mudstone.
This muddy limestone varies across the area it crops out:
grey, weathering brown
or - flaggy, variably sandy medium to coarsely bioclastic grainstone.
Environment of Deposition:
The Forest Marble is a complex association of vertically and laterally changing facies. These are interpreted to represent environments ranging from subtidal-intertidal fully marine sand shoals and hard substrates colonised by corals, to mud flats and tidal channels associated with coastal swamps.
Generally mudstone in the upper part of the Formation, overlain sharply and non-sequentially by ooidal shelly wackestone/packstone of the Cornbrash Formation.
A grey to brown variably sandy medium to coarsely bioclastic grainstone or packstone, resting with erosive, commonly channelled, disconformity on the White Limestone Formation.
Retrocostatum Zone to Discus Zone
Generally forms plateaux behind escarpments formed by basal limestone of the formation or underlying limestone formations.
See it in Oxfordshire:
Shipton-on-Cherwell Quarry (BGS reference section)
Wychwood Forest, Oxfordshire (i.e. the area approximately from Burford to Bladon, and northwards to the River Evenlode (Smith, 1812, unpublished stratal tables; see Arkell, 1933)
Some Key Words
Sedimentary facies are bodies of sediment that are recognizably distinct from adjacent sediments that resulted from different depositional environments.
A packstone is a rock with a grain-supported texture with the intergranular voids filled with a finer matrix.
A lime wackestone is a matrix supported carbonate rock containing less than 75% mud-grade (<32 μm) calcite.
Backscattered scanning electron microscope image of a rock sample from the Forest Marble Formation.
It comprises sub-rounded worn shell fragments composed of calcite (grey) with a matrix of fine microporous micritic calcite. Fine grained authigenic pyrite (white) is disseminated within the microporous matric.
[Taken from Milodowski and George, 1985, Harwell Borehole No. 3 342.7 m].