Geodiversity

Forest Marble Formation

forest marble.jpeg
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GEOLOGY | FOREST MARBLE FORMATION

(166.1–168.3 Ma)

Great Oolite Group

Forest Marble Formation

 

 

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: 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

GreenHill Quarry.jpeg

Description:

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. 

Upper Boundary:

Generally mudstone in the upper part of the Formation, overlain sharply and non-sequentially by ooidal shelly wackestone/packstone of the Cornbrash Formation.

Lower Boundary:

A grey to brown variably sandy medium to coarsely bioclastic grainstone or packstone, resting with erosive, commonly channelled, disconformity on the White Limestone Formation.

Index Fossils:

Retrocostatum Zone to Discus Zone

 

Landform contribution:

Generally forms plateaux behind escarpments formed by basal limestone of the formation or underlying limestone formations.

 

Thickness:

10-30 m

See it in Oxfordshire:

Shipton-on-Cherwell Quarry (BGS reference section)

Kirtlington Quarry

Type area:

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Oxfordshire (i.e. the area approximately from Burford to Bladon, and northwards to the River Evenlode (Smith, 1812, unpublished stratal tables; see Arkell, 1933)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

facies
Sedimentary facies are bodies of sediment that are recognizably distinct from adjacent sediments that resulted from different depositional environments.

packstone
A packstone is a rock with a grain-supported texture with the intergranular voids filled with a finer matrix.

wackestone
A lime wackestone is a matrix supported carbonate rock containing less than 75% mud-grade (<32 μm) calcite.

Scanning Electron Microscope image of White Limestone

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].