Geodiversity

Forest Marble Formation

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.

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].

Parent group:

Great Oolite

Age:

Jurassic/Bathonian

(166.1–168.3 Ma)

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.

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Oxfordshire (i.e. the area approximately from Burford to Bladon, and northwards to the River Evenlode (Smith, 1812, unpublished stratal tables; see Arkell, 1933)