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Northampton Sand Formation.jpg

Oxfordshire's Rocks

Cornbrash Formation

(Great Oolite Group)

 

Jurassic

Bathonian - Callovian  

[163.5–168.3 Ma*]

 

Image: Box weathering in Northampton Sand Formation (Ⓒ Crown Copyright)

* Ma is an abbreviation for million years

Description

A medium to fine-grained, predominantly bioclastic wackestone and packstone with sporadic peloids; generally and characteristically intensely bioturbated and consequently poorly bedded, although better bedded, commonly somewhat arenaceous units occur in places, particularly in the upper part. Generally bluish grey when fresh, but weathers to olive or yellowish brown. Thin argillaceous partings or interbeds of calcareous mudstone may occur.

 

The name of the rock relates specifically to its suitability for arable farming and was coined in the 18th century. Although of very limited value for building, the Cornbrash has been quarried for aggregate, for example at the Shorncote Quarry in the Cotswold Water Park

Environment of Deposition

The depositional environment for the Cornbrash represents a marine transgression at the close of the Bathonian.  An increasingly energetic environment.

Upper Boundary

The boundary between the shelly-based mudstone of the Kellaways Formation is usually sharp and comformable with the bioclastic limestone of the Cornbrash.

Lower Boundary

Generally sharp, disconformable non-sequence: bioclastic limestone resting upon mudstone (and bioclastic ooidal limestones, locally) of the Forest Marble Formation. 

Thickness

2 - 4 m

Contribution to landscape:

Gently rolling and undulating landscape.          

BGS Reference Section (not accessible by the public):

Shipton-on-Cherwell Cement Works Quarry, 4.4km northnorthwest of Kidlington, Oxfordshire. Full sequence exposed, up to about 3m.

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KEY MINERALS

CALCITE

Berthierine is a dark olive green to yellowish green mineral. It is an iron-rich, aluminous, 1:1-type layer silicate belonging to the serpentine group.

SMECTITE

Siderite is a dark to light brown (sometimes yellow/grey/white). It is iron-rich (Iron carbonate) and belongs to the calcite group of minerals.

 

Image 1:Berthierine mineral (© Modris Baum)

Image 2: exposure of the Northampton Sand Formation (© BGS)

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TECHNICAL WORDS

ARGILLACEOUS

A sediment containing clay

 

ARENACEOUS

A sandy sediment

 

BIOCLASTIC

Bioclasts are skeletal fossil fragments of once living marine or land organisms that are found in sedimentary rocks.  A sedimentary rock is described as bioclastic if they contain bioclasts.

 

BIOTURBATION

The disturbance of sedimentary deposits by living organisms.

 

PACKSTONE

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

 

WHACKESTONE

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

 Hand 

 Specimen  

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 Duston Pit 

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 MADMARSTON  HILL 

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 SIBFORD 

 FERRIS 

QUARRY