Oxfordshire Rock Types

Cornbrash Formation

Great Ooilite.        Jurassic/Bathonian-Callovian Age

Limestone, 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.

Forest Marble Formation

Great Ooilite.     Jurassic/Bathonian Age

Named by the Father of English Geology, William Smith, the Forest Marble is named after the type area: Wychwood Forest (Burford to Bladon and northwards to the River Evenlode).

White Limestone Formation

Great Oolite.     Jurassic/Bathonian Age

Previously known as the Great Oolite Limestone, the White Limestone Formation is a pale grey to off-white or yellowish limestone, peloidal wackestone and packstone with subordinate ooidal and shelly grainstones, recrystallised limestone and/or hardgrounds at some levels with rare sandy limestone, argillaceous limestone, marl and mudstone or clay. 

 

Lithogenesis: The depositional environment for the White Limestone Formation is a protected, shallow marine setting with periods of reduced sedimentation resulting in the formation of hardground.  Higher energy phases are recorded in the rock record by cross-bedded ooid limestones.

 

Vertically, the White Limestone Formation is found overlying the marls or fine-grained ooidal grainstones of the Hampen Formation.  The upper boundary is commonly a sharp, erosive boundary, with the Forest Marble Formation. Laterally, the White Limestone eventually passes into the Athelstan Oolite Formation south westward and the Blisworth Limestone Formation north eastward.

 

Spatial distribution:  The BGS determined 'type area' is the Cherwell Valley from Woodstock to Ardley, where it is commonly ca. 20 metres thick (it can be up to 30 m).

 

The type section and reference section are not accessible to the public (Shipton-on-Cherwell Quarry and Ardley Railway Cutting respectively).  There is a good exposure of the White Limestone at Kirtlington Quarry (SP 494 199).

 

Landform contribution: The White Limestone caps escarpments and valley slopes, and forms broad plateaux throughout the north Cotswolds.  The calcareous regolith contributes to the formation of the rendzina facet of the Cotswold catena.

 

         

Hampen Formation

Great Oolite.     Jurassic/Bathonian Age 

Previously known as the Marly beds (Woodward, 1894) this limestones with subordinate interbedded marls.  The limestone beds 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. The vertical thickness of this formation ranges from 4 to 11 metres.

 

Lithogenesis:  The Hampen Formation are interpreted as a crabonate facies deposited in a marine environment with some siliciclastic terrigenous input.

 

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.

 

The type section and reference sections are all located in 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.

Taynton Limestone Formation

Great Oolite.      Jurassic/Bathonian Age

The Taynton Limestone is a buff/brown to pale brown, rock. It is typically well-sorted, often medium  to coarse-grained and commonly it is described as a moderate to high  bioclastic, ooidal grainstone.  Typically it is cross-bedded. In some localities it can be fine to very coarse-grained or medium to thickly well-bedded. Thin shell-detrital marl seams and discrete calcareous sandstone beds are not uncommon.

Lithogenesis: A high-energy, shallow marine, transient ooid bank or bar.  In addition there is evidence of lower-energy environments with carbonate and terrigenous sedimentation.

 

Vertically, the conformable, lower boundary is with the Sharp's Hill Formation. The upper boundary is marked by the upward passage into the finer Hampen Formation.Spatial distribution: Burford and area, Oxfordshire.

 

Type section: Lee's Quarry, Taynton Down, Oxfordshire (SP 236 152).  7 m of the formation is exposed here, the chracteristic cross-bedded bioclastic/ooid grainstone facies. The thick beds are separated by thin. calcareous mudstones. The base of the formation is not exposed at this site. The upper boundary with the Hampen Formation is present.

 

Landform contribution: The Taynton Limestone Formation frequently forms ledges on valley sides and minor plateaux.

Sharp's Hill Formation

Great Oolite Group.     Jurassic/Bathonian Age

A varied sequence of greenish grey, silty, moderately shelly and calcareous mudstones, pale greenish grey shelly marls and fine-grained shelly limestones with marine and freshwater faunas. Rootlets are common, and some mudstones display seatearth textures and more variegated

colours, also indicating shallow, proximal conditions. Subordinate siltstone and sandstone beds are present near the base of the formation. 

 

Lithogenesis: Fluctuating very shallow marine/paralic/emergent environment. A steady input of fine terrigenous material is apparent.

 

Vertically, the Sharp's Hill Formation has a lower non-sequential boundary with the Chipping Norton Limestone Formation or the Horsehey Sand Formation.  At its upper boundary it is conformably overlain by ooidal and shelly-detrital limestones of theTaynton Limestone Formation. Laterally, the Sharp's Hill Formation passes in the south-west into the Fuller's Earth Formation.

 

Spatial ditribution: North Oxfordshire north-east of Milton-under-Wychwood and Minster Lovell.

 

Type section: Sharp's Hill Quarry, Sibford Ferris (SP 338 359). Reference section: Horsehay Quarry, Duns Tew. (SP 456 273).  At the reference section 4.6 metres of the fomation (clay, calcareous clay and fine-grained limestone facies) grounded on the Chipping Norton Limestone, is exposed.

 

Landform contribution: together with the Horsehay sands (if present) it generally forms hollows between the more resistant, competent units of the Chipping Norton Limestone (below) Formation and the Taynton Limestone Formation (above).

Horsehay Sand Formation

Great Oolite Group.      Jurassic/Bajocian - Bathonian Age

Unbedded to weakly bedded and cross-bedded, pale grey and brown to off-white, medium to fine-grained, quartzose sand, locally cemented into calcareous or weakly ferruginous sandstone with thin dark grey mudstone and siltstone beds in places, rootlets and lignitic debris common, shells and shell debris very rare.

 

ReferenceL Horsehay Quarry, Duns Tew.  (SP 456 273)

 

Chipping Norton Limestone Formation

Great Oolite Group.      Jurassic/Bathonian Age

This limestone ranges in thickness from 0-12 m (12.7 m at Chipping Norton). Lithologically it is a hard, slintery, buff/brown rock. Texturally it may be described as fine to medium-grained ooidal and coated peloidal grainstone. Fine burrows, medium to coarse-grained shell detritus, flakes of greenish grey mudstone, dark lignite and minor amounts of fine-grained sand elude to its near shore, marine depositional environment.

 

The Chipping Norton Limestone can be described as a thick-bedded and cross-bedded, weathering to flaggy or platy. Thin shell-detrital and ooidal marl and mudstone intercalations in places.

 

Vertically, the Chipping Norton limestone has a lower boundary  which is conformable/non-sequential with the Clypeus Grit member. Locally a mudstone bed (Roundhill Clay)  up to c.1 m thick occurs at the base. The upper boundary is with the Sharp's Hill Formation.   

 

Laterally the formation passes into the Horsehay Sand Formation to the north east.

 

Spatial distribution: North Oxfordshire Cotswolds.

 

Reference: Ditchley Quarry aka Town Quarry, Charlbury (SP 370 200)

Clypeus Grit Member

Inferior oolite.     Jurassic/Bajocian-Bathonian

A member of the Salperton Limestone Formation.

 

The Clypeus Grit is a pale grey to pinkish brown rock which can be either fine or coarse-grained.  It can be ooidal or peloidal, containing bioclastic fragments.

 

The Clypeus grit can therefore be described as either a packstone or a grainstone, with large orange-skinned peloids/pisoids and aggregate grains; common whole shells especially in upper part. Characteristic fauna includes the large echinoid Clypeus ploti, large myacean bivalves and terebratulid brachiopods Stiphrothyris tumida.

 

 

 

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