Geodiversity

Horsehay Sand Formation

Some Key Words

calcareous

Containing calcium carbonate.

disconformity

A disconformity is an unconformity between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition.

ferruginous

Containing iron oxides or rust.

Parent group:

Great Oolite

Period:

Jurassic

Age:

Bajocian/Bathonian

(166.1–170.3 Ma)

Description:

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.  

Thickness:

7 m

Lower Boundary:

Seen as the passage from lithologically similar sand and sandstone beds with marine shelly fauna, ooidal and shell-detrital limestones of the Northampton Sand Formation (Inferior Oolite Group) where present, or mudstone beds of the Whitby Mudstone Formation (Lias Group) into barren, sand-dominated beds. The non-sequential boundary with the Northampton Sand Formation, may appear conformable or gradational probably as a result of weathering effects, or locally channelled and disconformable

Upper Boundary:

Marked by the presence of mudstones, marls and limestone beds of the Sharp's Hill Formation where present, or ooidal and shell detrital limestones of the Taynton Limestone Formation. The boundary is non-sequential, and may be marked by an eroded or leached and rootleted surface.

Reference Site:

Horsehay Quarry, Duns Tew.  (SP 456 273)  

Spatial Distribution:

North-east Oxfordshire:  Sibford Gower, Hook Norton and Tackley [Horton, 1977, fig. 4], where it passes south-west (on BGS 1:50 000 sheet 218) into the Chipping Norton Limestone Formation