Geodiversity

Horsehay Sand Formation

Horsehay Sand.jpeg
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Great Oolite Group

Horsehay Sand Formation

BGS Key: (HYSA)

 

 

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: Tadmarton Pit 1963 (Ⓒ Crown Copyright)

 

 

 

* Ma is an abbreviation for million years

Lithology:

A pale grey/brown to off-white, medium to fine-grained, quartzose.  In other places is can appear unbedded, weakly-bedded or cross-bedded; cemented into a calcareous (or weakly ferruginous) sandstone; with occasional thin, dark grey mudstone and siltstone beds. Rootlets and lignitic debris common. Shelly material is rare.

Lithogenetic description:

Marginal marine or deltaic conditions depositing mainly sand.

 

Landform description:

Generally forms hollows between upstanding harder limestone or ironstone units. In some places  the hollows may be formed by this unit coated with mudstone of the Sharp’s Hill Formation.

Definition of upper boundary:

An upward change into mudstone, calcareous mudstone and limestone beds of Sharp's Hill Formation; or the ooidal and shell detrital limestones of Taynton Limestone Formation. Boundary non-sequential. May be marked by an eroded or leached and rootlet-bearing top.

 

Definition of lower boundary:

Change up from (lithologically similar) sand and sandstone beds with marine shelly fauna, but also including ooidal and shell-detrital limestones of Northampton Sand Formation (Inferior Oolite Group), where present, or mudstone beds of Whitby Mudstone Formation (Lias Group), into barren sand-dominated beds. Boundary on Northampton Sand Formation though non- sequential, is usually apparently conformable or even gradational (probably as a result of weathering effects), but locally may be disconformable or channelled (Horton et al., 1987, p. 56).

 

Thickness:

Up to 7 m.

 

Reference Site:

Horsehay Quarry, Duns Tew, Nr. Chipping Norton Oxfordshire.  (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