Salperton Limestone Formation

Salperton Limestone Formation

Age: 172-165 million years (Jurassic, Bajocian-Bathonian)

Overview

Continued sea level rise during the late Triassic meant that marine conditions were established over most of the UK during the Jurassic. The remaining land areas generated very little coarse sediment resulting in the fine grained lithologies characteristic of the Lower Jurassic. Occasional sporadic periods of uplift resulted in shallowing and led to the formation of shoals causing deposition of iron-rich ooids, sands, silts and shell debris making up the Marlstone Rock Formation. Substantial uplift at the end of the Early Jurassic caused a more extensive regression and an abrupt end to open marine conditions. This in turn caused the erosion of the top of the Lias Group. Mid-Jurassic times gave rise to a shallow shelf sea which, in conjunction with a warm climate and a low input of sediment from the land (terriginous sediment), led to the development of widespread carbonates. The Jurassic outcrop area is limited within the central graben of the Worcester Basin and is defined by major growth faults. Movement along the Lickey End fault on the eastern boundary of the Worcester Basin may have influenced sedimentation patterns.

The Salperton Limestone Formation forms the upper unit of the Mid-Jurassic Inferior Oolite Group. The Group is not very widespread, occurring only in the southeast corner of Worcestershire, where it forms outlying hills and escarpments in areas such as Bredon Hill and Broadway. The Salperton Limestone Formation is only seen to crop out at Broadway Quarry.

The entirety of the Inferior Oolite Group is made up of limestones, most of them oolitic, with the content of ooids varying locally. The presence of ooids in this Group suggests a shallow, above wave base, tropical sea as the depositional environment.

Lithology

The Salperton Limestone Formation consists of ooidal and peloidal, shelly and shell fragmented limestone. It is divided into two members:

Upper Trigonia Grit Member

This is a very hard and shelly limestone that is grey and brown in colour. It is a moderately peloidal and ooidal grainstone and packstone that forms rubbly beds.

Clypeus Grit Member

This pale grey to yellow or pink-brown rock is a fine to coarse grained, ooidal and peloidal packstone with subordinate wackestone. It has an abundant fauna including large bivalves and brachiopods.

Sites

Broadway Quarry, Worcestershire