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Aston Limestone Formation

Age: 172-168 million years (Jurassic, Bajocian)

Classification: Sedimentary

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 Aston Limestone Formation forms the middle 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 Aston Formation is almost exclusively seen in outcrop 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 Formation comprises grey and brown, rubbly, variably shelly, ooidal limestones with sandy and marly beds in parts. This formation is only exposed in Broadway Quarry and the surrounding area, and consists of four members.

Lower Trigonia Grit Member

This is a grey, very shelly, moderately sandy, peloidal and ooidal wackestone, packstone and grainstone.

Gryphite Grit Member

This is a grey and brown, rubbly, shelly, coarse grained, sandy, peloidal wackestone, packstone and grainstone interbedded with thin calcareous mudstones and sandstone beds.

Notgrove Member

This is a pale brown-grey, well-bedded, medium-coarse grained peloidal and ooidal grainstone. This is the only member exposed outside of Broadway Quarry and is found along the top of Broadway Hill.

Rolling Bank Member

This unit consists of rubbly, shelly, sandy, ferruginous, peloidal and ooidal packstone and wackestone, commonly containing pectiniid bivalves.

Sites

Broadway Quarry, Worcestershire


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