Birdlip Limestone Formation
Age: 175-171 million years (Middle Jurassic, Aalenian)
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 Birdlip Limestone Formation forms the basal 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 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.
The Birdlip Limestone Formation comprises mainly oolitic limestones, but also contains shell-fragmental limestones and sandy limestones. Within these lithologies there are occasional beds of sandstone and subordinate calcareous mudstone. The Formation consists of five members, which are hard to distinguish in the field. Contacts between the upper most three members (described below) can be seen at Broadway Quarry.
Cleeve Cloud Member
This is the thickest member and also the most extensively quarried. The unit is dominated by well bedded, well sorted medium to coarse-grained ooidal grainstones. The base of the Member is sandier than the rest of the unit. The Member also varies laterally, getting more ferruginous from west to east, with the exposures around Broadway being markedly more yellow than those seen at Bredon Hill. The only definitive exposure in Worcestershire is at the base of Broadway Quarry, where a hardground is developed at the top of the Member.
This unit is dominated by pale grey and brown, medium to coarse-grained, peloidal and ooidal packstone and grainstone interbedded with shelly limestone dominated by calcitic mud. It is known for its well preserved fauna which includes brachiopods, corals and bryozoa. The Member is also exposed at Broadway Quarry.
This is the uppermost of the Birdlip Limestone Formation. It comprises grey to orange-brown, fine to medium grained sandstone at the base, overlain by grey/brown, silty mudstones with variable sandy or shelly beds. This is topped by a pale grey and brown sandy, shelly, peloidal and ooidal limestone. The Member underlies an important disconformity resulting in a lack of exposure outside of Broadway Quarry or in boreholes.
Broadway Quarry, Worcestershire
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