The geology of the carboniferous limestone of leicestershire and south derbyshire.
2015-11-19T09:06:12Z (GMT) by
In northwest Leicestershire and South Derbyshire there are eight inliers of Carboniferous Limestone. The five Eastern Inliers consist mainly of thick to massive bedded dolomite of C.;S.;to age and has been namedthe Cloud Hill Dolomite Formation. The Formation has been divided into three ffiemberss (l) the I'lassive Bedded Dolomite Member; (2) the Red Bed Member.;(5) the Biohem Member. The three Western Inliers and the topmost beds in the southernmost of the Eastern Inliers consist of thin to thick bedded limestone and dolomitic limestone with interbedded shale and are D2 agej the beds have been named the Ticknall Limestone Formation and divided into two members: (l) the Limestone and Shale Member; (2) the Thick Bedded Limestone and Dolomite Member. The beds in the Eastern Inliers have been strongly dolomitized and are mostly medium to coarsely crystalline dolomite with fossil remains completely destroyed in most cases. The beds in the Western Inliers5 apart from patchy dolomitization of the topmost beds, are largely biomicrite and biomicrudite with interbedded quartz-rich shale. Fossil remains are abundant and well preserved. Dolomitization of the beds in.the Eastern Inliers is thought to have resulted from the invasion of magnesian brines originating from buried sub- Vise'an evaporites; the patchy dolomitization in the Western Inliers probably resulted from Mg∼rich groundwater penetrating the beds from the overlying Trias sic I-Iarl. The Massive Bedded Dolomite Member and the Red Bed Member appear to have been deposited mainly in intertidal conditions, while the Bioherm Member and Ticknall Formation are likely to have been deposited in subtidal conditions. A review of Kent's criteria for distinguishing between gulf and block facies suggests the need for a change in the southwest Widmerpool Gulf boundary. The beds in the eight inliers are highly disturbed structurally, with near vertical strata in the four northernmost Eastern Inliers intersected by normal, reverse, and strike-slip faults, the strike of the beds between the Eastern Inliers offset by sinistral faults, a small anticline through the Western Inliers, and both sets of inliers bounded by post-Carboniferous faulting. The uplift of the beds in the Eastern Inliers is thought to be the result of late Carboniferous to early post-Carboniferous diapirism involving concealed pre-Visean evaporites.