Geophysical investigations of the Carboniferous and pre-Carboniferous formations of the East Midlands of England.

A compiled Bouguer anomaly map of the southern half (Sheet 2) of Great Britain and the Irish Sea, after processing, shows a "regional" gravity gradient of about 0.08 mgal/km rising from east to west. One interpretation of the processed Bouguer maps shows a N-S "crustal ridge", of about 3 km in amplitude in the Irish Sea. The depth to Moho, in Hm in km, is related to regional Bouguer anomaly, by gm, by Hm = 30 - 0.07 gm. A number of the observed gravity anomalies of the East Midlands were interpreted, in some cases after the removal of the above regional field. The interpretations were used to divide the area into high and deep basement regions, depending on whether its depth was below or above 1 km. The axes of the high basement structures trend mainly NW-SE and sometimes N-S. The basement of the Derbyshire "Dome" is a N-S asymmetric structure of positive relief. There is a "ridge" of an undulating basement structure between the southern part of the "Dome" and Charnwood Forest, and this may be an extension of the Derbyshire "Dome" basement. The gravity study reveals that the Charnian rocks lie at a relatively shallow depth to the east and west of the present outcrops. A low density material, possibly granite, was postulated to lie at a relatively shallow depth below the Blackbrook Beds. The structure of the Carboniferous rocks may have been influenced by the basement structure. A considerable undulation in the surface of the Carboniferous Limestone around Breedon and Cloud Hill inliers was revealed from the gravity and seismic studies. Some of these undulations may have resulted from a series of normal faults. In a region of the East Midlands, centred around Nottingham, about 25 km in width in the northwest increasing to about 45 km in the southeast there occur high magnetic anomalies, the sources of which are thought to be intrusive granitic in origin. Seismic refraction work supports the existence of such an intrusive rock at a depth of approximately 1500 ft (457m).