New information on Neoproterozoic-Cambrian geology andthe Triassic unconformity around Groby, southern CharnwoodForest, UK

摘要

Detailed investigations utilizing borehole drilling, geophysical techniques and revision mapping have elucidated the geologi-cal evolution of southern Charnwood Forest. This study confirms that the South Charnwood Diorite was emplaced in latest Neoprotero-zoic times and has a broadly concordant intrusive contact with underlying volcaniclastic strata of the Charnian Supergroup. Following erosion that unroofed the diorite, an Early Cambrian marine transgression deposited an unconformable covering of Brand Group strata. In end-Silurian times, the region experienced orogenic compression and folding along a dominant NW–WNW tectonic grain. `Phase 1' structures with these orientations include the Charnwood Anticline and a transecting, penetrative cleavage that is mainly developed in the stratiform Charnian rocks. Further manifestations of this deformation were systems of NW-trending faults, the most important of which is the Groby Reverse Fault. This complex structure is described here for the first time; it features a narrow tectonic slice of Brand Group rocks, which was thrown down into the South Charnwood Diorite. The courses of younger, 'Phase 2' faults are also revealed by the present study; they are orthogonal to, and thus offset the earlier NW structures. During Permian times, the area was stripped of its Carboniferous cover, resulting in a rugged pre-Triassic topography. This study has shown that the South Charnwood Diorite was eroded into low-lying, NW-elongated, steep-sided domes (bornhardts). It has also revealed the configuration of an associ-ated palaeo-valley system, which suggests that regional tilting associated with development of the syn-Triassic Hinckley Basin to the south-west of Charnwood Forest may have influenced drainage directions before burial of the hill-range beneath desert sediments of the Mercia Mudstone Group.