Summary The study of latest Neoproterozoic fossils that comprise the Ediacara biota is a relatively new frontier of palaeontology. That it started when it did is due, in no small measure, to the discovery of macrofossils in Charnwood Forest in 1957, and their description in a paper to the Proceedings of the Yorkshire Geological Society by Trevor Ford in 1958. There is a prequel to this, however. We reveal newly found correspondence showing that as long ago as 1848 enterprising naturalists had seen these macrofossils and recognized them as such. The subsequent rejection of those finds by others mirrors the experiences of geologists elsewhere in the world, and is attributed to a previously received wisdom that macrofossils only occurred in Cambrian or younger strata.
Precambrian rocks in northwestern Chamwood Forest differ markedly
from their lateral equivalents to the east and south. They are subdivided into the
Whitwick Volcanic Complex, of massive to intensely brecciated high-silica andesites
and porphyritic dacites, and the Chamwood Lodge Volcanic Formation, which is a
thickly bedded sequence of mainly andesitic to dacitic volcanic breccias and lapillituffs.
Lithological elements common to both of these units are indicated by field,
petrographical and geochemical evidence, which suggests the existence of two
'genetic associations' of rock-types. These associations, and various other units that
are distinctive to this region, form the basis of a model that views the Whitwick
Complex as an aggregation of magmatic feeder bodies that supplied material, in the
form of blocks and lapilli, to a volcaniclastic apron represented by the Chamwood
Lodge Formation. The analogues for these rocks can be drawn from the axial
magmatic zones of modem or geologically very young volcanic arc systems. The
high-silica (dacitic and rhyolitic) Charnian magmas were intruded into
unconsolidated wet sediments, resulting in physical interactions that generated
peperitic lithologies and related breccias. By contrast, the andesitic magmas may
have extruded subaerially as lava domes that periodically collapsed, giving rise to
block and ash pyroclastic flows and lahars.
Summary Detailed investigations utilizing borehole drilling, geophysical techniques and revision mapping have elucidated the geological evolution of southern Charnwood Forest. This study confirms that the South Charnwood Diorite was emplaced in latest Neoproterozoic 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 associated 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.
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