OR/10/041 Guide to the geology of Bradgate Park and Swithland Wood, Charnwood Forest
INTRODUCTION AND GEOLOGICAL BACKGROUND
Charnwood Forest is one of the few parts of England where there are exposures of ‘basement’ rocks dating back to Precambrian time. Its locally rugged topography is caused by these highly resistant rocks protruding as craggy knolls through a surrounding cover of Triassic-age Mercia Mudstone strata and Quaternary deposits. Past workers have viewed Charnwood Forest as a ‘fossil’ hill range that was carved by erosion dating from the late Carboniferous (end-Variscan) block uplifts, was subsequently buried beneath Triassic and younger strata, and is now in the process of being exhumed. The ‘hard rock’ outcrops and distinctive scenery of Charnwood Forest, exemplified by Bradgate Park, have attracted much interest over the centuries, and Watts (1947) charts a number of publications stretching back to 1790. The lithostratigraphy of the Charnwood Forest succession was formalised after the detailed mapping and thesis of Moseley (1979), and a subsequent paper by Moseley and Ford (1985). Figure 1 shows that the Charnian Supergroup sensu stricto is divided into two principal groupings, of which the youngest, the Maplewell Group, will be visited today (localities and route are shown in Figure 2). In Swithland Wood, the strata to be visited belong to the Brand Group. This overlies the Maplewell Group, and as it is now referred to the Lower Cambrian, rather than to the Precambrian as previously thought, it is no longer included as part of the Charnian Supergroup. A Precambrian age (i.e. older than the start of the Cambrian Period, 543 million years ago) for the Charnian rocks was hinted at as long ago as 1865. It was finally confirmed following the work of Lapworth (1882), although his observation was based on similarities between Charnian rocks and the Caldecote Volcanic Formation, which is demonstrably overlain unconformably by Lower Cambrian strata at Nuneaton, 30 km to the west. Lapworth’s discovery had major implications for something that happened much later - the finding of fossils in the Charnian strata by a schoolboy, Roger Mason, when out climbing near Woodhouse Eaves in 1957. Since then, several more fossiliferous localities have been found in Charnwood Forest (eg. Boynton and Ford, 1995), including the important exposure in Bradgate Park. Their significance to Precambrian geology, and to the understanding of the early evolution of organized life, will be discussed later on. Much still remains to be clarified about the precise age of the Charnian Supergroup, in terms of a figure expressed in millions of years. Estimates of 560 - 566 Ma (Compston et al., 2002) have been determined for the fossil-bearing upper part of the Maplewell Group on the basis of isotopic analyses that measure the decay of uranium to lead in rock-forming minerals such as zircon. Given the exposed thickness of 3000 m for the Charnian sequence, however, it is clear that more isotopic determinations will be needed in order to constrain the entire age-range of the succession.
Mode of origin of the Charnian Supergroup
Evidence concerning the mode of formation of the Charnian sequence is at first sight contradictory. In the south and east - for example in Bradgate Park - the rocks are typically well stratified and of obvious sedimentary origin. When looked at under the microscope, however, the grain constituents – mainly volcanic rock fragments, crystals (plagioclase and quartz) and volcanic ash shards – point to a wholly volcanic, andesitic to dacitic source region. It is therefore accurate to say that the Charnian Supergroup is a volcaniclastic succession. This is an ‘umbrella’ term for bracketing strata containing varying proportions of grains derived from the erosion of pre-existing volcanic successions (epiclastic origin), as well as material incorporated into the rock directly from volcanic eruptions (pyroclastic origin). Pyroclastic material may consist of non-abraded volcanic ash shards (top right of Figure 6), crystals, or angular volcanic rock-fragments. The qualifying term tuffaceous is commonly used for sedimentary rocks that are a mixture of epiclastic and pyroclastic grains, where the latter’s abundance is more than 25 and less than 75 per cent of the rock. Evidence for the depositional environment of these rocks is provided by sedimentary structures seen in the stratified parts of the Charnian Supergroup. At Bradgate Park, for example, the absence of features such as cross-bedding or current and wave-ripple structures suggests that deposition occurred well below storm-wave base (ie. >50 m depth). Instead, sedimentary structures such as grading, loading and slump-induced disruption of bedding indicate that they accumulated by processes that involved the transport of volcanic detritus in sediment-laden submarine flows (turbidity currents). Earthquakes caused by tectonic or volcanic activity probably triggered individual sedimentary flow-events. Finally, marine (oceanic) environments are further suggested by the types of fossil seen in Charnwood Forest. The Maplewell Group strata contain the greatest volume of pyroclastic material, including ash fragments, and these ‘tuffaceous’ rocks were therefore formed during the time of maximum volcanic activity. As Figure 1 shows, this group displays a very important lateral change on going north-westwards, from the stratified and predominantly medium-grained tuffaceous rocks of the Beacon Hill Formation (to be seen at Bradgate Park) into the thickly-developed and very coarse volcanic breccias of the Charnwood Lodge Volcanic Formation, which have been interpreted as pyroclastic flow deposits (Carney, 2000a). This indicates that the Maplewell Group volcanoes (Figure 6) were located in the north-west of Charnwood Forest, where there are also rocks that could represent actual feeder zones or conduits to the vents. These rocks are typically massive (unbedded), and because they do not form part of