Chapter 3 The Mendip and South Wales massifs

M.J. Simms

Introduction

The Mendip Hills of Somerset form a major Palaeozoic inlier separating the predominantly Mesozoic rocks of the Severn Basin to the north and the Wessex Basin to the south. They comprise an en-echelon series of four asymmetric periclines developed in Carboniferous and older rocks, these periclines being markedly steeper, or even overturned, on their northern flanks and in several instances underlain by low-angle thrust planes (Williams and Chapman, 1986). Intensive subaerial erosion in Permo–Triassic times stripped away the Upper Carboniferous, and in places the Lower Carboniferous and underlying Devonian, rocks across the fold crests. Rising sea level during the Jurassic Period then saw the progressive submergence of the Palaeozoic inlier and its complete burial by late Jurassic times (Simms, 1997). Roughly along-strike to the WNW Triassic and Lias Group strata similarly onlap onto the Carboniferous basement of the Welsh Massif and thicken southwards into the Bristol Channel Basin, a north-westward extension of the Wessex Basin ((Figure 2.1), Chapter 2). The precise effect of proximity to these Palaeozoic massifs varies from locality to locality. The most widely observed effect is the development of carbonate-dominated, often bioclastic, marginal facies such as are seen at the Panty Slade to Witches Point and Viaduct Quarry GCR sites of south Wales and the southern Mendips respectively. In contrast the sequence to the north-east of the Mendips, on the Radstock Shelf is highly condensed but not necessarily of marginal facies. It is clear that, just as for the main Wessex Basin, the Palaeozoic rocks of the Mendip Hills experienced significant extension during the Mesozoic Era. The most graphic and spectacular evidence for this is the late Triassic and early Jurassic sediment-filled fissures that cut through the Carboniferous Limestone at the GCR sites of Cloford Quarry and Holwell Quarries.

The general lithostratigraphy of the sites discussed in this chapter is summarized in (Figure 2.3) (Chapter 2).

The Lias Group of South Wales

The Lias Group successions exposed at the two GCR sites in south Wales exemplify the relationship between facies and proximity to Palaeozoic basement rocks. The succession at the Pant y Slade to Witches Point GCR site shows clear lateral and vertical facies changes close to the underlying basement, whereas the succession at the Lavernock to St Mary's Well Bay GCR site is typical of more distal facies farther into the Bristol Channel Basin. The Welsh succession was documented by Trueman (1920, 1922b, 1930), who provided the basis for later sedimentological and palaeoecological investigations by Hallam (1960a, 1964a) and by Wobber (1965, 1966, 1968a,b). More recent accounts covering these sites are those of Waters and Lawrence (1987), Wilson et al. (1990) and Warrington and Ivimey Cook (1995).

References