history of the major rivers of southern Britain during the Tertiary, The

Journal of the Geological Society, Nov 2003 by Gibbard, P L, Lewin, J

In the Hampshire-Dieppe Basin the London Clay is succeeded by the Bracklesham Group sequences (Fig. 3); in particular by the coarse cross-bedded fluvial sands of the Poole Formation in the western part of the basin, e.g. around Wareham, that span the latest Ypresian (or 'Cuisian') to mid-Lutetian stages (sec Curry et al. 1977). These sands pass laterally eastwards into silts and fine sands of estuarine and marine origin. The basal sands, such as those exposed at Studland Bay, Dorset, together with those of the Bournemouth cliffs, Wareham and the Isle of Wight, characteristically occur in fining-upward cycles that pass upwards into lignitic sands and fine material rich in plant remains (Fig. 6; Flint 1983a; Daley 1999). Flint (1983a) interpreted these sequences as representing actively meandering sand-bed rivers, but with affinities to those of sandy braided streams. The progressive westward coarsening of these deposits is accompanied by a pebble composition that includes exotic clasts such as abundant vein quartz, black chert, silicified limestone and siltstone, all derived from Palaeozoic rocks but possibly reworked from the Wealdcn (Flint 1982). The architecture of the sequences suggests fluctuating, potentially seasonally varied discharge, and this accords well with the predicted tropical to subtropical climate (Smith et al. 1973) and a dense and diverse tropical to subtropical vegetation (Daley 1972, 1999).

Further west, beyond the margins of the Hampshire-Dieppe Basin, outliers of coarse gravels provide considerable insight into the early Palcocene drainage and environment of SW England. At Blackdown and Bincombc in Dorset, a conglomeratic facies of the Middle Eocene Bracklesham Group (Daley 1999) occurs. These crudely bedded, clast-supportcd gravels appear to be the upstream equivalent of those at Wareham (Flint 1982) and again include a range of exotic lithologies derived from Paleozoic source rocks. The coarse nature and bedding structures in the gravels have resulted in their being interpreted as high-energy fluvial deposits; Flint (1982) attributed them to alluvial fan-type deposits associated with local fault movements. However, if they were solely a local phenomenon, that would not explain their exotic pebble components (see Daley 1999). It is therefore more likely that they represent derivation from a gravel river-type accumulation tributary to a major eastward-flowing river, i.e. the Soient River, from Devon, possibly augmented by dissection of tributaries additionally supplying locally derived materials (Figs 6 and 7).

The Haldon Formation gravel in Devon apparently represents an upstream equivalent of the Poole Formation (Fig. 3). These gravels, described by Hamblin (1973) and Edwards & Frcshncy (1982), comprise two members. The lower, Tower Wood Gravel is a flint gravel comprising unabradcd clasts in a clay -dominated matrix, the latter being 'a well-ordered kaolinitc with little ball clay kaolinite and illite' (Hamblin 1973). It is interpreted as a weathering residue, derived from a former cover of Chalk that may have reached as much as 200-300 m thick. However, heavy minerals indicate that the clay was derived from the Dartmoor Granite, west of the Haldon Hills, and probably accumulated post-depositionally in the interclast voids.