Tertiary Rivers: late Early – early Middle Eocene
Fig.6 Palaeogeography of the Middle Eocene Lutetian/Bartonian stages.
London Clay deposition was brought to an end by a rapid expansion of fluvial delta complexes towards the east in the late Ypresian – Lutetian (early Middle Eocene) (Fig. 6). In the London Basin this transition is signalled by the sand-silt alternations of the Claygate Beds and the subsequent accumulation of the overlying Virginia Water - Bagshot Formation sands (Fig. 3). The latter are marine in the east, but to the west represent a vast delta complex. The sediments are similar to the Reading deposits, consisting of cross-bedded sands, with thin clay or silt partings and thin pebble beds, becoming considerably coarser towards their western limit. This transition from the London Clay to the overlying sands may reflect either depression of the basin, or uplift of the hinterland further west and north-west, which encouraged river incision and potentially increased discharges of coarser terrigenous sediments. No later Palaeogene fluvial or estuarine sediments are found in the London Basin.
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 middle Lutetian stages. 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 fines rich in plant remains (Fig. 6). These sequences are interpreted 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 Wealden. The architecture of the sequences suggests fluctuating, potentially seasonally- varied discharge and this accords well with the predicted tropical to subtropical climate and a dense and diverse tropical to sub-tropical vegetation.
Further west, beyond the margins of the Hampshire-Dieppe basin, outliers of coarse gravels provide considerable insight into the early Palaeocene drainage and environment of south-western England. At Blackdown and Bincombe in Dorset, a conglomeratic facies of the Middle Eocene Bracklesham Group occurs. These crudely-bedded, clast-supported gravels appear to be the upstream equivalent of those at Wareham and again include a range of exotic lithologies derived from Palaeozoic source-rocks. The coarse nature and bedding structures in the gravels have resulted in their being interpreted as high-energy fluvial deposits; they have been attributed 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. 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 Solent River, from Devon, possibly augmented by dissection of tributaries additionally supplying locally-derived materials (Figs 6, 7).
||Fig.7 Hypothetical three-dimensional
sequence model for point bars in the Middle Eocene Poole Formation of Dorset
(after Plint 1983).
The Haldon Formation gravel in Devon apparently represents an upstream equivalent of the Poole Formation (Fig. 3). These gravels, described by Hamblin and Edwards & Freshney, comprise two members. The lower, Tower Wood Gravel is a flint gravel comprising unabraded clasts in a clay-dominated matrix, the latter being “ a well-ordered kaolinite 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.
By contrast, the overlying Buller’s Hill unit is a clast-supported flint gravel in a sandy clay matrix. Its pebble assemblage includes a range of exotic lithologies including quartz, tourmaline and quartzite, as well as thermally-altered Carboniferous shale and chert. The associated heavy-mineral suite is particularly rich in tourmaline and is again of granitic origin. However, unlike that in the Tower Wood unit beneath, the clay occurs as both ordered and disordered kaolinite, with more illite. Taking the evidence altogether the gravels are interpreted as having accumulated under periodic high-energy sheet-floods, alternating with dry-seasonal conditions of low-energy flow or even complete dessication. These flash-type floods are often typical of arid or semi-arid areas. The Buller’s Hill unit therefore represents an eastward-flowing braided fluvial system; the material was derived from the Dartmoor Granite complex and its aureole, as well as weathered Palaeozoic rocks. In addition to the sedimentological evidence, the predominance of abundant kaolinite indicates that the contemporaneous weathering occurred under a savannah climate with intermittent dry periods since the clay mineral is characteristic of savannah lateritic soil processes. Similar sediments, overlying a laterite soil and an associated silcrete horizon, are known from the Sidmouth area.
Like the Buller’s Hill Gravel, the Aller Gravel found south of Newton Abbott, Devon, provides additional evidence of early Palaeocene fluvial activity in south-western England. This 25-30 m thick sequence of cross-bedded and channelled flint gravels and sands, the lenticular nature and horizontally bedding structure of the matrix-supported gravel and sand, together with the large clast size variability, frequent erosion surfaces etc. all indicate deposition in a braided fluvial complex. The pebble assemblage again confirms derivation from the south-west since it includes aureole as well as rocks from the Dartmoor Granite itself, together with unmetamorphosed Carboniferous rocks of north or north-western origin.