Tertiary Rivers: late Middle Eocene – Late Eocene
In south-west England a series of isolated, enclosed basins occur along the line of the major NW-SE-trending Sticklepath-Lustleigh wrench fault. They include the Bovey, Petrockstowe and the minor Dutson basin on land and the Stanley Bank Basin beneath the Bristol Channel. The Bovey Basin is the largest of these features, all of which are infilled by terrestrial, largely fluvial sediments. The predominently argillaceous sediments of the Bovey Formation are underlain by the Aller Gravel, discussed above. Here however, they are overlain by a complex, highly variable sequence that is over 1000 m thick. The upper 300 m of these sediments are certainly of Oligocene age (see below) and are exposed in quarries from which the koalinite-rich clay is extracted. However, the lower 700m thick unexposed part of the sequence (the lower Bovey Formation ) is of Eocene age and represents accumulation in the fault-bounded trough during which sedimentation kept pace with subsidence. The sediments comprise silts, sand, gravel and, often lenticular silty clay, comparable to similar sediments in the Petrockstowe basin, where the bulk of the deposits are Eocene. At Petrockstowe the basal sediments are again braided stream gravels, derived from the Dartmoor Massif and overlain by a series of upward-fining cyclic sequences of sands to fines, seatearths and deeply-weathered palaeosols. These channel to floodplain sediment cycles typify an active-meandering river, sands to fines-dominated active- to stable-meandering or even in part anastomosing system with stable floodplain surfaces, cut-off meander channel fills etc. However, the cyclicity is partly attributed to subsidence. Here the sediments fine in a north-westerly direction indicating that the river was flowing into the Stanley Bank Basin towards the Bristol Channel; the latter appearing as a landscape element for the first time at this stage. Comparable sediments are also known from the St.George’s Channel Basin beneath the Irish Sea (see below). In contrast the Bovey Basin stream flowed SSE-wards (Fig. 5) showing that by this time the upper headwaters of the Solent River had been severed by movement of the Sticklepath-Lustleigh fault. A comparable deep infilled Bassin de Rennes occurs in Brittany.
The later Middle – Late Eocene ( late Lutetian, Bartonian to Priabonian) saw the continued sedimentation of freshwater and estuarine sediments in the western part of the Hampshire-Dieppe Basin. The extensive estuarine Boscombe Sand (Barton Formation) in the Hampshire – Dorset area (Fig. 3), reflects deposition in a tidal channel environment that interfingers with beach conglomerates. Fluvial deposits of Bartonian age are apparently unknown but deposition of the Headon Hill Formation (Solent Group) above occurred in near-coastal situations, including brackish lagoons, freshwater stream and shallow alkaline lakes in a low-energy embayment (Fig. 8). The sediments include lacustrine freshwater limestones, silts, marls, occasional sands and local lignite. In places, fossil variegated palaeosols, formed under conditions of highly seasonal wetting and drying, also occur. Although local stream input is represented, no large-scale fluvial activity is apparently recorded.
Fig.8 Generalised palaeogeography of the western Hampshire-Dieppe basin during deposition of the Bracklesham Group.
The youngest Eocene sediment in the Hampshire-Dieppe Basin is the Bembridge Limestone. This highly fossiliferous muddy limestone complex represents deposition in a series of pools and lakes with a limited local catchment surrounded by dense forest under a subtropical or warm temperate climate. Although there is again no direct evidence of substantial fluvial activity at the Isle of Wight sites, it is highly probable that the Solent, and possibly a Hampshire (?proto-Avon) river, continued to flow at some distance since sedimentation continued into the early Oligocene Bembridge Marls (see below) which reflect deposition in a sluggish-water estuary. The lack of fluvial sediment implies either that discharges and/or sediment supply were low, which reinforces the conclusion that southern English landscapes remained subdued, with stable, densely vegetated surfaces during this interval.