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Tertiary Rivers: Oligocene

The Oligocene begins in the Isle of Wight area with sedimentation of the Rupelian-age Bouldnor Formation (Fig. 3), the lowest member of which, the Bembridge Marls Member, continues from the Eocene.  The Bouldnor sequence predominantly comprises black, green or grey silts that include plant and animal fossils of fresh to brackish water environments.  The overall sedimentology suggests deposition in a low-energy estuarine or floodplain complex, “the upper reaches of which were sufficiently river-influenced and isolated from the sea to have experienced near- or truly freshwater conditions” (Daley 1999, p.148) in what was in all probability the proto-Solent River.  The palaeobotanical investigations from the Bouldnor Formation are significant in that they demonstrate a distinct shift from the post-Bartonian subtropical/ very warm temperate conditions to cooler climate.  The flora records a remarkable assemblage of aquatic and paludal plants inhabiting a lush local environment, with a hinterland of more open vegetation with trees including Pinus and Sequoiadendron that contrasts markedly with the dense tropical-type forest of the Eocene.

To the west in the Irish/Celtic Sea region there are a series of downfaulted basin structures infilled with later Palaeogene to early Neogene sediments.  These basins owe their origin to basin inversion during the post-Maastrichtian (latest Cretaceous) to pre-Middle Eocene period. They include the St.George’s Channel Basin, the North and South Celtic Sea basins, the Bristol Channel Basin, the Stanley Bank Basin and the Cardigan Bay basin. They were infilled, particularly in the north and east mainly by fluvial and related deposits.  The sequences are best known from the Mochras borehole, in the Cardigan Bay basin and from the well-known onshore basins at Petrockstowe and Bovey Tracey in Devon.  The repeating sand, silt, clay and lignite fining-upward sequences represent fluvial floodplain complexes (Fig. 9).  These fining-upward cycles are thought to represent relatively short periods of sedimentation punctuated by long phases of pedogenesis under moist swampy forested conditions.  Seasonal watertable variation is invoked to account for the formation of characteristic gley soils. Lateral reworking resulting from active stream channel meandering is rare.  The sediment cycles include sharp-based sand beds, interpreted as crevasse splays, and basal coarsening upward sheets immediately predating a fining-upward accumulation, interpreted as levée deposits.  Overall this facies architecture resembles those of sand-bed anastomosing or meandering channel model.    These sequences have been compared to those infilling the substantial Lough Neagh basin in northern Ireland which is also thought to be part of the same river system.

Associated with these predominantly fine sediment sequences in north Wales is a characteristic facies of conglomerates interpreted as representing mass flow deposits or alluvial fans (Fig. 9).  These fans initiated on steep slopes, possibly generated by movement on the adjacent faults, involved transport of debris flows of varied energy types.  Some of the flows were sufficiently fluid to have resulted in transport of soft, deeply weathered clasts periodically derived from the adjacent massif and extending out onto the river floodplain.  Although deposition in the Irish Sea region basins was predominantly terrestrial, the South Celtic Sea Basin includes micaceous, glauconite-rich sands and in the basal part carbonaceous materials interpreted as of marginal-marine origin, implying deposition at the mouth of a substantial Irish Sea river system (Fig. 10).

Fig. 9  Simplified reconstruction and vertical sequence sediment profiles from the late Oligocene – early Miocene of Cardigan Bay.  The depositional model represents a high-sinuosity river system with swamp-dominated floodplains, formed by overbank deposits.  Course modification arose from crevasse splays.  The sediment facies assemblages represent fining-upward, channel to floodplain cycles 1-2m thick.  They terminate as follows: 1. perrennial swamps with gleyed soils; 2. penecontemporaneous swamps with seasonal watertable variation; 3. fine-sediment accumulations associated with coarse-grained conglomeratic material (facies 4), thought to represent well-drained floodplains.  Facies 3 - 4 are restricted to the basal part of the succession (after Dobson & Whittington 1987).

As noted above, the upper 300 m of the Petrockstowe and Bovey Basin infills ( the middle and upper Bovey Formation) are certainly of Oligocene age and comprise cyclic sequences of sand and carbonaceous clay complexes deposited predominantly on river floodplains, remarkably similar to those described from Cardigan Bay, and short-term lakes.  The sands represent channel fills, occasionally associated with local brecciated horizons, thought to result from reworking of desiccated floodplain clays.  In parts, channels cut into underlying clay beds include intraformational breccias of clay clasts in a clay matrix, interpreted as a result of channel bank undercutting.  Meanwhile, presumably floodplain vertically-accreted accumulations of koalinite-rich clays, associated with rootlet beds and local soils and backswamp lignite units are frequent.  But most of the 'lignite' is of detrital origin being composed of reworked Sequoia macrofossils derived from the surrounding dense upland forests.   Overall the streams are thought to have adopted a meandering mode, since vertical-accretionary fine-sediment sequences predominate but there is also distinct evidence of point-bar, crevasse splay and levée facies.  Some of the accumulations, particularly those in the actively-subsiding basins, may have adopted an anastomosing form.   However, the predominance of clays in the Petrockstowe and Bovey basins again suggest that the South-West Peninsula landscape was relatively subdued, although they could have been derived from older sequences.  The clays were derived from the decomposition of the Dartmoor Granite. The rivers flowed in opposite directions (Fig.10); the Petrockstowe river continuing to flow north-westwards whilst the Bovey basin river flowed south-south-eastwards towards the Channel.

These deposits were laid down under a seasonally wet, generally hot climate which was of subtropical aspect, although cooler conditions may have occurred on the higher areas such as Dartmoor.  However, marked climate change at the onset of the late Oligocene Chattian Stage is indicated by substantial cooling in North Sea molluscs.  This is accompanied by a eustatic sea-level fall of over 100 m (Fig.2), related to glaciation in Antarctica.  This gave rise to a major regression phase which saw large previously submerged areas exposed subaerially, allowing the extension of rivers such as the Thames and the Irish Sea river, mentioned above (Fig.10).

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