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Tertiary Rivers: Early Eocene

Fig.5  Palaeogeography of the Early Eocene Ypresian Stage.

The Eocene opens with renewed significant marine transgression from the east resulting in deposition of Thames Group sediments, the thickest and most extensive of which is the London Clay Formation (Fig. 3), deposited during the Early Eocene Ypresian Stage.  During this period, submergence was most extensive towards the west and linked the London and Hampshire-Dieppe basins (Fig. 5).   At least 5 transgression-regression cycles are represented, reflecting periods of sea-level rise followed by shallowing and coastline progradation.  Thin, marginal sand and silt-dominated sediments are rare in the London Basin but are found in the Hampshire-Dieppe Basin in Dorset and western Hampshire, where they were subjected to pedogenesis.  

During deposition of the London Clay, the regional climate was very warm, the surrounding land to the north, west and south supporting dense tropical to sub-tropical and warm-temperate forests, with mangrove swamps on the coasts.  Inland an ‘upland flora’ may have flourished, but temperate elements were also well represented in the shallow marine sediments.   The overall character of the flora shows affinities with the present-day Malay Pensinsula, characterised by high levels of precipitation and environmental stability. 

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Modern river channels in southern Thailand (Malay Peninsula). Photographs by P. Gibbard (1999).

The nature of the land surface during London Clay times is poorly known but appears to have continued to be subdued with wide plains and no mountains in Wales.  The major erosion phases seen earlier had ceased and instead had given way to overall slow degradation of exposed surfaces subjected to intense chemical weathering.  This is seen in the vast volumes of clay which may have been produced by extended earlier weathering and supplied to the basinal areas by the rivers from the north and west.  Only limited evidence of actual channel systems is known at present.   However, continual influx of river water has been invoked to explain the low diversity foraminiferal assemblage in the Hampshire – Dieppe basin.  In addition, incised fluvial channels, formed during local sea-level lowstands and then infilled by estuarine deposits during subsequent marine transgression, are recorded from the Portsmouth and Whitecliff members across the basin.  The basal pebble beds of the Hampshire basin region are thought to have been were derived from the south-west, presumably by rivers (Fig. 5).  They envisage several smaller streams flowing northwards.

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