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Regional Studies |
British Geological Survey, Keyworth, Nottingham NG12 5GG, UK
Depositional sequences in the Palaeocene and early Eocene of southern England are grouped into three unconformity-bounded composite sequences, corresponding to the Ormesby Clay and Thanet Sand formations, the Lambeth Group (including the Woolwich and Reading formations) and the Thames Group (including the Harwich and London Clay formations).
A refined chronostratigraphical framework, coupled with a revised Palaeogene chronometry, reveals a clear relationship between long-term Palaeocene to early Eocene sea-level trends and radiometrically dated volcanic events in the North Atlantic Province. In gross terms, it is possible to relate the progressive Palaeocene uplift of NW Europe to the rise of the proto-Icelandic mantle plume beneath the continental crust of East Greenland, and the early Eocene subsidence of NW Europe to the transfer of the plume to the developing NE Atlantic mid-ocean ridge. However, both the sedimentary and volcanic record indicate that the uplift of NW Europe took place in three stages, expressed in the North Sea region as long-term regressive-transgressive facies cycles. The first uplift phase was associated with the onset of Hebridean volcanic activity and with a short-lived influx of coarse clastic sediments into the North Sea Basin (basal Ekofisk Formation). No sediment associated with this cycle is preserved in southern England. The second uplift phase was associated with the main period of Hebridean activity and led to substantial uplift of Scottish source areas. It was followed by regional subsidence and transgression of the basin margins (Ormesby-Thanet composite sequence). The third uplift phase was associated with the onset of rifting and volcanism along the North Atlantic rift zone. Eastward tilting initially led to sedimentation in eastern and southern England (Lambeth composite sequence), but continued uplift of southern and western Britain eventually restricted sedimentation to the central and northern parts of the North Sea basin, with major progradation of the Dornoch delta complex taking place along the eastern margin of the Scottish land mass. This period marked the maximum isolation of the North Sea Basin, with reduced salinities being reflected in a near-absence of marine fauna and a highly restricted microflora.
The third uplift phase is believed to mark the culmination of plume-related tectonism in NW Europe and was associated with a resurgence of volcanic activity in the British Tertiary Igneous Province, including intrusion of the Cleveland-Blyth-Acklington dyke system along the uplifted Sole Pit inversion zone. A sharp reduction in intrabasinal activity, coupled with the initiation of a long period of regional subsidence (Thames composite sequence), is interpreted as representing thermal subsidence following the onset of seafloor spreading between Greenland and Scotland. Oceanic crust generation was initially subaerial and was associated with intense, largely basaltic, pyroclastic activity that affected the whole of NW Europe (Balder Formation and equivalent tuffs).
The cause of the episodic nature of the plume-related uplift is uncertain. It appears that the mechanics and configuration of plume emplacement were either intrinsically more complex than previously recognized, or were influenced by changes in the regional stress pattern within the NW European crust.
