Marine to fresh water: the sedimentology of the interrupted environmental transition (Ludlow-Siegenian) in the Anglo-Welsh region

Abstract

The transition occurred in a period of approximately 15-20 Ma, in broad association with the closure of the Iapetus Ocean, and as thick marine sediments in the Welsh Basin (?small fore-arc basin) became welded on to the Midland Microcraton and East Anglian Foldbelt to the east, with a consequent inversion of relief. The earlier Ludlow sediments are sharply differentiated between the basin, where deep-water turbidites accumulated, and the microcraton, on which calcareous shallow-marine deposits formed. Slope facies, in association with canyons, mark the basin margins. Environmental differences became increasingly less marked as the Ludlow advanced, but the distinction between basin and microcraton was never entirely eradicated. By the end of the Ludlow, shallow and restricted marine conditions prevailed, but only in central Wales was deposition apparently continuous into the Downton. The earliest Downton sediments, in the areas to the south and east, give evidence of either non-deposition or the temporary withdrawal of the sea. Later Downton sediments spread widely but environmentally are somewhat enigmatic. Except in southwest Wales, where a valley had been alluviated before being transgressed, they point to the replacement of a shallow-marine shoal and barrier complex by extensive and uniform coastal mudflats, influenced for a substantial period by both rivers and the sea. The distant northerly complex of regionally metamorphosed rocks which furnished the Downton sediments became isolated from the Anglo-Welsh area in the early Gedinnian, as deformation in the region of the Irish Sea brought Lower Palaeozoic (including Downton) rocks of the Welsh Basin into the zone of weathering. A major consequence of this shift of sediment source and rearrangement of drainage, marked by an extensive spread of unusually well developed and closely spaced palaeosols, was the sudden appearance and subsequent rapid southward advance of wholly fluvial environments. Under the continued pressure of deformation nearby to the northwest, comparatively stable and frequently meandering streams (Gedinnian) were replaced by larger and more unstable sand-bed rivers of low sinuosity (Siegenian-Emsian).