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Mapping and detailed sedimentological study of a group of geosynclinal formations of Llandoverian age (Lower Silurian) around Rhayader, central Wales, have been undertaken jointly by the writer and M. A. Woollands (University College, London). These formations occur near the southeastern margin of the Welsh trough and consist essentially of bodies of sandstone and conglomerate, lenticular through distances of several kilometers, and enclosed within a framework of graptolite-bearing argillaceous sediments.
Marked lateral and vertical variations in the geometry, internal features, and transport patterns of the coarser intercalations may be demonstrated and are attributed to differences in the mechanisms and sites of deposition. On the southeast, an argillaceous facies with a few thin, laterally derived tractional sandstone beds and calcareous bands, displaying slumps, is interpreted to represent a slope environment. This facies contains a few longitudinally derived distal turbidite units which appear in more profusion on the northwest in an argillaceous facies of inferred basin-plain origin.
Mantling the pelagic sediments are broadly lenticular bodies of coarse sandstone interpreted as proximal turbidites. Sole marks in these sandstone beds present a complex but essentially longitudinal transport pattern but ripple marks and other depositional features indicate modification of the northeast-southwest longitudinal pattern by lateral tractional currents (from the southeast). Such proximal turbidites grade both laterally and vertically into more distal turbidites and into coarse granule-gritstone and boulder conglomerate.
The rudites occupy successive, steep-sided channels excavated in the pelagic basin floor and slope sediments, and display features characteristic of fluxoturbidite associations. The orientation of channels, sole marks, ripples, and pebble imbrication consistently indicates current movement toward the north-northwest whereas the internal features and fabric of the coarse sediments suggest emplacement by a sand-avalanche
mechanism in which shearing processes were dominant.
The rudites are interpreted as a complex of submarine channel fills, excavated in base-of-slope sediments and possibly representing the lower ends of small submarine canyons. The more extensive proximal turbidite bodies may then be regarded as fan accumulations fronting the canyon mouths and feathering out into the distal representatives. One of the most interesting features of this sequence is the abruptness of the change from channel rudite through proximal facies to distal turbidites, which in several localities may be shown to occur within less than 8 km down-current. The short-lived and unusual character of the turbidity currents in this example may be linked to the clean, mud-deficient nature of the original supply of gravel and sand.
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