Evidence for relative sea-level falls during deposition of the Upper Carboniferous Millstone Grit, South Wales

Detailed sedimentological facies analysis of the fluvio-deltaic Millstone G rit succession (Upper Carboniferous) of South Wales reveals that a number o f cyclothems bounded by marine flooding surfaces (marine bands) in these st rata exhibit facies architectures that represent erosion, non-deposition an d/or deposition during periods of falling relative sea-level. A major fluvi al complex below the Subcrenatum Marine Band, the Farewell Rock, lies withi n an incised valley, with a regional unconformity (sequence boundary) at it s base. This unconformity is marked by deep erosional relief; an identifiab le time gap and an angular discordance in bedding. The Cumbriense Quartzite , a correlative unit containing several mature palaeosols, records a deposi tional hiatus on a terrace-like interfluve that lay beyond the margins of t he coeval Farewell Rock valley. Cyclothems in the underlying Middle Shares contain additional surfaces and units of subtler character. Beneath the Can cellatum Marine Band, a thin (15 cm), calcareous siltstone bed (the 'Amroth Granule Bed') that directly overlies prodelta shales contains reworked bio clasts, bored phosphorite clasts and quartz granules. Quartz granules in th is bed are interpreted to represent relict lowstand, fluviatile? deposits, which were reworked during later transgression. Three further cyclothems co ntain sharp-based, storm-reworked mouth bars that record an abrupt lowering of wave base, most probably during periods of falling relative sea-level. One of these cyclothems also contains a distributary channel complex, which records an abrupt influx of coarse-grained sediment of ambiguous origin. T he significance of these subtle surfaces and units for intracyclothem strat igraphy has rarely been considered; their prevalence in the Middle Shales p rovides evidence for numerous, high-frequency relative sea-level falls, whi ch were previously unrecognized. These relative sea-level falls appear to a lternate coherently with the widespread sea-level rises recorded by the mar ine bands, suggesting that glacio-eustasy is their most likely driving mech anism. The notion of glacio-eustatic sea-level falls is supported by the co rrelation of the basal Farewell Rock sequence boundary with sequence bounda ries documented in adjacent basins. The angular unconformity and a change i n sediment provenance at the base of the Farewell Rock, however, suggest an additional tectonic control on stratigraphic architecture here, namely a s hort-lived phase of rifting or inversion prior to widespread fluvial incisi on. (C) 1998 John Wiley & Sons, Ltd.