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

Abstract

Detailed sedimentological facies analysis of the fluvio-deltaic Millstone Grit succession (Upper Carboniferous) of South Wales reveals that a number of cyclothems bounded by marine flooding surfaces (marine bands) in these strata exhibit facies architectures that represent erosion, non-deposition and/or deposition during periods of falling relative sea-level. A major fluvial complex below the Subcrenatum Marine Band, the Farewell Rock, lies within an incised valley, with a regional unconformity (sequence boundary) at its base. This unconformity is marked by deep erosional relief, an identifiable time gap and an angular discordance in bedding. The Cumbriense Quartzite, a correlative unit containing several mature palaeosols, records a depositional hiatus on a terrace-like interfluve that lay beyond the margins of the coeval Farewell Rock valley. Cyclothems in the underlying Middle Shales contain additional surfaces and units of subtler character. Beneath the Cancellatum Marine Band, a thin (15 cm), calcareous siltstone bed (the ‘Amroth Granule Bed’) that directly overlies prodelta shales contains reworked bioclasts, bored phosphorite clasts and quartz granules. Quartz granules in this bed are interpreted to represent relict lowstand, fluviatile? deposits, which were reworked during later transgression. Three further cyclothems contain 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. The significance of these subtle surfaces and units for intracyclothem stratigraphy has rarely been considered; their prevalence in the Middle Shales provides evidence for numerous, high-frequency relative sea-level falls, which were previously unrecognized. These relative sea-level falls appear to alternate coherently with the widespread sea-level rises recorded by the marine bands, suggesting that glacio-eustasy is their most likely driving mechanism. The notion of glacio-eustatic sea-level falls is supported by the correlation of the basal Farewell Rock sequence boundary with sequence boundaries documented in adjacent basins. The angular unconformity and a change in sediment provenance at the base of the Farewell Rock, however, suggest an additional tectonic control on stratigraphic architecture here, namely a short-lived phase of rifting or inversion prior to widespread fluvial incision. Copyright © 1998 John Wiley & Sons, Ltd.