External controls on sedimentary sequences : a field and analogue modelling-based study

The Carboniferous Central Pennine Basin provides an ideal testing ground to examine the effects of tectonic activity, climate variation, sea-level changes and evolving bathymetric conditions upon continental to marine strata. During deposition of the glacio-eustatically controlled Millstone Grit Group the bathymetry of the area changed, tectonic activity has been invoked to explain basin-margin unconformities and high frequency climate variations have been interpreted as a driver of small-scale cyclicity. Tectonic activity does not appear to have affected the stratigraphic character of the Millstone Grit Group significantly. The inference of a major tectonic unconformity on the northern margin of the Central Pennine Basin is re-interpreted through recognition of an incised valley. The influence of active tectonics is minor but tectonic lineaments provide loci for syn-depositional structural activity. Facies analysis of Gilbert-type deltas within incised valley fills indicates a highly variable flow regime. Contrastingly, Gilbert-type deltas during sea-level fall are formed under constant, low flow conditions. This difference is tentatively linked to variable monsoonal discharge. Bathymetric differences combined with sea-level variations strongly influence stratigraphic development. Shelf height is inferred as a control on valley incision based on analogue modelling, detailed field investigation of the oldest part, and literature review of the entire Millstone Grit Group. The deepest incised valleys occur where fluvial systems incised into the highest shelf margins. Analogue modelling indicates that deep incised valleys are associated with increased sediment supply to the slope relative to incised valleys formed on lower shelf margins during the same magnitude sea-level falls (in agreement with field data). Additionally, lateral variations in shelf-margin height appear to have steered the positions of fluvial systems, increasing the likelihood of valley incision in specific locations. Integrating basin depth and basin-margin morphology in sequence stratigraphic models as a controlling factor on the behaviour and position of fluvial systems might thus improve insight into the position and size of incised valley systems and associated turbidite lowstand fans.