THE APPLICATION OF SEQUENCE STRATIGRAPHY TO UPPER CARBONIFEROUS FLUVIO-DELTAIC STRATA OF THE ONSHORE UK AND IRELAND - IMPLICATIONS FOR THE SOUTHERN NORTH-SEA

Stratigraphical correlations and facies interpretations of Upper Carbo niferous fluvio-deltaic strata have been based traditionally on cyclot hems bound by marine flooding surfaces (marine bands). The recent reco gnition of major, regionally extensive erosional unconformities (Exxon -style sequence boundaries) within selected cyclothems questions their validity as units of genetically related strata. Using examples from the Carboniferous of the onshore UK and Ireland, we present sedimentol ogical criteria for the recognition of sequence boundaries, placing pa rticular emphasis on the regional context of these surfaces. Sequence boundaries comprise widespread, deeply eroded surfaces at the base of major fluvial sandstone complexes, and laterally equivalent palaeosols developed on interfluves at the margins of the fluvial complexes. The se sequence boundaries define units of genetically related strata (seq uences) which contain other key surfaces of time-stratigraphic signifi cance, including marine bands and regionally extensive coals. The reco gnition of key surfaces enables the construction of a high resolution stratigraphic framework within which coeval facies relationships can b e interpreted. Sequence boundaries can be correlated between individua l basins in the onshore UK, by reference to their position in relation to a particular marine band. For example, the sequence boundary at th e base of the Farewell Rock in the South Wales Basin can be correlated with that at the base of the Rough Rock in the Pennine Basin, norther n England, since both these sandstone bodies are directly overlain by the Subcrenatum Marine Band. Interbasinal correlations of this nature imply that potential fluvial sandstone reservoirs within major incised valley fills in the Upper Carboniferous strata of the southern North Sea can be predicted by correlation with the onshore UK. The stratigra phical framework can be extended and tested using core and well-log da ta, particularly spectral gamma-ray data, which are able to identify k ey sequence stratigraphic surfaces.