ELLIOTT, TREVOR, Department of Earth Sciences, Univ. of Liverpool, Liverpool, England; and ANDREW J. PULHAM, British Petroleum, Colombia
Upper Carboniferous strata exposed in western Ireland include fine-grained, fluvial-dominated, unstable delta systems which exhibit slumps, growth faults and mud diapirs. Shelf- and shelf-edge deltas can be distinguished, both closely resembling Tertiary deltas in the sub-surface Gulf of Mexico. High resolution sequence stratigraphic analysis of a regional scale depositional strike section (400 m thick, 40 km wide) through the delta systems reveals a complex range and hierarchy of key stratigraphic surfaces. Surfaces formed during periods of falling sea level include down-dip sequence boundaries represented by widespread syn-sedimentary deformation horizons in the prodelta region, up-dip sequence boundaries represented by erosional unconformities at the base of incised valley systems nd forced regression surfaces. Transgressive surfaces include thin, faunal concentrate horizons interpreted as initial and maximum flooding surfaces and intensely bioturbated surfaces which bound parasequences and parasequence sets. Recognition of this hierarchy of surfaces permits the effects of two superimposed orders of fluctuation in relative sea level to be discriminated within the delta systems. Assessment of the relative importance of systems tracts preserved in the strike section reveals that lowstand tracts (prograding lowstand wedge and incised valley fills) and, to a lesser extent, transgressive systems tracts predominate. In contrast, highstand systems tracts are poorly represented. The down-dip, shelf-margin location of the strike section and the high frequency/high magnitud , glacially forced character of the sea level curve are felt to be the dominant factors in explaining this bias.
AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.