The Upper Carboniferous in northern Europe was characterized by a humid--tropical climate, high magnitude--high frequency fluctuations in sea level forced by the Gondwanan glaciation, and a copious sediment supply. Faunal concentrate condensed horizons (marine bands) produced during periods of rising sea level provide a high-resolution chronostratigraphic framework in which intervals of approximately 60,000 yr can be analyzed at a regional scale. These conditions render Upper Carboniferous successions an ideal testing ground for the methods and concepts of high-resolution sequence stratigraphy. Extension in the Lower Carboniferous was followed by thermally-driven subsidence in the Upper Carboniferous, and in the early part of the Upper Carboniferous the basins were characterized by a stepped bathymetry that was inherited from the extensional phase. The basins comprised river-fed deltas, a basin slope, and deeper basin turbidite systems. Re-examination of classic sections in these basins using an integrated sedimentological and sequence stratigraphic approach has challenged many long-held views of Upper Carboniferous sedimentation.
These points can be illustrated by reference to a well exposed remnant of an Upper Carboniferous basin in western Ireland, the Clare Basin. The basin comprises a 1.6-km succession that commenced with a deepening, sediment-starved phase and was followed by a basin-fill succession of turbidites, slope, and deltaic deposits. Examination of the deltaic deposits has led to the recognition of regional-scale incised valleys dominated by sand-rich fluvial deposits, lowstand shelf-edge deltas that differ sedimentologically from highstand and transgressive systems tract deltas, and a pronounced bias in preserved systems tracts to those associated with lowstand conditions. In the slope an ordered, sea level--driven stratigraphy that includes the creation and filling of slope channels in a single sea level cycle has been recognized. In the deeper basin, discrete sand-rich lowstand turbidite units can be identified between regionally extensive condensed horizons. Although this succession accumulated in an intracratonic basin, it shares many similarities with the stratigraphy and sedimentation of clastic-rich, delta-driven passive margins such as the Gulf of Mexico, Nigeria, and Angola.
AAPG Search and Discovery Article #90938©1997-1998 AAPG Distinguished Lecturers