The Sequence Stratigraphy of Upper Carboniferous Deltas, Western Ireland

ELLIOTT, TREVOR, University of Liverpool, Liverpool, U.K., and ANDREW PULHAM, BP America, Houston, TX

Upper Carboniferous deltaic cyclothems across northern Europe are defined by distinctive faunal concentrate horizons termed "marine bands" that represent widespread transgressions. Cyclothems in the Namurian Clare basin are approximately 200 m in thickness and comprise an initial thick (up to 150 m) progradational deep-water delta front overlain by smaller scale (5-10 m) bay fills and thick (30-70 m), composite fluvial channels, capped by small to moderate scale (5-35 m) shoreline and/or shallow-water delta fronts.

Sand bodies in the delta fronts include isolate, 2-3 km wide mouth-bars, implying that distributary channels were narrow and widely spaced. This contradicts the evidence of the fluvial channel complexes, which are widespread, multilateral bodies up to 25 km wide. In view of these contrasts, the fluvial channels are interpreted as incised valley fills resulting from a relative fall in sea level. The bases of these complexes are interpreted as sequence boundaries; the fluvial channels as lowstand wedges; the overlying shorelines and shallow-water deltas as transgressive systems tracts; and the marine bands as maximum flooding surfaces. Subdivision of the thicker delta fronts into highstand and lowstand components can be made in several ways. Intepretations are dependent on the significa ce attached to various facies surfaces in these deep-water, possibly shelf-edge delta fronts.

The recognition of lowstand, Type 1 sequence boundaries extends the traditional view of the cyclothems as being defined by Frazier-Galloway type flooding surfaces and argues that erosional phases, nondepositional interfluves, and lowstand deep basin deposits should be sought throughout these Upper Carboniferous successions.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)