Abstract: Gamma-Ray Characterisation of High Resolution Key Surfaces and Systems tracts in Shelf-Margin Delta Systems: Upper Carboniferous of Western Ireland

Sarah J. Davies, Trevor Elliott

High Resolution sequence stratigraphic analysis through shelf-margin delta systems from the Upper Carboniferous of the Clare basin, western Ireland, has recognised key sequence stratigraphic surfaces bounding prograding low-stand wedges, incised valley fills and transgressive systems tracts. The hierarchy of significant surfaces and the depositional systems tracts identified by the sedimentological analysis have been examined using a portable gamma-ray spectrometer to produce total and spectral (K, U, and Th) profiles. Key surfaces can be differentiated by both the absolute radiochemical values and the relative differences in their gamma-ray response. Erosional unconformities at the base of incised valley complexes are characterized by a marked decrease in total gamma-ray counts. Non- epositional interfluves, laterally correlative to these unconformities, are significant radiochemical markers with high Th\K ratios. Condensed faunal concentration horizons, representing the maximum flooding surfaces, are represented across the basin by high U values with higher-order surfaces characterized by an overall increase in the total gamma-ray counts. The application of the integrated sedimentological and radiochemical data will aid the sequence stratigraphic analysis of subsurface gamma-ray responses.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994