Schuman Wu, Paul Weimer, Gareth Taylor
The recognition and correlation of stratigraphic sequences and sequence boundaries in complexly faulted and folded regions can be difficult tasks. Stratigraphic correlations across faults are often equivocal, and fault offsets and deformation can obscure reflection relationships critical for interpreting seismic facies and systems tracts. By sequentially retrodeforming a seismic profile, stratigraphic relationships and correlations can be revealed, along with an accurate record of the depositional and deformational events.
A multifold seismic profile from offshore Kushiro, Hokkaido Island, Japan was interpreted and restored to illustrate this procedure. The seismic profile displays Cretaceous and Cenozoic strata that have been deformed by a reverse fault. Three major unconformities are recognizable based upon erosional truncation and onlap of strata. Detailed stratigraphic correlation across the faults is problematic and sequence boundaries are obscured in the deformed state. An initial interpretation of the seismic data was made and then iteratively modified and validated using geologic cross section interpretation and validation software. Two restored-state sections indicate that prior to the development of the second unconformity, none of the preexisting sequences had been folded or faulted. Immediat ly overlying the oldest unconformity, transgressive sediments onlap from northwest to southeast. Reverse faulting started shortly after the development of the second unconformity and produced two wedge-shaped piggyback basins. The youngest unconformity truncates the two older folded sequences, and the overlying sequence is not deformed.
Seismic reflections were then traced on this validated interpretation. The traced seismic profile was restored to different stages in the geologic history by a flexural-slip mechanism, in which both bedlengths and unit areas are conserved. Sequence boundaries were picked from the restored-state sections rather than on the faulted and folded seismic section. The depositional history, sequence stratigraphy, and structural evolution are clearly revealed by this process.
AAPG Search and Discovery Article #90982©1994 AAPG International Conference and Exhibition, Kuala Lumpur, Malaysia, August 21-24, 1994