Abstract: Sequence Stratigraphic Interpretations in Fold-Fault Terrains: an Example from Offshore Hokkaido, Japan

Schuman Wu, Paul Weimer

The recognizing and correlating 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 section with traced reflections, cryptic stratigraphic relationships and correlations can be revealed, along with a precise record of the depositional and deformational events.

A multifold seismic profile from offshore of Kushiro, Hokkaido Island, Japan was interpreted and restored to illustrate this procedure. The seismic profile displays shallow marine Cretaceous and Cenozoic strata that has been deformed by a reverse fault and all units below the upmost unconformity are folded. Detailed stratigraphic correlation is problematic and sequence boundaries are obscure in the deformed state. An initial interpretation of the seismic data was made and then iteratively modified and validated using GEOSEC, a geologic cross section interpretation and validation program. Two restored-state sections indicate that prior to the development of the second unconformity, none of the pre-existing units had been folded or faulted. Immediately overlying the oldest unconformity, transgressive sediments onlap from northwest to southeast. Reverse faulting started shortly after the second unconformity and produced two wedge-shaped piggyback basins. The youngest unconformity truncates the two older folded sequences and the sequence above is not deformed.

Seismic reflections were then traced on this validated interpretation. The traced seismic profile was restored to different stages in the depositional history by a flexural-slip mechanism, in which both bed-length and unit area were 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 #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994