Abstract

Unconformities are key elements of sequence stratigraphy, yet seismic imaging of unconformities is not straightforward. Subtle differences in the dip of bedding planes are well recorded in seismics. However, the tool has less resolution than the eyes of the field geologist and seismics often fails to "see" the difference between genuine unconformities and rapid transitions between sediment packages of different dip. Seismic models of outcrops indicate that pseudo-unconformities - seismic lap-outs that cross time lines and correspond to rapid lateral changes in dip or facies - are common features. Of ten seismic outcrop models at least five showed pseudo-unconformities, including false downlap, onlap and toplap. Differentiation between unconformities and pseudo-unconformities in seismic data remains difficult. Higher frequencies as well as certain reflector configurations provide some clues. Furthermore, certain depositional settings are more prone to pseudo-unconformities than others. The flanks of reefs and carbonate platforms are a case in point: pseudo-unconformities are common in this setting because platform sediment and basin sediment mix there; the materials are brought in by different transport mechanisms, causing rapid changes in facies and in dip of bedding. With direct seismic recognition of pseudo-unconformities in its infancy, we must accept that not all seismic unconformities correspond to unconformities in outcrop.