Discovery of a 400 km2 honeycomb structure mimicking a regional unconformity on three-dimensional seismic data
Résumé
Recognition of seismic unconformities is crucial for interpreting basin history from seismic reflection data sets in both siliciclastic and carbonate settings. While it is well established that non-erosional changes in sedimentary facies can create seismic reflections that mimic seismic unconformities (i.e., pseudo-unconformities), these features are generally considered to be localized and uncommon, and, therefore, are largely overlooked during interpretation. Diagenetic alteration of strata can also affect the morphology of seismic reflectors and mislead seismic interpreters. This study is based on a three-dimensional (3-D) seismic data set and documents a 400 km2 honeycomb structure (HS) masquerading as a regional erosional unconformity in the Oligocene–Miocene carbonate strata of Australia’s North West Shelf. This HS is located at the transition between the topsets and the foresets of clinoforms of carbonate to marly composition. The HS expression in 3-D seismic data cross sections is irregular, giving the HS the appearance of a truncated surface that could erroneously be interpreted as a regional seismic unconformity. Closer examination reveals that the HS crosscuts chronostratigraphic clinoform reflectors, and frequency extraction processing shows that the HS dominantly falls within a lower-frequency band than the clinoform reflectors. The morphology of the HS (i.e., continuous with densely packed cells) and its time-transgressive nature suggest that it has a burial diagenetic origin. This suggests that creation of pseudo-unconformities at basin scale by burial diagenesis may lead to surface misidentification, with negative consequences for paleoenvironmental studies and petroleum exploration activities.