Seismic-scale funnel-shaped collapse features from the Paleocene–Eocene of the North West Shelf of Australia

A multi-tiered cluster of funnel-shaped seismic anomalies can be observed over 11,000 km2 in the lower Tertiary of the Carnarvon basin, NW shelf of Australia. The lithology is dominated by deep-water carbonates and marls. Individual funnel-shaped depressions typically have a diameter exceeding 1 km and a depth of a few hundred meters. The strata inside some of the anomalies can be correlated with surrounding layers. In the best documented example, the correlation shows that the infill has foundered into the funnel-shaped depression. The collapse appears to have removed about 1 km3 below sedimentary cover. The anomalies were emplaced in deep water, when gas was actively migrating in the basin and above a structural high. Based on this context, and using recently described hydrate pockmarks as live analogs, the following model is proposed for the emplacement of these collapsed features: 1) thermogenic gas migrated up into the fine-grained Paleogene series and “froze up” into hydrates upon entering the gas hydrate stability zone; 2) the correlative volume increase resulted in conical failure of the overburden and gas migrating along the failure impregnated the surrounding sediments with hydrates; 3) as the hydrate stability zone moved up, hydrate dissolution/dissociation expelled gas and associated liquefied sediment from the cone, while the non-impregnated core foundered towards the bottom of the cone. Three such episodes are recorded in the study area during the Paleogene. Two of them lie in close temporal vicinity to the main hyperthermal events of this period. Volume calculations indicate that the amount of gas released in our study area is two orders of magnitude lower than what would be needed to trigger the Paleocene–Eocene Thermal Maximum event suggesting that the anomalies are rather developed as a consequence of global hyperthermal events.