Seismic Characteristics of Paleo-Pockmarks in the Great South Basin, New Zealand

Abstract

Globally, a wide range of pockmarks have been identified onshore and offshore. These features can be used as indicators of fluid expulsion through unconsolidated sediments within sedimentary basin-fills. The Great South Basin, New Zealand, is one such basin where paleo-pockmarks are observed at around 1,500 m below the seabed. This study aims to describe the characteristics of paleo-pockmarks in the Great South Basin. Numerous paleo-pockmarks are identified and imaged using three-dimensional seismic reflection data and hosted by fine-grained sediments of the Middle Eocene Laing Formation. The paleo-pockmarks are aligned in a southwest to northeast direction to form a fan-shaped distribution with a high density of around 67 paleo-pockmarks per square kilometre in the centre of the study area. The paleo-pockmarks in this area have a similar shape, varying from sub-rounded to a rounded planform shape, but vary in size, ranging from 138 to 481 m in diameter, and 15–45 ms (TWT) depth. The origin of the fluids that contributed to the paleo-pockmark formation is suggested, based on seismic observations, to be biogenic methane. The basin floor fan deposits beneath the interval hosting the paleo-pockmark might have enhanced fluid migration through permeable layers in this basin-fill. This model can help to explain pockmark formation in deep water sedimentary systems, and may inform future studies of fluid migration and expulsion in sediment sinks.

The diverse morphology of pockmarks around Aotearoa New Zealand

“…The megapockmarks on the Chatham Rise are likely the result of multiple formation processes interacting, for example an initial venting episode creating a small pockmark that is then significantly altered and enlarged by the erosive action of currents (Hillman et al, 2018;Klaucke et al, 2018). The larger pockmarks on the South Westland Margin and the Chatham Rise are therefore not considered to be monogenetic, since there are likely to have been multiple stages of their formation and in some cases they may be composites of multiple features amalgamating to form one large pockmark (Judd and Hovland, 2007;Karaket et al, 2021).…”

“…This compilation and analysis form the basis of the first national database of pockmarks worldwide. Furthermore, whilst paleopockmarksthose that are buried in the subsurfacehave been observed in locations such as offshore Taranaki (Chenrai and Huuse, 2017), the Chatham Rise, and Great South Basin (Davy et al, 2010;Stott et al, 2019;Karaket et al, 2021), and offshore Canterbury and Otago (Hillman et al, 2015;Hoffmann et al, 2019;Micallef et al, 2022) (Figure 1), here we focus only on pockmarks imaged at the present-day seafloor for which other datasets are readily available to investigate modern drivers. This study is unique as it compiles pockmarks across a range of geological and geomorphological regions (Figure 1D), from the tectonically active Hikurangi and South Westland margins to the passive Otago and Taranaki margins (Figure 1A).…”