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.
Highlights
Sediment remobilization and fluid flow features have been discovered on the present-day surface and subsurface in several geological settings worldwide (Pecher et al, 2001; Loncke et al, 2004; Cartwright, 2007; Plaza-Faverola et al, 2011; Anka et al, 2013; Luo et al, 2014)
Fluid flow features recording fluid expulsion at the paleo-seabed are manifested in the Great South Basin as paleo-pockmarks that formed during the Middle Eocene
The paleo-pockmarks are observed along a trend that can be related to the distribution of underlying basin floor fan sediments in the ancient sedimentsink area
Summary
Sediment remobilization and fluid flow features have been discovered on the present-day surface and subsurface in several geological settings worldwide (Pecher et al, 2001; Loncke et al, 2004; Cartwright, 2007; Plaza-Faverola et al, 2011; Anka et al, 2013; Luo et al, 2014) Fluid flow features, such as pipes, mud volcanoes, pockmarks, and polygonal fault systems on the seabed, and in the subsurface, have gained significant attention over the last few decades because of their relevance to hydrocarbon exploration and production, local biodiversity, and as a source of greenhouse gases in the atmosphere (Berndt, 2005; Judd and Hovland, 2007; Huuse et al, 2010; Andresen, 2012; Karstens and Berndt, 2015). Various types of fluids are expelled in different ways during basin burial and subsidence, and are inferred to involve different geological processes, such as the development of overpressure, abnormal gravitational loading, differential compaction, and eo- and meso-diagenesis
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