Research paper Evidence for the biotic origin of seabed pockmarks on the Australian continental shelf R.J. Mueller Fugro Survey Pty Ltd, Australia article info Article history: Received 28 April 2014 Received in revised form 8 December 2014 Accepted 16 December 2014 Available online 3 March 2015 Keywords: Unit pockmark Seabed pit Seabed fluid flow North West Shelf Timor Sea Epinephelus coioides abstract The generally accepted formation mechanism of pockmarks worldwide is the expulsion of fluid at the seafloor, but such a mechanism does not explain the close association between pockmarks and seabed infrastructure such as pipelines and wellheads within the Stag oil field on the North West Shelf of Australia. Furthermore, certain characteristics of the pockmarks, such as conical mounds of sediment positioned around their perimeters, are strongly suggestive of a biotic origin. Pockmarks in this case are typically 5 m in diameter and 1 m deep, excavated within a sandy seabed in 45 m water depth. Inspection of ROV footage acquired during oilfield operations within the Stag field supports but does not entirely confirm without doubt the proposition that the pockmarks are created by fish of the genus Epinephelus. Having determined the characteristic features of pockmarks within the Stag field which mark them as biotic excavations, data from commercial seabed surveys at 11 other sites on the North West Shelf, all of which reveal numerous pockmarks, was reviewed for evidence of similar pockmark characteristics. Based on the review, it appears likely that the majority of pockmarks on the shallow North West Shelf (between 40 m and 130 m water depth) are representative of biological rather than geological activity. The probability that pockmarks less than approximately 10 m in diameter throughout the remainder of the Australian continental shelf are also the result of purely biological activity is high, as demonstrated by the analysis of data from three further sites. Close inspection of seabed survey data from further afield could extend the findings of this paper throughout not only the tropical Indo-West Pacific (the range of the particular fish species implicated on the North West Shelf), but potentially worldwide if other species can be shown to display similar behaviour. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Pockmarks are circular or oval-shaped depressions in the sea- floor. Dimensions may range from a few metres to several hundreds of metres in diameter, with a corresponding depth range, relative to the surrounding seabed, of a few decimetres to tens of metres. The preponderance of scientific study to date, as collated for example by Judd and Hovland (2007), has concluded that pockmarks are formed by the seafloor escape of gas and/or water. In such a sce- nario, the escaping fluid carries sediment particles with it into the water column, thus leaving behind a crater-like depression in the seafloor. The fluid escape may be continuous, whereby pockmarks form slowly over the course of hundreds or thousands of years, or catastrophic, whereby pockmarks form during a single event, such as an earthquake or storm (Kelley et al., 1994). A more recent theory proposes an intermittent outburst triggered when trapped gas overcomes a capillary seal of fine grained sediments and forces pore water upwards, which in turn quickens or liquefies the seabed sediments allowing them to be dispersed by seabed currents (Cathles et al., 2010). Hovland et al. (2010) built on this theory to explain the formation of small pockmarks on the Norwegian shelf and slope, by the cyclic ‘pumping’ of pore water by tides acting on shallow pockets of free gas. In carbonate environments, submarine fresh water discharge may create circular structures through car- bonate dissolution (Land et al., 1995; Whiticar, 2002), while freshwater ice rafting has been proposed as a formation mechanism in high latitude shelf environments (Paull et al., 1999). However, an explanation based on fluid escape for the pock- marks which exist on the southern North West Shelf of Australia is problematic. As an open, high energy carbonate ramp with little terrestrial sediment input and a low rate of sedimentation since at E-mail address: r.mueller@fugro.com. Contents lists available at ScienceDirect Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo http://dx.doi.org/10.1016/j.marpetgeo.2014.12.016 0264-8172/© 2015 Elsevier Ltd. All rights reserved. Marine and Petroleum Geology 64 (2015) 276e293