Automated feature extraction and spatial organization of seafloor pockmarks, Belfast Bay, Maine, USA

Abstract

Seafloor pockmarks occur worldwide and may represent millions of m 3 of continental shelf erosion, but few numerical analyses of their morphology and spatial distribution of pockmarks exist. We introduce a quantitative definition of pockmark morphology and, based on this definition, propose a three-step geomorphometric method to identify and extract pockmarks from high-resolution swath bathymetry. We apply this GIS-implemented approach to 25 km 2 of bathymetry collected in the Belfast Bay, Maine USA pockmark field. Our model extracted 1767 pockmarks and found a linear pockmark depth-to-diameter ratio for pockmarks field-wide. Mean pockmark depth is 7.6 m and mean diameter is 84.8 m. Pockmark distribution is non-random, and nearly half of the field's pockmarks occur in chains. The most prominent chains are oriented semi-normal to the steepest gradient in Holocene sediment thickness. A descriptive model yields field-wide spatial statistics indicating that pockmarks are distributed in non-random clusters.Results enable quantitative comparison of pockmarks in fields worldwide as well as similar concave features, such as impact craters, dolines, or salt pools. Published by Elsevier B.V. IntroductionFirst identified in muddy sediments of the Scotian Shelf (King and MacLean, 1970), pockmarks are seafloor depressions that are found worldwide in a variety of geologic settings (Hovland and Judd, 1988;Judd and Hovland, 2007). These craters can measure hundreds of meters in diameter, may occur in chains kilometers long and, where present in extensive fields, may dominate the seafloor surface (Fader, 1991;Rogers et al., 2006;Pilcher and Argent, 2007). Despite global distribution and general association with seafloor fluid escape, the mechanisms for pockmark formation and evolution remain uncertain (Ussler et al., 2003).Analysis of pockmark morphology and spatial distribution relative to antecedent geology and subsurface fluids (e.g., methane) can provide insight into fluid-migration pathways, pockmark field evolution, and possible mechanisms for pockmark generation and maintenance. In the absence of high-resolution seafloor bathymetry data, previous characterizations of entire pockmark fields relied upon visual interpretation of acoustic backscatter data for pockmark delineation, size statistics and spatial distribution (Fader, 1991;Kelley et al., 1994;Gontz et al., 2002;Rogers et al., 2006). Although acoustic backscatter data were the best available in the cited studies, interpreting size dimensions of concave features, such as pockmarks from these data is often ambiguous (Song, 2007). High-resolution bathymetry data collected by multibeam echosounder and swath sonar technologies enable the study of seafloor morphology to reach scales and resolutions similar to studies in subaerial geomorphology based on digital elevation models (DEMs) (Hughes Clarke et al., 1996). With one exception ) these new technologies have not been applied to an entire pockmark field. Instead, whole-field spatial and morphologic analysis has ebbed, r...

Shallow stratigraphic control on pockmark distribution in north temperate estuaries

“…Bathymetry data were collected and processed using techniques described in Andrews et al (2010) and by the Ocean Mapping Group (2012a). Pockmark locations and size were extracted from the bathymetry data as point features, and the fields spatially examined, using methods presented by Andrews et al (2010). Where seismic data were available, we generated isopach maps of the Holocene sediment by subtracting interpreted surfaces from each other (Fig.…”

“…Their frequency and size varied among embayments (Table 3). Because pockmarks in Belfast Bay have a consistent depth-to-diameter ratio (Andrews et al, 2010) we use the depth of a pockmark's base below the background seafloor as an indication of its size. In Passamaquoddy Bay pockmarks are the most numerous (4515), smallest (mean depth below seafloor = 3.6 m), and vary the least in size (standard deviation ± 2.6 m).…”

More than a century of bathymetric observations and present-day shallow sediment characterization in Belfast Bay, Maine, USA: implications for pockmark field longevity

“…Although bedrock-channeled rivers transport sediment during periods of high discharge in the region (Fenster et al 2001;Brothers et al 2008), sediment introduction in Belfast Bay is much less than the available accommodation space. Pockmarks typify the embayment's bathymetry and occupy 5.8 km 2 , or 24% of the mapped seafloor (Andrews et al 2010). Three-dimensional models of Penobscot Bay's circulation suggest that near-bottom currents in Belfast Bay are minimal, and generally do not exceed 0.2 ms −1 in any direction (Xue et al 2000).…”

“…2). The pockmarks occur only within Holocene mud, and do not penetrate below the Holocene/ Pleistocene unconformity, thus confining pockmark formation to the Holocene (Rogers et al 2006;Andrews et al 2010).…”

“…Rogers et al (2006) proposed that lakes, marshes, and bogs that existed offshore when sea level was lower than at present provided the organic Fig. 2 An interpreted CHIRP sonar profile in the northern sector of the study area with minimal pockmarks (after Andrews et al 2010;see Fig. 1 for location).…”