POCKMARKS: SELF-SCOURING MUD LANDFORMS?

Pockmarks, or seafloor craters, occur worldwide in a variety of geologic settings and usually within cohesive fine-grained sediment. Pockmarks may appear in fields numbering thousands in areas of petroleum production shelf basins, continental slopes and rises, deltas, fjords and previously glaciated estuaries. Associated with fluid escape, the mechanisms and timescale of pockmark formation are not well constrained and several hypothesis for their formation and maintenance have been proposed including seismic activity, cetacean feeding, and even meteorites. This study combines morphologic analysis and numerical modeling to assess the role of nearbed currents in pockmark evolution.

In 2006, 2008 and 2009 the US Geological Survey Seafloor Mapping Group collected 35 km² of high-resolution swath bathymetry and Chirp sonar data in the Belfast Bay, Maine pockmark field. Bathymetry data were gridded at 2.5 and 5-m resolution and indicate that the field contains over 2,000 pockmarks representing over 15 million cubic meters of displaced sediment and pore fluid. Morphology varies throughout the field and indicates scour in certain locations. Though most pockmarks in the northern portion of the Bay are circular and concave, the largest pockmarks located at the center of the field have extensive flat bottoms. Back-scatter and seismic-profile data reveal that these pockmarks terminate in coarse grained sediments that characterize the Holocene/Pleistocene unconformity. Pockmarks in the southern portion of field, where the Bay transitions to a channel, are elongate and current-aligned. Recent work in the Oslofjord pockmark field observed upwelling currents within pockmarks and suggested the possibility of rotational flow. Our simulations of flow over a depression in both 2 and 3-dimensions corroborate these findings. Further, our simulations produce rotational flow within the depression whenever flow passes above the pockmark. In the simulations, zones of shearing occur at the depression’s base and rim, suggesting that vortical flow may play a role in pockmark areal and vertical erosion, even in perfectly circular, concave pockmarks. From these findings we construct a novel working hypothesis that pockmarks result from initial seafloor perturbations that become modified and grow by vortical flow.