Shallow stratigraphic control on pockmark distribution in north temperate estuaries

Abstract

Pockmark fields occur throughout northern North American temperate estuaries despite the absence of extensive thermogenic hydrocarbon deposits typically associated with pockmarks. In such settings, the origins of the gas and triggering mechanism(s) responsible for pockmark formation are not obvious. Nor is it known why pockmarks proliferate in this region but do not occur south of the glacial terminus in eastern North America. This paper tests two hypotheses addressing these knowledge gaps: 1) the region's unique sea-level history provided a terrestrial deposit that sourced the gas responsible for pockmark formation; and 2) the region's physiography controls pockmarks distribution. This study integrates over 2500 km of high-resolution swath bathymetry, Chirp seismic reflection profiles and vibracore data acquired in three estuarine pockmark fields in the Gulf of Maine and Bay of Fundy. Vibracores sampled a hydric paleosol lacking the organic-rich upper horizons, indicating that an organic-rich terrestrial deposit was eroded prior to pockmark formation. This observation suggests that the gas, which is presumably responsible for the formation of the pockmarks, originated in Holocene estuarine sediments (loss on ignition 3.5-10%), not terrestrial deposits that were subsequently drowned and buried by mud. The 7470 pockmarks identified in this study are non-randomly clustered. Pockmark size and distribution relate to Holocene sediment thickness (r 2 =0.60), basin morphology and glacial deposits. The irregular underlying topography that dictates Holocene sediment thickness may ultimately play a more important role in temperate estuarine pockmark distribution than drowned terrestrial deposits. These results give insight into the conditions necessary for pockmark formation in nearshore coastal environments.Published by Elsevier B.V.

Control of Quaternary sea‐level changes on gas seeps

“…Indeed, the role of sedimentation rate in pockmark morphology has been revealed in Belfast Bay where Brothers et al . [] found that the largest pockmarks occur where Holocene sediment is thickest. Seismicity and faulting can cause an overpressure to initiate gas seeps but, as presented previously, the study area is relatively stable, not disturbed by faults, and shows very low seismic activity.…”

Pockmarks in the Witch Ground Basin, Central North Sea

“…Microbial methanogenesis may lead to accumulation of free gas due to the soft, cohesive, low permeable sediments of the Witch Ground Basin instead of migrating out of the sediments (Boudreau et al, 2001;Boudreau et al, 2005). Such a scenario has been proposed for organic-rich sediments in the Arkona Basin, Baltic Sea, (Abegg & Anderson, 1997;Mogollón et al, 2012;Orsi et al, 1996), and Belfast Bay, Maine, USA (Brothers et al, 2012). There, pockmark formation is dependent on the sediment thickness, flux of organic matter, and sedimentation rates.…”

Occurrence, characteristics and formation mechanisms of methane generated micro-pockmarks in Dunmanus Bay, Ireland

“…They occur in the majority of aquatic environments including lakes, deltas and estuaries (e.g. Berkson and Clay, 1973;Hovland et al 1997;Brothers et al 2012). In most reported cases of pockmarks around the globe the fluid involved is methane gas and thus they are frequently referred to as 'gas pockmarks' (Judd and Hovland, 2007).…”