Large‐scale elongated gas blowouts along the U.S. Atlantic margin

Abstract

[1] In May 2000 we surveyed a series of en echelon, asymmetric depressions along the outer shelf off Virginia and North Carolina using high-resolution chirp and side-scan sonar. The features, which are elongated parallel to the shelf edge and have steep landward walls, are $4 km long, 1 km wide, and up to 50 m deep. On the basis of internal stratal geometry interpreted from chirp profiles, the depressions do not appear to result from simple, down-to-the-east, normal displacement along deep-seated faults or structure. Rather, the depressions seem to have been excavated primarily by gas expulsion, creating large-scale asymmetric gas escape structures that have been termed ''gas blowouts.'' Gas appears to have been trapped beneath a shelf edge delta that is a few tens of meters thick and exhibits internal soft sediment deformation suggestive of progressive downslope (seaward) creep. These new data suggest the blowouts occurred when thin-skinned deformation and creep of the surficial deltaic sediment layers combined with updip/ upslope gas migration, ultimately leading to gas pressure in excess of the overburden. The location of the expulsion craters along the shelf edge and their elongated, asymmetric shapes strongly suggests a causal relation between the downslope creep of the delta and the expulsion event. We suggest a positive feedback between upward migration of gas-rich fluids through the low-stand delta and the downslope creep processes. While the complex interplay between differential permeability, overpressure, and upslope fluid migration remains poorly understood, we suggest such interactions may play an important role in controlling slope stability.

Insights into methane dynamics from analysis of authigenic carbonates and chemosynthetic mussels at newly-discovered Atlantic Margin seeps

“…Such expulsion may also have a thermogenic origin at depth (Hill et al, 2004;Newman et al, 2008;Brothers et al, 2014). However, the geochemical analyses presented here yield carbonate δ 13 C values < −40h.…”

“…This agrees with earlier work by Newman et al (2008) that demonstrated the microbial origin of pore fluid DIC δ 13 C values along the US mid-Atlantic shelf break. Hill et al (2004) argued that microbial gas flowing updip from dissociating gas hydrates is responsible for the distribution of gas blowouts in the region, and Skarke et al (2014) make the same argument for the distribution of hundreds of seeps on the continental slope updip of the present-day hydrate stability limit, particularly on the mid-Atlantic part of the margin. Recent multi-channel seismic profiles on the upper continental slope below the Baltimore Canyon seep field do not reveal clear evidence for strata that could be laterally channeling gas updip into the seeps (Ruppel et al, 2015b), but these observations are equivocal.…”

Methane hydrate stability and anthropogenic climate change

“…Seismic studies often show "wipeout zones" where the bubble zone beneath the hydrate stability zone is missing, and all of the layered structure of the sediment column within the stability zone is smoothed out. These are interpreted to be areas where gas has broken through the structure of the sediment to escape to the ocean (Riedel et al, 2002;Wood et al, 2002;Hill et al, 2004). Bubbles associated with seismic wipeout zones are observed within the depth range which should be within the hydrate stability zone, assuming that the temperature of the sediment column is the steady-state expression of the local average geothermal gradient (Gorman et al, 2002).…”

“…The sediment surface of the world's ocean has holes in it called pockmarks (Hovland and Judd, 1988;Hill et al, 2004), interpreted to be the result of catastrophic or continuous escape of gas to the ocean. Pockmarks off Norway are accompanied by authigenic carbonate deposits associated with anerobic oxidation of methane (Hovland et al, 2005).…”

Norwegian margin outer shelf cracking: a consequence of climate-induced gas hydrate dissociation?

“…Outer shelf cracks and elongated depression features were first discovered along a 40-km-long section of the U.S. Atlantic margin (Driscoll et al 2000;Hill et al 2004). The individual cracks are several km long, 1 km wide and up to 50 m deep.…”