n May 2000 we surveyed several, $\sim$5 km-long, $\sim$1-km wide, crack-like features offshore Virginia and North Carolina using side-scan and chirp sub-bottom sonar onboard the R/V Cape Hatteras. The features, which are up to 50-m deep, are arranged in en-echelon fashion along the outermost shelf. Collection of the new sonar data was undertaken with the aim of using the plan-form and sub-bottom geometry of the crack system to test our explanation that these features represent incipient failure of the outer shelf/upper slope. The cracks appear to have a complex history with the excavations in large part resulting from the expulsion of gas through the seafloor. On the basis of internal stratal geometry and surficial morphology, the cracks are not simple normal faults, but are better described as large-scale asymmetric gas escape structures, or elongated pockmarks. Shallow, trapped gas is easy to recognize in the chirp data as bright, high-amplitude reflections that obscure any deeper signals. The gas is trapped beneath a thin (few tens of meters) veneer of stratified sediment draped across the outermost shelf/upper slope, which we interpret as a low-stand marine delta. Inboard of the deltaic sequence there is no sign of trapped gas, and thus it appears that gas percolates freely though the seafloor in this region. We do not yet know the origin of the gas, whether it is sourced from shallow or deep parts of the margin. The deltaic strata exhibit internal deformation suggestive of progressive downslope creep. We speculate that permeable pathways provided by internal deformation have facilitated escape of the trapped gas. In addition, it appears that gas is presently escaping through the landward walls of the blowouts, and thus recharge is occurring. Side-scan sonar images of the steep, landward sides of the blowouts reveal highly lineated patches of very high backscatter associated with outcropping deltaic strata. These bright patches might indicate present escape of gas through the landward blowout wall or that biological communities have become established around fluid expulsion sites.
Large-scale, Elongated Gas Blowouts Along the Outer Shelf off Southern Virginia/North Carolina
L TOSI
2000
Abstract
n May 2000 we surveyed several, $\sim$5 km-long, $\sim$1-km wide, crack-like features offshore Virginia and North Carolina using side-scan and chirp sub-bottom sonar onboard the R/V Cape Hatteras. The features, which are up to 50-m deep, are arranged in en-echelon fashion along the outermost shelf. Collection of the new sonar data was undertaken with the aim of using the plan-form and sub-bottom geometry of the crack system to test our explanation that these features represent incipient failure of the outer shelf/upper slope. The cracks appear to have a complex history with the excavations in large part resulting from the expulsion of gas through the seafloor. On the basis of internal stratal geometry and surficial morphology, the cracks are not simple normal faults, but are better described as large-scale asymmetric gas escape structures, or elongated pockmarks. Shallow, trapped gas is easy to recognize in the chirp data as bright, high-amplitude reflections that obscure any deeper signals. The gas is trapped beneath a thin (few tens of meters) veneer of stratified sediment draped across the outermost shelf/upper slope, which we interpret as a low-stand marine delta. Inboard of the deltaic sequence there is no sign of trapped gas, and thus it appears that gas percolates freely though the seafloor in this region. We do not yet know the origin of the gas, whether it is sourced from shallow or deep parts of the margin. The deltaic strata exhibit internal deformation suggestive of progressive downslope creep. We speculate that permeable pathways provided by internal deformation have facilitated escape of the trapped gas. In addition, it appears that gas is presently escaping through the landward walls of the blowouts, and thus recharge is occurring. Side-scan sonar images of the steep, landward sides of the blowouts reveal highly lineated patches of very high backscatter associated with outcropping deltaic strata. These bright patches might indicate present escape of gas through the landward blowout wall or that biological communities have become established around fluid expulsion sites.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
