Abstract

Widespread soft-sediment deformation (SSD) in upper-bathyal continental-margin sediments has been explained by compaction-related dewatering and related physical processes that can destabilize margin sediments. We present the hypothesis that SSD in such facies resulted in part from instability of gas hydrates (clathrates), leading to overpressurization in sediments and resulting fluid flow and brecciation. Clathrate-related deformation must be widespread in upper-bathyal continental-margin methanogenic sequences at depths where clathrates are destabilized by changes in bottom-water temperatures and sea level. A close association exists between an array of SSD features with clathrate-hosting sediments in ocean drill cores. Similar features are conspicuous in the partly laminated, upper Neogene upper bathyal sediments of the Sisquoc Formation in southern California, suggesting that clathrates may have been responsible for abundant and well-developed SSD in these units. The lithology, methane-producing organic content, and depositional environment of the Sisquoc Formation are similar to those features in modern clathrate-hosting settings. These characteristics, in conjunction with the upper bathyal paleobathymetry of the unit, the geothermal gradient of the region, and δ13C values of benthic foraminifera and dolomite concretions suggest the occurrence of clathrates in the shallow subsurface of these sediments. We suggest that the analyzed parts of the Sisquoc Formation represent an exhumed zone of SSD resulting from clathrate instability in the shallow subsurface of the continental margin. Sedimentary rocks exhibiting an association of SSD features that resulted from clathrate formation and dissociation are named clathrites.

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