Relationship of clathrate instability to sediment deformation in the upperNeogene of California

Widespread soft-sediment deformation (SSD) in upper-bathyal continental-mar gin sediments has been explained by compaction-related dewatering and relat ed physical processes that can destabilize margin sediments. We present the hypothesis that SSD in such facies resulted in part from instability of ga s hydrates (clathrates), leading to overpressurization in sediments and res ulting fluid flow and brecciation. Clathrate-related deformation must be wi despread in upper-bathyal continental-margin methanogenic sequences at dept hs where clathrates are destabilized by changes in bottom-water temperature s and sea level. A close association exists between an array of SSD feature s with clathrate-hosting sediments in ocean drill cores. Similar features a re conspicuous in the partly laminated, upper Neogene upper bathyal sedimen ts of the Sisquoc Formation in southern California, suggesting that clathra tes may have been responsible for abundant and well-developed SSD in these units. The lithology, methane-producing organic content, and depositional e nvironment 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 delta(13)C values of benthic foraminifera and dolomite concreti ons suggest the occurrence of clathrates in the shallow subsurface of these sediments. We suggest that the analyzed parts of the Sisquoc Formation rep resent an exhumed zone of SSD resulting from clathrate instability in the s hallow subsurface of the continental margin. Sedimentary rocks exhibiting a n association of SSD features that resulted from clathrate formation and di ssociation are named clathrates.