LITHOSTRATIGRAPHY AND SEDIMENTOLOGY OF SHALLOW PISTON CORES FROM PGC2008-007 EXPEDITION; BULLSEYE AND AMNESIAC VENTS, CASCADIA MARGIN

During the summer of 2008, a two-week long geophysical and shallow-piston-coring expedition was conducted on the Vancouver Island accretionary wedge along the Cascadia Margin. One of the primary goals of this expedition was to further describe and constrain the controls of fluid flux and methane hydrate occurrence at cold-vent sites at ~1300 meters water depth and to better understand their relationship with the host geologic system. During the expedition, 13 shallow piston cores were recovered from within multiple vent sites with a maximum recovery of 7.1 meters below sea-floor.

Two of these sites, referred to as Amnesiac and Bullseye Vents, are the focus of this study. Bullseye Vent has been the focus of previous expeditions including Integrated Ocean Drilling Program Expedition 311. Previous studies at Bullseye Vent have collected bulk mineralogy, sedimentology, geochemical, geophysical and microbiological properties from cores at the vent. The only hydrate recovered during PGC2008-007 was in a Bullseye Vent core at 4.43 mbsf. Amnesiac Vent was detected during seismic profiling prior to coring during PGC2008-007. Isotopic analysis of carbonates formed from the anaerobic oxidation of methane (AOM) from this site will be the focus of future study. Of the six cores recovered from Amnesiac Vent, one was unable to penetrate past ~40cmbsf due to a carbonate crust. This core returned gravel sized carbonate. XRD analysis has shown aragonite concentrations of over 50% in subsamples from cores within the Amnesiac site.

Our work aims to complement ongoing studies that attempt to characterize and understand the geologic controls on these vent systems. As part of this effort, detailed sedimentologic analysis of sediments from the shallow piston cores recovered during PGC2008-007 has been completed including visual core and smear slide descriptions, XRD/XRF measurements, and grain size quantification. Sediments recovered from both vent sites are predominantly fine grained silts to clayey silts, with rare turbidite sand beds up to 3 cm thick. Based on our measurements, primary minerals include quartz, feldspar, mica and clay minerals. Authigenic precipitates are common throughout many of the cores including iron sulfides and authigenic carbonates with a wide array of compositions.