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doi:10.2204/iodp.proc.311.211.2009

Introduction

Among the major microfossil groups routinely used for biostratigraphy in deep-sea drilling projects, diatoms have the most diverse ecology occupying open-marine to terrestrial nonmarine environments. Therefore, diatoms can be used to determine the extent of any terrestrial inputs to ocean sites. Shipboard diatom analysis of turbidite sediments collected from Sites U1325–U1329 during Integrated Ocean Drilling Program Expedition 311 at the continental slope in the Cascadia margin, an accretionary complex, revealed unexpectedly abundant and highly fluctuating occurrences of shallow-marine and nonmarine diatoms for the first time (see the "Expedition 311 summary" chapter). This suggests that the turbidites originated from shelf and inland water areas andat the fluctuations may be related to the global glacioeustatic sea level changes in the Pleistocene. Previously no reports of such abundant occurrences of these diatoms have been documented, although many holes were drilled in the region during Deep Sea Drilling Project Leg 18 and Ocean Drilling Program Legs 146, 168, and 204. Schrader (1973) noted the abundant occurrences of allochthonous diatoms at Leg 18 Site 174 in the Astoria Fan, but he did not distinguish between shallow-marine and nonmarine diatoms. Fourtanier (1995) analyzed the diatoms from Leg 146 Site 892, from which no nonmarine diatoms were recorded. More recently Watanabe (2006) reported diatoms from Leg 204 sites, where only very scarce and sporadic occurrences of nonmarine diatoms were recognized.

In addition to shipboard diatom analysis, we carried out shore-based diatom and foraminifer analyses in Holes U1328B and U1328C in order to clarify their depositional environment and/or mechanism in more detail, which might be important to characterize the organic materials in the sediments generating hydrocarbons and/or gas hydrate.

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