The results of a calcareous nannofossil biostratigraphic investigation of the North Fork Cottonwood Creek section of the Budden Canyon Formation (BCF; Hauterivian-Turonian) in northern California are summarized using the Boreal - cosmopolitan Boreal Nannofossil Biostratigraphy (BC) - Upper Cretaceous Nannofossil Biostratigraphy (UC) nannofossil zonal schemes of Bown etal. and Burnett etal. Sixteen intervals, ranging from the BC15 to UC8 zones, were established in the section. Combined biostratigraphic and magnetostratigraphic studies suggest a Hauterivian to mid-Turonian age for the studied sequence. The Hauterivian-Barremian, Barremian-Aptian, Aptian-Albian, Albian-Cenomanian, and Cenomanian-Turonian stage boundaries were delineated near the top of the Ogo Member, below the Huling Sandstone Member, within the upper Chickabally Member, in the upper portion of the Bald Hills Member and within the Gas Point Member, respectively. Unconformities probably exist at the base of the Huling Sandstone Member and the upper part of the upper Chickabally Member. The nannofossil assemblage in the North Fork Cottonwood Creek suggests that the study area was under the influence of cold-water conditions during the Barremian to Lower Aptian interval, shifting to tropical/warm-water conditions during the Albian to Turonian interval as a result of the mid-Cretaceous global warming. Although oceanic anoxic events have not yet been reported in the BCF, preliminary total organic carbon, along with nannofossil data, suggest the presence of the global Cenomanian-Turonian boundary oceanic anoxic event 2.
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