Biostratigraphic implications for the turbidite depositional processes on the Monterey deep-sea fan, Central California

Abstract

Recognition of the Pleistocene/Holocene boundary and biohorizons on the continental margin of California is complicated by turbidite deposition. The coiling ratio of Neogloboquandrina pachyderma (Ehrenberg), the relative frequencies of subarctic and transitional planktonic foraminifers, radiolarian number, and the radiolarian-foraminiferal ratio all show variability in upper Quaternary sections that appear related to downslope transport processes, in addition to the environment of surface waters and carbonate dissolution. We examined the distribution of microfossils in ten turbidites from the western levee of the Monterey Fan to determine 1) how tubidite microfossil assemblages differ from those of hemipelagic intervals, 2) if turbiditic muds can be distinguished from hemipelagic muds based on fossil content, and 3) if microfossils used to define biostratigraphic zones are significantly affected by turbiditic processes. Using cluster analysis, samples were successfully divided into hemipelagic and turbiditic groups based on the relationship among five of 16 variables (cluster A): sand content, the relative frequency of shallow-water benthonic Foraminifera, benthonic foraminiferal number, adjusted benthonic foraminiferal number, and the relative frequency of dextral Neogloboquadrina pachyderma (Ehrenberg). The variables jointly characterize a transitional assemblage transported from a bathyal or neritic source. Both Radiolaria and planktonic Foraminifera are used to mark biohorizons on the California continental margin. In this study, it was found that both fossil groups vary significantly in concentration and relative freqeuncy between turbiditic and hemipelagic muds of the monterey Fan, and microfossil assemblages are measurably different between tops and bases of turbidites. The coiling ratio of Neogloboquadrina pachyderma (Ehrenberg) varies the least and may be most resistant to bias by hydraulic sorting and differential carbonate preservation related to transport processes.