The Surface sediment types in the North Pacific region are characterized by grayish mud in the hemipelagic marginal area of the continent, red clay in the pelagic deep sea, and calcareous ooze on the seamounts shallower than calcite- compensation depth (CCD). While the hemipelagic gray clay is covered by thin (less than 10m) oxidized brown clay on its surface, the red clay in the pelagic seafloor is usually several tens of meters thick and underlain by bedded chert in the North Pacific. Thickness and stratigraphy of the pelagic (red) clay and bedded chert were compiled from the lithologic descriptions of the Deep Sea Drilling Project (DSDP) cores in the North Pacific region.
As a general scheme in the North Pacific, several tens of meters of pelagic clay are underlain by the Eocene to Cretaceous interbedded unit of bedded chert and pelagic clay of less than 100m, the Cretaceous chalk with occasional occurrence of flint nodules, and the basement basalt in the descending succession. The thickness of the pelagic clay is thinnest at the peripheral area of the East Pacific Rise, and the pelagic clay is overlain by thick gray clay at the hemipelagic margin, as possible results of the subduction of pelagic clay beneath hemipelagic gray clay. The thickness of the interbedded sequences of bedded chert and pelagic clay does not vary at the wide areas of the North Pacific; however, the geographical distributions of the Eocene-Cretaceous unit of bedded chert-pelagic clay interbeds and the Cretaceous unit are restricted as belts and change older in a northwestern direction from the equatorial belt. A schematic cross section of these deep-sea stratigraphic units across the Northwest Pacific may demonstrate lateral and vertical facies changes due to the lateral spreading of the Pacific plate in ages.
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