Distinctive Miocene diatomites and genetically related porcellanites form a remarkably widespread lithofacies within bathyal marine sequences around the margin of the North Pacific Ocean from Mexico to Korea. The Monterey Shale of California and the Onnagawa Formation of Japan typify these deposits. Planktonic correlations, radiometric dates, and oxygen isotope records indicate that the initial accumulation of the diatomites commenced in the early middle Miocene (15 Ma), coincident with initiation of a major climatic event marked by massive ice accumulation in Antarctica, steepening of the latitudinal temperature gradient, and increasingly vigorous surface circulation and primary productivity. Microfaunal and sedimentologic evidence demonstrates that the diatomites were c mmonly deposited in subsiding marginal basins characterized by sills intersecting intensified oxygen minima allowing preservation of laminated muds. Moreover, the diatomites occur within strikingly similar stratigraphic successions that suggest closely parallel tectonic, sedimentologic, and oceanographic development of Neogene marginal basins in this region. Typically, four major events can be recognized in these sequences: (1) deposition of Oligocene-lower Miocene volcanic rocks and continental and/or neritic marine clastics followed by (2) rapid margin subsidence, development of silled basins, and deposition of middle and upper Miocene diatomaceous sediments in essentially empty depocenters, (3) climatically induced elevation of the carbonate compensation depth and increasing influx of terrigenous debris in latest Miocene-earliest Pliocene time producing a carbonate-poor mudstone facies, and (4) introduction of rapidly deposited wedges of coarse terrigenous clastics during Pliocene-Pleistocene time that cap the underlying diatomites. This widespread and correlative stratigraphic pattern appears to be the combined product of (1) a major middle Cenozoic readjustment of Pacific plate margins resulting in synchronous development of Neogene marginal basins around the North Pacific rim and (2) coincident acceleration of diatom productivity in this region in response to severe deterioration of Neogene climate.

End_of_Article - Last_Page 940------------