Neogene Siliceous Sediments and Rocks off Southern California and Baja California, Deep Sea Drilling Project Leg 63

Abstract

The basal parts of Neogene diatomaceous deposits encountered off Southern California and Baja California during Leg 63 of the Deep Sea Drilling Project are diagenetically altered to chert, Porcellanite, and siliceous mud rock. This depth-related lithologic trend coincides principally with mineral transformation of opal-A to opal-CT. At DS DP Site 471, off Baja California, the boundary between the opal-A zone and the underlying opal-CT diagenetic zone corresponds to a pronounced bottom-parallel seismic reflector that can be traced at least 80 km to the northwest along the coast of Baja California. At DSDP Sites 467 and 469 in the outer California Continental Borderland and at Site 471 the d(101)-spacing of opal-CT decreases from about 4.10-4.12 Å to about 4.07 Å with increasing depth of burial. Chemical analyses and X-ray diffraction of some sedimentary rocks below the opal-CT zones at Sites 467 and 471 show significant amounts of quartz, suggesting the presence of underlying quartz diagenetic zones. Using present thermal gradients and measured depths of burial, the approximate range in the present subsurface temperature of the opal-A to opal-CT transition at Leg 63 sites isl3°Cto52 c C. The depth to this transition is generally shallower at those sites in areas of high present thermal gradients. The vertical sequences of facies encountered at Leg 63 sites are diagenetically and lithologically similar to some Neogene sequences in California, the Bering Sea, Kamchatka, Sakhalin, Japan, Peru, and Baja California. In contrast, Miocene quartz chert and spherulitic jasper intercalated with diabase and pillow basalt at Sites 469 and 471 probably represent small accumulations of thermally altered siliceous sediments deposited on active volcanic crust. Similar cherts occur in igneous rocks in the Franciscan complex in California and other Mesozoic sequences in the northwestern Cordillera.

Can Opal-A/Opal-CT BSR Be an Indicator of the Thermal Structure of the Yamato Basin, Japan Sea?

“…Hein et al (1978) first demonstrated the existence of an opal-A/opal-CT transition BSR on seismic reflection profiles in the Bering Sea. Grechin et al (1981) and Hubbard et al (1985) also mentioned the silica phase change off Baja California and off central California, respectively. There have been no observations since then except for that recorded in the Japan Sea (Tamaki, Pisciotto, Allan, et al, 1990;Ingle, Suyehiro, von Breymann, et al, 1990).…”

High‐resolution seismic stratigraphy of the Yamato Basin, Japan Sea and its geological application

“…Unlike gas hydrate BSR, which are common along continental margins worldwide, BSR resulting from the opal‐A/CT phase change are rare and mostly found in the North Pacific, mainly in the Bering Sea (Hein et al 1978), along offshore California (Grechin et al 1981; Hubbard et al 1985) and in the Japan Sea (Ingle et al 1990; Tamaki et al 1990). Based on oxygen isotope and microfossil studies, major cooling of the middle and high latitudes of the Pacific in the middle and late Miocene may have caused an increase in upwelling and then expansion of biosiliceous sediments in the North Pacific (Ingle 1981).…”

Composition and depositional environment of the siliceous Odanah Member (Campanian) of the Pierre Shale in Manitoba

“…Such transformations have been recorded in the study of deep-sea porcelanites and siliceous deposits of the Pacific region (including the Monterey Formation of California) and elsewhere (Heath and Moberly, 1971;Heath, 1973;Wise and Weaver, 1973;Keene, 1975;Hein et al" 1978;Grechin et al, 1981;Iijima and Tada, 1981;Larue et al, 1987;Stamatakis et al, 1989). A number of factors control the transformation of porous biogenic sediments into hard, brittle rocks such as chert, porcelanite, and siliceous mudstone.…”