BAJA CALIFORNIA, DEEP SEA DRILLING PROJECT LEG 631
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 tr...
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| Format: | Text |
| Language: | English |
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| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.492.6833 http://www.deepseadrilling.org/63/volume/dsdp63_17.pdf |
| Summary: | 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 cor-responds 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°Cto52cC. 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 Cor-dillera. |
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