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Karl, S.M., 1982

Geochemical and depositional environments of upper Mesozoic radiolarian cherts from the northeastern Pacific Rim and from Pacific DSDP cores

Bibliographic Reference

Karl, S.M., 1982, Geochemical and depositional environments of upper Mesozoic radiolarian cherts from the northeastern Pacific Rim and from Pacific DSDP cores: Palo Alto, California, Stanford University, Ph.D. dissertation, 245 p., illust. (some color), maps.


Detailed sedimentologic and geochemical analyses of cherts can be combined with biostratigraphic and paleomagnetic information to infer sedimentary source components and depositional environments. A pilot study of measured and well-dated chert sections from the Franciscan assemblage of California coordinated geochemical, sedimentologic, and petrologic evidence illustrating that distinct chemical patterns characterize cherts with different lithologic associations. Patterns of cherts from the Marin Headlands, California, reflect a change up-section from a site of pelagic deposition to a site within terrigenous influence. Multivariate statistical analysis of 268 Upper Mesozoic and Tertiary Deep Sea Drilling Project and northeastern Pacific rim orogenic belt chert samples determined that the same elements proven effective for discriminating sediment source components and depositional provinces for Recent Pacific sediments are also effective for ancient cherts in spite of diagenetic and paleo-oceanographic processes which distinguish them from unlithified sea-floor sediments. Five primary sediment sources contribute to the Pacific basin, including detrital material, hydrothermal precipitates, authigenic precipitates, biogenic tests, and biogenic dissolution residue. Relative proportions of these sediment components distinguish major depositional provinces in the Pacific basin, including equatorial, volcanic arc, equatorial and mid-latitude active ridge, and northern and southern Pacific low-accumulation provinces. Important elements which define unique ratios for identifying chert sediment sources include Al, Fe, Mn, Ni, Ba, Cu, K, Ti, Zr, Pb, V, Y, Mg, and B. Chemical patterns of DSDP cherts, backtracked to sites of deposition, compared favorably with modeled provincial chemical patterns. A simple procedure was developed from the results of statistical analyses, whereby element ratios plotted on Fe-Ba-Ni, Cu-Ba, and Fe-Mg-K discriminant diagrams designate samples in distinct sediment source fields. When applied to Pacific rim cherts, the method delineates sediment sources and depositional provinces consistent with observed local geology, and emphasizes differences in the provenance of cherts and associated lithologies for various rock units. Chemical parameters combined with other geologic data suggest that some Alaskan Chugach terrane cherts and Californian Franciscan cherts were initially deposited near, but north of, the equator. Other Franciscan and Chugach terrane cherts were deposited in marginal basins associated with volcanic arcs. Differences in northern and southern Pacific depositional provinces in Late Mesozoic time may explain observed differences between Mesozoic DSDP and orogenic belt cherts.

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