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Nd isotopic signatures and stratigraphic correlations : examples from western Pacific marginal basins and Middle Jurassic rocks of the southern Canadian cordillera Mahoney, J. Brain


The purpose of this investigation is twofold: 1) to evaluate the applicability of Nd and Sr isotopic analyses of fine grained clastic sediments to basin analysis and stratigraphic correlation; and 2) to document the lithostratigraphic, biostratigraphic, geochemical and isotopic characteristics of Lower to Middle Jurassic strata in tectonostratigraphic terranes of the southern Canadian Cordillera in order to evaluate potential terrane linkages. Isotopic analyses of Neogene strata from three western Pacific marginal basins (Shikoku Basin, Sea of Japan, Sulu Sea) permit evaluation of isotopic analyses to basin discrimination and stratigraphic correlation. The isotopic signatures of the Sulu Sea and Sea of Japan demonstrate that modern marginal basins have an isotopic signature that varies within limits defined by the geology of its source regions. The highly evolved (εNd < (-8)) isotopic signature of the Shikoku Basin, however, strongly overlaps that of the Sea of Japan, and contrasts with the juvenile character of the crustal domains on the basin margins. This anomalous signature is interpreted to be the result of cratonal aeolian influx. Temporal isotopic fluctuations in the Shikoku Basin are roughly synchronous across 5600₂ of basin floor, and the pattern of isotopic fluctuations can therefore be used to correlate strata throughout the basin. Isotopic fluctuations are interpreted to result from changes in the relative contribution of each crustal domain within the source region to the basins’ total sediment budget, which is a function of tectonism, volcanic episodicity, climatic factors, and basin hydrology. Isotopic fluctuations in a stratigraphic sequence may therefore prove to be important as both tools for stratigraphic correlation and as a monitor of basin evolution. Lithostratigraphic data indicate that Lower to Middle Jurassic strata of the Harrison, Cadwallader, Bridge River, and Methow terranes each contain six strikingly similar, regionally consistent lithostratigraphic variations. Biostratigraphic data indicate that each terrane contains Aalenian to Bajocian strata with identical mixed fauna of Boreal, East Pacific and Tethyan faunal realms. Isotopic data indicate that the Harrison, Cadwallader, and Methow terranes contain coeval isotopic fluctuations of similar magnitude. Volcanic geochemical data indicate that the Harrison and Methow terranes constitute separate volcanic arc systems flanking a basin containing back arc basin basalts. In addition, volcanic geochemistry and isotopic data suggest that the Harrison terrane represents the youngest eastern fades of the eastward migrating Bonanza- Harrison arc system, which provides an Early Jurassic link between Wrangellia and Harrison terranes. Results of this investigation strongly suggest that Lower to Middle Jurassic strata of the Wrangellia, Harrison, Cadwallader, Bridge River, and Methow terranes comprise a single marginal basin floored by trapped oceanic crust of the Bridge River terrane, and flanked by volcanic arc systems to the east and west. Wrangellia, Harrison, Cadwallader, Bridge River, and Methow terranes were amalgamated by the Early Jurassic, and have behaved as a coherent crustal block since that time.

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