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Murchey, B.L., 1989

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Murchey, B.L., 1989, Late Paleozoic siliceous basins of the western Cordillera of North America (Nevada, California, Mexico, and Alaska): Three studies using radiolarians and sponge spicules for biostratigraphic, paleobathymetric, and tectonic analyses: University of California, Santa Cruz, Ph.D. dissertation, 195 p., illust., maps.


Radiolarians and siliceous sponge spicules are the most abundant fossils in upper Paleozoic siliceous rocks of the Cordillera of western North America but insufficient documentation hampers their use as biostratigraphic (radiolarians) and paleobathymetric (sponge spicules) indicators. Studies of the siliceous microfossil faunas in the Cordilleran regions of Mexico, Nevada, California, and Alaska were undertaken to correct this deficiency. The purpose for the radiolarian study was to document and describe the radiolarian biostratigraphy of three Cordilleran regions, to test the feasibility of interbasin correlations, and to define the precision with which radiolarians can be used for age determinations in the Cordillera. All three regions (Havallah sequence, Nevada; Northern Sierra sequence, California; Barita de Sonora sequence, Sonora) have some radiolarian taxa in common, permitting correlation between regions. The occurrences and ranges of 41 biostratigraphically useful radiolarian taxa were documented. Late Paleozoic sponge spicule faunas from Mexico, Nevada, California, and Alaska were analyzed quantitatively (percent spicules in total microfossil population) and qualitatively (types of spicules in fauna). Systematic variations of spicule populations related to lithology and tectonic setting prompted the development of a paleobathymetric model using the relative depth ranges of ten types of spicules as well as total spicule abundance. In the model, shallower environments (<150 m) are characterized by abundant demosponge spicules and few or no radiolarians; deeper water environments are characterized by a few hexactinellid spicules and abundant radiolarians; intermediate depths have abundant, diverse spicule faunas as well as abundant radiolarians. The radiolarian biostratigraphic zonation and the sponge spicule bathymetric model were used, in the last study, to analyze the depositional and tectonic history of one part of the Havallah sequence in Nevada. The refined biostratigraphic zonation was sufficient to detect upsection stepping of the sole thrust with accretion of each successive unit and to correlate sedimentary events within the basin. The paleobathymetric model suggests that structurally lower lithostratigraphic units were shallower than higher more basinal units.

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