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Early and middle Jurassic Radiolarian biostratigraphy, Queen Charlotte Islands, B.C. Carter, Elizabeth Sibbald


Radiolarian biostratigraphy is used to construct an informal zonation for sediments of upper Pliensbachian to lower Bajocian age from the Maude and Yakoun Formations, Queen Charlotte Islands, B.C. Paleoenvironmental studies assess depositional environment and the effects of changing facies relationships on the fauna. Seven distinctive assemblages are recognized comprising 167 species of spumellarian and nassellarian Radiolaria. The first well established middle Toarcian radiolarian assemblages are documented and both these and upper Toarcian assemblages are highly diverse and contain many new and unusual forms. Five genera and 89 new species are described many of which have restricted biostratigraphic ranges. A chlorophyte algal cyst appearing in all lowest Bajocian samples may, with further study, prove to be a significant marker for the lower Bajocian in the Queen Charlotte Islands. Although Toarcian Radiolaria have been little studied, the assemblages compare with others from North America, the Mediterranean and Japan, and are distinctly Tethyan in aspect. This is consistent with the position of Wrangellia, which paleomagnetic and ammonite biogeographic evidence indicates was in the northern hemisphere within 30° of the equator during the Early to Middle Jurassic. Studies of the abundance of spumellarians vs. nassellarians indicate thatnassellarians predominate, are abundant and diverse in deeper-water deposits (middle Toarcian and lower Bajocian shales) whereas spume Marians, particularly those with multi-layered or spongy tests, dominate in shallow-water deposits (upper middle Toarcian to Aalenian sandstones). Shallow-water nassellarians are much less diverse but a few species (all multicyrtids with thickened tests) are very abundant. Depth appears to be the major factor controlling radiolarian distribution patterns in this relatively shallow-water setting. Studies of eustatic sea-level changes throughout the Jurassic have indicated that major phases of sea-level rise occurred in the early to mid Toarcian and in the early Bajocian with a major phase of sea-level lowering in the late Toarcian to Aalenian; detailed study of the radiolarian faunal succession in the Queen Charlotte Islands appears to confirm this major worldwide trend.

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