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Lower Jurassic radiolarian faunas recovered from the sandstone member of the San Hipolito Formation, Baja California Sur, indicate a younger age for much of this member than previously assumed. The San Hipolito Formation, exposed at Punta San Hipolito on the Vizcaino Peninsula, consists of 2,400 m (7,875 ft) of marine sedimentary rocks. The oldest member, a Halobia-bearing green chert, lies on pillow basalts and is successively overlain by limestone, breccia, and sandstone members. Upper Triassic (upper Karnian? and/or lower Norian to lower upper Norian) radiolarians have been described from the chert member. The hemipelagic pelecypod Monotis cf. M. subcircularis (Gabb) of Late Triassic (late Norian) age is found in the upper part of the limestone member.
The uppermost member of the San Hipolito Formation, a volcaniclastic sandstone 1,840 m (6,035 ft) thick, previously yielded only poorly preserved radiolarians, and was inferred to be of Late Triassic age. In this study, well-preserved radiolarians were not recovered from the sandstone and tuff beds. Thin limestone beds at the very base of the sandstone member contain a unique radiolarian fauna with some forms similar to Triassic radiolarians described from Austria and Italy. Discoidal limestone concretions and thin limestone beds from the top of the lower part of the sandstone member yielded abundant, well-preserved Lower Jurassic radiolarians.
No ammonites are found in association with the San Hipolito Lower Jurassic radiolarians, but the radiolarians are similar to Lower Jurassic radiolarian faunas from British Columbia and Oregon, for which excellent ammonite biostratigraphy is available. Based on the occurrence of the radiolarian genera Canoptum, Droltus, Bagotum, Canutus, Hsuum, Lupherium and Pseudoheliodiscus, a Pliensbachian and/or Toarcian age is assigned to the top of the lower part of the sandstone member of the San Hipolito Formation. The continuation of radiolarian studies in British Columbia, Oregon, and Baja California will provide a basis for further elaboration and refinement of a Lower Jurassic radiolarian zonation.
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