SIGNIFICANCE OF EDIACARAN CYCLOMEDUSIDS AND OTHER PACIFIC RIM BIOTA IN THE YREKA TERRANE, EASTERN KLAMATH MOUNTAINS, CALIFORNIA

The closely related, Vendian-Lower Devonian Yreka (YT) and Trinity terranes (TT) are the oldest in the Klamath Mountains. Forms that resemble genera allied with the Cyclomedusa plexus dominate an assemblage of fossil medusoids from the YT Antelope Mt. Quartzite (AMQ). These cosmopolitan genera are common elements of the late Proterozoic (Vendian) Ediacaran fossil assemblage. The AMQ forms lack radial ornamentation typical of the genus Cyclomedusa, instead showing a basic tri-partite subdivision of the ex-umbrellar surface by one or more furrows, and a raised marginal zone of varying width, strongly resembling Ediacaria. Some specimens belong to the genus Beltanella sp. which has a smooth ex-umbrellar surface with no central subdivisions, some have a raised marginal rim. Some of the AMQ Beltanella have the central papilla characteristic of this group. Our specimens fall within the accepted range of variation previously reported for these genera. Thin sections of cross sectional views of the medusoids reveal that the primary laminations of the host sediment underlying the fossils are undisturbed, supporting the view that they are primary biogenic features rather than sediment compaction or dewatering structures.

Reinterpreting published data, we place YT fossils into three groups: 1) Laurentian interior (Great Basin); 2) Panthalassic (proto-Pacific) rim adjacent to Laurentia (Arctic Canada, Baltic, Siberia, Kazakhstan, China, Australia); 3) a few unique to the YT. Thus, early Paleozoic tectonic elements partly separated the YT from interior Laurasia while facilitating faunal migration along the Panthalassic rim. The AMQ Ediacaran biota strongly supports an Australian connection for these two terranes. The TT, and presumably the YT as well, was located at about 7º N latitude in late Vendian time (Mankinen et al., 2002). This position is consistent with a Vendian location of Australia west of Laurentia (e.g., SWEAT, AUSWUS). A Middle Devonian overlap sequence that dates the juxtaposition of the YT and the TT also yields 7º N latitude. We suggest that the YT and TT began forming in the Vendian Panthalassic ocean basin at some unknown distance from both Australia and Laurentia, and did not travel across latitudes until after their mid-Devonian amalgamation.