EDIACARAN CYCLOMEDUSOIDS AND THE PALEOGEOGRAPHIC SETTING OF THE NEOPROTEROZOIC-EARLY PALEOZOIC YREKA AND TRINITY TERRANES, EASTERN KLAMATH MOUNTAINS, CALIFORNIA

Abstract

Newly recognized fossil cyclomedusoids from the Yreka terrane include Ediacaria sp. and Beltanella sp. They are typical of the Ediacaran fossil assemblage, range from 640 to 575 Ma, and thus are latest Neoproterozoic (Vendian) in age. The Yreka terrane structurally overlies the Trinity terrane, which also includes Vendian rocks. The Yreka terrane is a polygenetic stack of sedimentary and metasedimentary thrust sheets consisting of the Vendian Antelope Mountain Quartzite, Siluro-Devonian turbidites (trench fill), lower Paleozoic mélanges (accretionary complexes), and the Lower Devonian Gazelle Formation (trench-slope basin deposits). The Trinity terrane is a polygenetic mafic-ultramafic complex consisting of multiple mantle tectonite blocks and two ophiolitic crustal sequences, one Vendian and one Siluro-Devonian. Multistage textures and structures within the Trinity terrane indicate Vendian or Cambrian ductile deformation in the mantle blocks, followed by pre-Early Ordovician amalgamation, then regional uplift and brittle deformation. The Siluro-Devonian crustal sequence developed on this polygenetic composite basement in a supra-subduction zone setting. The Trinity and Yreka terranes formed close together, with some Yreka terrane components receiving Trinity terrane detritus. The Lower Devonian Gregg Ranch Complex was the active accretionary wedge on which the Gazelle Formation trench-slope basin formed, accompanied by minor near-trench volcanism. Kinematic analysis of the Gregg Ranch Complex indicates convergence directed from the Yreka terrane toward the Trinity terrane, combined with a strike-slip component, probably during Early Devonian collision of outboard terranes. The Yreka terrane-Trinity terrane composite terrane was then stitched together by Middle Devonian dike swarms that fed overlapping lava flows. Because of the close spatial and temporal proximity between the Yreka and Trinity terranes, paleopoles from the Trinity terrane can be used to suggest paleolatitudes where Yreka terrane biota may have originated, and biogeography of Yreka terrane fossils limits the paleogeographic setting of both terranes. © 2006 Geological Society of America.