Sedimentology, petrology, and structure of the Moffet Creek Formation, Eastern Klamath Mountains, Northern California

Gary Michael Gin, University of Nevada, Las Vegas


The stratigraphy and tectonic significance of the Late Silurian-Early Devonian(?) Moffett Creek Formation are poorly understood because of structural complexity and the dearth of fossils. Elsewhere, the Moffett Creek Formation has been interpreted as melange, because abundant sandstone phacoids are surrounded by a sheared pelitic matrix. In contrast, field mapping in the western part of the Moffett Creek Formation and subsequent structural analysis indicate a coherent succession of folded turbidites that experienced younger thrust faulting. A major flat-lying thrust overlies a northeast-vergent footwall syncline indicating southwest to northeast thrusting. Similar fold vergence in the overlying Gazelle Formation may be the result of a single regional deformational event throughout the Yreka terrane; Petrographic analysis of sandstones in the Moffett Creek revealed two petrofacies; one rich in metamorphic rock fragments (Lm) and one rich in sedimentary rock fragments (Ls). Provenance discrimination diagrams indicate that the Lm-rich petrofacies was derived from a crystalline cratonic source, whereas the Ls-rich petrofacies was derived principally from a subduction complex, with minor contributions from a volcanic arc and a crystalline cratonic source. Petrography of olistoliths of the Antelope Mountain Quartzite indicates derivation from a crystalline cratonic source. Furthermore, petrographic analyses of turbidites in the Moffett Creek reveal that the Antelope Mountain Quartzite supplied recycled crystalline cratonic detritus to both the Lm-rich and Ls-rich petrofacies; Independent geologic evidence from previous studies in the Eastern Klamath and Central Metamorphic Belts indicates the Moffett Creek Formation was deposited either in an abyssal plain or trench-floor setting. Sedimentological and petrographic evidence from the Moffett Creek is inconsistent with an abyssal plain setting. Additionally, no evidence was observed that was inconsistent with deposition in a trench-floor setting. Sedimentological and petrographic aspects of the Moffett Creek Formation are most consistent with a trench-floor setting during the Late Silurian-Early Devonian (?).