INTRACALDERA SEDIMENTATION IN THE MIOCENE COTTONWOOD CREEK CALDERA, NEVADA, USA

The Cottonwood Creek Caldera of Coble and Mahood (Geology, 2012) is the youngest of four silicic collapse structures in the Miocene High Rock Caldera Complex of northwestern Nevada. The caldera contains a post-collapse sedimentary record of an intracaldera lake. Fine-grained lacustrine sediments comprise the majority of the intracaldera sedimentary package. These deposits are generally non-calcareous and are composed of volcaniclastic siltstones, shales, and rare claystones. These fine-grained deposits contain minimal biogenic sediment. Coarse-grained clastic deposits are rare in the caldera lake sedimentary succession and only comprise isolated sediment gravity flow deposits in the surficially exposed portions of the package. Silicified wood fragments and leaf molds are common in the epiclastic deposits. The Cottonwood Creek Caldera intracaldera package also includes pyroclastic deposits that appear to have been emplaced in the caldera lake from external eruptions. These deposits also include abundant petrified wood fragments. Phreatomagmatic deposits within the caldera document continued post-collapse volcanism within the caldera lake. This intracaldera sedimentary package as a whole documents a post-collapse volcanic lake system that saw abundant, wide-spread fine-grained sedimentation with episodic coarse-grained epiclastic input as well as internal and external primary volcanic input.