Volcaniclastic Sediments of the Miocene Pickhandle Formation in the Northern Calico Hills: A Sedimentological and Volcanological Perspective
Abstract
The early Miocene Pickhandle Formation in the central Mojave records fluvial and lacustrine sedimentation and significant volcanic deposition related to extensional faulting. Understanding the geologic history and volcanic activity of the Pickhandle Formation is significant because it was deposited coeval with Miocene extensional faulting in the Mojave, and it records recurrent, explosive, rhyolitic volcanism associated with a source that was likely in the region of the Mojave Desert. The Pickhandle Formation was deposited in an extensional basin; it is approximately 1500 meters thick, unconformably overlain by the Barstow Formation, and conformably overlies the Jackhammer Formation near the type section in the Calico Hills north of Barstow, CA. Deposition of the Pickhandle Formation began 23.7 Ma, and occurred synchronously with the development of the Waterman Hills Detachment Fault. Repeated volcanic eruptions, originating from an unknown source near the Calico Mountains, occurred 23.7 to 21.7 Ma ago and all sedimentation ended when extension ceased at 18.9 Ma. Dacite lavas related to the Yermo Dacite Volcanic Field are interbedded with the volcanic sediments in the Pickhandle Formation. The dacite domes and the volcanic ash of the Pickhandle Formation may share the same source. Our preliminary fieldwork identifies a significant ignimbrite deposit, and the 50 cm diameter size of the pumice blocks strongly implies that the source was nearby, possibly near the Calico Mountains. Generally speaking, the Pickhandle Formation can be divided into two units; primary volcanic deposits and reworked volcaniclastic deposits. The goals of this research are to 1) determine the relationship between the Pickhandle Formation volcaniclastic sediments in the Calico Mountains and the Yermo Dacite domes using field observations and geochemical analysis, and 2) to unravel the volcanic history of the Calico Hills region using the volcaniclastic sediments in the Pickhandle Formation. We conducted detailed geologic mapping of the region and constructed five stratigraphic columns in order to document facies changes and determine the nature of the pyroclastic and fluvial deposits. Our field research shows that the deposits are primarily composed of volcanic ash and clastic sediments. Evidence from the correlated columns (300 meters) shows a transition from a lacustrine environment to fluvial system as the lake gradually filled with volcanic material. Freshwater gastropods, lacustrine laminae, fluvial imbrication, and alluvial fan conglomerates are consistent with this hypothesis. Several times during the deposition of the formation, large pyroclastic flows were deposited in the basin, indicating that large eruptions were continuing. Based on stratigraphic evidence, we conclude that the Calico Hills records deposition in the volcaniclastic unit of the Pickhandle Formation. Primary volcanic ash deposits are found interspersed with volcaniclastic layers, however, the paucity of the primary volcanic deposits lead us to conclude that they were largely destroyed by faulting and/or lacustrine and fluvial reworking. The Yermo dacite interfingers with the Pickhandle Formation and preliminary geochemical results may indicate that the volcanic ash and the dacite domes are related to each other.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMEP13C0854C
- Keywords:
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- 8404 VOLCANOLOGY / Volcanoclastic deposits