HR: 1340h
AN: GP13A-0048    [Abstracts]
TI: Anisotropy of Magnetic Susceptibility (AMS) and Sedimentary Fabric Studies of Phreatomagmatic Surge Deposits, Hopi Buttes, Navajo Nation, NE Arizona
AU: * Newkirk, T T
EM: newkit73@gmail.com
AF: Northern Arizon University, Department of Geology PO Box 4099, Flagstaff, AZ 86011 United States
AB: The Hopi Buttes volcanic field is a group of late Mio-Pliocene volcanic vents characterized by hydrovolcanic features. The volcanism at the Hopi Buttes produced ~300 maar and diatreme volcanic landforms scattered within an area of 50km in diameter. The maars in the area formed from phreatomagmatic explosions involving the interaction of the rapidly ascending monchiquitic/nephelinitic magmas and liquefied lower Bidahochi sediments, and groundwater. Phreatomagmatic eruptions produce a spectrum of pyroclastic density currents (i.e. "pyroclastic flows" or "pyroclastic surges/hydrovolcanic surges"). The direct products of these violent eruptive events are dilute gravity driven gas charged pyroclastic density currents. Which, over distances gain and/or lose competency due to a decreasing energy budget and/or paleotopographic control. Paleotopographic reconstructions of the Hopi Buttes volcanic field reveal a sub-horizontal playa type environment. This affords the opportunity to study individual surge deposits on the micro- to macroscopic scales to determine the emplacement dynamics of individual eruptive events without the complications of paleotopographic interference. Anisotropy of magnetic susceptibility and sedimentary fabric analysis has been used to examine the micro- to macroscopic fabrics of individual surge deposits at proximal, medial, and distal locations from the eruptive vent. These techniques give insight on the flow and depositional processes of the transient hydrovolcanic surges. Data shows a distinct correlation of microscopic AMS fabric changes to macroscopic sedimentary facies changes. At the proximal-medial interface ~150-200m AMS fabrics turn from chaotic/lack of orientation to a lineated fabric long axis perpendicular to flow direction. This fabric is interpreted to be transitional debulking interface from a highly concentrated rapid depositional flow to a more dilute transient flow, thus allowing the internal sorting and individual particles to "roll" in transport. Macroscopically this can be observed in the transition from a massive facies to a laterally contiguous stratified/sandwave facies. The transition of medial to distal facies also shows a distinct segregation at ~650-770m. This transitional fabric turns from a long axis perpendicular to flow lineation to foliation. This is interpreted as the continual waning and dilution of the depositional flow to form a microscopic imbrication. Macroscopically this can be observed in the transition from a sandwave facies to a laterally contiguous planar parallel facies. These observations are attributed to downcurrent flow transformations, variable flow regimes, and degrees of wetness result in complex lateral and vertical facies changes. These results indicate a favorable outcome for new techniques to examine flow and depositional processes. With further work on the Hopi Buttes surge deposits we may find a proxy for bridging the gap between theoretical modeling and physical volcanology.
DE: 1518 Magnetic fabrics and anisotropy
DE: 8404 Volcanoclastic deposits
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8428 Explosive volcanism
DE: 8488 Volcanic hazards and risks
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005