Decoding PDC Flow Direction and Shear Conditions in the Flow Boundary Zone via Particle-Fabric Analysis
Abstract
The deposits of pyroclastic density currents (PDCs) record complex transportation and depositional histories, which can be used to extract information about flow dynamics. Prior to deposition, particles must pass through the flow boundary zone where they are subjected to shear forces, leading to the development of particle-shape fabric. In this study, we lithify and extract samples from unconsolidated PDC deposits for fabric analysis. Nineteen samples were taken from PDC deposits generated by the 18 May 1980 eruption of Mount St. Helens, WA (USA). These samples were partially consolidated in the field by coating with sodium silicate, then further lithified in the lab by impregnating with sodium silicate using a vacuum chamber. The samples were cut in plan-view, photographed, and analyzed using computer software that automatically measures the orientation of particles and produces statistics that demonstrate the strength of particle-shape fabric (FabricS). We demonstrate that fabric development in PDC deposits effectively encodes shearing conditions at the flow boundary zone and can be used to constrain PDC flow direction. We find that fabric strength varies greatly at a decimeter to meter scale within massive lipilli tuff deposits. We interpret this variability to reflect current unsteadiness and fluctuation between shearing and non-shearing conditions, resulting in, deposition of discrete packages of material. This style of deposition supports a step-wise aggradation model for the deposition of the 18 May 1980 afternoon PDCs. Our work demonstrates how particle-fabric studies can be used to determine flow directions and provide insight into current dynamics. The methods and results also have promising implications for interpreting PDC deposits with poor preservation and less geologic context.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.V44A..08Z
- Keywords:
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- 8404 Volcanoclastic deposits;
- VOLCANOLOGYDE: 8428 Explosive volcanism;
- VOLCANOLOGYDE: 8445 Experimental volcanism;
- VOLCANOLOGYDE: 8486 Field relationships;
- VOLCANOLOGY