Particle shape-fabric analysis of PDC deposits: Extracting flow direction and shear conditions
Abstract
Deposit characteristics of pyroclastic density currents (PDCs) can be used to extract detailed information about flow dynamics. Shear forces in the basal region of PDCs result in grain alignment known as particle-shape fabric. In this study, we use particle-shape fabric observed in samples extracted from unconsolidated PDC deposits generated by the 18 May 1980 eruption of Mount St. Helens, WA (USA) to test three hypotheses: (1) particle-shape fabric will align with previous PDC flow direction estimates; (2) samples extracted above the contact between debris avalanche hummocks and PDC deposits will have stronger fabric relative to samples extracted well above flow-substrate contacts; and (3) proximal bedded deposits will have stronger grain alignment relative to the massive deposits due to the evidence of greater shear. Nineteen samples were collected and partially consolidated in the field, then further lithified in the lab. The samples were cut in plan-view parallel to the estimated bedding plane, photographed, and analyzed using computer software that measures the orientation of particles (FabricS). Our results demonstrate that: (1) measured fabric orientations are consistent with estimated flow directions delineated by previous field studies; (2) samples extracted just above contacts with debris avalanche deposits do not exhibit a greater degree of fabric development relative to samples taken well above the contacts; and (3) fabric development in samples extracted from the proximal bedded deposits is comparable to samples extracted from the massive pumice plain deposits. Furthermore, we observe fine scale variability in particle-fabric which suggests unsteady flow conditions at the time of deposition and supports step-wise aggradation for these deposits. Our work demonstrates how particle-fabric can be used to provide insight into current flow direction, flow dynamics, and depositional processes.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V13E0219Z
- Keywords:
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- 8428 Explosive volcanism;
- VOLCANOLOGY;
- 8434 Magma migration and fragmentation;
- VOLCANOLOGY;
- 8445 Experimental volcanism;
- VOLCANOLOGY;
- 8486 Field relationships;
- VOLCANOLOGY