Data for: Decoding pyroclastic density current flow direction and shear conditions in the flow boundary zone via particle-fabric analysis
Description
One of the greatest challenges in volcanology is extracting information from volcanic deposits that inform the conditions at the time of emplacement. Here we explore particle shape-fabric within samples from the column-collapse pyroclastic density current (PDC) deposits generated by the 18 May 1980 eruption of Mt. Saint Helens (MSH). Particle shape-fabric is the mutual alignment of particles within the samples, which is dependent of shearing conditions at the time of deposition. Here, particle shape-fabric is used to address the following hypotheses: (1) particle shape-fabric will align with previous interpretations of PDC flow direction during the 1980 event; (2) samples extracted from PDC deposits immediately above a basal contact will show higher extents of fabric development (i.e. clast alignment) relative to samples extracted well above the contact; and (3) samples extracted from the proximal bedded deposits, located on the steep flanks of the volcano, will have higher extents of fabric development relative to samples extracted from the massive facies found in the gently dipping pumice plain. We find that fabric orientations approximate previous flow direction interpretations well for massive PDC deposits, validating the method of using particle-shape fabric to determine flow direction. However, while samples extracted immediately above the PDC deposit-substrate contact do not show a higher measure of clast alignment relative to samples taken well above flow contacts, the typical clast bimodality suggests some degree of traction transport and thus higher shearing conditions relative to when the currents deposited massive lapilli tuffs. Samples extracted from the proximal bedded deposits do not show higher extents of fabric development relative to the massive deposits, supporting previous interpretations that these deposits formed from a rapidly sedimenting, concentrated flow boundary zone that was thin enough to be frequently interrupted or influenced by the overriding turbulent portion of the flow. Finally, observations of fine scale variability in measured fabric within a single outcrop indicates unsteady flow conditions and step-wise aggradation during deposition. This work demonstrates that particle shape-fabric analyses can aid in decoding PDC flow conditions, and future, complementary experimental work may allow for the extraction of quantitative information regarding the magnitudes of shear within the lower flow boundary zone of PDCs.