Quantitative Characterization of Billowed Structures Along the Margins of Basaltic Intrusions to Determine Process and Timing of Formation: Case Studies from Big Bend National Park, Texas and 71 Gulch, Idaho
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By constraining the conditions in which billowed structures form, the conditions of explosive vs. non-explosive magma-water interaction can be further revealed, improving our understanding of how magma propagates and interacts with water in the subsurface. Billowed structures are likely formed at the arrival of the intrusion due to cooling of intruding magma while experiencing Kelvin-Helmholtz instabilities. Aligned plagioclase laths and stretched vesicles along margins are evidence that the formation of billowed structures occurred while the margins were fluid, deformable, and flowing. Linear billow crests in BBNP were commonly subvertical in orientation, while at the shallower system of 71 Gulch, no organization of billow orientations was observed. The inclusion of sedimentary particles in the glassy margins of the intrusion support the claim that the magma experienced Kelvin-Helmholtz instabilities, as external sediment was able to be transmitted through the magma-sediment interface and become incorporated into the magma. Basaltic intrusions exhibiting unique billowed structures that record magma and water-bearing sediment interaction have been examined and reconstructed in three dimensions using structure for motion (SfM) in Big Bend National Park, Texas (BBNP) and 71 Gulch, Idaho. Exposures in this study formed at approx. 400-500 m below the pre-eruptive surface at BBNP and at ≤24 m below the pre-eruptive surface at 71 Gulch. Evidence that the host sediment was wet and unconsolidated at the time of formation include nearby desiccation cracks in sediment, peperites and phreatomagmatic deposits, as well as the transitioning of glass from sideromelane to tachylyte along the exterior margins of intrusions, resembling quench margins of pillow lavas. To characterize the diversity of the billowed structures, a classification system has been developed utilizing 21 SfM-derived high-resolution three-dimensional models of exposed billowed intrusions across both locations. Seven structure types were recognized: linear, sinuous, teardrop, bulbous, circular, transitional, and overprinted. The scale of identified structures is similar between the two localities with insignificant variation in mean wavelength and amplitude of crests and troughs (≤6 cm). Relative abundances of each structure type are also similar, varying by <7% for each structure type between the two localities. Each site hosts at least three structure types, no one billowed structure type is dependent on another, and dimensions of the structure types are similar at both sites, suggesting that depth is not a major influence in the occurrence of different structure types. Because billowed structures are evidence of non-explosive magma-water interaction, they serve as a platform to study the conditions in which the interaction can become explosive. Future research could involve a more detailed investigation into host-sediment water content, hydrostatic pressure, and the thermodynamics of the system.
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Introduction -- Methodology -- Results -- Discussion -- Conclusions -- Appendix
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M.S. (Master of Science)