INFERRING THE PROCESS OF FORMATION FOR BILLOWED STRUCTURES ALONG MARGINS OF BASALTIC INTRUSIONS EMPLACED INTO WET SEDIMENT AT BIG BEND NATIONAL PARK, TEXAS, USA AND 71 GULCH, IDAHO, USA

Billowed structures provide an opportunity to further our understanding of magma and wet sediment interaction. Billowed structures are curved, ellipsoidal, or elongate cm- to m-scale undulations preserved along the glassy margins of basaltic intrusions. Field observations and thin sections of intact margins of basaltic intrusions in proximity to phreatomagmatic diatremes at Big Bend National Park, Texas, USA (BBNP) and 71 Gulch, Idaho, USA (71G) were used to determine the manner in which billowed structures form. Digital elevation models derived from Structure from Motion photogrammetry of billowed margins at 11 study sites at BBNP and nine study sites at 71G were used to quantitatively assess billow geometries. Significantly, we found no relationship between paleodepth of intrusion (200-500 m at BBNP, ≤24 m at 71G) and billowed structure morphologies or dimensions.

The presence of aligned plagioclase laths and elongate vesicles at both localities suggests billowed structures formed while intrusion margins were fluid, deformable, and flowing. Cracking due to expansion of glassy margins is not evident at either locality, leading to the conclusion that magma likely does not expand outward to form billowed structures. These observations indicate billowed structures likely form during initial stages of intrusion while margins experience Kelvin-Helmholtz instabilities as cooling magma intrudes through unconsolidated wet sediment and velocity shear occurs between the soft, fluid-like mediums. The rapid cooling of intruding magma then preserves structures of varying geometries and scale along the glassy margins of the intrusion. A thin vapor film is likely formed during this process to insulate the intruding magma and prevent explosive interaction.

Magma in the subsurface is often susceptible to interaction with water in some manner. Billowed structures offer insight into how magmas can migrate towards the surface through wet sediment in a non-explosive fashion, while explosive interaction may still occur nearby. The more we understand about conditions of explosivity vs. non-explosivity during migration of magma through the subsurface, the more prepared we will be for monitoring and hazard mitigation in volcanic environments.