Formation of Billowed Structures Along Margins of Basaltic Intrusions: Insights from Big Bend National Park, Texas and 71 Gulch, Idaho

Abstract

Billowed margins of basaltic intrusions provide opportunity to study the conditions of non-explosive magma-pore water interactions and how magma propagates and interacts with unconsolidated sediment in the subsurface. Basaltic intrusions exhibiting billowed structures that formed at different paleodepths in Big Bend National Park, Texas (200–500 m) and 71 Gulch, Idaho (≤24 m) have been examined and reconstructed in three dimensions (3D) using structure for motion (SfM) techniques. To characterize billowed structure diversity, a classification system was developed utilizing 21 high-resolution 3D models of outcrops from both locations. Seven structure types are recognized: linear, sinuous, teardrop, bulbous, circular, transitional, and overprinted. Relative abundances of each structure type are similar between both localities. All structures identified were recognized along dikes, apart from one sill. Each site hosts ≥3 structure types, no one structure type is dependent on another, and the dimensions of structure types are similar at both localities with insignificant variation in mean wavelength and amplitude of crests and troughs (≤6 cm), suggesting that depth of intrusion is not a major influence in the occurrence of different structure types. Aligned plagioclase laths and stretched vesicles along margins are evidence that billow formation occurred while margins were fluid, deformable, and flowing. These observations indicate billowed structures likely form during the initial stages of intrusion while the margins are experiencing Kelvin-Helmholtz instabilities during cooling. As billowed intrusions from varying paleodepths at the two localities both occur near phreatomagmatic diatreme structures, changes in the rate or flux of magma intrusion or local heterogeneities in the host sediment and/or pore fluid content may be more responsible for explosive vs. non-explosive interactions rather than simple arrival of magma.