BILLOWED STRUCTURES OF BASALTIC INTRUSIONS: A COMPARISON OF BIG BEND NATIONAL PARK, TEXAS AND 71 GULCH, IDAHO

Basaltic intrusions exhibiting unique billowed structures that record magma and water-bearing sediment interaction have been examined and reconstructed in 3D using structure for motion (SFM) in Big Bend National Park, Texas (BBNP) and 71 Gulch, Idaho (71G). Exposures in this study formed at approx. 400-500 m below the pre-eruptive surface at BBNP and at <12 m below the pre-eruptive surface at 71G. Evidence that the host sediment was wet and unconsolidated at the time of formation include 1-6 cm wide desiccation cracks along the margins of the dikes (BBNP), nearby peperites, and phreatomagmatic deposits (71G).

To characterize the diversity of the billowed structures, a classification system has been developed utilizing 21 SFM-derived high-resolution 3D models of exposed billowed intrusions across both locations. Measurements from SFM-derived models were compared with field measurements to test the validity of each model and to analyze geometries of the structure types. Six structure types were recognized: linear, sinuous, teardrop, bulbous, circular, and overprinted. A total of 131 structures were documented at BBNP and 87 were identified at 71G. Model measurements reveal that despite the difference in formation depth at each site, the dimensions of the billows and the quantity of each structure types are similar. Amplitude measurements range from 1-34 cm (mean: 6 cm) at BBNP and from 1-39 cm (mean: 8 cm) at 71G. Wavelength measurements range from 5-110 cm (mean: 20 cm) at BBNP and from 6-72 cm (mean: 24 cm) at 71G. Bulbous structures are most common across both sites, representing 46% of the structure type population at BBNP and 51% at 71G. Linear structures are the second-most common at both sites but were more common at BBNP (32%) than at 71G (17%). The remaining structure types are minor in population, representing a combined 22% at BBNP and combined 32% at 71G.

Formation of the billowed structures is likely due to the cooling of intruding magma while experiencing interfacial instabilities. The structures may represent different phases of the instability interaction. Each site hosts multiple structure types, no one 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.