CLASSIFICATION OF BILLOWED STRUCTURES IN MARGINS OF EOCENE BASALTIC INTRUSIONS USING PHOTOGRAMMETRIC ANALYSIS AT BIG BEND NATIONAL PARK, TEXAS, USA

Big Bend National Park, Texas is host to numerous Eocene (47-42 Ma) basaltic intrusions formed at 400-500 m below the original eruptive surface with unique billowed structures that record magma and water-bearing sediment interactions. To reconstruct the conditions that form billowed intrusions, a classification system for the reoccurring structures has been developed using photogrammetry-derived high-resolution three-dimensional models of exposed billowed dikes at twelve field sites. Nine of the twelve field sites were along a 4-km-long dike where billowed structures were displayed on most of its extent. Measurements from photogrammetry-derived models were compared with field measurements to test the validity of each model and to compare geometries of the proposed structures. Six structure types were recognized: linear, sinuous, teardrop, bulbous, circular, and overprinted. Field measurements reveal that the billows have varying amplitudes (2-30 cm, mean: 9 cm) and wavelengths (6-46 cm, mean: 18 cm). Larger individual features can be traced up to 1.2 m in length, and smaller features extend only 6 cm. Desiccation cracks in sediment were also evident along the margins of the dikes in at least two field locations. These cracks produce 1-6 cm wide hexagonal columns varying in length from 12 cm to approximately 60 cm.

Kelvin-Helmholtz instabilities created between the interface of the basaltic magma and the water-bearing sediment likely contribute to the formation of six proposed structures. Comparison of the wavelengths and amplitudes of the structures and the characteristics of the desiccation cracks can be used to make estimates of sediment saturation and heat transfer between the magma and the host sediment. The quantification of these structures will constrain the conditions in which these interactions occur and how they differ from the conditions that result in explosive sediment-magma interactions.