Fabrics, facies, and flow through a large-volume ignimbrite: Pampa De Oxaya, Chile

Abstract

The Miocene Oxaya Formation, exposed along the western Andean slope in northern Chile, represents one of the largest ignimbrite provinces on earth. In this study, magnetic fabric data were acquired from a ~ 1-km-long core drilled vertically through a single cooling unit of the Oxaya Formation the ca.22 Ma Cardones ignimbrite. Samples for magnetic analysis were obtained every 20 m from the fine-grained matrix of the core. Detailed measurements of the variation in bulk magnetic properties, including natural remanent magnetization (NRM), susceptibility, and anisotropy of magnetic susceptibility (AMS), were used to monitor changes in magnetic mineralogy as well as changes in the strength and orientation of the magnetic fabric throughout the flow. AMS ellipsoid orientation and shape reflect rapid deposition from a concentrated granular fluidized flow and constrain both the location of the source caldera for this catastrophic eruption and processes of transport and deposition in this large-volume ignimbrite. After utilizing the magnetic remanence to correct for rotation about the core axis, well-grouped and imbricated petrofabric orientations reveal a well-defined SW (247°) transport direction down a proto-Western Andean slope indicating syn- or post-welding flow and confirming the deformed Lauca caldera as the likely source of the eruption. Systematic variations in fabric shape (T) and intensity (P) with depth reveal a predominately oblate fabric toward the top and base of the flow and predominately prolate fabrics in the center. These vertical changes in fabric reveal that this massive, apparently homogeneous, deposit has a systematic layering. This layering reflects depth-dependent temperature variations, temporal changes in the flow boundary zone during deposition and post-depositional processes.