Textural features as indicators of debris avalanche transport and emplacement, Taranaki volcano

Abstract

The Pungarehu debris avalanche deposit was emplaced by the largest known collapse of the proto–Taranaki volcano, ca. 25,000 calibrated (cal.) years ago. This debris avalanche deposit displays a highly contrasting sedimentary character between its proximal and distal reaches. Examination of the deposit granulometry, sedimentary structures, and microscopic particle attributes provides new insights into debris avalanche transport and internal evolution processes. Initial collapse of the proto–Taranaki volcano during this event occurred near the Last Glacial Maximum, with snow and ice cover and substantial groundwater present. The collapsing, sliding large blocks of edifice material, “megaclasts,” were highly fractured by the landslide generation and the depressurization event, forming pervasive jigsaw textures. As the megaclasts moved, shear was focused in softer domains between the hardest, lava-dominated lithologies. These crush and shear zones developed a complex pattern of relative motion between horizontal and vertical parts of the landslide, rather than a simple basal shear zone that supported an upper pluglike mass. The sheared zones, concentrated in soft, pyroclastic lithologies, were areas of intense synflow fragmentation, producing a proto-interclast matrix between large blocks of coherent (albeit jigsaw fractured) lavas. Down flow, the interclast matrix component increased to become pervasive by ~23–25 km from the source, enveloping and preserving large megaclasts out to at least 30 km. The most distal exposures, limited by coastal erosion to ~25–27 km, show that the matrix was not completely water saturated, with only superficial penetration of the sand-dominated material into the margins of fractured lava domains, which still contained central void space. Evidence of multiple generations of particle fracturing is seen under scanning electron microscopy of sand-grade clasts, with initial decompression fractures crosscut by later cracks, pits, and scratches produced by collisional and frictional processes during transport. The findings from this study help to explain the formation of the highly irregular topography of debris avalanche deposits, with chaotically distributed (and probably temporary) zones of shear developing where softer lithologies occur in a collapsing mass, thus leading to differential velocity profiles of portions of the flowing mass in vertical and horizontal planes.