Fragmentation of debris avalanche material during flowage — Evidence from the Pungarehu Formation, Mount Egmont, New Zealand

Abstract

The Pungarehu Formation, Mount Egmont, New Zealand (Neall, 1979) is a debris avalanche deposit (Crandell et al., 1984) characterized by two components, block and matrix. The percentage and size of blocks together with the number and width of jigsaw cracks, and the roundness of volcanic clasts contained in the matrix were measured at 44 exposures. Our results show that the percentage of block is low at marginal and distal exposures than in the central exposures of the Formation. The width of jigsaw cracks in the block and the roundness of volcanic clasts in the matrix are high at the marginal and distal exposures. The data obtained in this study support the concept of blocks being gradually loosened and split apart during transportation, into deformed smaller blocks which eventually become isolated clasts. It appears that jigsaw cracks are formed in larger blocks during the early stage of flowage due to collision against either the ground surface or other blocks. The matrix, together with smaller blocks, flowed further than the block component, leaving large hummocky hills in the central portion of the flow comprised of clusters of blocks. Abrasion of isolated volcanic clasts within the matrix also appears to have occurred during flowage.