Compaction of the Bishop Tuff, California

Abstract

The foliation in welded tuffs is defined by planar alignment of glass shards, platy crystals, and flattened pumice fragments. The textural features and interrelationships of these elements are clearly the result of the deformation of ash-flow material. The strain involved in developing the alignment can be determined by measuring deformed objects of known original shape. The orientation and shape of the two-dimensional finite strain ellipse, computed from measured bubbles and Y-shaped shards, demonstrates that the long axis parallels the foliation and that there is a close correspondence between the ratio of the principal strains and the bulk density. The measured strain is inhomogeneous on at least two scales. Single cooling units show a regular and continuous vertical variation in deformation: the upper and lower portions are nearly undeformed, whereas the middle portion of the sheet is strongly deformed. On a small scale, the strain varies systematically around rigid lithic fragments and crystals and reaches high ratios at the tops and bottoms of these objects, while pressure shadow zones develop at the sides, which may have complex strain histories. All the lines of evidence point to compaction as the only mechanism involved in the production of the observed characteristic features of the Bishop Tuff, a Pleistocene ash-flow sheet in eastern California. Deformation in the tuff is defined by (1 + e 1 ) = 1.0. During flow and the earliest stages of compaction, pumice lapilli behave as rigid bodies. At about 50 percent porosity, the pumice collapses with the matrix, but at a more rapid rate, and the final forms are flattened in the plane of the foliation. Final shape ratios may reach 25 by simple volume loss; further flattening by essentially volume constant deformation may account for the relatively high mean ratios in fully compacted tuff, and for large ratios reported by others. A comparison of these results with selected specimens of other tuff units and published data strongly suggests that compaction is the dominant mechanism in producing the strong parallel alignment of textural components in all these welded tuffs. In cases where late or postcompactional deformation has also taken place, its effects are superimposed on the earlier compactional features; even so, the results appear to be more closely related to processes in the compactional stage. Extensive flow at or near final tuff densities does not explain most of these features.