DEFORMATION BANDS IN NONWELDED IGNIMBRITES: PETROPHYSICAL CONTROLS ON FAULT-ZONE DEFORMATION AND EVIDENCE OF PREFERENTIAL FLUID FLOW

The impact of faults on vadose-zone fluid flow and transport through ignimbrite sequences depends in part on the nature of fault-zone deformation. Both fractures and deformation bands (0.1 to10 mm-wide tabular zones within which grain and pore size have been reduced by cataclasis) have been found at Los Alamos, NM, and Busted Butte, NV—locations at which an understanding of the hydrogeology of faulted ignimbrites is of great societal concern. We have investigated the distribution of these structures within variably welded and crystallized (devitrified and/or vapor-phase crystallized), ash-dominated ignimbrite units, and have begun to evaluate their impact on fluid flow.

The distribution of faults and fractures in ignimbrite sequences is determined by degree of welding and crystallization of individual units. Welding (compaction and fusion of matrix glass and pumice) increases the mechanical strength of a rock by decreasing porosity and increasing grain-contact area and strength. Welding influences mode of failure: we have documented fractures in faults in welded ignimbrite units, whereas faults in glassy, nonwelded units contain only deformation bands. Postdepositional crystallization can decrease porosity and increase grain-contact area and strength in nonwelded units via occlusion of porosity and production of a fine-grained, crystalline rock. Crystallization appears to be secondary to welding in affecting rock strength, as both fractures and deformation bands are found in crystallized, nonwelded units. These relationships between fault-zone structure and protolith petrophysical properties provide a basis for predicting fault-zone character from rock type, which is useful for fluid flow and transport models.

Fractures are considered to be vadose-zone fluid-flow conduits in welded ignimbrites. However, the effect of deformation bands on nonwelded ignimbrite permeability has not been previously considered, as the structures were not previously recognized. Widespread alteration and cementation of deformation bands record preferential vadose-zone fluid transport, suggesting that deformation bands enhance unsaturated fault-zone permeability via pore-size reduction. The magnitude of this increase and the density of deformation bands necessary to create significant flow pathways have not yet been determined.