Experimental evidence for fluidization processes in breccia pipe formation

Abstract

Results of small-scale fiuidization, spouting, and channeling experiments suggest that similar processes may have been involved in the formation of many breccia pipe features. Various cylindrical, parallel plate, and boxlike model containers were filled with mixtures of particulate materials ranging from clay-sized (powdered kaolinitc) to 1-cm gravel and each container was attached at its base to a large-volume compressed air source. Features generated during the course of the various experimental runs are analogous to the majority of features observed in breccia pipes.Complex branching fracture systems developed in models containing abundant cohesive material appear to be similar to incipient crackle breccias. Steep-walled, funnel- to cone- shaped conduits formed readily, and in some models, inward-dipping fractures (ring fractures), marginal shear folds and/or downwarps, and/or high-angle normal faults developed in adjacent material. Thorough mixing of heterogeneous particulate materials from throughout the stratigraphic column commonly was achieved and blocks of more cohesive materials were readily rounded and comminuted. Convective particle motion (spouting) was most effective in the mixing process and was accompanied by downwarping and fragmentation of adjacent strata, downward migration of wall-rock blocks along conduit margins, and generation of annular zonation of conduit-filling material; in some systems, annular-size classification resulted in fine layering parallel to conduit walls. Crude, centroclinally, nested saucer bedding developed in some experiments where air-fall ejecta were generated and these beds commonly were transported essentially intact downward within the pipe during a bubbling fiuidization event. A gas-streaming or channeling process, in which finer material was elutriated or cleared out of coarser material, led to the formation of channelways or chimneys which in natural systems would provide excellent avenues for introduction of later magma or hydrothermal solutions. In model systems enriched in strongly cohesive clay-sized particles, accretionary pelletal spherules commonly were generated.Fluidization and related processes have been defined to describe highly controlled industrial systems. Problems of scale and strength of materials are significant when comparing experimental with geologic systems, but the striking similarity of many features observed in both experimental and natural breccia pipe systems suggests that a similar genetic process is reasonable. Although many pipes clearly are not formed by fiuidization and related processes, these processes may be important in the development of, or some developmental stage of, some breccia pipes.