Rhizoconcretions in vitric ash-fall tuff, Nye County, Nevada

Small, vertically elongate concretionlike structures, here called rhizoconcretions, are found in scattered localities in and near the southern part of the Belted Range of Nye County, Nevada. The rhizoconcretions occur within a stratigraphically restricted zone in bedded vitric ash-fall tuff of late Tertiary age. The rhizoconcretions differ in mineralogy from their host rock in that they are cemented by a binder of undetermined composition, possibly a zeolite, whereas the host tuffs are un-cemented. These structures appear to have formed during diagenesis of the tuff by means of capillary movement of subsurface waters that reacted with volcanic glass within the conduits to form the cementing material.