Plastic Deformation and Dissolution of the Hutchinson Salt Member in Kansas

Abstract

The Hutchinson Salt Member of the Permian Wellington Formation is preserved throughout south-central Kansas at subsurface depths of between 60 m and 760 m (197 ft and 2,494 ft), as a more-or-Iess contiguous body having maximum net thicknesses on the order of 170 m (558 ft). Within Kansas, the margins of the salt are depositional except for the updip eastern edge, which is dissolutional. Basinward of the present-day main eastern edge of the Hutchinson Salt, localized dissolution of both natural and anthropogenic origin has occurred. Such leaching has resulted in the formation of both surface sinkholes (e.g,. Janssen sink, Panning sink) and subsurface cavities (e.g., Knackstedt cavity).

Janssen sink is an example of slow and gradual surface subsidence, characterized by the predominantly ductile deformation of remnant salt. On seismic data, the feature is manifested as an upwards-expanding zone of measurable subsidence. Panning sink, in contrast, is an example of catastrophic surface subsidence. In cross section, the feature is presented as an inverted coneshaped, vertically migrated collapse cavity (chimney).

The Knackstedt void is a bridged cavity, characterized by a vertically migrating collapse chimney, the roof of which is presently bridged above the original top of the salt. It is anticipated that this void will eventually close and that a surface sinkhole will develop on site. The nature of the sinkhole (i.e., gradual subsidence or catastrophic collapse) will be partially dependent upon: 1) the rate of continuing dissolution, 2) the rate of effective cavity closure due to creep and stoping, and 3) the shear strength of the post-salt strata.