Salt Dissolution in the Permian Flowerpot and Blaine Formations Defines Limits of the Syracuse Basin in Western Kansas and Eastern Colorado

Abstract

The Syracuse Basin is a large region of about 8,100 mi2 (21,000 km2) in western Kansas and eastern Colorado that is underlain by Permian-age salts in the Flowerpot and Blaine Formations of the Nippewalla Group. Originally thought to be a structural or depositional basin, detailed study around the perimeter of the basin shows that it is a dissolutional remnant wherein the salt beds are dissolved at all places around the basin’s margins. The two main salt units, the Flowerpot salt and the middle Blaine salt, consist mainly of displacive halite in red-brown shales and siltstones (mudstones). The Flowerpot salt is generally 200–300 ft (61–91 m) thick within the basin, but where most or all of the salt is dissolved outside of the basin, equivalent strata are 50–150 ft (15–46 m) thick. The younger middle Blaine salt is typically 45–60 ft (14–18 m) thick in the basin, and equivalent strata are 5–10 ft (1.5–3 m) thick where the salt is dissolved. Five areas selected for detailed study of the dissolution zone around the perimeter of the Syracuse Basin show that removal of about 250 ft (76 m) of Flowerpot salt occurs within horizontal distances ranging from about 930 ft (283 m) to as much as 14 mi (23 km). Structural cross sections show that sub-salt strata dip gently and uninterrupted beneath the dissolution zone, whereas strata above the salt are disrupted and are flexed down by an amount roughly equal to the amount of dissolved salt. This supports the thesis that the salt deposits are a dissolutional remnant and not a structural or depositional basin. In most areas, descending unsaturated groundwater dissolves the shallower middle Blaine salt first and then dissolves the deeper Flowerpot salt. But in two areas, unsaturated groundwater is sourced from a sub-salt aquifer, causing dissolution of the Flowerpot salt first and then the shallower middle Blaine salt. Salt dissolution occurred at different times in different parts of the Syracuse Basin. In most areas, it occurred mainly during the Pliocene–Pleistocene–Holocene Epochs, but locally it started before deposition of the Cretaceous or even from Late Permian through Early Cretaceous time. The original extent of the Flowerpot and middle Blaine salts went far beyond the current extent of the Syracuse Basin. Remnants of both salt units are present in six large regions that extend from the Denver Basin in northeast Colorado and western Nebraska on the north to the Anadarko and Palo Duro basins in Oklahoma, Texas, and New Mexico on the south, a total area of about 115,800 mi2 (300,000 km2). In all these regions, the two salt units have dissolutional limits like those at the perimeter of the Syracuse Basin. Dissolution of subsurface salt units can cause problems when or if underground cavities become so large that the roof of the cavity collapses and the cavity rises to the land surface to form a sinkhole or an area of ground subsidence. Problems can also arise when seismic-reflection surveys cross a dissolution boundary and false images of phantom structures are created in strata below the dissolution zone. Also, drilling through salt units must be done with care so that unsaturated drilling muds and formation waters do not cause cavity development in the salt. Dissolution of salt also can affect the quality of groundwater: Salt-dissolution brine can migrate into fresh groundwater aquifers and even render the water unusable for most purposes.