A NOVEL APPROACH TO CHRONOSTRATIGRAPHY OF THE WELLINGTON FORMATION EVAPORITE DEPOSITS BY U-PB DATING OF ZIRCON FROM CRYPTOTEPHRA

The Hutchinson Salt Member is the stratigraphically lowest evaporite deposit within the Wellington Formation (Early Permian) consisting of argillaceous and evaporitic rocks that lack precise radiometric dates. Radiometric chronostratigraphy of sedimentary units has relied mainly on dating discrete volcanic ash beds and youngest detrital minerals in clastic sediments. This study tests if there are hidden traces of volcanic ash, or cryptotephra, in the clastic sedimentary interbed layers of the Hutchinson Salt Member and whether the evaporite units can be dated using these traces. Weathering-resistant minerals found in volcanic ash can be stored in the interbeds of these evaporite deposits, allowing chronostratigraphic correlations using zircon geochronology via laser ablation - inductively coupled plasma - mass spectrometry (LA-ICP-MS). The samples were taken from various clastic interbeds of salt layers in the STRATACA salt mine in Hutchinson, KS, and then processed for heavy mineral separation to extract zircon.

The feasibility of obtaining heavy minerals datable by the U-Pb method from the clay-rich samples has been the focal point of this project so far. Separation of heavy minerals from the clay-rich material has entailed a repetitive process of grinding, decanting, sonication, and dissolution before heavy liquid separation. The literature on clay-mineral extraction helped guide the rationale behind these methods.

Currently, a stratigraphic age of ca. 275 Ma has been assigned to the Wellington Fm. (lower part of the Leonardian Series of the Permian) based on the assemblage of fossils. However, the existence of a magmatic arc system in the Western margin of North America around the time of deposition of this evaporite deposit makes it feasible to assume that volcanic ash was carried downwind. Euhedral zircons — indicative of volcanic origin — have been procured from these samples. If these zircons yield consistent early Permian appropriate ages, a more precise technique of dating evaporite deposits will have been discovered.