Authigenic Clay Minerals in the Rustler Formation, WIPP Site Area, New Mexico
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AUTHIGENIC CLAY MINERALS IN THE RUSTLER FORMATION, WIPP SITE AREA, NEW MEXICO. D. G. Brookins,* S. J. Lambert,"* and D. B. Ward* "Geology Department, University New Mexico, Albuquerque, NM 87131 "*Sandia National Laboratories, Albuquerque, NM 87185 ABSTRACT Transuranic waste is planned for disposal in the Late Permian evaporites of the Delaware Basin, southeastern New Mexico, at the WIPP Site. The disposal horizon is located in the bedded halite of the Salado Formation, which is overlain by the impure halite-anhydrite (gypsum)-siltstone-mudstone of the Rustler Formation. The Rustler Formation also contains two dolomite members, the Magenta and Culebra, which transmit water. The Culebra Member is suspected to have actively interacted with waters at time(s) from the Late Permian to the present, and it is important to assess the reactivity of these waters in conjunction with WIPP stability. We have investigated the Rb-Sr systematics of clay minerals from the Culebra Member and elsewhere in the Rustler Formation. By separating the less than 0.125 Am size material we are able to deal with presumed true authigenic clay minerals. The authigenic fraction is especially sensitive to chemical and isotopic exchange with waters, and an episodic exposure to a large amount of water will re-set the clay minerals to such a time. Our data yield 259 ± 22 Ma Rb-Sr isochron, which is consistent with the Late Permian age of the Rustler Formation. This age demonstrates that age-determining cations in these clay minerals have preserved their isotopic and chemical integrity since the Late Permian. INTRODUCTION The marine evaporites of the Delaware Basin consist of thickly bedded sequences of anhydrite-halite (Castile Formation), halite (Salado Formation), halite with mudstones and sulfates (Rustler Formation), and sandy red beds (Dewey Lake Formation), all of which were deposited in the Late Permian. The Castile Formation is gypsified near the surface. The WIPP repository is located 2150 feet below the surface in the halites of the Salado Formation. Overlying the Salado Formation is the Rustler Formation which contains two dolomite members, the Culebra and Magenta; both are low-permeability water-bearing units. From previous studies it is thought that the Culebra dolomite has contained meteoric water at various times since the Late Permian [1]. This is of concern because if this inflow has been in geologically recent time, then a potential pathway might exist for meteoric water to come into contact with the repository rocks. We have chosen to investigate a response of minerals to groundwater using the Rb-Sr geochronologic method on size fractions of clay minerals separated from Rustler Formation rocks and especially from Culebra Member samples. Samples of clay mineralbearing mineral from aones of suspected evaporite dissolution in the Rustler Formation [2] were investigated as well. In previous studies, we examined selenite veinlets from these same cores, and presented Sr isotopic evidence for their in situ origin as opposed to some extrin
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