Geochemistry of the Dakota Formation of northwestern New Mexico: relevance to radioactive waste studies

The Dakota Formation of the San Juan Basin in northwestern New Mexico consists predominantly of well-cemented sandstones and arenaceous mudstones. Clay mineral-rich rocks, derived from volcanic ash, are mapped as bentonites. The likely physical conditions during burial were temperatures between 35 to 600C and a pressure of about 0.5 kbar. X-ray studies reveal a mixture of montmorillonite, kaolinite, illite, and mixed layer clay minerals. The typical cation exchange capacities range from 20 to 40 meq/100 g for most samples. Radiometric age determinations of clay minerals by the K-Ar method yield 90 to 94 millions of years before present (MYBP) and Rb-Sr ages yield 93 + or - 8 MYBP. These dates agree with paleontological ages and indicate closed-system conditions for potassium, argon, rubidium, and strontium in these rocks. Closed-system conditions for cesium are inferred based on its greater retentivity than rubidium and potassium in clay-rich rocks. Neutron activation analysis (NAA) of the Dakota samples indicates normal lanthanide abundances and distribution in the bentonitic rocks; local lanthanide enrichment is noted where local uranium accumulations are noted. The uranium has been derived from several sources and fixed in the Dakota Formation at various times from roughly 60 to near 0.25 MYBP. The chalcophile elements copper, antimony, and lead are often fixed with uranium in organic-rich rocks and apparently have not migrated since fixation, even under oxidizing conditions. No mobilization for the lanthanides and barium is noted either. Collectively, the radiometric ages and NAA data indicate the bentonite and bentonite-sand mix to be suitable for overpack in radioactive waste repositories