U-PB DATING OF AN EARLY CRETACEOUS CALCRETE, CEDAR MOUNTAIN FORMATION, UTAH

A laminar calcrete from the basal Cedar Mountain Formation northwest of the Paradox Basin, east-central Utah, has been analyzed for U-Pb dating. Phosphor imaging of samples from this horizon show significant heterogeneous U enrichment associated with micrite. Carbon and oxygen isotopic trends indicate vadose zone chemistry for the micrite used in U-Pb dating. Synchrotron XRF mapping shows U enrichment associated with micro veins that connect to the larger areas of enrichment and an inverse relationship to Fe and Mn. U-XANES shows that the U is entirely in the reduced state consistent with this relationship as U is generally immobile in the reduced state while Mn and Fe are soluble. Thus stable isotopic analyses and petrography indicate that the sequestration of U is located in fabrics associated with laminar calcrete formation and not those related to later diagenesis, although minor mobility may be indicated by the micro veins. While all of the 14 Pb isotope analyses form a tight line with a MSWD of 0.57, the calculated age of 119 ± 26 is slightly (but not outside the uncertainty) older than the ²³⁸U-²⁰⁶Pb age. The coherent Pb systematics suggests that any mobility of U or Pb has to have been quite recent. The U/Pb ratios are scattered in a predictable way and a planar fit to the data gives a lower intercept that is indistinguishable from today. By avoiding areas of discoloration a subset of samples give a Total Pb-U isochron age of 115.94 ± 0.62 Ma; MSWD = 1.16. Inclusion of all 14 data points yields an indistinguishable age of 112 ± 15 with a MSWD of 214 reflecting the visible scatter in some of the U/Pb data. The Pb-U age of 115.94 ± 0.62 Ma is our best age determination for this calcrete. Furthermore, since this laminar calcrete caps a succession of three calcrete profiles beginning at the basal contact of the CMF; the age may represent continued pedogenesis and diagenetic alteration at sites of catena development along paleotopographic highs. These ages are stratigraphically plausible and may aid in constraining the developing biostratigraphy of the CMF.