ABSTRACT

Isotopic variations of Nd in sedimentary rocks provide a new tool for provenance determination. We have compared the isotopic technique with classic petrographic indicators of provenance in sediments from two Tertiary basins of the Rio Grande rift, north-central New Mexico. The siliciclastic sediments in these basins were derived from varying proportions of Precambrian basement and Oligocene volcanic rocks, which are isotopically and petrographically distinct. In the analyzed sediments, a good correspondence exists between petrographic and isotopic provenance indicators. The data also suggest that the isotopic tracer is more sensitive to minor detrital input from a second source than is the framework petrology. The similar Sm/Nd ratios for all analyzed sediments and the identical isot pic values of interlayered mudstone and sandstone indicate that no significant fractionation of Sm and Nd occurs during the formation of the sediments. Thus, the Nd model ages of the sediments preserve the crust-formation age of the source. The Sr isotopic and concentration data show trends generally similar to the Nd data, but they do not indicate the sedimentary provenance as clearly. This ambiguity is a consequence of strong Rb-Sr source heterogeneities and Rb-Sr fractionation during the sedimentary cycle.