USING THE U-PB SYSTEM OF CALCRETES TO DATE THE TIME OF SEDIMENTATION OF ELASTIC SEDIMENTARY-ROCKS

The time of sedimentation of rapidly deposited elastic sedimentary roc ks in fluvial environments may be directly dated with an uncertainty o f less than three million years using U-Pb dating of pure micritic cal cite from calcretes developed in overbank deposits. This conclusion is based on results obtained for calcretes (soil calcite, caliche) forme d in the late Triassic New Haven Arkose, Hartford Basin, Connecticut, USA. The paragenesis of calcrete samples from the New Haven Arkose was determined using plane-polarized light and cathodoluminesence petrogr aphy, uranium fission track analysis, as well as trace element and sta ble isotope geochemistry. These calcretes contain an abundance of pale osol microfabrics and diagenetic calcite. The first-generation micriti c calcite and second-generation blocky calcite have characteristics co nsistent with soil calcite. The third generation blocky calcite is a l ater diagenetic calcite (post-soil calcite). The U-Pb data for pure mi critic calcite (first generation) in a horizontal sheet calcrete in se dimentary rocks of Norian age gives a U-238/(207)pb-(206)pb/(207)pb is ochron age of 211.9 +/- 2.1 Ma (2-sigma, and used hereafter for ail ag es). This age and the stratigraphic position for this sample are in ex cellent agreement with the ages proposed by Gradstein et al. (1994) fo r the Norian/Rhaetian boundary of 209.6 +/- 4.1 Ma and the Carnian/Nor ian boundary of 220.7 +/- 4.4 Ma. The U-Pb data for two samples of fir st generation micrite in rhizoliths with about 15% insoluble residues give ''ages'' of 7 +/- 66 Ma and 20 +/- 36 Ma. These results suggest t hat relatively recent events disturbed the U-Pb system of these detrit al rich samples, perhaps due to redistribution of U during weathering or during chemical dissolution for analysis. The U-Pb data for a sampl e of pure third generation blocky calcite cement in a rhizolith yields a (206)pb/U-238-(207)pb/U-235 isochron age of 81 +/- 11 Ma. This age suggests that this sample of third generation blocky calcite precipita ted during the late Cretaceous perhaps over an extended period. These results suggest that by carefully selecting samples using petrographic and geochemical characteristics U-Pb dating of pure calcite found in calcrete horizons in elastic sediments can be used to precisely date t he times of sedimentation and times of later diagenesis. Copyright (C) 1998 Elsevier Science Ltd.