Dating the depositional ages of clastic sedimentary rocks: Uranium-lead systemics of calcretes.

摘要

The main goal of this research is develop techniques to allow for direct determination of the absolute ages of sedimentary rocks with an uncertainty less than 3 million years. U-Pb system of calcretes has been chosen to evaluate obtaining precise ages of sedimentation.; Calcrete samples from three sections in later Triassic New Haven Formation, Hartford Basin, Connecticut, USA, have been investigated using field, petrographic, geochemical, fission track mapping and geochronologic techniques. In order to date the time of sedimentation (not diagenesis) of calcrete host-sedimentary rocks, criteria have been established to select those samples that can be used and analyzed to obtain precise U-Pb ages of sedimentation.; Calcretes in the New Haven Formation mainly occur as three fabrics: horizontal sheets, rhizoliths and nodules. These fabrics contain abundant pedogenic (paleosol) microfabrics including micritic root cylinders, alveolar structures, massive micrites, micritic breccia, ribbon spar, coated grains, desiccation cracks, non-isopachous spar and peloids. These microfabrics are composed of three major generations of calcites. G1, the first generation, is dull luminescent micritic calcite. G2, the second generation, is non-luminescent blocky calcite. And G3, the third generation, is bright luminescent blocky calcite. G1 and G2 have high U, Pb, Mg, low Mn, and typical soil carbonate δ 13C and δ18O values with δ13C from −6.42‰ to −8.61‰ (PDB) and δ18 O from −4.56‰ to −6.53‰(PDB). G3 has high Mn, low U, Pb, Mg, and wider ranges in δ13C and δ 18O with δ13C from −6.55‰ to −9.39‰ (PDB) and δ18O from −5.00‰ to −10.67‰(PDB). These data in conjunction with fabrics indicate that G1 and G2 precipitated in oxygenated, well-drained, unsaturated vadose to capillary fringe environments, and G3 precipitated in reduced, saturated phreatic zone to deep burial environments. Fission track mapping has revealed that U is enriched (relative to Pb) in G1 and G2, but depleted in G3. Calcites in G1 and G2 were selected for U-Pb dating of the time of sedimentation. However G3 can not be used for such purpose. Its age represents the time of diagenesis.; The pure micritic calcites (G1) from the horizontal sheet calcretes of Norian age yielded a 238U/207Pb-206Pb/ 207Pb isochron age of 211.9 ± 2.1 Ma (2-sigma). This age is in excellent agreement with the most recently published geological time scale (Gradstein et al., 1994). (Abstract shortened by UMI.)