Evaluation of uranium distributions and uranium-lead dating of authigenic sedimentary materials: Correlation of terrestrial and marine sedimentary sequences.

摘要

A fundamental problem in stratigraphy is the correlation of a well-constrained marine sequence with biostratigraphic control to poorly constrained terrestrial or evaporative sections. Correlation of sections could be improved dramatically if we were able to date the time of sedimentation directly. This study evaluates the potential of dating non-marine materials and evaporite deposits that have relatively high U/Pb ratios. These include: sedimentary phosphate (francolite) formed in black shales of lake deposits; lacustrine (lake) calcite, which has undergone pedogenesis; organic-rich calcite from an evaporite sequence; and tufa calcite formed at a lake margin. Precise U-Pb ages of 294 ± 6 Ma (2σ) and 302 ± 3.7 Ma (2σ) are reported from the non-marine Late Paleozoic central Appalachian Basin, in southeast Ohio and western Pennsylvania. These results question the currently accepted biostratigraphic correlation of the Appalachian Basin with the midcontinent and places radiometric age constraints on a fossil-poor fluvial setting. A precise U-Pb age of 251.6 ± 2.5 Ma (2σ) for organic-rich calcite from the Castile Formation, west Texas and southeastern New Mexico—an evaporite sequence with no previous radiometric control—allows a precise correlation to the marine type section for the Late Permian and Permian-Triassic boundary. A hydrologic mixing model between Ca-rich spring water with a low U/Ca ratio and Ca-poor saline alkaline lake water with a high U/Ca ratio is proposed to explain cyclic variations in U concentrations (42 to 169 ppm) observed in the Miocene Barstow tufa, Mojave, California.