Uranium-Series Dating of Quaternary Deposits

A wide variety of Quaternary deposits can be dated by U-series methods, over the time range from 400 ky to 1 ky. In order to be datable, the deposits must contain material which:. a) crystallized at the time of deposition; b) initially contained some U (0.1 ppm or more) but no Th or Pa; c) has remained a chemically-closed system since the time of deposition. In continental regimes the best such materials are travertines formed either in caves (speleothems) or as spring deposits. The latter are often intercalated with detrital clastic deposits containing vertebrate fossils, or relatable to glacial stratigraphy. The travertines may have been porous at the time of deposition, and subsequent filling of pores by later carbonate can result in spuriously young ages. Calcrete (caliche) formed in soil profiles can be dated; however. calcreted horizons grow continuously over long periods, and material must be selected to be representative of discrete depositional events. Speleo-them is often intercalated with fluvial sediments in the deep interior of caves, marking periods of flooding. In cave-mouth deposits, windblown detritus is also present, as well as cultural materials to which ages can be assigned by dating of speleothem. Lacustrine marls are datable if the carbonate component is dominantly authigenic. Molluscan carbonate, present as snail or clam shells in lacustrine sediments, appears to give less reliable ages. Bone, although containing the highest concentrations of U (up to 1000 ppm) has given questionable results in some cases, evidently due to "open system", continued uptake of U from groundwater. U-series dates of marine, biogenic carbonates have been used to establish high sea-stands related to eustatic fluctuations. Low sea-stands can be established by dating speleothems found in caves that have been drowned by rise of sea level. These dating methods can be related to climatic fluctuations in various ways. Speleothem, dated by U-series methods contains an isotopic record of paleotemperature and variation in isotopic composition of rainfall. Dated travertines in arid regions mark ancient periods of higher rainfall and lake levels. Calcretes deposited intermittently in soil sequences or at varying depths may reflect changes in past rainfall intensity. Glacial outwash deposits in some localities in the U. S. Rockies have been buried by or interstratifled with travertines formed by warm-water springs. Speleothem formed near cave mouths can entrap pollen which can be used to define the climatic conditions at the time of deposition. This would permit the construction of dated pollen profiles beyond the range of ¹⁴C. © 1984.