Test of thermoluminescence dating with coastal sediments from northern California

Abstract

Application of the thermoluminescence (TL), fine-grain (4–11 μm) dating method to modern coastal deposits and to tectonically displaced sediments near the Mendocino Triple Junction in northern coastal California demonstrates the suitability and limitations of dune sand, buried soil, estuarine mud, and beach deposits for TL dating. TL measurements on modern estuarine mud, modern beach sediments, and radiocarbon-dated dune sand and soils show that these deposits are suitably zeroed for TL dating under some conditions. A weighted mean TL age of 16.4 ± 1.3 ka was obtained for four samples (two from a buried soil and two from the straddling terrace dune sand), in close agreement with a radiocarbon age of 16.84 ± 0.06 ka B.P. on charcoal from this paleosol. Application to older sediments and soil profiles of similar origin may therefore provide useful geological ages. Deposits associated with deformed marine terraces yielded TL ages of 176 ± 33 ka for a shallow marine mud at the Mad River site and 119 ± 31 ka for nearshore deposits at the Southport Landing site. These age estimates are consistent with the stratigraphic positions of the TL samples relative to deposits having previously inferred ages based upon soil profile correlations to local age assignments, which are in turn based upon uplift rates relating to eustatic sea level stands. These single TL age results suggest that more detailed TL dating applications in this region, and in similar geological settings, could provide accurate estimates of geologic ages and rates of deformation. However, the polymineralic fine grains from two other nearshore deposits (at the Table Bluff and Centerville Beach sites) yielded ambiguous plateau plots and thus can not be considered datable without additional TL experiments. This difficulty may be due to a high quartz/feldspar ratio in sediments from this area.