The Applicability of Multiparameter, Relative Dating Techniques to Beach Ridge Correlation in Baja California Norte, Mexico

Abstract

Beach ridges record shoreline position through time but are often discontinuous. If they can be correlated, beach ridges may have impact on paleogeographic understanding. The applicability of several relative dating techniques to the correlation of discontinuous beach ridges was, therefore tested. Two spatially distinct sites in Baja California Norte, Mexico provided the necessary data. Both were at the mouths of adjacent arroyos and both contained well-preserved, shingle beach ridges that had been formed by coastal progradation. Two of the beach ridges have been assigned overlapping radiocarbon dates of 1,035 ± 250 (?) and 1,150 ± 230 years BP; the beach ridges increase in age away from the present coastline. Data from relative dating parameters show that beach ridges also become progressively more weathered with distance from the shore. Such predictable, local variation did not discourage attempts to correlate the spatially discrete beach ridges. Surficial roughness, existence of a rind, and tendency to disintegrate when struck were the relative dating parameters used in the correlation analysis; each was time dependent process-independent and capable of responding to the full range of time represented by the beach ridges. Statistical tests were applied to the weathering data and successfully differentiated beach ridges known to be time-unequal and correlated those known to be modern. Two separate grouping techniques combined the parameters differently and yielded broadly similar correlation results. These were checked by limited radiocarbon control; two beach ridge segments that had been statistically correlated on the basis of their weathering parameters also had overlapping dates. Correlations of beach ridges older than the controls could not be verified, but most seemed reasonable from field evidence. In all, four pairs of beach ridges seemed reasonably correlated by the methodology and have time-stratigraphic importance.