DEVELOPMENT OF A CALIBRATED CARBONATE RIND CHRONOSEQUENCE

Determining surficial deposit ages is necessary to estimate rates of tectonic and geomorphic processes. An important use of surface-age determinations is in earthquake hazard assessment. Reconnaissance surveys produce slip rates estimates using semi-quantitative methods to estimate surface age. The slip rate values estimated using these methods typically span an order of magnitude or more. A better method of surface age determination may result in more robust estimates of seismic hazard. While many methods have been developed that yield numerical ages of alluvial deposits and bedrock; they are expensive and many have a long turn-around time. Carbonate rind chronosequence may be a better tool for reconnaissance seismic assessments.

Carbonate rind measurements have previously been used to construct an age-constrained soil chronosequence in the Lost River Valley, Idaho. I am expanding on this idea by developing a calibration data set to make numerical age estimates of Quaternary deposits in the Mojave Desert portion of the Basin and Range. The method involves measuring carbonate rind thickness profiles beneath surfaces with numerical ages (found in the literature) to produce a calibrated carbonate rind chronosequence. The calibration points are selected for good geographic coverage of the areas where soil carbonate is present in the Mojave Desert. With the calibration, the maximum measured rind thickness (averaged over 20 cm) in a soil profile would yield a numerical age range estimate. This approach has been tested in a paleoseismic investigation of the Hurricane fault in northern Arizona. The carbonate rind chronosequence was calibrated using surfaces dated using ³He cosmogenic isotope geochronology. The age of ruptured Quaternary surfaces were estimated using the calibrated chronosequence; the slip rate of the fault was determined using the age and vertical surface displacement. Diverse methods used during the paleoseismic study - including fault trenching, morphologic dating of the fault scarp, as well as the carbonate-rind chronosequence all produced similar slip rates suggesting that this is a viable method in surface age estimation.