TESTING BOULDER MATRIX FOR LUMINESCENCE DATING

The application of optically stimulated luminescence (OSL) dating has significantly contributed to quaternary geochronology where sand lenses within fluvial deposits were widely used. This is because of abundance of the target minerals (quartz and feldspar), getting properly bleached and the heterogeneous radiation sources are absent. So, researchers working in the quaternary period have been using the OSL dating technique in the lower energy sandy horizon and discriminated against the higher energy coarser systems, such as the matrix within the boulders. However, finding the age of these events could be the key to understanding the episodic release of sediments which has implications in alpine settings like Himalaya where the deformation driven by tectonics and the paleohydrology driven by the atmosphere is highly entangled. The problem in dating the matrix between boulders lies in defining the dose rate. Typically, the dose rate is evaluated either from the concentration of radioactive elements (geochemistry) or by directly measuring the radioactivity of the sediment. However, in the boulder matrix system there are different radiation sources i.e., matrix and clasts and so the dose rate evaluation is not straightforward. So, there is an obvious problem of finding suitable sand lenses. The present study aims to formulate a technique for calculation of dose rate in a boulder matrix system. To gather the most appropriate material and sedimentary units we studied an alluvial fan system in the Northwest Bengal, India. Here, five sections of the alluvial fan system are studied and sampled. Two types of samples were collected- (1) boulder matrix and (2) sand lenses adjacent to the boulder matrix system. We measured the dose rate of the boulder matrix and surrounding boulders independently to quantify the apparent and theoretical dose rate received by the matrix by considering the different radiation sources. Then we calculated the ages. Whereas the ages obtained from the sand lenses were used to validate the theoretical ages of the boulder matrix system. This study provides a quantitative perspective on how to estimate the equivalent dose stored within a boulder matrix system by considering the heterogeneous radiation sources to produce the correct age.