OSL and IRSL characteristics of quartz and feldspar from southern California, USA

Southern California comprises of a wide range of diverse landscapes and environments, from high mountains with glacial and periglacial sediments to deserts with large sand dunes, extensive alluvial fans and ephemeral playas. Highly active tectonic processes has exposed ancient (c. 2 Ga) plutonic and metamorphic basement from deep within the crust, while similar Palaeozoic, Mesozoic and Cenozoic rocks are also common. A rich array of volcanic lithologies extending into the late Quaternary complement many thick sedimentary sequences that formed in equally diverse ancient environments typical of an accreting active continental margin. In some locations, notably in the Coachella Valley close to Palm Springs and the Salton Sea, low OSL sensitivity and poor characteristics restrict the application of the quartz SAR protocol to date late Pleistocene and Holocene fluvial sediments. In other locations such as the Malibu coastline, high sensitivity of the quartz OSL signal is observed, despite local source rocks being dominated by volcanic lithologies. Problems of poor quartz characteristics, along with uncertainty in predicting quartz OSL behavior for future dating campaigns poses a significant problem for projects, in particular for neotectonic contexts. While K-feldspar has been used extensively to date eolian and fluvial sediments in southern California, little information regarding signal stability is available. We explore the characteristics of both quartz and feldspar sub-samples from eolian, fluvial, lacustrine environments, in order to help develop mineral selection criteria for optical dating applications and clarify these issues. The importance of radiation quenching in quartz grains recently eroded from bedrock and the role of fires in enhancing OSL sensitivity are considered. The relative bleachability of quartz and feldspar fractions, along with thermal stability considerations is discussed. A simple test for quartz OSL signal contamination based on thermal quenching and assistance, and the susceptibility of the OSL signal to IR bleaching is introduced.