LUMINESCENCE DATING OF SEDIMENTARY DEPOSITS ASSOCIATED WITH OROGENIC SETTINGS

Optically stimulated luminescence dating of quartz sedimentary deposits associated with orogenic settings has been previously shown to be difficult, and sometimes impossible, due to low sensitivity of the quartz signal obtained through blue light stimulation (BSL). Alternative luminescence dating approaches employing infrared stimulated luminescence (IRSL) measurements can be used to target greater-sensitivity signals arising from feldspar grains. However, the feldspar luminescence signal measured at low temperatures (e.g., IR₅₀) is prone to anomalous fading, which may lead to IRSL age underestimation. A more stable feldspar signal may be obtained through infrared stimulation at elevated temperatures following low temperature IR exposure, referred to as post IR-IRSL (pIRIR). However, the luminescence clock of feldspar is less readily reset (bleached) than that of quartz, especially when measured at elevated temperatures (e.g., pIRIR₂₂₅, pIRIR₂₉₀). This may lead to overestimation of the depositional age obtained through pIRIR approaches, particularly in fluvial and glacial deposits and in sediments that have experienced relatively short transit from source to sink. These issues with sensitivity, fading, and bleaching, make luminescence dating a tricky and underused chronology tool in orogenic settings and their proximal basins.

Here, we explore different luminescence dating approaches of sedimentary deposits in two highly-challenging settings located on active tectonic margins: the Ganges-Brahmaputra megadelta (Bangladesh) and the eastern outlet glacier valleys of the Northern Patagonian Ice Field (Chile). We employ a multi-pronged approach to test the feasibility of luminescence dating of quartz and feldspar sediments of different grain-sizes in these settings. This is facilitated through polymineral, multiple-signal measurements that yield data on both the sensitivity and differential bleaching of BSL, IRSL, and pIRIR signals. We show that confident dating of Holocene-aged Ganges-Brahmaputra quartz silt is possible with our protocols, and we explore the application of these findings to the Patagonian glacial and outwash deposits. This work contributes to extending the tool of luminescence dating to settings in which it has been historically underused.