UNMIXING EUSTATIC AND TECTONIC SIGNALS IN SEDIMENTARY SYSTEMS FROM DETRITAL GEOCHRONOLOGY (Invited Presentation)

The record of changing environmental conditions over geologic time, including varying tectonism, climate, sea level, and now anthropogenic effects, is preserved in clastic sedimentary sequences. Detrital geochronology has emerged as a leading approach for investigating changes in the provenance of sand- and silt-sized sediment, which may be driven by exogenic forcing of source-to-sink systems. To the extent that sediment source areas produce unique detrital age distributions, the relative contributions from each source area may be resolved by unmixing calculations. However, approaches to unmixing detrital geochronologic data have received little attention until recently, and more work is needed to develop these approaches.

We demonstrate that end-member modelling analysis, an approach commonly applied to grain size data, is also a powerful tool for unmixing detrital geochronologic data. This “bottom-up” approach does not require independent assumptions about parent age distributions and complements the more widely used “top-down” approach that models daughter samples as mixtures of user-specified parent age distributions. Synthetic experiments demonstrate one such “bottom-up” unmixing algorithm produced median modeled daughter end-member abundances typically within 10 percentage points of the true abundance. We demonstrate the utility of both approaches to sediment unmixing using two case studies. First, we present results from coastal California to highlight the influence of sea level state on how sediment is routed from coastal systems to submarine fans. Second, we present a study of Cretaceous-modern units in the Gulf of Mexico sedimentary basin that highlights tectonic controls on spatial patterns of sediment mixing around the northern margin of the basin. These case studies demonstrate sediment unmixing as a powerful tool for analyzing detrital geochronologic data, with implications for resolving changes in sediment source areas that may be driven by tectonic, climatic, or other external controls.