Fractionation of Detrital Zircon Grains in Ancient Deltaic, Lacustrine and Ocean Basin Settings and Implications for U-Pb Age Zircon Provenance Studies

Recent studies suggest that detrtial zircon age populations can be fractionated by grain size in modern sand sediments. If this is true, could provenance information recorded by zircon age populations also be biased by grain size? Does the grain size bias on detrital zircon age also exist in ancient sedimentary rocks? Do other detrital zircon grain properties (shape, density, source) contribute to the age bias? To address these questions, this research compared the detrital zircon age distribution in ancient mudstone and its facies equivalent sandstone in three different depositional environments in three different sedimentary basins. In the first case study, Late Cretaceous Trinidad sandstone and Pierre shale deposited within a delta-front to prodelta environment in the Raton Basin (Colorado, USA) were sampled. Both coarse and fine grain sizes of the Trinidad sandstone and Pierre shale all shared overlapping major peaks in their age spectra. However, their relative percentages of age peaks were quite different. Zircon ages were not evenly distributed at various grain sizes. Here, zircons of 70 Ma and 150 Ma were enriched in the finer grain sizes and the 1460/1420 Ma and 1690 Ma zircons were enriched in the coarse grain sizes. This suggests that detrital zircon age bias caused by hydrodynamic grain size sorting exists in ancient sandstone and shale. The different age distribution of the Trinidad sandstone and Pierre shale indicates that studies only focusing on detrital zircon geochronology of sandstone might underestimate or overestimate the age provenance signature of a depositional environment. Shale/mudstone need also be considered. In the second case study, the Eocene Wasatch Formation, Luman Tongue Member and Tipton Member of the Green River Formation deposited in a lacustrine environment were sampled in the Green River Basin (Wyoming, USA). Differences in percentages of age peaks between two Wasatch Formation sandstones were controlled mainly by grain size sorting, followed by sorting by grain shape (aspect ratio) and source differences (Th/U). Wasatch Formation mudstone lacks large peaks of 2460 and 2720 Ma in Wasatch Formation sandstone, and its zircon age population is controlled by grain size and source (Th/U). This indicates that only using Wasatch Formation sandstone for provenance may overestimate the 2460 and 2720 Ma age sources. The relative percentage differences in peak zircon ages for the two Luman Tongue Member sandstones were controlled mostly by grain size sorting, but also sorting by shape (aspect ratio) and U concentration (grain density or source). The most pronounced hydrodynamic sorting is for zircons in Luman Tongue Member mudstone, which have a much higher proportion of 175-750 Ma age zircons than in sandstones, caused by grain size sorting and different sources (Th/U). This indicates that only using Luman Tongue Member sandstones for provenance study may miss or underestimate a very important Appalachian/Ouachita source. In the third case study, the Mississippian Upper Exshaw and Lower Banff Formations, deposited into a marine shelf and basin environment were sampled in the Alberta Basin (Alberta, Canada). Pairs of coarser and finer-grained samples from each Formation had overlapping prominent age peaks, but two important differences were noted – (1) the relative percentages in peaks abundances were different; (2) some minor peaks only existed in one sample of the pair. In the two samples from the Lower Banff Formation, their relative percentage differences in peak ages appeared to be controlled mostly by grain size sorting. The presence and absence of some minor age peaks were controlled mostly U concentrations, reflecting differences either in grain density orsource. Two samples of the Upper Exshaw Formation had percentage differences in age peak influenced mostly by grain size sorting, then source differences (Th/U) and shape sorting (aspect ratio). For provenance implication, the coarser sample in in Lower Banff Formation has a larger Yavapai-Mazatzal Terrane provenance than the finer sample. The coarser sample in the Upper Exshaw Formation has a larger Midcontinent Granite- Rhyolite Terrane provenance than the finer sample. This thesis clearly underscores the desirability of conducting more detailed research into the impact of detrital zircon grain properties in different lithologies and the necessity to include as many grain sizes as possible in provenance investigations.

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