Grain‐Size Control on Detrital Zircon Cycloprovenance in the Late Paleozoic Paradox and Eagle Basins, USA

Abstract

Detrital zircon U‐Pb data are routinely used to assess provenance in sedimentary systems. However, recent studies have suggested that zircon provenance interpretation may be biased by grain‐size‐dependent selective transport and deposition. In this study, we present zircon grain‐size and U‐Pb data from 11 sandstone and 11 mudstone samples from the late Paleozoic Eagle and Paradox basins to test whether apparent provenance changes between sandstone and mudstone samples can be explained by changes in zircon grain‐size alone. The Eagle and Paradox basins formed in association with the Ancestral Rocky Mountains (ARM). These basins provide an excellent system to test grain‐size/provenance relationships because they are filled with cyclic stratigraphy that records, via grain size and facies changes, flooding/exposure driven by interglacial/glacial cycles. These cycles are interpreted to have substantially altered sediment transport pathways and provenance of ARM‐related sedimentary rocks. Data presented here suggest that although zircon grain size does correlate to sample grain size in most samples, age spectra appear to be independent of grain size. As such, differences in age spectra between sandstone and mudstone samples appear to be the result of changes in relative inputs of two endmember sources: (1) grains eroded from adjacent basement‐cored ARM uplifts and (2) far‐traveled sediment sourced from outside the ARM system. The data presented in this study also provide a successful proof‐of‐concept test for a manual method of determining zircon grain size. This method is simple and straightforward and can be applied to any sample analyzed using the Chromium off‐line targeting software.

Significance of U-Pb detrital zircon geochronology for mudstone provenance

“…The broader significance and implications of our results are that U-Pb zircon provenance of mudstone reflects availability of volcanic and other fine-grained source rocks rather than simply distal sources. Mudstones will not record distal sources dominated by coarse-grained source rocks, as shown by the results of Leary et al (2020). U-Pb zircon provenance studies should routinely include mudstone because it may identify zircon derived from fine-grained source rocks more clearly than associated sandstones.…”

“…Sláma and Pedersen (2015) reported that U-Pb ages for fine-grained detrital zircon from metamorphosed siltstone and mudstone of the Caledonides in southern Norway were derived from far-traveled (∼2000 km) distal sources and thus complement the record of coarser detrital zircon from associated sandstone, which reflects the U-Pb zircon age profile of local sources. However, Leary et al (2020) found that only some mudstone units in the late Paleozoic Paradox and Eagle Basins, southwestern USA, contained substantial populations of finegrained zircon derived by long-distance transport from the Grenville and Appalachian orogens.…”

“…In contrast, mudstone and its metamorphic equivalents, which make up two-thirds of the sedimentary rock record (Schieber et al, 1998), have been dated only rarely by U-Pb detrital geochronology (e.g., Sláma and Pedersen, 2015;Leary et al, 2020). This is because mudstone commonly contains silt-sized detrital zircon grains (Totten and Hanan, 2007) that can be difficult to separate from clay mineral-rich rock matrices using heavy liquids (Hoke et al, 2014).…”

Two‐Dimensional Quantitative Comparison of Density Distributions in Detrital Geochronology and Geochemistry

“…Here we have demonstrated the utility of 2D quantitative comparison methods using paired zircon U‐Pb and Hf data, as it has not only become common in sediment provenance studies (e.g., Gehrels & Pecha, 2014) but it has also become a major lens to interpret secular changes in crustal growth, global erosion, and changes in tectonic styles (e.g., Belousova et al., 2010). The future is bright for 2D quantitative assessment, as incorporating a second dimension of information for detrital minerals is becoming more common using thermochronology (Reiners et al., 2005; Saylor et al., 2013), trace element geochemistry (e.g., Anfinson et al., 2016; McKenzie et al., 2018), as well as physical characteristics of zircon such as grain roundness (Decou et al., 2013; Sundell et al., 2018) or grain size (Leary et al., 2020). Moreover, the 2D methods described above are not limited to detrital applications.…”

“…Zircon is the mineral of choice in detrital geochronology as it is physically and chemically robust, refractory, can survive multiple erosional and/or tectonic cycles, and incorporates abundant U with little initial Pb (Cherniak et al., 1997; Speer, 1980; Stacey & Kramers, 1975). In addition to being a useful mineral for geochronology, zircon can also be paired with secondary information such as (U‐Th)/He or fission track thermochronometry for determining thermal histories (e.g., Reiners, 2005; Stockli, 2005), trace element geochemistry for fingerprinting sediment sources (e.g., McKenzie et al., 2018; cf., Hoskin & Ireland, 2000), Hf analysis for understanding crustal evolution and the rate of crustal growth through time (e.g., Belousova et al., 2010; Roberts & Spencer, 2015), or physical characteristics of zircon such as grain roundness (Decou et al., 2013; Sundell et al., 2018) or grain size (Leary et al., 2020) for detailed analysis of sediment recycling and provenance.…”