TESTING FOR THE EFFECTS OF SEDIMENT SORTING ON DETRITAL-ZIRCON AGE SPECTRA: CASE STUDIES FROM THE WOOD CANYON FORMATION (CAMBRIAN) AND STIRLING QUARTZITE (NEOPROTEROZOIC), SOUTHEASTERN CA

Detrital-zircon age spectra from sands collected in modern streams show an unexpected degree of variability, even at the scale of a single bedform, which prompts a concern of how representative the age spectrum from a single sample would be for characterizing the provenance of a stratigraphic unit. To explore this, two samples from the upper member of the Stirling Quartzite (uSQ) and four samples of the middle member of the Wood Canyon Formation (mWCF), were collected to test whether sedimentary rocks with different grains sizes and sorting characteristics that were collected from individual sets of cross-stratification within single alluvial channels have similar detrital zircon age populations. These stratigraphic units were selected because they show differences in provenance, fluvial channels contain abundant easily identified sets of cross-stratified fluvial sandstone with a range of grain size and sorting values representative of deposition in shifting flow regimes, and the depositional ages of both units are known. Sandstone samples vary in textural characteristics: grain sizes range from fine-to-coarse sand, sorting ranges from moderately well to poor, and sedimentary structures vary in scale and geometry. Analyzed whole detrital zircons in samples of the uSQ range in long-axis dimension from 67 - 288 μm; analyzed mWCF samples contain detrital zircons with similar long-dimension measurements from 80 - 300 μm. Pairs of age spectra from each unit were compared using Kolmogorov-Smirnov testing, overlap, and likeness, all of which classify samples from each channel as statistically similar. Comparing the size, sphericity, and roundness of individual whole detrital zircons to their ages, yields no correlation. Differential sorting and other textural parameters do not effect the age populations in either unit. The results indicate that the while the separation of zircons by age may occur from sorting during transport in some alluvial systems, that is not a generally applicable rule. From the perspective of the Laurentian margin, mWCF strata likely contain detrital zircon derived from uplift of the Ouachita rift-flank (located in present-day Texas and Oklahoma), which may have generated a pulse of Grenvillian sediment entering braided-alluvial systems flowing toward the Cordilleran margin.