Grain size and transport biases in an Ediacaran detrital zircon record

Detrital-zircon records of provenance are used to reconstruct paleogeography, sediment sources, and tectonic configuration. Recognition of biases in detrital-zircon records that result from grain-size-dependent processes adds new complexity and caution to the interpretation of these records. We begin by investigating possible size-dependent biases that may affect interpretation of detrital-zircon provenance records in an idealized sedimentary system. Our modeling results show that settling and selective entrainment can differentially affect detrital-zircon spectra if an initial size variation between source zircon populations exists. We then consider a case study: a detrital-zircon record from Ediacaran to Terreneuvian strata of Death Valley, USA, with a focus on the Rainstorm Member of the Johnnie Formation. The detrital-zircon record of the Rainstorm Member shows that despite its unusual, heavy-mineral-rich character, the provenance of the unit is like other units in the succession. Size and density measurements of the grains of the deposit suggest that its enriched heavy-mineral suite is best explained through concentration by selective entrainment and winnowing. The relationship between detrital-zircon grain size and age for samples from the Johnnie Formation are consistent with grain-size influence on the interpretation of provenance, especially for large Grenville-age (1.0-1.2 Ga) zircons. Grain size can exert significant bias on a provenance interpretation and must be accounted for in provenance studies.