Primarily based on biostratigraphy, Wilcox Group strata along the eastern Gulf Coastal Plain are bracketed from the late Paleocene to early Eocene. However, reported detrital zircon (N=33; n≈100) from Wilcox samples sporadically contain Eocene grains, raising the possibility that the entire group may be Eocene. We present a high n-value (n=355) LA-ICPMS detrital zircon U-Pb sample from the basal Wilcox Group (Nanafalia Formation, correlative strata to the Gravel Creek Sand Member) in northeastern Mississippi to determine the maximum depositional age (MDA), advance regional stratigraphic correlations, and integrate age data with geologic settings. Stratigraphic architecture suggests that basal Wilcox strata represent paleovalley fill following a period of lowstand incision, and that material was derived via a continental-scale drainage. The youngest single grain (YSG) yields an Eocene MDA of 53.8 ± 2.6 Ma, while weighted mean averages of the two youngest overlapping populations (YGC1σ 2+ and YGC2σ 3+) yield Paleocene ages of 56.3 ± 1.1 Ma (MSWD = 0.52) and 58.5 ± 0.7 Ma (MSWD = 0.89), respectively. Youngest probability peak (YPP), youngest statistical population (YSP), and maximum likelihood age (MLA) determinations all produce Paleocene MDAs that align well with biostratigraphy and previously reported glauconite K-Ar ages. Our results show that as the number of analyses per sample increases, MDAs based on YSG, YGC1σ, and YGC2σ are shifted to younger ages that deviate from the true depositional age. The MLA (59.1 ± 1.36 Ma) represents a reliable, impartial approach to determining the MDA and can be corroborated by the YPP and YSP.
Jackson et al. (Wed,) studied this question.