Earliest Arikareean (later early Oligocene) Iniyoo local Fauna from Chilapa Formation of Santiago Yolomécatl area in northwestern Oaxaca, southern Mexico

Abstract

Strata containing the Iniyoo Local Fauna (LF) crop out in the outskirts of Santiago Yolomécatl in the Yolomécatl Subbasin of the southeastern Tlaxiaco Basin in northwestern Oaxaca, southern Mexico. The assemblage is from a 325-m-thick fluviolacustrine unit previously assigned to the Yanhuitlán Formation and then the Yolomécatl Formation, which was restricted to the Yolomécatl Subbasin and its immediate vicinity. However, recent field investigations and laboratory analyses indicate the unit is, instead, a marginal facies of the dominantly lacustrine Chilapa Formation, which unconformably overlies the Yanhuitlán Formation where intervening volcanic units are absent. Consequently, Yolomécatl Formation is abandoned in favor of Chilapa Formation on the bases of synonymy and priority. Similarly, San Marcos Andesite and Nicananduta “Group” were applied to the same exposures of an andesitic unit interfingering with the Chilapa Formation. Accordingly, Nicananduta “Group” is abandoned and replaced by San Marcos Andesite for the same reasons.The Iniyoo LF is earliest Arikareean (Ar1) (i.e., later early Oligocene) in age, not latest Uintan to late Chadronian (i.e., later early middle to latest Eocene), based on the first appearance datum for the amphicyonid Mammacyon and the last appearance datums of the tayassuid Perchoerus probus and the rhinocerotid Subhyracodon. That age assignment is supported by (1) corrected potassium-argon (K–Ar) dates of 28.939 ± 0.6 and 30.376 ± 0.6 Ma for the Yucudaac Andesite's lowermost flow or lower tongue, which underlies the Chilapa Formation north of the Yolomécatl Subbasin, (2) a corrected 40argon/39argon (Ar/Ar) date of 27.829 ± 0.716 Ma for the correlative Nicananduta “Group” (= San Marcos Andesite), which overlies the Chilapa Formation in the Yolomécatl Subbasin, and (3) uranium-lead (U–Pb) maximum depositional ages of 30.6 ± 0.77 and 30.62 ± 0.67 Ma for detrital zircon grains from sandstone beds occurring above the bottom and top, respectively, of the nearly 88.4-m-thick vertebrate fossil-bearing interval. The K–Ar and Ar/Ar dates further constrain the age of the Iniyoo LF to an interval spanning about 27.83–28.94 Ma ago.A corrected Ar/Ar date of 40.561 ± 1.009 Ma for potassium-feldspar crystals in a bed originally identified as a silicified tuff sheet and later determined during the current investigation to be a sandstone bed was bracketed by the two sandstone beds subjected to U–Pb dating analyses. However, the last Ar/Ar age is much too old when compared with the two U–Pb dates for the Chilapa Formation and the three K–Ar and Ar/Ar for volcanic units bracketing the formation. Presumably, the dated material represented volcanic debris reworked from a substantially older unit. Consequently, the Ar/Ar date and any land mammal age assignment based on it are considered highly unreliable.