40Ar/39Ar Sanidine Chronostratigraphy of K-Pg Boundary Sediments of the San Juan Basin, NM

Abstract

New 40Ar/39Ar dating of a key ash horizon and detrital sanidines from the San Juan Basin, NM (SJB) demonstrate that the lower part of the Ojo Alamo formation (Naashoibito) is not older than 66.5±0.2 Ma thereby revising down some earlier age estimates by ~2-3 Ma. In addition, a 65.70±0.03 Ma sanidine date from the Nacimiento Formation that stratigraphically overlies a recently reported U/Pb date of a dinosaur bone at 64.8±0.9 Ma challenges the accuracy of the U/Pb system to directly date bones. The SJB strata holds a globally important record of mammal and dinosaur evolution and extinction, but the lack of a precisely and accurately determined chronostratigraphy limits complete understanding of this record. We use a combination of high precision detrital sanidine dates from Cretaceous and Paleocene strata and a newly discovered ash layer to advance our understanding of this record. Three samples of the dinosaur-bearing Naashoibito member yields variable detrital sanidine age populations. The youngest group of one sample is 66.5±0.2 Ma and variations between age populations could indicate unconformities or variable provenances in the Naashoibito. A depositional age at no greater than 66.5 Ma demonstrates that the Naashoibito's mammal fauna, the Alamo Wash local fauna, are significantly younger than earlier suggestions of 69 Ma. Three samples (2 sandstones, 1 ash) at stratigraphically similar levels within the Nacimiento Formation in the De-na-zin Wilderness area record concordant ages. The ash is 65.70±0.03 Ma (n=59 of 61 sanidines). The fact that the two sandstone samples contain minimum detrital populations that are equal in age (65.78±0.11, 65.62±0.07 Ma) to the ash demonstrate that detrital sanidine can yield the depositional age of sedimentary rocks. These units are stratigraphically above a U/Pb dated dinosaur bone (64.8 ± 0.9 Ma) and thus are not compatible. We suggest that the U/Pb date is not accurate and the dinosaur bone cannot be younger than 65.7±0.03 Ma. Importantly, this 65.7 Ma ash occurs between the fossil horizons that yield middle and late Puercan age fossils. This places significant and surprisingly old age constraints on Puercan faunas if the K-Pg boundary is 66.0 Ma. At Mesa de Cuba, detrital dating of a layer ~61 m above the Ojo Alamo (OA) contact gives a maximum depositional age of 64.24±0.30 Ma. This age and error suggests a depositional rate between 17 and 36 m/m.y. and would place the top of the OA between 65.9 and 67.8 Ma and consistent with the youngest found detrital sanidines in the OA. This further indicates that the OA is significantly less than 69 Ma. Ar/Ar data are at 1σ, relative to a 40K total decay constant of 5.543-10/a and Fish Canyon sanidine at 28.294 Ma.