NEW Ar40/Ar39 DETRITAL SANIDINE GEOCHRONOLOGY FROM THE EOCENE SAN JOSE FORMATION, EASTERN SAN JUAN BASIN, NORTHWESTERN NEW MEXICO

The San Juan basin (SJB) in NW New Mexico has been extensively studied to determine the timing of Laramide deformation, provenance, and sediment dispersal trends from Jurassic-to earliest Paleogene strata. However, little is known about the sources and mechanisms driving deformation and erosion during the deposition of the Eocene synorogenic strata of the San Jose Formation. The San Jose Formation comprises four units: (1) the basal Cuba Mesa Member (sand- and gravel-dominated), (2) the silt-dominated Regina Member, (3) the sand-dominated Llaves Member, and (4) the sand- and silt-dominated Tapicitos Member. Here, we present new ⁴⁰Ar/³⁹Ar detrital sanidine data from N=8 samples of the San Jose Formation.

The Cuba Mesa Member shows detrital sanidine peaks from 180-160 Ma and 80-63 Ma, with primary peaks at 74 Ma and a secondary peak at 81 Ma; the youngest grain is 61 Ma. The Regina Member exhibits peak ages between 180-220 Ma, 78-82 Ma, and 74-76 Ma, with the youngest grain at 65 Ma. The Llaves Member peaks between 260-280 Ma, with the youngest grain at 130 Ma. The Tapicitos Member peaks between 380-480 Ma, with the youngest grain at 360 Ma. Both Llaves and Tapicitos show a lack of late Mesozoic grains present in the lower members. Additionally, all members contain ages between 500-1000 Ma, with the youngest grain at 61 Ma in the Cuba Mesa Member.

Detrital sanidine ages older than 300 Ma likely represent other K-feldspar varieties such as microcline and orthoclase. Ages between 300-150 Ma overlap with the Sevier fold-and-thrust belt, while ages between 150-90 Ma overlap with the Cordilleran arc. Ages younger than 90 Ma likely derive from the Cordilleran arc and Laramide volcanism. The absence of grains from syn-depositional Eocene sources could be due to: (1) unfavorable paleoriver and climate patterns for sanidine deposition; (2) insufficient sample size due to low amounts of sanidine in the basin; (3) overwhelming presence of basement K-feldspars; or (4) a combination thereof. Younger grains are also less common in upper members, suggesting regional sources in the Cuba Mesa and Regina Members (Cordilleran arc, Sevier fold-and-thrust belt) and local sources in the Llaves and Tapicitos Members (microclines and orthoclases from nearby basement Laramide uplifts). These data support a model where the Llaves and Tapicitos Members were increasingly sourced from local Laramide uplifts.