New Mexico Geological Society Annual Spring Meeting — Abstracts

The San Juan basin (SJB) of northwest New Mexico has received a considerable amount of study focused on determining the timing of Laramide deformation, provenance, and basin-scale sediment dispersal trends from Jurassic-earliest Paleogene strata. However, little is known about the sources and driving mechanisms for deformation and erosion that resulted in the deposition of Eocene synorogenic strata of the San Jose Formation in the SJB. The San Jose Formation has been subdivided into four units that include: (1) the basal Cuba Mesa Member (sand- and gravel-dominated facies), (2) the overlying silt-dominated Regina Member, (3) the sand-dominated Llaves Member, and (4) the youngest (sand and silt dominated) Tapicitos Member. Presented here are N=8 new ⁴⁰Ar/³⁹Ar detrital sanidine samples from the San Jose Formation.

The basal Cuba Mesa Member of the San Jose Formation contains a variety of peaks ranging from 180-160 Ma and 80-63 Ma, a primary peak at 74 Ma, and a secondary peak at 81 Ma. The youngest grain obtained in the Cuba Mesa Member is at 61 Ma. The overlying Regina Member contains peak ages between 180-220 Ma, 78-82 Ma, and 74-76 Ma, with the youngest grain at 65 Ma. The Llaves Member has a peak between 260-280 Ma with the youngest grain at 130 Ma. The Tapicitos Member has a peak between 380-480 Ma with the youngest grain at 360 Ma. Both the Llaves and Tapicitos Members have a noticeable lack of late Mesozoic grains that are present in the two lower members. In addition to the peak ages, all four members also contain occurrences of ages that fall between 500-1000 Ma. The youngest grain in these members is in the Cuba Mesa Member, with an age of 61 Ma.

Detrital sanidine ages that are older than 300 Ma are likely other varieties of K-feldspar, such as microcline and orthoclase. Ages between 300-150 Ma overlap with the Sevier fold-and thrust belt. Ages between 150-90 Ma overlap with the Cordilleran arc. Ages that are younger than 90 Ma likely represent detritus from the Cordilleran arc and Laramide volcanism. The lack of grains from syn-depositional Eocene sources could be explained through numerous means: (1) paleoriver and climate patterns did not favor the deposition of sanidines into the basin; (2) there are a small amount of sanidines in the basin and a larger sample size is required; (3) basement K-feldspars overwhelm the sanidines in the samples; or (4) a combination thereof. Younger grains are also increasingly less common in the upper members of the San Jose Formation, and more common in the lower members. This could represent more regional sources in the Cuba Mesa and Regina Members (Cordilleran arc, Sevier fold-and-thrust belt) and more local sources in the Llaves and Tapicitos Members (microclines and orthoclases sourced from nearby basement Laramide uplifts). These data support a model where the overlying Llaves and Tapicitos Members were sourced increasingly from local Laramide uplifts.