PROVENANCE OF OLIGOCENE-AGED VOLCANIC CLASTS FROM THE SAN LUIS BASIN, NEW MEXICO, USA: REGIONAL DISTRIBUTION AND GEOLOGIC IMPLICATIONS

Understanding the paleogeography of a region provides insight into the timing of geologic events, such as uplift of mountain ranges, changes to drainage patterns, and the timing and source(s) of basin sedimentation. One of the most efficient methods of paleoreconstruction is through provenance determination. This study aims to determine the provenance of a suite of Oligocene-aged volcaniclastic deposits from the San Luis Basin (SLB) in New Mexico as part of a larger study conducted by the USGS in Denver to fully reconstruct the Cenozoic history of the Southern Rocky Mountains.

Two competing hypotheses currently exist regarding the source of SLB clasts. Previous studies suggest that clasts from the Oligocene-aged Abiquiu Formation (AF) are solely derived from the Latir Volcanic Field (LVF) based on ⁴⁰Ar/³⁹Ar geochronology and paleogeographic interpretations. However, additional studies argue that AF clasts are primarily derived from the San Juan Volcanic Field (SJVF) based on additional intermediate-composition clasts that could indicate mixed provenance. Proposed research will test the competing hypotheses on the origin of SLB clasts, and the sole presence of Latir-derived clasts will be confirmed or denied.

To determine the provenance of these clasts, a petrographic, mineral chemistry, and geochronological study of volcanic clasts will be completed following detailed field sampling of the SLB. These data will then be compared with published data on volcanics from the proposed source terranes in the SJVF, the San Luis Hills (SLH), and the LVF.

If volcanic clasts from the SJVF/SLH are identified in the AF, paleodrainage patterns during the late Oligocene and early Miocene flowing south from the San Juan Mountains followed a path either through or around the Tusas Mountains and contributed to early basin-fill deposits of the Rio Grande Rift (RGR) instead of flowing solely west onto the Colorado Plateau (CP). If SJVF/SLH clasts are present within the interior of the Tusas, then the paleohigh was not an impenetrable geographic barrier throughout the Oligocene/Miocene. If SJVF/SLH clasts are not present within the interior of the Tusas but are in the Abiquiu Embayment, then a paleodrainage flowing onto the CP was redirected back into the RGR. Additionally, SJVF/SLH clasts would indicate ages older than the proposed 25 Ma.