New Mexico Geological Society Annual Spring Meeting — Abstracts

STATEMAP geologic mapping northeast of Santa Fe elucidates stratigraphic and structural relationships recording extensional foundering of the local Laramide uplift by middle Oligocene. Central to this hypothesis are new data on the age and provenance of the Bishop's Lodge Member (BLM) of the Tesuque Fonnation (Santa Fe Group rift-basin fill). The BLM consists of at least two volcaniclastic intervals (each 10-60 m thick) within non volcaniclastic alluvium of the Nambe Member (NM). Typically interpreted as the distal equivalent of the Abiquiu and Picuris Frns. derived from northern New Mexico and Colorado, new observations demonstrate that the BLM is locally derived and correlates to the upper Espinaso Fm.: (1) 60-cm clasts are too large for distal Abiquiu and Picuris Fms.; (2) clast sizes coarsen southward; (3) clast mineralogy is consistent with late-stage alkalic Espinaso magmatism; (4) tephra with 3-cm-diameter lapilli requires a local source; (5) 30.45±0.16 Ma tephra(⁴⁰Ar/³⁹Ar date on biotite by NM Geochronology Laboratory, NMIT) demonstrates correlation to the Espinaso Fm. Olivine basalt in the NM above the BLM may correlate to post-Espinaso mafic lava near La Cienega. Furthermore, the BLM locally overlies at least 400 m of NM indicating significant Oligocene deposition of rift-basin fill. These lower NM strata thicken on the hanging walls of east-facing normal faults and rest on Precambrian rocks; Paleozoic strata are locally preserved on the footwalls, suggesting inversion of Laramide reverse faults. The lowest NM strata have abundant Paleozoic clasts (25-60%, typically 50-60%) compared to NM strata between BLM volcaniclastic intervals (25-35%) and NM beds above the BLM (<10%, typically <1 %). Although possibly recording unroofing of Paleozoic strata now absent from the Santa Fe Range, there are also arguments for prior removal of Paleozoic rocks during Laramide uplift. I speculate that most Paleozoic clasts originated east of the Picuris-Pecos fault (PPF) and suggest the following testable scenario. Middle (or early?) Oligocene inversion of Laramide compressional/transpressional structures initiated Santa Fe Group sedimentation. East-side-up motion on the PPF provided Pennsylvanian clasts to west-flowing drainages; onset of tectonic inversion permitted northward progradation of volcaniclastic aprons and spread of mafic lava from the La Cienega (and Cerrillos?) areas. Significant late Oligocene and later westward tilting of the Espanola Basin was accommodated on the east by west-side-up motion on the PPF, forming the headwaters of the Pecos River and abruptly terminating westward transport of Paleozoic detritus to the Espanola Basin.