CONSTRAINING DEPOSITIONAL HISTORY, STRATIGRAPHIC RELATIONSHIPS, AND PROVENANCE FROM EOCENE SYNOROGENIC STRATA OF THE SAN JOSE FORMATION, EAST-CENTRAL SAN JUAN BASIN, NORTHWESTERN NEW MEXICO

Although there has been extensive prior work on Jurassic–Paleocene strata in the San Juan Basin, little is known about the youngest, Early Eocene strata of the San Jose Formation which record the final deformational phase of the Laramide orogeny in northwestern New Mexico. Presented here are new sedimentologic and stratigraphic data (e.g., measured stratigraphic sections and facies analysis) as well as provenance data (e.g., U-Pb detrital zircon geochronology, sandstone modal composition, clast counts, and paleocurrent trends) from four previously identified members of the San Jose Formation.

The basal San Jose Formation is marked by the Cuba Mesa and Regina members, both of which consist primarily of lenticular, amalgamated, sandstone bodies (with isolated pebble and cobble beds) that are bound above and below by fossil-wood/tree-bearing siltstone and claystone. The Regina member is distinguished from the Cuba Mesa by a distinct, fine-grained succession of interbedded red, pink, and yellow siltstone and mudstone. Clast counts from both members consist primarily of quartzite, lithic volcanic clasts, and fossil wood debris with isolated occurrences of quartz, chert, and granite. Paleocurrent data from the Cuba Mesa and Regina members show paleoflow primarily to the south/southeast. Primary peak U-Pb detrital zircon ages from both units fall between 1689–1693, 158–185 Ma with secondary peaks occurring between 1404–1406 Ma and at 231, 188, and 86 Ma.

The two youngest members of the San Jose include the Llaves and Tapacitos members, both of which are made up of lenticular, amalgamated sand bodies that range in size from medium-to coarse-grained sandstone to as large as granules. Amalgamated sandstone units are bound above and below by thick siltstone and claystone successions. Both members are dominated by quartz granules and neither member appears to have evidence of fossil wood occurrences. The Tapacitos member contains abundant mica (including outsized flakes of muscovite). Paleocurrent trends of both members show paleoflow to the west/northwest. Primary peak U-Pb detrital zircon ages from both of the younger units fall between 1702–1708 Ma and at 163 and 66 Ma, Secondary peaks occur at 1426, 205, 96 and 66 Ma. Both upper units also contain occurrences of ages that fall between 650-225 and 1200-1000 Ma.