NEW STRATIGRAPHIC AND PROVENANCE CONSTRAINTS FROM LOWER(?) EOCENE SYNOROGENIC STRATA OF THE SAN JOSE FORMATION, SOUTHEASTERN SAN JUAN BASIN, NEW MEXICO

Authors: Thomas Valenzuela, Brian Hampton, Kevin Hobbs

New Mexico Geological Society Guidebook, 75 th Fall Field Conference

Date: October 11, 2025

Presentation Slides

Abstract: Sedimentologic, stratigraphic, and provenance data from siliciclastic nonmarine strata of the lower(?) Eocene San Jose For- mation provide new insights on the depositional style, stratigraphic relationships, and sandstone modal composition trends across four key members of the San Jose Formation that crop out in the southeastern part of the San Juan Basin in northwestern New Mexico. Presented here are new measured stratigraphic sections and sandstone modal composition trends from N = 18 samples (n = 7,200 point counts) from the Cuba Mesa, Regina, Llaves, and Tapicitos Members. New measured sections record near-continuous sedimentation during deposition of the basal Cuba Mesa Member and overlying Regina Member as well as in the younger Llaves and overlying Tapicitos Members. The Cuba Mesa Member marks the base of the San Jose Formation and shares similar sedimentologic characteristics with the overlying Regina Member. Both members consist largely of laterally extensive lenticular and tabular fluvial sandstone bodies (up to 10 m thick) that are encased in tabular floodplain siltstones that preserve isolated fossil wood including entire fossil tree trunks near the base of the Regina. The younger Llaves and Tapicitos Members exhibit laterally extensive lenticular and tabular fluvial sandstone bodies that are similar to the Cuba Mesa and Regina Members with thicknesses not exceeding 5 m. Both upper members preserve thick succession (up to 15 m) of floodplain siltstone and isolated claystone. The sandstone bodies within all four members of the San Jose Formation have regular occurrences of horizontal and ripple cross-stratification, small-scale planar cross-stratification (<0.25 m thick), and soft-sediment deformation. We note that occurrences of larger-scale cross-stratification and evidence for in-channel bar forms throughout the San Jose Formation are extremely rare as are occurrences of conglomerate (not including mudstone rip-up clasts). The Cuba Mesa and Regina Members share similar detrital sandstone modes and consist primarily of arkosic to lithic-arkosic sandstone (quartz [Q]: 67%, feldspar [F]: 26%, lithic fragments [L]: 7%), whereas the Llaves and Tapicitos Members record higher relative percentages of quartz and consist largely of subarkose to sublitharenite sandstone (Q: 85%, F: 9%, L: 6%). Based on new data presented here, we favor a model for the San Jose Formation where sedimentation was uniform and characterized primarily by vertically aggrading channels (with little to no lateral migration) that avulsed onto adjacent well-developed, vegetated floodplains. Detritus in the Cuba Mesa and Regina Members was likely derived from a combination of recycled orogen and arc sources (Sevier fold-and-thrust belt and Cordilleran arc) and Laramide basement uplifts. Quartz-rich detritus in the Llaves and Tapicitos Members has very little arc signature and was likely derived primarily from Precambrian basement-cored Laramide uplifts.