New Mexico Geological Society Annual Spring Meeting — Abstracts

The fluvial San Jose Formation was deposited unconformably across pre-Eocene rocks during early Eocene Laramide tectonism in the San
Juan Basin and the San Juan, Brazos-Sangre de Cristo, and Nacimiento uplifts. The basal sheet sandstone of the San Jose, the Cuba Mesa Member is overlain by the mudrock-dominated Regina, and sandstone-dominated Llaves members. Most San Jose sarydstones have
sheet-like geometries. The sheets are composed of crosscutting multistoried channels that contain basal intraformational mudrock and
quartzite-conglomerate and very coarse-grained, large-scale trough cross stratified sandstone. Small-seale- and epsilon-cross stratification and mudrock-filled channels are locally preserved.

Sediment-dispersal patterns were derived from paleocurrent analysis of trough cross-stratified sandstones throughout the formation's 8500 km² outcrop area. Data for the Cuba Mesa and Regina members indicate paleotransport towards the south and southwest, away from the San Juan and Brazos-Sangre de Cristo uplifts, and around a subdued Nacimiento uplift. Llaves Member paleotransport directions are to the west and southwest, away from the Brazos-Sangre de Cristo uplift.

The Cuba Mesa Member is interpreted to have been laterally accreted by south-flowing streams during base-level stability following
tectonism and erosion. Disconnected sandstone bodies and angular unconformities along the eastern margin of the basin in the Regina Member, and west-directed paleoslopes in the Llaves Member suggest basin subsidence concurrent with uplift of the Brazos-Sangre de Cristo
and Nacimiento blocks. This early Eocene tectonism may have been a precursor to middle Eocene Galisteo-El Rito basin-initiation during
segmentation of the southern Sangre de Cristo uplift.