Stratigraphy, sedimentology, and paleontology of the Lower Eocene San Jose Formation in the central portion of the San Juan Basin, northwest New Mexico

The lower Eocene San Jose Formation in the central portion of the San Juan Basin of New Mexico consists of the Cuba Mesa, Regina, Llaves, and Tapicitos Members. Well-log data indicate that, from its 100 m thickness, the Cuba Mesa Member thins towards the center of the basin and pinches out to the northeast by latitude 36 deg. 40' N, longitude 107 deg. 19' W. The Regina Member crops out most extensively in the study area and decreases in sandstone-to-mudrock ratio to the north. The Llaves and Tapicitos Members occur only in the highest locales of the area, are thin due to erosion, and are not mappable as separate units.Well-log data and 1275 m of measured sections in the Regina, Llaves, and Tapicitos Members indicate that strata in the study area are made up of approximately 35% medium- to coarse-grained sandstone and 65% fine-grained sandstone and mudrock. Sedimentology and sediment-dispersal patterns in the study area suggest deposition by generally south-flowing streams that had sources to the northwest, northeast, and east. Low-sinuosity, sand-bedded, braided(?) streams shifted laterally across approximately 1 km wide channel belts to produce sheet sandstones that are prominent throughout the formation. Levees separated channel environments from floodplain and local lacustrine areas. Avulsion relocated channels periodically to areas on the floodplain, resulting in the typically disconnected sheet sandstones within muddy overbank sediments of the Regina Member.Fossil plants from lacustrine strata of the Regina Member at Santos Peak are the only flora described from the San Jose Formation. They indicate a humid, forested environment in the depositional basin. Fossil vertebrates from the Regina Member include the mammal taxa Phenacolemur praecox, Oxyaena forcipata, Ectoganus gliriformis, Esthonyx bisulcatus, Coryphodon molestus, Hyopsodus miticulus, Phenacodus primaevus, and Hyracotherium angustidens. These taxa support correlation with the Almagre local fauna of the Regina Member in the east-central San Juan Basin. They are thus indicative of a middle Wasatchian (Lysitean) age, about 53 Ma.