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

Abstract

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.

Progradational‐to‐retrogradational styles of Palaeogene fluvial fan successions in the San Juan Basin, New Mexico

“…Redrawn from Dickinson et al (1988), Lawton (2008) and Cather (2004). (c) Palaeogene stratigraphy and time constraints, modified from Smith and Lucas (1991). (d) Geological map of the SJB, areas selected for detailed investigations and palaeocurrent measurements.…”

“…The San Jose Formation is the most extensively preserved and exposed Eocene stratigraphic unit in New Mexico (Smith, 1992; Smith & Lucas, 1991). Like the underlying Nacimiento Formation, it is composed of terrestrial fluvial deposits with a prevailing north‐to‐south palaeoflow (Figure 1d; Smith, 1988).…”

“…The basal sandstone of the Cuba Mesa Member is continuous nearly basin wide over an area of 8000 km 2 (Smith, 1988). In some parts of the basin, the contact between the Nacimiento Formation and the Cuba Mesa Member of the San Jose Formation can be difficult to distinguish because the Escavada Member of the Nacimiento Formation contains arkosic sandstones that are petrographically similar to those of the overlying Cuba Mesa Member (Baltz, 1967; Smith & Lucas, 1991) and are composed of similar channel‐fill facies (Zellman et al., 2020). Like the underlying Escavada Member, no age‐diagnostic fossils or dateable materials have been discovered in the Cuba Mesa Member, leaving the deposits of both the upper part of the Nacimiento Formation and the lower part of the San Jose Formation poorly constrained in time (Figure 1c).…”

“…It has long been propagated in the literature that there is a large unconformity spanning a time interval ≥5.6 m.y. (Fassett et al., 2010) separating the Nacimiento and San Jose Formations in at least the southern SJB (Baltz, 1967; Barnes et al., 1954; Cather et al., 2019; Smith & Lucas, 1991). Some researchers have interpreted the contact as an angular unconformity (Baltz, 1967; Cather et al., 2019; Smith & Lucas, 1991; Williamson & Lucas, 1992) due to the abrupt change from mudrock‐dominated lithofacies to sandstone‐dominated lithofacies and localized observations of angular discordance between the Nacimiento Formation mudrock layers and the base of the overlying Cuba Mesa Member channels.…”

“…It has also been suggested that the Nacimiento–San Jose Formation contact may be transitional in nature due to interpretations that the contact is conformable in the northern SJB (Barnes et al., 1954; Cather et al., 2019; Smith & Lucas, 1991; Stone, 1983) and either unconformable or disconformable in the southern SJB (Barnes et al., 1954; Cather et al., 2019; Smith & Lucas, 1991; Stone, 1983) (Figure 1c). Evidence cited for a conformable contact in the northern SJB include inter‐tonguing of the Nacimiento Formation and lowermost part of the San Jose Formation (Cuba Mesa Member) with the uppermost part of the upper Palaeocene Animas Formation, where the presence of Tiffanian strata (61.7–56.8 Ma; Alroy et al., 2000) indicates that the upper Palaeocene strata are preserved below and/or laterally adjacent to the San Jose Formation (Figure 1c; Smith, 1988; Smith & Lucas, 1991; Cather et al., 2019). Evidence cited for an unconformable or disconformable contact in the southern SJB include a fossil gap of at least 6 m.y.…”