DETRITAL SANIDINE AND PALEOCURRENT CONSTRAINTS ON DEPOSITION OF THE PALEOCENE NACIMIENTO AND ANIMAS FORMATIONS, SAN JUAN BASIN, NEW MEXICO

In the southern San Juan Basin, the Paleocene Nacimiento Formation is a mudstone-dominated fluvial succession sandwiched between the lower Paleocene Kimbeto Member of the Ojo Alamo Sandstone and the Cuba Mesa Member of the lower Eocene San Jose Formation, both sandstone-dominated fluvial deposits. The Nacimiento consists of three members (ascending): the Arroyo Chijuillita, the Ojo Encino, and the Escavada. In contrast to the southerly paleoflow exhibited by the Ojo Alamo and the San Jose, the Nacimiento Formation exhibits evidence of diverse paleoflow (based on ~750 measurements). During deposition of the Arroyo Chijuillita Member, a major east-flowing paleoriver entered the basin south of Nageezi. Paleocurrents from this member in adjacent areas appear to reflect contributory drainages to this paleoriver. Paleoflow was northward during deposition of the Ojo Encino Member, but switched to southeastward during Escavada deposition. In the central and northern San Juan Basin, paleoflow in the Nacimiento and the partly time-equivalent Animas Formation was generally toward the south. An important exception is where a newly discovered, east-flowing drainage exited the northeastern part of the basin ~15 km south of Dulce. There, thick basal sandstones of the Animas Formation record a major fluvial system that filled a broad paleovalley carved into the Upper Cretaceous Lewis Shale. The paleovalley faced a structurally low area in the Archuleta anticlinorium to the east. Age constraints for the Animas in this region are sparse. However, ⁴⁰Ar/³⁹Ar dating shows the presence of a young population of 65.38 ± 0.08 Ma detrital sanidines (DS) from mudstones intercalated with the basal Animas sandstone. These DS grains are indistinguishable in age and K/Ca ratio from sanidines of the Horseshoe ash, which is exposed 13 m above the base of the Arroyo Chijuillita Member in the southern part of the basin. The DS maximum depositional age suggests the basal Animas sandstone near Dulce may represent the fluvial exit from the basin during Arroyo Chijuillita and/or Ojo Encino time. Despite extensive reconnaissance, no other potential exit points for these fluvial units have been found. Alternatively, the basal sandstone may be correlative with the Ojo Alamo Sandstone. If so, the Ojo Alamo must be time-transgressive (younger near Dulce).