Abstract

The Tertiary sedimentary rocks of the San Juan Basin comprise the Paleocene Ojo Alamo Sandstone, Nacimiento Formation, and Animas Formation (excluding the Cretaceous McDermott Member) and the Eocene San Jose Formation. The oldest Tertiary rock unit in the basin is the Ojo Alamo Sandstone. The Ojo Alamo is late early Paleocene age in the southern part of the San Juan Basin where it rests on Campanian age rocks indicating a hiatus of as much as 11 million years. The hiatus diminishes in magnitude northwestward across the basin but appears to be significant there also where at least the upper part of the Mastrichtian is absent. The pre-Ojo Alamo Sandstone erosion episode resulted from basin-wide uplift. This uplift was the first major tectonic event of the Laramide Revolution in the San Juan Basin area following 25 to 30 million years of nearly uninterupted deposition of marine and continental rocks as the Late Cretaceous epicontinental sea repeatedly transgressed and regressed across the basin area.

The Animas and Nacimiento Formations are apparently time-equivalent. The Animas is present only in the northern part of the basin and the Nacimiento is present in the remainder of the basin. The Animas contains an abundance of volcaniclastic material derived from the San Juan Mountains area. Downwarping of the northern part of the basin began as the Animas Formation was being deposited. The Nacimiento Formation represents deposition in the southern part of the basin of the finer-grained component of the clastic load carried by the same streams that deposited the Animas in the north.

The San Jose Formation was deposited as the basin continued to downwarp in Eocene time and the eastern and western rims of the basin started to form. Younger Cenozoic rocks were probably deposited on top of the San Jose as the basin achieved its final structural configuration, but these younger rocks are no longer present because of erosion accompanying uplift of the Colorado Plateau in late Tertiary time.