The Blanco Basin Formation (Eocene), San Juan Mountains region, Colorado and New Mexico
Brian S. Brister
A new principal reference section and four additional sections measured from the Eocene Blanco Basin Formation illustrate the variability of the depositional environments, composition and sedimentation patterns. Lithofacies associations show a wide range of depositional environments, from rare proximal massflow breccias to abundant pedogenically modified basinal overbank deposits. Conglomerate clast counts and sandstone point counts indicate a laterally and vertically variable composition influenced by both local provenance and progressive onlapping of the basin margins. Paleocurrent studies of Blanco Basin outcrops demonstrate that the San Juan sag, a Laramide foreland basin of the San Juan Mountains region, was a major pathway for sediment input into the San Juan Basin during late Paleocene-Eocene deposition of the San Jose Formation. Stratigraphic relationships and tectonic implications suggest that the upper part of the Blanco Basin Formation may be coeval with the Eocene El Rim Formation, but that the two were deposited in different basins separated by a drainage divide near Chama, New Mexico:
- Brister, Brian S., 1992, The Blanco Basin Formation (Eocene), San Juan Mountains region, Colorado and New Mexico, in: San Juan Basin IV, Lucas, Spencer, G.; Kues, Barry S.; Williamson, Thomas E.; Hunt, Adrian P., New Mexico Geological Society, Guidebook, 43rd Field Conference, pp. 321-331.