Laramide Structural Evolution of the San Juan Basin, New Mexico and Colorado

By

James E. Fassett

U.S. Geological Survey, Scientist Emeritus/Independent Geologist, Santa Fe, NM

Laramide tectonic activity in the San Juan Basin of northwest New Mexico and southwest Colorado began in Late Cretaceous time following deposition of rocks associated with the final regression of the Western Interior Seaway in late Campanian-early Maastrichtian time. The basin area was uplifted and tilted - down to the northwest - resulting in the beveling of 1200 feet of strata to the southeast. Following this tectonic event, the entire basin area began to subside again in earliest Paleocene time resulting in deposition of the Paleocene Ojo Alamo Sandstone and the Nacimiento and Animas Formations and the Eocene San Jose Formation. The basin’s present structure began to form after Ojo Alamo Sandstone deposition in mid Paleocene time and continued through the time that the Paleocene Nacimiento and Animas Formations and the Eocene San Jose Formation were being deposited. Detailed geologic mapping of these rock units clearly shows progressive, episodic, truncation of rocks on the basin rim through time during deposition of these rocks as the basin center was down-warping. The basin structure, as we know it today, was thus formed from about mid-Paleocene through Eocene time over 25-30 m.y. The Chuska Sandstone is present as a flat-lying unit overlying tilted younger strata in the Chuska Mountains west of the San Juan Basin indicating that the basin’s structural formation was completed prior to deposition of the Oligocene Chuska Sandstone. Reservoir creation and trapping of hydrocarbons in the basin was clearly related to the structural evolution of the basin.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming