Archuleta Mesa panorama. Photo credit: Chris Carroll, former Colorado Geological Survey geologist.
Archuleta Mesa (Archuleta County, Colorado – near the New Mexico/Colorado border) has exposures of a diorite sill approximately 10.5 square miles in size. This sill intruded the Tertiary Animas Formation. The harder igneous rock of the sill resisted erosion to form the mesa. The Tertiary Animas Formation was deposited above the Tertiary Ojo Alamo Sandstone.
The Colorado Geological Survey mapped the coal beds of the Fruitland Formation and also mapped the extent of the Ojo Alamo Sandstone in Colorado: this provided insight into the extent of the large sill on the surface of Archuleta Mesa.