What happed during Late Cretaceous time in the Raton and San Juan Basins--With some thoughts about the area in between
— James E. Fassett

The Raton and San Juan basins are in northern New Mexico and southern Colorado (Fig. 1); the Raton straddles the state line and the San Juan is mostly in northwest New Mexico. Both structural basins are asymmetric: the Raton has a steeper, east-dipping, west flank and the San Juan has a steeper, west-dipping, east flank. The structural axis of the Raton basin is arcuate and trends from northeast in New Mexico and northwest in Colorado; the axis of the San Juan basin trends northwest. In this paper the Raton basin is defined (Fig. 1) by the outcrop of the youngest marine sandstone (Trinidad Sandstone) and by the overlying Vermejo Formation. The San Juan basin is outlined by the outcrop of the marine Pictured Cliffs Sandstone. The distance between the basins from the westernmost outcrop of the Trinidad andVermejo, to the easternmost outcrops of the Pictured Cliffs is about 90 miles (145 km).

 

The basins are presently separated by a structurally complex area—the Chile High—including fault-block mountains, grabens, platforms and embayments. Passing west from the Raton basin to the San Juan basin, the major structural elements traversed are the Sangre de Cristo Mountains (fault-block range), the San Luis-Espanola basin (part of the Rio Grande graben), the Chama basin and the San Pedro-Jemez uplift (Kelley, 1954). Most of these tectonic elements trend north. Physiographically, the area between the basins is the south part of the Southern Rocky Mountain Province (Fenneman, 1946).

 

Both the Raton and San Juan basins contain sedimentary rocks ranging in age from Paleozoic through Mesozoic to Cenozoic. In the Raton basin the oldest known sedimentary rocks, as revealed by electric logs, are Pennsylvanian in age; in the San Juan basin the oldest sedimentary rocks are Cambrian. The total thickness of Cambrian and younger sedimentary rocks in the Raton basin is 8,000+ ft (2,440 m) and in the San Juan basin, 14,000+ ft (4,270 m) (Jensen, 1972).