Geologic Setting

The San Juan Basin (SJB) an asymmetric structural basin formed during Cenozoic and Mesozoic time periods straddles the New Mexico - Colorado border (Appendix B: Maps and Cross-Sections 1). Within the stratigraphic section of the Basin lie three distinct Cretaceous Age fluvial-lacustrine-marine sequences of sediments averaging 5,000 feet in thickness. The northern and western edges of the Basin are formed by a structure known as the Hogback Monocline (Appendix B: Maps and Cross-Sections 2; Figure 1, below).

Figure 1: Hogback Monocline near Durango, Colorado

Along the Monocline, Cretaceous and Tertiary age depositional horizons dip steeply into the Basin. Along this structural margin, all of these sedimentary units are either exposed at the surface or subcrop beneath a thin layer of alluvium-colluvium or moraine deposits.