Before The San Juan Basin: An Overview Of The Upper Cretaceous Western Interior Seaway
— Bruce S. Hart

Although exposed around the margins of the San Juan Basin, Upper Cretaceous rocks of northwestern New Mexico were
deposited in a much larger predecessor basin that owes its origin to the breakup of Pangea and an acceleration of Mid-Atlantic Ridge spreading
rates. This paper presents a high-level overview of the Upper Cretaceous Western Interior Seaway, an inland sea that extended from the
current Gulf of Mexico to the Arctic Ocean and from Utah to Kansas. I link deposits exposed in the San Juan Basin area with time-equivalent
rocks deposited elsewhere in the Western Interior Seaway to illustrate the extent of the seaway and controls on sediment deposition.
Westward movement of North America, initiated in the Jurassic, accelerated during the Late Cretaceous. Retroarc foreland basins along
the western margin of the continent deepened. Flat-slab subduction created dynamic topography that widened the basin. At the same time,
mid-ocean ridge swelling raised eustatic sea level. That combination of subsidence and eustasy led to the formation of the Western Interior
Seaway. Upper Cretaceous rocks of New Mexico and elsewhere record a relatively rapid flooding of the basin during the Cenomanian
followed by a more gradual retreat of the seaway throughout the rest of the Cretaceous. Superimposed on that second-order trend were
third-order transgressive-regressive cycles formed in response to shorter-term (tens of thousands to millions of years) fluctuations in subsidence,
eustatic sea level, and sediment supply. That combination of short- and long-term cyclicity explains stacking patterns and facies
transitions associated with the Dakota and Gallup sandstones, Mesaverde Group, and other Upper Cretaceous units in New Mexico and
elsewhere in North America’s interior.