Stratigraphic Architecture of The Mesaverde Group in The San Juan Basin Area
— Bruce S. Hart

For more than 80 years, outcrop- and subsurface-based stratigraphic analyses of the San Juan Basin’s Mesaverde Group have
helped geologists establish relationships among sediment supply, accommodation, and stratigraphic architecture. Different types of depositional
systems develop along regressive and transgressive coastlines. In typical regressive systems, shoreface sandstones gradationally
overlie offshore shales with cm- to dm-scale interbeds of sandstone and shale representing a transition zone between those two depositional
systems. Most outcrops of the Point Lookout Sandstone present some variant of this stratigraphic succession. Shoreface sandstones can be
particularly thick if the shoreline trajectory included aspects of both progradation and aggradation. Sharp-based shorefaces indicate forced
regression, a situation where a fall in sea level causes the shoreline to move basinward. In transgressive systems like that represented by
the Cliff House Sandstone, sandstones deposited in barrier island complexes typically erosionally overlie nonmarine deposits. Those sandstones
can be thin or even absent in purely transgressive systems. They can become thicker and interfinger laterally with nonmarine and
offshore deposits when transgression slowed or paused while subsidence continued. The nonmarine Menefee Formation is heterolithic,
with abundant fine-grained and carbonaceous deposits. These characteristics, along with the formation’s wedge-shaped geometry, indicate
that much of the deposition occurred in a rapidly subsiding foreland basin setting, landward of the time-equivalent Point Lookout and Cliff
House shorelines.