Unconformities, Diastems, and Condensed Section in Upper Cretaceous Series of San Juan Basin: ABSTRACT

Abstract

The Upper Cretaceous series of the San Juan basin consists dominantly of terrigenous clastics derived from the Sevier orogenic belt to the west. The marine and marginal marine deposits range in age from middle Cenomanian to Campanian. Several distinct chrono- and lithostratigraphic breaks occur within this succession, the origin of which must be precisely determined to correctly interpret the genetic stratigraphy of the basin fill. Major chronostratigraphic breaks are produced both by subaerial erosion on the basin flank during relative sea level fall (depositional sequence boundaries) and in the form of omission surfaces in the marine section. The most distinctive lithostratigraphic breaks form where depositional systems representing great differences in physical energy become juxtaposed. These two categories of surfaces may or may not coincide. Depositional sequence boundaries are best identified where paleosols have formed on marine sediments or where an abrupt shallowing of facies can be recognized (equivalent to the seismic stratigraphic pattern of downward shift in onlap). Sequence boundaries are associated with regional incision of the coastal plain, while their marine portions generally are conformable. Omission surfaces record intervals when marine sedimentation was absent. These may be firmgrounds, hardgrounds, or erosional surfaces, often recording significant biostratigraphic breaks. The mostmore » significant omission surfaces in the Cretaceous rocks of the San Juan basin may have formed at times of rapid rise of relative sea level, when terrigenous sediment supply to the marine environment was effectively shut off. Such surfaces, therefore, represent the top of marine condensed sections and probably formed sometime between maximum rate of sea level rise and highstand.« less