Subsidence and Eustatic Sea Level Records in the Stratigraphic Architecture of the U.S. Cretaceous Western Interior Basin*

Research over the past 15 years have established that the Cretaceous subsidence history of the Rocky Mountain region reflects a complex temporal and spatial interplay of long-wavelength dynamic subsidence related to mantle flow above the subducting Farallon slab, intermediate-wavelength subsidence across the foreland basin of the Sevier orogenic belt and short-wavelength and spatially complex subsidence and uplift related to the Laramide orogeny. Combined, these driving forces have created a Cretaceous sedimentary wedge that extends eastward more than 1500 km from the orogenic belt, generally thins eastward, and can be subdivided into megasequences by regional unconformities that closely relate to tectonic episodes in the thrust belt. Individual megasequences range in duration from 5 to 7 million years. Superimposed on these regional tectonic drivers were global sea level fluctuations, which also imparted their signature on the stratigraphic architecture. Long-term global sea level rises and falls, now documented well in oxygen isotope data, are reflected in some major regressions and transgressions but appear to exert only a secondary role relative to temporal changes in regional subsidence rates. In contrast, sea level changes on the scale of Milankovitch cycles (104 to 106 years) are prominently expressed in the shallow and marginal marine strata