ABSTRACT

Lower and lower Upper Cretaceous strata of the Montana portion of the Western Interior sedimentary basin contain several types of conformable and unconformable stratigraphic surfaces that can be related in a sequence stratigraphic framework. Some surfaces are interpreted as sequence boundaries; others are contained within sequences, such as transgressive surfaces of erosion, local scour surfaces, and condensed sections associated with maximum flooding. Together, these surfaces record the sedimentary response of terrestrial (southwest Montana) and marine (central Montana) environments within the basin to relative sea-level changes.

Four sequence boundaries (SB1 through SB4) in upper Aptian, Albian, and Cenomanian rocks can be recognized and correlated regionally within Montana, based on stratigraphic, paleontologic, and limited radiometric data. SB1, SB2, and SB4 also probably correlate interregionally within the Western Interior sedimentary basin. The position of these sequence boundaries, and of other types of surfaces within the sequences, is based principally upon facies analysis and on recognition of specific criteria that reflect specific conditions of the depositional environment The stratigraphic model for these associated facies and surfaces is based on a Holocene Atlantic coast analog and on modification of the existing continental-margin sequence stratigraphy model to a cratonic basin setting.