A Sequence Stratigraphic Framework for the Middle to Late Jurassic of the Sundance Seaway, Wyoming: Implications for Correlation, Basin Evolution, and Climate Change

Well-exposed Middle to Upper Jurassic (Bajocian to Oxfordian; approximate to 170-155 Ma) strata of the Gypsum Spring and Sundance Formations in Wyoming and South Dakota, United States, preserve seven third-order depositional sequences on the distal flank of a retroarc foreland basin. Some of these depositional sequence boundaries can be correlated with previously recognized regional unconformities, but other correlations are ambiguous, and some of these sequence boundaries are not recognized in previous studies. The strata preserve 18 facies associations in carbonate ramp, wave-dominated siliciclastic shelf, siliciclastic tidal coast, and mixed evaporite-siliciclastic desert systems. The widespread extent of these depositional sequences indicates their potential utility as a correlation framework for the Jurassic of the western interior. Facies in these sequences show a consistent northwestward trend toward more open marine conditions, consistent with previous interpretations of depositional dip. Although the thicknesses of depositional sequences likewise tend to thicken northwestward, they also show marked changes both laterally and vertically, suggesting patterns of accommodation more complicated than a simple northwestward increase. The regional extent of the depositional sequences points to a eustatic origin, but the thickness and distribution of individual sequences were substantially modified by dynamic topography, which caused the great width of this basin across Wyoming and Montana compared with regions to the north and south. These strata record a gradual climatic shift from predominantly arid environments to semiarid, winter-wet conditions, which is reflected by a gradual change from a carbonate-evaporite system to a siliciclastic system. This shift was promoted by northward drift of the region out of the global subtropical arid belt and filling of the foreland basin by the northward-prograding Morrison coastal plain.