HIGH-RESOLUTION CHEMOSTRATIGRAPHIC CORRELATIONS OF MID-CRETACEOUS STRATA DEPOSITED FROM THE EASTERN MARGIN TO THE FOREDEEP OF THE WESTERN INTERIOR BASIN, NORTHWESTERN IOWA TO NORTHEASTERN WYOMING

In this study, chemo- and lithostratigraphic techniques are used to perform high-resolution, parasequence-scale correlations of mid-Cretaceous paleosol-bearing sections from the eastern margin of the Western Interior Basin to coeval marine strata in the basin center. The goal of these high-resolution correlations is to apply well-constrained marine chronostratigraphy to poorly dated amalgamated paleosol in order to quantify parasequence cyclicity and further understand processes that affected alluvial-plain stratal architecture and pedogenesis during the mid-Cretaceous. A previous study set up a framework to reach this goal by distinguishing between the influence of base level change and climate signals in a stack of amalgamated mid-Cretaceous paleosols located in present-day northwestern Iowa using oxygen isotopic values obtained from paleosol siderite.

Data for this study was collected from a composite outcrop section in northwestern Iowa (overlapping with the isotopic study), two cores in eastern and western South Dakota (BHP 92-4 and Phillips #1 Harrington A), and a composite section from northeastern Wyoming consisting of one core (#1 Patrick Walsh Federal) and two outcrops. These sites are correlated along a northwest transect from an area of predominantly non-marine deposition in the east to an area of mostly marine deposition in the west. The data generated in this study includes lithological descriptions, palynology, carbonate content (%CaCO3), total organic carbon content (%TOC), and total sulfur content (%TS). Fluctuations within the data are interpreted to reflect changes in relative sea level and provide the basis for applying a parasequence model for the development of geochemical facies. In this model, low TOC-CaCO3-TS horizons are usually interpreted to represent terrestrial progradation, whereas horizons with higher TOC, CaCO3, and TS values represent marine flooding. Parasequence correlation is built within this model by pattern matching techniques applied to the geochemical profiles. Various bentonites that have ages are correlated to the paleosol sequence. Initial results suggest 300,000 year cyclicity in parasequence distribution. While this does not match Milankovitch periodicity, changes in paleoclimate most likely played a role parasequence stacking patterns.