STRATIGRAPHY AND PALEOECOLOGY OF THE MIDDLE PIERRE SHALE ALONG THE MISSOURI RIVER (CENTRAL SOUTH DAKOTA)

A study of the stratigraphy of the middle Pierre Shale was undertaken, concentrating on two sections located along the eastern side of the Missouri River, in the vicinity of Fort Thompson. The members of the Pierre Shale represented within these sections are, from oldest to youngest, the Gregory, Crow Creek, DeGrey and Verendrye. Micropaleontologic samples were analyzed with factor analysis performed to classify the assemblages. Four foraminiferal biofacies are recognized: two predominantly agglutinated, Boreal in origin and indicative of cooler, less oxygenated marine conditions, one shallower and the other deeper; one composed of calcareous benthic foraminifera, Tethyan in origin, indicative of warmer, more oxygenated and shallower waters; and a fourth, that is interpreted as a mixed assemblage, transitional between the others. ⁸⁷Sr/⁸⁶Sr age analysis yielded twelve useable results. Samples from near the Baculites compressus biozone gave an age of approximately 72.43 +/- 2 Ma, a number relatively close to Obradovich’s 1993 time scale ⁴⁰Ar/³⁹Ar value for this horizon of 73.35 +/- 0.39 Ma. An average ⁸⁷Sr/⁸⁶Sr date of 72.41 Ma +/- 2 Ma was determined from fossil shell at a significant paleontologic find – the globular-toothed mosasaur, Globidens sp. More equivocal dates of 74.58, 74.63 and 75.09 +/- 2 Ma were determined for the basal Crow Creek Member, a calcareous sandy siltstone within the upper Gregory Member and the B. gregoryensis biozone located at the base of the section, respectively. Based on the data from this study, the Crow Creek Member is interpreted as the result of the Bearpaw transgression (TST) and the DeGrey/Verendrye Member’s boundary is a maximum flooding surface. The Verendrye Member is interpreted as an HST; the basal Gregory Member is interpreted as an HST but part of the regressive pulse of the Claggett depositional cycle. A preliminary correlation with the eustatic record from the late-K coastal plain of New Jersey places the basal Crow Creek unconformity coincident with an unconformable bounded surface at the base of the Marshalltown Formation linked to Haq’s UZA-4.4 onlap cycle. A distinct, calcareous sandy siltstone unit in the upper Gregory Member is suggested to represent a storm-dominated deposit or, perhaps, a tsunamtite resulting from the approximately coeval Manson Impact.