CHARACTERIZATION OF SEDIMENTS DEPOSITED ON THE SIOUX RIDGE OF SOUTHEASTERN SOUTH DAKOTA DURING THE LATE CRETACEOUS PERIOD

In the Late Cretaceous Period, the Western Interior Seaway (WIS) inundated the midcontinent region of North America. Deposition of the typical stratigraphic sequence in southeastern South Dakota was disrupted by the topographic high created by the Sioux Quartzite Ridge. The Sioux Quartzite is a 1.6-1.7 billion-year-old, extremely well-indurated unit that extends from south-central Minnesota to south-central South Dakota. Typical Cretaceous stratigraphy for the WIS in southeastern South Dakota, from oldest to youngest, includes the Dakota Sandstone, which comprises sandstones, siltstones, and shales; the Graneros Shale; the Greenhorn Limestone; the Carlile Shale; the Niobrara Formation of chalks and marls; and the Pierre Shale. Cretaceous deposition in the immediate vicinity of the Ridge has been correlated with the typical stratigraphy of the WIS by microfossil analyses and stratigraphic data, particularly in embayments around the ridge. One of these unique deposits, the Split Rock Creek Formation, was deposited in a protected embayment at the southeastern part of the Sioux Ridge in South Dakota. It is a sequence of sandy diamictites and quartz sandstones overlain by laminated organic claystones, grading upward into interbedded opaline sediments and massive cherts. The purpose of this investigation is to evaluate paleoenvironmental conditions along the crest of the ridge itself rather than in embayments. Sediments atop the body of the Sioux Ridge may represent the same time period, but were deposited under a range of different conditions. These deposits, which may be distinct from both the traditional stratigraphy and the sheltered embayment deposits, include marls, sandy shales, highly organic siltstones, petroliferous claystones, and sandstones. These deposits have typically been called “undifferentiated sediments of Cretaceous age” or “Cretaceous, undifferentiated.” Cores collected by the South Dakota Geological Survey were analyzed for their lithologic characteristics, and microfossil analyses are ongoing to attempt to correlate these undifferentiated sediments of Cretaceous age to a particular time period and set of depositional conditions.