Depositional History of the Cretaceous Sediments in Embayments along the Sioux Ridge

During the Late Cretaceous, a large sea known as the Western Interior Seaway covered the mid-continent of North America. Within this seaway were deposited thick sequences of non-marine sandstones overlain by marine shales, siltstones, and limestones. At its eastern margin, large portions of a Precambrian high, the Sioux Ridge, remained above sea level, forming a series of islands. Steep-sided canyons, termed embayments, are present along the Sioux Ridge. Within these embayments, typical Upper Cretaceous sediments grade into sequences of interbedded sandstone and claystone, laminated black organic-rich shale, interbedded opaline spiculite, and massive chert. Based on microfossil studies, these unusual sediments are believed to be restricted marine, near shore facies equivalents to perhaps all of the Upper Cretaceous stratigraphy present elsewhere in the region (eastern South Dakota, northwestern Iowa, and southwestern Minnesota).

Preliminary research has been carried out on these sediments within one of these embayments (located in southeastern Minnehaha County, South Dakota), where a formal designation, the Split Rock Creek Formation, has been assigned. However, work carried out by the Geological Survey Program of the South Dakota Department of Environment and Natural Resources indicates that sections of the sedimentary sequence in each embayment may be unique to that embayment. Thus, sediments in each embayment may be only partially stratigraphically equivalent to the Split Rock Creek Formation. To fully understand the depositional history of these Upper Cretaceous sediments proximal to the Sioux Ridge, considerably more sedimentological, stratigraphic, and paleontological work must be done. To learn more about the opportunities to collaborate with the Geological Survey Program on this topic, contact Dennis Tomhave (dtomhave@usd.edu),  Kelli McCormick (kmccormi@usd.edu), or Sarah Chadima (schadima@usd.edu).

Benefits to South Dakota

The information collected through the study of embayments may result in the delineation and characterization of a greater number of viable aquifers for public use. These studies would be directed at characterizing sedimentary rocks around the Sioux Ridge, an area that extends through at least nine counties. Some of the sedimentary rocks are potentially viable aquifers for local communities or public water systems. Because we do not know how these sedimentary rocks differ among the embayments, we are currently unable to confidently extrapolate results from one project in one embayment to the other embayments.