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14 Hay Butte marker Saddle Pass marker Heck Table marker Cedar Pass white layer Poleslide Member Scenic Member Lower Lower Middle Middle Upper Arikaree Channel Rockyford Ash Chamberlain Pass Formation Chadron Formation Brule Formation Fox Hills Formation Pierre Shale Cactus Flat marker Sharps Formation Peanut Peak Member Crazy Johnson Member Ahearn Member Chadron Formation Lower Lower Middle Middle Upper Upper Poleslide Member Scenic Member Brule Formation 0 50 100 150 190 m 0 50 100 130 m claystone beds tan sandy siltstone beds well bedded silty sandstone beds massive very light gray siltstone beds with abundant vertically elongated carbonate nodules massive tan siltstone beds with scattered silty sandstone blankets brown mudstone beds brown mudstone beds green gray clayey mudstone beds with thin white limestone beds brown mudstone beds with nodules light gray muddy sandstone blankets separated by widespread mudstone units Top of Sheep Mountain Table Top of Norbeck Ridge W E 2.1. Stratigraphic units of the North Unit of Badlands National Park. The sequence in the west includes the rocks of the Red River paleovalley of Clark, Beerbower, and Kietzke (1967) and the rocks exposed on Sheep Mountain Table. The sequence in the east is a composite of features from the Dillon Pass area east to Norbeck Ridge. 15 2 Sedimentary Geology of the Big Badlands The rocks of Badlands National Park record the end of the great Western Interior Seaway near the end of the age of dinosaurs (Mesozoic Era) and, after a 30-millionyear gap, record some of the features on land during the greatest volcanic eruptive intervals and one of the greatest climatic changes in the age of mammals (Cenozoic Era). To understand the geologic history of the sedimentary rocks exposed in the South Dakota Badlands, we need to see how the rocks are distributed in space and time (stratigraphy) and how they were deposited (sedimentology). We start our story by building the framework of the rocks in space and time by discussing the stratigraphy of the Badlands. The rocks in the Badlands were deposited in the sea, in river deltas, by rivers, in lakes, and by the winds, and we will discuss the evidence for these depositional environments. To understand the ancient environments and how they changed through time, we need to start by understanding the origins of the sediments that now make up the 190 m thick sequence of rocks exposed in Badlands National Park (Fig. 2.1). Stratigraphy: Geologic Framework of the White River Badlands Sedimentary geologists recognize formal stratigraphic units, or distinct packages of rocks that are carefully described and named for permanent geographic features and their associated lithologies. These formal stratigraphic units include formations, members, and groups. The fundamental stratigraphic unit is a formation, which is a package of sedimentary rock with unique lithologic (rock) features that separate it from other formations above and below it. Formations must be widespread and thick enough to be easily plotted on a map. There are six formations recognized in the Badlands, including, from the oldest to the youngest, the Pierre Shale, the Fox Hills Formation, the Chamberlain Pass Formation, the Chadron Formation, the Brule Formation, and the Sharps Formation. Formations of similar origins and closely related through time can be combined to create groups. The Chamberlain Pass, Chadron, and Brule formations are part of the White River Group, the subject of most of this discussion . The 365 m thick Pierre Shale that lies below the Fox Hills Formation has traditionally been considered to be a formation, but Martin and Parris (2007) have subdivided the Pierre Shale into seven formations and elevated the unit to the Pierre Shale Group. (We will continue to call this unit simply the Pierre Shale in the following discussion.) Formations can be subdivided into members, which are thinner, less distinctive, but widespread units within a formation. The history and nomenclature of the White River Group and its formal subdivisions is presented in Table 2.1. These names all refer to distinct packages of rocks that have unique features and contain fossils that record changes in environments through time. Our discussion will focus on the features of the White River Group, the rocks, and the main source of fossils in the intricately carved Badlands. Sedimentary geologists also recognize a variety of informal stratigraphic units. These tend to be widespread distinctive individual layers, too thin to be named as members but nonetheless important markers for stratigraphic positioning. Such marker beds can include volcanic tuff beds, widespread limestone beds, and, in the case...

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Additional Information

ISBN
9780253016089
Related ISBN
9780253016065
MARC Record
OCLC
909368973
Pages
240
Launched on MUSE
2015-05-21
Language
English
Open Access
No
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