Pennsylvanian Subsystem and (Absaroka Sequence) W. John Nelson and Russell J. Jacobson 10

Introduction The Absaroka sequence was named for the Absaroka Mountains in Wyoming (Sloss et al. 1949). Originally the The term “” or “Pennsylvania ” sequence included Pennsylvanian rocks, but the upper limit was introduced by Williams (1891, p. 83) in reference to was not specified. Sloss (1963) defined the upper contact the coal-bearing rocks in Pennsylvania. The Pennsylvanian as the that underlies Middle strata was a subdivision of the System, defined in the and Gulf Coast. The sequence is by Conybeare and Phillips (1822) in England and Wales. divided into Absaroka I, II, and III subsequences (Sloss Weller (1906) was the first to use “Pennsylvanian” in Il- 1988). The Absaroka I extends from latest , linois, although “Pennsylvanian,” “Coal Measures,” and approximately 330 million before present, through “Upper Carboniferous” were used interchangeably in early Wolfcampian (Early Permian) time 268 million years ago. reports. North American geologists gradually adopted the The rock record of the Absaroka sequence in Illinois is en- Pennsylvanian as a system, recognizing that Mississippian tirely Absaroka I, comprising Pennsylvanian sedimentary and Pennsylvanian rocks differ in fundamental ways and rocks and Lower Permian intrusive igneous rocks (Fig- are separated by a regional unconformity. By international ure 10-1). This chapter covers Absaroka I only; Chapter agreement, the Pennsylvanian was reclassified in 2004 as 11 deals with Absaroka II and III and younger sequences the Pennsylvanian Subsystem of the Carboniferous System identified by Sloss (1963). (Heckel 2004). The Pennsylvanian Subsystem in the United States commonly is divided into five series: Morrowan (oldest), Geological History Atokan, Desmoinesian, Missourian, and Virgilian (Fig- Near the end of Mississippian time, glaciation in the ure 10-1). Also, the Pennsylvanian is divided into Lower southern hemisphere caused the sea to withdraw from (equivalent to Morrowan), Middle (Atokan and Desmoine- the Illinois Basin. Rivers eroded a series of deep, parallel, sian), and Upper (Missourian and Virgilian) Series (Pep- southwest-trending valleys into the exposed sea floor. The pers 1996). ancestral Rocky Mountain Orogeny commenced around Pennsylvanian rocks contain the bituminous coal re- the same time, having impacts as far east as Illinois (Mc- sources of Illinois, the largest of any state (Chapter 14, Bride and Nelson 1999). Many faults and folds developed in Coal). They also have yielded 13% of Illinois’ cumulative Illinois, strongly influencing sedimentation. oil and gas production (Chapter 15, Oil and Gas) and are an As the sea level rose in early Morrowan time, incised important source of and of clay and shale used in valleys of Illinois were drowned and filled with sediment. brick-making (Chapter 17, Industrial Minerals). Initial sediments included large volumes of pebbly, quartz- The Permian Period was named by British geologist R. rich sand carried by rivers from the northeastern United I. Murchison in 1841. The period takes its name from the States and southeastern Canada. Apparently derived from city of Perm near the of Russia, near which older sedimentary rocks, these deposits consti- rocks of this age are extensively exposed. Permian sedi- tute the (Nelson 1989). Sea level rose mentary rocks are widespread in the southern U.S. midcon- and fell repeatedly in concert with tectonic activity in Il- tinent, especially in west , where the North American linois, producing multiple episodes of valley cutting and standard Permian section was assembled. The only known filling. Permian rocks in Illinois are igneous rocks in the southern By Atokan time, sediments had accumulated in most of part of the state, but evidence suggests that sedimentary Illinois, and the drowned-valley topography was gradually layers of this age formerly covered much of Illinois. The leveling. crystalline rocks were being eroded cyclic character of Permian strata, which is comparable to in the northern and central Appalachian Mountains, sup- underlying Pennsylvanian rocks, is not consistent with de- plying mica, feldspar, rock fragments, and clay to Illinois position in a small basin (Kehn et al. 1982). sediments. Peat-forming swamps, which developed along