H. B. Willman
The term "Paleozoic" means ancient life.
Paleozoic Erathem --the rocks deposited during the Paleozoic Era (Sedgwick, 1838)-- originally included only the rocks now referred to the Cambrian, Ordovician, Silurian, and Devonian Systems, but the definition of the name was expanded in 1840 to include the Carboniferous (Mississippian and Pennsylvanian) and Permian Systems (Phillips, 1840), and present usage follows that definition.
The Paleozoic rocks of Illinois (fig. 1) are separated from the older Precambrian and younger Mesozoic rocks by great unconformities. The basal unconformity is marked by the absence of rocks of late Precambrian and early and middle Cambrian age, an interval approximately as long as all the time that has passed since then- more than half a billion years. The unconformity at the top of the Paleozoic represents more than 125 million years and is marked by the absence of the youngest Pennsylvanian, the Permian, and all the Mesozoic rocks older than late Cretaceous. Between these unconformities the Paleozoic Erathem in Illinois contains sediments representing the oldest six of the seven systems.
Above the thick sandstones of the Cambrian System, carbonate sediments are dominant through the middle Mississippian, whereas shale and sandstone dominate the younger deposits. Sandstones in the Cambrian and up through the Devonian rocks consist mainly of well rounded quartz grains and a minor amount of stable heavy minerals of a limited suite. Many are almost entirely quartz sand, but others contain some clay, silt, and more varied heavy minerals. The sand of the early Paleozoic sandstones was reworked largely from late Precambrian sandstones and has passed through several cycles of erosion and transportation. The Mississippian and early Pennsylvanian sandstones are largely quartz sandstones but are not as well rounded or sorted or as pure as the older sandstones, which suggests that the sources of the older sandstones were being covered by the Paleozoic sediments. The later Pennsylvanian sandstones are noticeably angular, poorly sorted, silty, feldspathic, and micaceous, and have a larger suite of only moderately stable heavy minerals. They appear to be first-cycle sediments derived from more recently exposed or accessible granitic and metamorphic rocks.
Fossils of the Paleozoic rocks represent the oldest abundant life. Algal-like fossils and burrows and trails of worm-like organisms are found in the Precambrian rocks of other regions, but only rarely. A great variety of marine invertebrate animals characterized the Paleozoic Era. Marine vertebrates (fishes) developed during Ordovician time, but it was not until Devonian time that plants became abundant enough on land to furnish the food that permitted fresh-water and land-dwelling animals to develop. Although fossils are scarce in Illinois Cambrian rocks, they are abundant in many other Paleozoic strata. By late Mississippian time plants were abundant, and during the Pennsylvanian Period they flourished profusely. In swampy areas they accumulated and eventually formed coal beds. Amphibious vertebrates became more common late in the Paleozoic, but invertebrates continued to be the dominant form of animal life throughout the era.
The Paleozoic rocks are dominantly marine sediments, representing repeated invasions of the sea into the interior of the continent. The seas generally advanced from the south, and the land-derived sediments deposited in them came mainly from the north, northeast, and east, with smaller amounts from sources in the southeast and northwest. Very small contributions came from the Ozarks to the west.
PHILLIPS, J., 1840, Penny cyclopedia: v. 17, p. 153-154.
SEDGWICK, A., 1838, Geological Society of London Proceedings, v. 2, no. 58, p. 84-85.
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