H. B. Willman and T. C. Buschbach
The name "Ordovician" is derived from Ordovices, the Roman name for a people who inhabited Wales, where these rocks are well exposed.
Before "Ordovician" was introduced, these strata were included in the Silurian System, and they were called Lower Silurian in the early reports on Illinois stratigraphy.
Extent and Thickness
The Ordovician System (Lapworth, 1879, p. 12-14) underlies all of Illinois except a few areas in the northern part of the state where it has been eroded and Cambrian rocks are at the surface (fig. O-1). The Ordovician System, where overlain by the Silurian System, ranges from 700 feet thick in extreme northeastern Illinois to more than 5000 feet in southern Illinois (fig. O-1). The variations in thickness are related to major unconformities that occur at the top and within the Ordovician System and to original southward thickening of many of the formations. The structure of the system is shown by the map of the top of the Galena Group (fig. O-3).
Ordovician rocks are widely exposed in northern Illinois, particularly along the upper Illinois Valley, the Rock Valley, the Pecatonica Valley, and the Upper Mississippi Valley and its tributaries north of Savanna. Elsewhere in Illinois they are exposed only along truncated anticlines-- in Calhoun and Jersey Counties, Monroe County, and Alexander County. Typical exposures are shown in figure O-2.
The Ordovician System is subdivided into three series based on faunal correlations and on lithologic tracing from Illinois to their type regions. The Canadian Series and the Champlainian Series have been traced to New York State, and the Cincinnatian Series to Cincinnati, Ohio.
The Ordovician System in Illinois is dominantly dolomite and limestone. It is differentiated into 26 formations and 66 members (fig. O-4). Four of the formations are dominantly sandstone, four are shale, and 18 are limestone or dolomite. Most of the formations extend throughout the state and many throughout the Mississippi Valley. Some even have lithologic continuity eastward to New York and Virginia, southward to Tennessee, westward to Colorado, and northward to northern Michigan and Ontario (Templeton and Willman, 1963).
The Ordovician System is separated from the systems above and below it by unconformities, but both unconformities are difficult to recognize in parts of the state, particularly in the subsurface. In the southern part of Illinois, the lowermost Ordovician dolomites are not readily separable from the Cambrian dolomites that underlie them, especially in areas where the basal Ordovician Gunter Sandstone is missing. The upper Ordovician (Cincinnatian) shales in some areas contain dolomitic siltstones that are not readily separable from similar early Silurian sediments that fill erosional channels cut into the Cincinnatian strata.
Within the Ordovician System other unconformities occur, particularly at the base of the Champlainian (the sub-Tippecanoe unconformity) and the base of the Cincinnatian
Series. Within the Champlainian Series, a lesser but widespread unconformity occurs at the base of the Trentsnian Stage. The Champlainian rocks contain numerous minor diastems that reflect interruptions in sedimentation but do not indicate withdrawal of the sea. Corrosion surfaces, which truncate only a few inches of strata, represent temporary cessation of carbonate deposition and solution of the sea floor. Current-produced features include wavy scour surfaces, which generally have a relief of less than 6 inches, and giant ripple marks.
The St. Peter Sandstone, at the base of the Champlainian Series in most of Illinois, is an unusually extensive, very pure, uniformly fine-grained, and well sorted quartz sandstone, which appears to have been deposited by a sea advancing northward across the Illinois Basin (Dapples, 1955). South of Illinois it grades into sandy carbonates. Across north-central Illinois the final episode of St. Peter deposition was a broad, east-west, offshore bar of medium-grained sandstone (fig. O-16). North of the bar, and partly contemporaneous with it, the poorly sorted sandstone, impure dolomite, and shale of the Glenwood accumulated, and south of the bar the shaly limestone of the Dutchtown and the sandy and argillaceous dolomite of the Joachim were deposited. These impure sediments join the St. Peter to form the Ancell Group.
In the Champlainian Series overlying the Ancell, dominantly carbonate rocks are characterized by the persistence of thin units that are identified by relative purity, texture, bentonite beds, and fossils. The argillaceous materials in the Champlainian carbonate rocks in northern Illinois came largely from a northerly source, as is shown by the gradual change in Trentonian rocks from nearly pure carbonate in Illinois to calcareous shale at the northern margin in Minnesota (fig. O-23). The thin shale, sandstone, and siltstone beds in the Blackriveran and lower Trentonian rocks in southwestern Illinois thicken onto the Ozarks, which the overlapping relations of the basal Trentonian strata show were at least intermittently above sea level. The red-brown shales in the Guttenberg Formation thicken to the south in Illinois and the red color may be from the residuum on Cambrian and early Ordovician carbonate rocks in the Ozarks. Although the amount of shale also increases northward from northern Illinois, the red color does not extend far north of the state boundary.
Seven thin beds of bentonite are widely present in the Ordovician section in Illinois, most of them in the Champlainian Series. Bentonites have been found locally at several other horizons, and others may occur at distinctive, smooth-surfaced bedding planes like those commonly associated with bentonites. Although the bentonites in Illinois are generally less than an inch thick, a few, possibly thickened by squeezing, are 3 or 4 inches thick. Most of them are correlated with much thicker beds in Kentucky, Tennessee, and Virginia (Templeton and Willman, 1963).
The rocks of the Canadian Series contain few fossils, but the Champlainian and Cincinnatian rocks have large faunas, and fossils are abundant in many beds (fig. O-5). An occasional high-spired gastropod is the most prevalent invertebrate fossil found in the Canadian rocks, but algal domes are common, particularly in the Shakopee Dolomite. In the Champlainian Series the Chazyan strata and the Ancell Group strata have few fossils other than worm borings, except in the Dutchtown, which is not exposed in Illinois. The Platteville and Galena Groups contain a variety of fossils, among which brachiopods, bryozoans, gastropods, and corals are most abundant. Crinoidal debris is abundant in some beds, particularly in southern Illinois. Pelecypods and trilobites are common in a few beds, and conodonts are numerous in the limestones. Fucoids of several types, possibly impressions of marine plants, are abundant and widely distributed in many beds. Several faunal zones based on abundance of one or more species of invertebrates are recognized (fig. O-24) and are widely useful for identification of the formations. The base of the Trentonian rocks is well defined by an abundance of several species that appear for the first time.
In the Cincinnatian strata, brachiopods and many species of bryozoans are abundant in most limestones and calcareous shales. Fossils are not common, however, in the very argillaceous limestone, the brown shales and siltstones, or the light greenish gray shales. One to three beds at or near the base of the Scales Shale and another bed 60-70 feet above the base contain a distinctive fauna of small fossils, mostly gastropods and pelecypods less than a quarter of an inch in greatest dimension. These beds are called depauperate beds, and the intervals in which they occur depauperate zones. The beds are mainly phosphatic and pyritic and are generally only an inch or two thick, although locally they are as much as a foot thick. The lower zone is very persistent, but the upper has been found only in northeastern and central Illinois.
During most of Ordovician time, Illinois was in a broad sea, far from the shores and from the sources of clastic sediments. Most of the sediments were moderately deep-water, fine-grained carbonate. During mid-Champlainian Blackriveran time, shallow-water argillaceous carbonates and evaporites (Joachim, Glenwood) were deposited; later, during Trentonian time, coarse calcarenites (Kimmswick) accumulated in the southern part of Illinois. The sand in the Canadian and early Champlainian sandstones came from the north. Only during Cincinnatian time, when large deltas were being built in the Appalachian region, did clay and silt from the east dominate the clastic sediments in Illinois.
During Canadian time the sea covered the entire state, and sedimentation was essentially similar to that of the Cambrian. The Canadian sediments were deeply eroded before the early Champlainian (Chazyan) sediments-largely dolomite, sandy dolomite, and sandstone were deposited in Illinois. These sediments have much the same character as the Canadian rocks, but in Illinois they were eroded from all except the extreme southwestern part of the state before St. Peter deposition. Their composition suggests they could originally have been as extensive as the Canadian sediments.
DAPPLES, E. C., 1955, General lithofacies relationship of St. Peter Sandstone and Simpson Group: American Association of Petroleum Geologists Bulletin, v. 39, p. 444-467.
LAPWORTH, C., 1879, On the tripartite classification of the lower Paleozoic rocks: Geological Magazine, New Series, Decade II, v. 6, p. 1-15.
TEMPLETON, J. S., and H. B. WILLMAN, 1963, Champlainian Series (Middle Ordovician) in Illinois: Illinois State Geological Survey Bulletin 89, 260 p.
|Stratigraphic Code||Geo Unit Designation|