H. B. Willman and Elwood Atherton
The Marcus Formation is named for Marcus, Carroll County, 4 miles northwest of the type section.
The type section of the Marcus Formation is in an abandoned quarry in the southern part of Mississippi Palisades State Park (SW SE SE 33, 25N-3E).
The Marcus Formation was differentiated as the "Pentamerus beds" in early studies and later was named "Waukesha" (Savage, 1926).
The abundance of Pentamerus suggests correlation of the Marcus with the Schoolcraft in Wisconsin and the Reynales in New York.
Extent and Thickness
The Marcus Formation is 40.3 feet thick in the type locality. The Marcus Formation occurs in northwestern Illinois both north and south of the Savanna Anticline, along which it is eroded. North of the anticline it occurs only in the deepest part of the syncline, but it extends eastward to Shannon, Carroll County, and is well exposed north of Mt. Carroll, Carroll County (SE NW NW 25, 25N-4E). South of the anticline it is well exposed in quarries east of Fulton (SW NE NW 19, 22N-4E). The Marcus is uniformly close to 40 feet thick.
The Marcus Formation (Willman, 1973, p. 37) is the basal Niagaran unit in northwestern Illinois (fig. S-4).
The Marcus Formation consists of pure, vesicular, brown, massive dolomite (fig. S-2C). The Marcus occurs at the position occupied by the Joliet Formation farther east and south, but it does not resemble it lithologically, and strata equivalent to the Marcus may be missing at the corrosion surface that marks the base of the Joliet.
The lower 10-20 feet of the Marcus is generally crowded with the shells of Pentamerus oblongus (fig. S-12), which is also common in the upper part along with other fossils, especially corals.
SAVAGE, T. E., 1926, Silurian rocks of Illinois: Geological Society of America Bulletin, v. 37, p. 513-533.
WILLMAN, H. B., 1973, Rock stratigraphy of the Silurian System in northeastern and northwestern Illinois: Illinois State Geological Survey Circular 479, 55 p.
|Stratigraphic Code||Geo Unit Designation|