Historical:Moccasin Springs Formation
Lithostratigraphy: Hunton Limestone Megagroup >>Moccasin Springs Formation
Chronostratigraphy: Paleozoic Erathem >>Silurian System >>Niagaran Series
Allostratigraphy: Tippecanoe Sequence
H. B. Willman and Elwood Atherton
The Moccasin Springs Formation (Lowenstam, 1949, p. 16) (fig. S-6) overlies the St. Clair Limestone in southern Illinois and is named for the village of Moccasin Springs, which is 9 miles north of Cape Girardeau, Cape Girardeau County, Missouri, and 3 miles north of the type section.
The type section of the Moccasin Springs Formation is in a ravine in the Mississippi River bluffs (SE SE NW 24, 32N-14E).
The interreef facies of the Moccasin Springs Formation is similar to the Mississinewa Shale Member of the Wabash Formation in Indiana and the Henryhouse Formation in Arkansas and Oklahoma.
Extent and Thickness
The formation is 100-130 feet thick in the outcrop area, and a thick section is exposed along Sammons Creek (Salamans on some maps), Alexander County (cen. S line 2, 15S-3W) (Pryor and Ross, 1962). The formation thickens northeastward and is commonly 160-200 feet thick, but it is as much as 400 feet thick bordering the reefs. The Moccasin Springs contains numerous reefs (fig. S-1), some as much as 1000 feet thick (Lowenstam, 1949) (fig. S-11).
The position of the St. Clair--Moccasin Springs contact in the northern Illinois Niagaran is uncertain, and the Moccasin Springs may be equivalent to only the upper part of the Racine Formation, including and above the "Lecthaylus shale" that is approximately 200 feet above the base of the Racine. Ross (1962b) reported a lower Ludlovian Monograptus from about 50 feet below the top of the Moccasin Springs, which makes it possible that much of the formation above that position may be Cayugan in age.
The Moccasin Springs is dominantly red, or red- and gray-mottled, very silty, argillaceous limestone and calcareous siltstone, with shale common near the top. Beds that lack the red mottling become more abundant upward and northward. The Moccasin Springs reefs are dominantly limestone, have well defined flank structures, and many have been large producers of oil. The interreef Moccasin Springs is differentiated into three members that have not been formally named. The lower is red- and gray-mottled shale and silty limestone that grades eastward from the outcrop area into alternating beds of mottled argillaceous limestone and red calcareous siltstone that have a characteristic and widely traceable electric log curve described as the "2-kick, 3-kick zone" (fig. S-16) throughout much of the basin and bordering the reefs. The overlying member is more uniformly red to greenish gray limestone, but it has some argillaceous and silty beds. The uppermost member is dominantly green shale, but it contains beds of red-brown siltstone and limestone.
LOWENSTAM, H. A., 1949, Niagaran reefs in Illinois and their relation to oil accumulation: Illinois State Geological Survey Report of Investigations 145, 36 p.
PRYOR, W. A., and C. A. ROSS, 1962, Geology of the Illinois parts of the Cairo, La Center, and Thebes Quadrangles: Illinois State Geological Survey Circular 332, 39 p.
ROSS, C. A., 1962b, Silurian monograptids from Illinois: Paleontology, v. 5, p. 59-72; Illinois State Geological Survey Reprint 1962-N.
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