Historical:Mt. Simon Sandstone
T. C. Buschbach
The Mt. Simon Sandstone (Ulrich, in Walcott, 1914, p. 354) is named for Mount Simon, an escarpment near Eau Claire, Wisconsin.
The lower part of the formation has been correlated with Cambrian and Precambrian sandstones in the Lake Superior region, in particular with the Bayfield Sandstone of northern Wisconsin, the Fond du Lac Sandstone in Minnesota, and the Jacobsville Sandstone of northern Michigan, and also with lower and middle Cambrian sediments in Kentucky and Tennessee. However, because there is no evidence of formational breaks within the Mt. Simon, it is retained as a unit in the Croixan Series.
The type section of the Mt. Simon Sandstone is an escarpment near Eau Claire, Wisconsin, which consists of 234 feet of coarse-grained, partly conglomeratic sandstone overlying Precambrian granite and overlain by fine-grained sandstone of the Eau Claire Formation.
Extent and Thickness
The Mt. Simon Sandstone, although not exposed, underlies all of Illinois except in local areas where it failed to cover hills on the Precambrian surface. It ranges from less than 500 to 2600 feet thick, with the greatest thickness in northeastern Illinois (fig. Є-6).
The base of the Mt. Simon is the sub-Sauk unconformity. The contact with the overlying Eau Claire Formation is conformable. Relatively fine-grained units alternate with coarse-grained, pebble-bearing units to divide the Mt. Simon into seven members: Crane Member, at the base (fine to medium grained), Kenyon Member (pebbly), Lovell Member (medium grained), Mayfield Member (interbedded pebbly and medium grained), Lacey Member (pebbly), Gunn Member (medium grained), and Charter Member (pebbly). These units are recognized in borings in north-central Illinois (Templeton, 1950), but have not been traced throughout the state. Only the upper three have been recognized in northeastern Illinois (Buschbach, 1964).
The Mt. Simon consists of fine- to coarse-grained, partly pebbly, friable sandstone, most of which is coarser grained, more poorly sorted, and more angular than younger Cambrian and Ordovician sandstones. The pebbles, mostly quartz, are as much as 8 mm in diameter in central northern Illinois and only 3.5 mm in northeastern Illinois. Most of the sandstone is white, but in some areas much of it is red, particularly the lower part, and in other localities some of it is yellow to light greenish gray. A basal zone, as much as 350 feet thick, is strongly arkosic. Beds of red and green micaceous shale, in some places 15 feet thick, occur, especially in the upper 300 feet and the lower 600 feet of the formation. The Mt. Simon Sandstone is an important aquifer in northern Illinois, and on several major structures it is used for the storage of natural gas.
The Mt. Simon is probably entirely marine, but its characteristics are unknown in a large part of the state.
BUSCHBACH, T. C., 1964, Cambrian and Ordovician strata of northeastern Illinois: Illinois State Geological Survey Report of Investigations 218, 90 p.
TEMPLETON, J. S., 1950, Mt. Simon Sandstone in northern Illinois: Illinois Academy of Science Transactions, v. 43, p. 151-159.
WALCOTT, C. D., 1914, Cambrian geology and paleontology: Smithsonian Miscellaneous Collections, v. 57, p.
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