Abstract
The composite Cambrian sequence of the Lower Peninsula of Michigan consists, in ascending order, of the Mt. Simon Sandstone; Munising Group, composed of the Eau Claire Formation, Galesville Sandstone, and Franconia Formation; St. Lawrence Formation; Lodi Formation (a minor unit); and the Jordan Sandstone. They compose an estimated 3,000 ft (915 m) of section at the approximate depocenter, near or possibly in Ogemaw County.
These rocks long have been an enigma to Michigan geologists because of a lack of both drilling information and outcrops. When viewed against the broad, regional stratigraphic framework and defined rigorously in terms of lithology, they are clearly identifiable as representing sedimentary facies deposited in or near the margins of a transgressive-regressive sea.
The Mt. Simon Sandstone represents the high-energy shoreline facies of the northerly transgressing Cambrian sea. The glauconitic Eau Claire and Franconia Formations of the Munising Group are composed of sandstone, siltstone, and minor dolomite and shale interbeds which reflect a lower energy, near-shore marine environment. These two units can be distinguished from one another only where separated by the Galesville Sandstone, a regressive unit restricted to southern and western Michigan. Outside the depositional limits of the Galesville, there is present within the Munising Formation interbedded sandstone, dolomite, shale, and nodular anhydrite which, taken together, strongly suggest a shallow to supratidal environment of deposition. Known only in central Michigan, the areal extent of this facies is undetermined.
Essentially a dolomite unit, the St. Lawrence Formation contains many chert and cherty sandstone interbeds in southwestern and western Michigan. The term St. Lawrence includes those stratigraphic elements called Trempealeau and Prairie du Chien in previous investigations. The St. Lawrence grades upward into the Jordan Sandstone, a regressive sandstone unit that documents the gradual infilling of the Michigan basin area. The Jordan Sandstone has been identified erroneously in prior reports as the St. Peter Sandstone of the Mid-Continent region, but the St. Peter was not recognized anywhere within the area of study.
The top of the section is bounded by an erosional unconformity of regional extent on which rocks of the Glenwood Formation (Middle Ordovician) were deposited.
The Mt. Simon through Jordan rocks form an essentially continuous sequence of deposition beginning with marine transgression probably in the Late Cambrian and ending with regression and regional uplift in the Early Ordovician. The sedimentary record clearly places the formation of the Michigan basin back at least to Late Cambrian time.
In the light of the growing energy crisis in the United States, the clarification of the stratigraphic relations of these rocks should lead to increased interest in their petroleum potential.
