Jacobsville Sandstone is a red sandstone formation, marked with lightly-colored streaks and spots, primarily found in northern Upper Michigan, portions of Ontario, and under much of Lake Superior. Desired for its great durability and aesthetics, the sandstone was used as an architectural building stone in both Canada and the United States. The stone was extracted by thirty-two quarries throughout the Upper Peninsula of Michigan State. Approximately between 1870 and 1915. The sandstone has been variously called redstone, brownstone, Lake Superior Sandstone, and Eastern Sandstone. In 1907, the Jacobsville Formation was given its current classification and the name Jacobsville, in honor of Jacobsville, Michigan, a town known for its production of the sandstone. The sandstone was deposited within terrestrial fluvial environments early in the Neoproterozoic Era.
Geology
Classification The earliest geologic studies of southern Lake Superior were made in the early 1800s. Many studies used the term Lake Superior Sandstone to describe a number of different geologic formations. Differentiation of the sandstone largely centered on an east-west division across the Keweenaw Peninsula or between the lower red and upper grey sandstones. The current definition for the Jacobsville Formation was made in 1907 by A. C. Lane and A. E. Seaman, in which the various Lake Superior sandstones were divided into the Jacobsville, Freda, and Munising. The Jacobsville reflects the sandstones described as either lower red or Eastern. They also introduced the name Jacobsville in honor of Jacobsville, Michigan, a town notable for its production of the sandstone including the "famous Portage Redstone". Similarities in lithology and heavy mineral suites between the Jacobsville Sandstone and the Bayfield group, a similar sandstone located in northern Wisconsin, suggest they may be correlated. Jacobsville Sandstone is most similar to the group's uppermost formation, the Chequamegon Sandstone. Despite their similarities, it is unknown as to whether the two formations are one and the same. A correlation between the Jacobsville and the Freda Sandstone (instead of the Bayfield) has been suggested on the basis on similar mineralogy, texture, and stratigraphic position. The formation is similar to the Middle Run Formation in southwestern Ohio. Early studies also suggested a correlation between the Jacobsville and the Potsdam Sandstone in New York. A red clastic layer in Putnam County, Ohio, was tentatively termed Jacobsville Sandstone in 1948 by Charles Fettke.
Description Jacobsville Sandstone is generally red, due to small quantities of iron oxides, mottled with various pinks, whites and browns. The sandstone exhibits many white streaks and spherical spots, caused by leaching and bleaching, which form sharp boundaries with surrounding colors. Its lithology varies from quartz-rich to feldspar-rich sandstone and shale. The formation consists of four distinct facies: conglomerate, lenticular sandstone, massive sandstone, and red siltstone. The sandstone is well-cemented and contains no glacial deposits and no fossils. The grains of Jacobsville Sandstone range from .25 to .5 mm (0.0098 to 0.0197 in) in size. Based on an average from samples taken in Marquette and Alger counties, Jacobsville Sandstone is composed of: 27.4% nonundulatory quartz, 27.0% undulatory quartz, 23.0% potassium feldspar, and 12.3% silicic volcanic clasts. Smaller constituents are 3.8% polycrystalline quartz, 2.4% metamorphic, 1.4% sedimentary, 1.3% opaque, 0.8% mafic volcanic, and 0.1% plagioclase. Jacobsville Sandstone's heavy mineral suite is 84% ilmenite, 4–5% leucoxene, 4% garnet, 3% apatite, 3% zircon, and 1% tourmaline.
Extent and stratigraphy
The formation lies southeast of the Keweenaw Fault in the Keweenaw Peninsula, and south of Lake Superior further east in the Upper Peninsula. In Ontario, Jacobsville Sandstone underlies Sault Ste. Marie and extends discontinuously along the lake shore from Bar River in the south to near Wawa in the north. The formation also has a small extent in Iron County, Wisconsin. Due to the highly irregular surface on which it was deposited, the formation varies in thickness from 5 ft (1.5 m) to over 1,800 ft (550 m) in Michigan. The sandstone also underlies most of Lake Superior, west of Munising in particular, at a maximum thickness of over 3,000 ft (910 m). The Jacobsville Formation lies unconformably beneath the Munising Formation and above the Oronto Group. The formation is an upper member of the Keweenawan Supergroup. Jacobsville Sandstone varies from horizontal to gently inclined and paleocurrents point toward the Lake Superior basin. The Jacobsville Formation outcrops significantly throughout the Upper Peninsula, with lesser exposures in Ontario and Wisconsin. Cliffs of the sandstone tend to have blocky talus that supports little vegetation. Jacobsville Sandstone, rising only a few feet above lake level, is the oldest formation exposed in Pictured Rocks National Lakeshore. The sandstone features prominently on Grand Island in Hiawatha National Forest.
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