Sandstone Geology
Jacobsville Geology
The Jacobsville Sandstone is an example of “red bed” sandstone formations which have formed from rivers flowing from mountains toward the rift. The red color comes from hematite (Fe oxide) which formed after water flowed through the pores of the sandstone after deposition. The formation is made up of pieces of rock that were once part of the Huron Mountains and other ranges which rose near where the UP/Wisconsin border is today. We know this must be true by “reading the rocks”--looking very carefully at them and explaining how they must have formed by analogy with similar modern sediments which are forming now. Rock reading is one of the geologist’s main activities--how we make a living.
Jacobsville Sandstone near Rabbit Bay
Jim Belote
Michigan Geo Map sources:
excerpt from: GEOLOGIC MAP OF THE KEWEENAW PENINSULA AND ADJACENT AREA, MICHIGAN By William F. Cannon and Suzanne W. Nicholson
The correlation, if any, between the Jacobsville and units of the Oronto Group is not known. Details of the Jacobsville Sandstone are summarized by Kalliokoski (1982). From its erosional edge on the south and east, the Jacobsville thickens northwestward toward the Keweenaw fault and reaches a maximum thickness of about 3 km. The Jacobsville is mostly feldspathic sandstone, a considerably more mature unit than the Oronto Group. It lies with a low-angle unconformity on basalt flows of the Siemens Creek Volcanics indicating a period of gentle tilting and erosion between the early rift phases and deposition of the Jacobsville. The base of the formation onlaps over older rocks of the Michigamme Formation and Late Archean gneisses in the southeastern part of the map area.
