This chapter cites two localities, Paxton Quarry and Partridge Point (Fig. 1), and combines field observations fromboth to illustrate important stratigraphic relations and principles.Focus is on litho- and biostratigraphyof Middle and Late Devonian rocks as they relate to the Michigan Basin (Fig. 2).
Black organic-rich shales deserve our attention due to their energy potential asource rocks and fracture reservoirs for petroleum and natural gas. The Paxton Quarry offers the opportunity to examine an exceptional exposure of black shale (large area in Fig. 3; thick section in Fig. 4) as part of the formational sheet that continues into the Michigan Basin subsurface. These special rocks offer achallenge to observe and test their physical, chemical, and organic composition and structureto decipher origin, paleoenvironments, diagenesis, history, and economic value. A spectacular display of numerous, largecalcareous concretions, in situ and free of shale matrix, is present in the quarry. How werethey formed?
Partridge Point has an exposed limestone sequence that underlies Antrim black shales beneath Squaw Bay. Applying Walther’s Law, one can go up section and down dip from fossiliferous shallow-water oxygenated Middle Devonian carbonate platform rocks at the shelf marginof the Michigan Basin into Upper Devonian transitional deeper water oxygen-deficient pelagic limestones followed by deep-wateranaerobic black shales of a euxinic basin.
Figures & Tables
North-Central Section of the Geological Society of America
One hundred field guides, with area maps, to locations in Illinois, Indiana, Iowa, Michigan, Minnesota, Missouri, Nebraska, Ohio, and Wisconsin.