Abstract: Diagenesis at a Modern Unconformity: Pennsylvanian Sandstones, Grand Ledge, Michigan

Jason R. Price, Michael A. Velbel

Petrographic and mineralogical comparison of the subaerially exposed Eaton Sandstone [Conemaugh] with subsurface Pennsylvanian sandstones gives insight into epidiagenetic processes associated with unconformities. (1) Detrital framework modes have been altered, transforming sublitharenite into diagenetic quartz arenite. (2) Leaching of carbonate cement has rejuvenated primary porosity. (3) The formation of diagenetic kaolinite is unrelated to the present unconformity, and the early formed kaolinite is presently undergoing dissolution. These observations call into question the utility of kaolinite as an indicator of unconformity related diagenesis. (4) Iron has been redistributed from joint-block interiors to the faces of the joint blocks, forming a well-indurated, ferruginous-cemented one roughly 1-2 cm thick along joint faces (groundwater ferricrete). The amount of iron in a unit volume of groundwater ferricrete required removal of iron from 16 unit volumes of joint-block interior sandstone. Iron was mobilized from the interiors of the joint blocks and accumulated at the joint faces, evidently oxidized and precipitated by oxygenated water flowing along the fractures. This alteration took place after the sandstones were sufficiently well-indurated to develop systematic joint sets, but before the sandstones were uplifted into the vadose zone. The presence of indurated zones along joint faces should have profound effects on aquifer and reservoir properties of the sandstone.

AAPG Search and Discovery Article #90984©1994 AAPG Annual Convention, East Lansing, Michigan, September 18-20, 1994