Weathering of the Eaton Sandstone (Pennsylvanian), Grand Ledge, Michigan: Geochemical Mass-Balance and Implications for Reservoir Properties Beneath Unconformities

Abstract

The Eaton Sandstone (Pennsylvanian), which crops out near Grand Ledge, Michigan, has been modified by weathering. Qualitative and quantitative modifications in detrital and diagenetic mineralogy are identified by comparing outcrop samples with subsurface counterparts collected from depths ranging from 22 to 41 meters below surface grade. Mass-balance calculations suggest that most major elements are removed from the sandstone during weathering, but iron appears to be conserved within the sandstone, with local Fe redistribution occurring on the scale of meters from joint-block interiors to 1-2 cm thick zones of ferruginous cementation (groundwater ferricretes) at joint faces. Dissolution of detrital schistose lithic fragments has converted a sublitharenite to a quartz arenite. Two possible sequences of weathering events for the Pennsylvanian Eaton Sandstone are proposed: (1) simultaneous cement dissolution and case hardening involving formation of groundwater ferricretes simultaneously with weathering of ferrous sulfides and carbonates; dissolved iron generated during degradation of these phases diffused to the joint faces for groundwater ferricrete formation; and (2) sequential cement dissolution and case hardening where weathering of ferrous carbonates and pyrite resulted in replacement by ferric oxyhydroxides initially, followed by the dissolution of the ferric oxyhydroxides in dilute reducing waters. The latter step liberated iron, which contributed to the formation of the groundwater ferricrete at the joint faces. The subaerially exposed and weathered Eaton Sandstone serves as a modern analog of weathering products commonly preserved beneath unconformities in sedimentary sections. Carbonate cements have been dissolved from the subaerially exposed sandstone, resulting in rejuvenation of primary intergranular porosity. Kaolinite in the Eaton Sandstone is an early burial diagenetic mineral and is unrelated to subaerial weathering of K-feldspar; K-feldspar appears to be stable in outcrop. Kaolinite is presently dissolving, with the Al3+ being mobilized into the aquifer. These observations suggest that: (1) kaolinite may be destroyed as well as formed beneath an unconformity, and (2) kaolinite is not a reliable indicator of an unconformity. The ferruginous-cemented joint faces have the potential to create seals with respect to fluid flow within the sandstone. The seals have strong implications for aquifer and reservoir compartmentalization.