Abstract

Iron content and cathodoluminescence define six regionally extensive calcite cement zones over 100,000 sq. km of study area in the Burlington-Keokuk Formation of Illinois and Missouri. The regional cement stratigraphy is composed of mappable zones, analogous to lithostratigraphic units, which represent diagenetic episodes during which calcite with distinct cathodoluminescent properties precipitated in a given time interval relative to other diagenetic events such as cement fracturing and calcite dissolution. Individual cement zones may not be strictly isochronous throughout the study area but are not strongly diachronous. Two different, but related, cathodoluminescent sequences have been identified within pre-Pennsylvanian cements in the Burlington-Keokuk Formation: 1) within lower strata, early, nonferroan cement consists of both nonluminescent and luminescent calcite (zones II-IV); 2) within upper strata, early, nonferroan cement consists of entirely luminescent calcite (zone II'). Petrographic relationships indicate that zone II' is time equivalent with zones II-IV in lower Burlington-Keokuk strata. The occurrence of cement zones II-IV encased in Pennsylvanian shale and quartz sand within pre-Pennsylvanian karst cavities constrains the age of early, nonferroan calcite as pre-Pennsylvanian. In all cases, pre-Pennsylvanian cements are followed by ferroan, dull to moderately luminescing calcite (zone V) which, in turn, is overgrown by nonferroan, moderate to brightly luminescing calcite (zone VI). Cross-cutting relations of Burlington-Keokuk syntaxial calcite with Mississippi Valley-type mineralization phases constrains the minimum age of cement zones V and VI as Early Permian. Pre-Pennsylvanian cements formed in a shallow, meteoric aquifer characterized by vertically segregated chemical facies. Pre-Pennsylvanian cements within lower strata precipitated from waters of varying Eh, while those in upper strata formed from reduced waters. Post-Mississippian/pre-Early Permian cements precipitated under less than 1 km of overburden from reducing meteoric waters or subsurface brines.

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