Carbonate-Cement Stratigraphy of Burlington Limestone (Osagean) of Iowa: Evidence for Eh Gradients in a Regional Mississippian Paleogroundwater System: ABSTRACT

Abstract

Cathodoluminescent petrography of calcite overgrowth cements in crinoidal grainstones from the Burlington Limestone reveals compositional zoning, which suggests that a chemical gradient was effective during cementation. These syntaxial calcite cements have as many as 7 luminescent zones. On the basis of these zones, a regional cement stratigraphy has been established among 22 measured sections within a 5,000 mi2 area in southeastern Iowa. Overgrowths are interpreted as freshwater phreatic-zone precipitates. Comparison of the characteristic luminescent signature of Burlington calcite cements with cements in underlying Kinderhookian and overlying Meramecian limestones suggest the Burlington formed during, or before, the regional mid-Meramecian (sub-St. Louis Lime tone) hiatus. Early nonferroan calcite cements show a pronounced basinward change in luminescent zoning. Updip cements consist of 4 distinct compositional zones. In contrast, downdip cements contain only a single luminescent zone. Zonal distribution may have formed when cementation occurred along an Eh gradient within a fresh or phreatic-water system during exposure in mid-Meramecian time. Updip recharge areas were characterized by alternating oxidizing and reducing conditions, resulting in a sequence of luminescent (low Eh) and nonluminescent (high Eh) cement zones. Downdip, away from recharge areas, contemporaneous cements have only a single luminescent zone, which formed under continuously reducing conditions. The recognition of Eh gradients that were effective during carbonate cementation enabl s paleoflow paths to be determined, and aids in the prediction of diagenetic trends. End_of_Article - Last_Page 263------------