The origin of mottled dolomitic limestones in the Platteville group (Ordovician) of Northern Illinois
Mottled dolomitic limestones are a characteristic feature in rocks of Ordovician age throughout the Upper Mississippi Valley. Several hypotheses have been presented concerning the origin of these limestones and recent theories favor a combination of processes. Previous work dealing with mottled dolomitic has been summarized to note similarities and differences in hypotheses of origin. From this summary an "idealized" mottled dolomitic has been constructed. Recent theories favor a burrow origin for the mottles. The burrows later influenced dolomiting fluids through a permeability control. Data collected from a suite of samples in the Platteville Formation of northern Illinois lead to similar conclusions. Samples were analyzed by serial sectioning, thin sections, microprobe analysis and X-ray analysis of insoluble residue. From this analysis the following hypothesis is presented. Mottles are of organic origin and probably represent burrows of sediment-eating organisms. Synsedimentary lithi- fication of the sediment produced a marked heterogeneity between sediment filling the burrows and sediment surrounding burrows. Burrows during this time also became outlined with pyrite. During late diagenesis movement of dolomitizing fluids passing through the sediment was influenced by the burrows. Being less permeable the burrows were dolomitized first.