Mudstone facies of the Upper Ordovician Maquoketa Group of Indiana and their paleoenvironments.

摘要

The Upper Ordovician Maquoketa Group is a mixed carbonate-clastic succession found in Indiana, the basal portion of which is laterally equivalent to the Utica Shale of Ohio. Previous work on this succession had largely ignored small-scale (i.e., cm-scale) compositional heterogeneities within the fine-grained mudstones. The purpose of this study is to define a set of facies, and subsequently link these textually and compositionally defined facies with depositional environments to assess their paleogeographic distribution. These facies are defined using more detailed sedimentary criteria, from the hand sample to thin section scale, and are used to arrive at environments of deposition.;Detailed descriptions were made of three cores, from northeast, north central, and northwestern Indiana, parallel with the Kankakee Arch. The characteristics used for describing the rocks include Bedding, with emphasis on lamina and lamina sets; Color; Physical Sedimentary Structures, such as ripples, trace fossils, nodules or concretions; Bioturbation; Fossils; Effervescence (reaction to HCl); and Diagenetic features. Paleoenvironmental variables are the inferred position on the marine shelf, water depth, the current activity of the environment (how energetic it was), the sedimentation rate, and the amount of diffused oxygen that was present. In the distal setting of Fulton and Lake Counties, Indiana, the variety of mudstone facies present diverges from the more common facies found in Wells County or changes into different facies.;The study shows that a range of mudstone facies can be differentiated, that these can be traced across a depositional basin, and that environments of deposition can be inferred to a reasonable degree. Stacking patterns of mudstone facies change when moving westward: new mudstone facies appear, several disappear, and the thicknesses of various packages change. This could indicate a combination of bed thinning, erosion, or non-deposition. The overall matrix composition of the group changes with regards to calcite cement, becoming dolomitic distally and less silt-rich. This probably reflects major changes in depositional environment, indicating a shift to a shallower marine setting with less detrital input.