Document Type

Student Paper

Publication Date

Winter 1-7-2025

Disciplines

Geology

Description, Abstract, or Artist's Statement

During the Devonian Period, eastern to central Iowa experienced multiple transgressive and regressive intervals. This study focuses on the Cou Falls and Gizzard Creek Members of the Coralville Formation, deposited during the Eifelian Stage. Sometime after the formation of the limestone, the interiors of various brachiopods, gastropods, and other fauna began to weather out, creating vugs within the limestone, which gave rise to the first of three growth phases of calcite in the form of rhombohedrons and scalenohedrons. After these primary calcites were formed, new fluids flowed through these vugs, depositing a previously misidentified iridescent film on the calcite crystals, observed in multiple locations in the region. Prior to this study, this iridescent film was being classified as stilpnosiderite, a pseudomorph of limonite. After the thin-film formed, two more phases of calcites grew: a secondary growth of large, white rhombohedrons and a tertiary growth of large scalenohedrons. These differences in morphologies are probably related to progressive changes in elemental concentrations of the groundwater. The focus of this research is to determine the thin-film composition and identify geochemical conditions of precipitation on the primary crystals. Based on SEM analysis, the film’s composition is a mix of aluminum, calcium, nitrogen, silicon, and sulfur. When vugs are exposed to the natural elements (sunlight, rain, and groundwater), the film begins to disappear. Through experiments to simulate the natural environments of sunlight, rain, and groundwater for 100 days, it is evident that the film does not deteriorate within that time frame.