A PRELIMINARY STUDY OF THE EXTENT OF DIAGENETIC ALTERATION IN THE SPRING GROVE MEMBER OF THE WAPSIPINICON FORMATION (MIDDLE DEVONIAN, IOWA, USA)

The Spring Grove Member is a Middle Devonian carbonate deposit present in Eastern Iowa. Its lithology varies between dolostone and limestone, but is most commonly referenced as a dolostone. Where it is present as limestone, it is sometimes quarried for high quality aggregate used in pavement construction. In several areas of Eastern Iowa, the Spring Grove could potentially fill in high quality aggregate production gaps- however, its quality is unknown overall. In at least one quarry where it produces high quality limestone aggregate, the Spring Grove exhibits a complex diagenetic history. To map rock properties that are important to aggregate production (e.g., low clay, pure limestone or dolostone, and low microporosity), the logged wells from Iowa were mapped according to the chemistry and thickness of the Spring Grove. This mapping showed two lobes of dolomitized Spring Grove, one along the Mississippi River and one along Iowa’s southern border. Lime lithologies are found between the dolomitic areas, and were thickest in Central, Eastern, and Northeast Iowa.

Following this, Spring Grove rock cores were sampled for thin sectioning as well as petrophysical, and geochemical analyses. Thin sections revealed a complex diagenetic history of dolomitization, dedolomitization, karstification, fracturing, and brecciation. There are fine and coarse dolomites with high and low porosities, and one with a stylolite preserved. The limestones vary from mudstones with clear depositional textures preserved to crystalline calcite with dolomoldic porosity. The calcite cement varies from isopachous and sparry to fibrous. In all the thin sections, only one fossil is present—an ostrocod in a peloidal lime mudstone from central eastern Iowa. The most northern and eastern samples have rare well-rounded quartz grains. Helium porosity data ranges from 4% to 50%. The overall trends in chemistry, porosity, and texture are still not well understood, thus adding uncertainty to predictions of aggregate quality within the Spring Grove.