Devonian, fossils, minerals, silicification
Diagenesis of fossils contained in middle-Devonian limestones was studied at the Troy Mills and Robins quarries in Linn County, Iowa and at the Four County and Ernst quarries in Johnson County, Iowa. Fossils located at weathered bedrock surfaces were preferentially silicified, and the silicification affected corals, stromatoporids and, to a limited extent, brachiopods, but not other fossils or the host rock. The siliceous horizon extends no more than a few centimeters below the weathered surfaces, suggesting that silicification was constrained by them. Silica for silicification might have come from a variety of sources, including sponge spicules or radiolarian tests, insoluble residues (fine-grained quartz and clay minerals) contained within the host carbonate rock, sediment deposited over the area that was subsequently eroded, or from weathering and alteration of clay minerals contained in paleokarst-hosted Pennsylvanian fluviatile sediment. A change in pH or other chemical variable(s) in upward- or downward-moving fluids, at the interface between Devonian carbonate rock and adjacent Pennsylvanian fluviatile sediment, could have created an environment favorable for silica precipitation. The extent of silicification of different fossils was likely controlled by variations in permeability of skeletal material. Mineral diagenesis began with precipitation of euhedral microcrystalline calcite, followed by three types of quartz (megacrystalline quartz, chalcedony and microcrystalline quartz), and ended with anhedral coarse-grained calcite. The position in the paragenetic sequence of rare chert and dolomite could not be determined.
Journal of the Iowa Academy of Science
© Copyright 2008 by the Iowa Academy of Science, Inc.
Majeski, Adam L. and Garvin, Paul L.
"Silicification of Corals, Stromatoporids, and Brachiopods at the Weathered Surface Within the Devonian-Age Little Cedar Formation (Solon and Rapid Members) in Eastern Iowa,"
The Journal of the Iowa Academy of Science: JIAS: Vol. 113:
, Article 6.
Available at: http://scholarworks.uni.edu/jias/vol113/iss3/6