Interpretation of the Geologic History of the Rock Strata as Observed at Robins Quarry in Linn County, IA

April 12th, 2003

Sedimentary rock strata at Robins Quarry were sampled and examined in an attempt to interpret the history of the geologic processes in the area. An expedition was made to the quarry in order to take samples from each lithology. Samples were taken from the Spring Grove limestone, the Davenport limestone, the Solon limestone, and the Pennsylvania Karst. The three limestone layers were lain down in the Devonian period (410-360 million years ago), while the shale was deposited in the Pennsylvanian (325-285 million years ago). Laboratory analysis of the samples included x-ray diffraction of insoluble residues, categorization of fossils, and microscopy. In addition x-ray difraction data from previous research was studied. Lithologies were also examined for diagenetic characteristics. X-ray diffraction revealed the mineral composition of each sample. The varieties of brachiopods, corals, echinoderms, sponges, and bryozoans gave an indication of the undersea environment of the time. Microscopy and analysis of the mineral size in the samples gave an indication of what conditions the sediment was lain down in. Diagenetic characteristics observed in the lithologies were oxidation, dolomitization, chertification, vugs, brecciation, karst cave-fill, pyrite and marcasite precipitation, formation of calcite crystals, silicification, and kaolinite recrystallization, which indicate what geologic processes acted on the sediment after deposition. The sum of the collected data infers that in the Devonian period what is now southeastern Iowa was submerged beneath a warm, shallow epicontinental sea.

Michael Lommler, ’06 Springfield, IL
Major: Undecided
Amelinda Webb, ’06 Grand Island, NE
Major: Undecided
Mark Wegner, ’06 Northfield, MN
Major: Undecided

Sponsor: Paul Garvin

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