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| The Dunleith Formation, exposed in Winneshiek county, Iowa, is Middle Ordovician in age. Its fresh surface is a light gray color with somewhat visible grains of calcite within. Weathered surfaces add a greenish tint to the gray color. In hand sample, the matrix appears to be sparry, but on further thin sample investigation, it is observed to be a micritic matrix. The thin section of this formation is dominated by well preserved echinoderms, but also contains bivalves, brachiopods, bryozoans, and trilobites, in descending order of appearance. The Dunleith Formation is classified as a biomicrite under the Folk system due to its micritic matrix, with significant biological components. It is classified as a packstone under the Dunham system due to its mud-based matrix and grain-supported nature. The Dunleith formed in a medium energy marine environment below wave base, as it still contains significant mud content, but is poorly sorted. |
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This photograph centers on a well-preserved bryozoan, cut at an oblique angle. Notice the detail of the fossil as well as the large size of the fossil relative to the surrounding fragments. These unidentifiable fossil fragments are found throughout the sample. Width of view: 3.0mm (40x)
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In this photograph is a recrystallized bivalve fossil. The micritic nature of the matrix is very obvious in this view. The micritic matrix shows that the Dunleith formed below wave base, or else this mud would have been washed away. Width of view: 3.0mm (40x)
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This photograph shows a columnal echinoderm, the dominant component of this formation. The cut of this sample is perpendicular to it width, showing the external structure of the echinoderm well. Notice the crinulations along the outer edges, indicating the external structure, as well as the cut angle. Width of view: 3.0mm (40x)
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Here in the center of this photograph is a well-preserved trilobite. Cut along its length, its visible structure is characteristic of trilobites. Width of view: 3.0mm (40x) |