**Stewartville Formation**
**Definition**
The Stewartville Formation is a geologic formation primarily composed of sedimentary rock layers dating to the Ordovician period. It is recognized for its fossil content and stratigraphic significance in the Midwestern United States, particularly within Minnesota.
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## Stewartville Formation
The Stewartville Formation is a notable geologic unit within the sedimentary rock record of the Upper Midwest, especially prominent in southeastern Minnesota. It represents a sequence of sedimentary deposits laid down during the Ordovician period, approximately 450 million years ago. The formation is significant for its well-preserved fossil assemblages, which provide valuable insights into marine environments and biotic communities of the Ordovician seas. It also plays an important role in regional stratigraphy, serving as a key marker for correlating rock units across the region.
### Geological Setting
The Stewartville Formation is part of the larger sedimentary sequence that characterizes the Paleozoic rock record of the Upper Midwest. During the Ordovician period, much of what is now the central United States was covered by a shallow inland sea. This marine environment facilitated the deposition of carbonate and clastic sediments that would eventually lithify into the rock units observed today.
The formation is typically found underlying younger Silurian units and overlying older Cambrian or Lower Ordovician formations, depending on the local stratigraphic context. Its thickness varies but generally ranges from several meters to over 30 meters, reflecting changes in depositional environments and sediment supply.
### Lithology
The Stewartville Formation is predominantly composed of limestone and dolostone, with interbedded shale layers in some localities. The carbonate rocks are often fossiliferous, containing abundant marine fossils such as brachiopods, trilobites, bryozoans, and cephalopods. The limestone is typically fine- to medium-grained and may exhibit features such as bioturbation, cross-bedding, and stromatolitic structures, indicative of shallow marine depositional settings.
Dolomitization is common in parts of the formation, reflecting post-depositional chemical alteration processes. Shale layers, where present, are generally dark gray to black and may contain organic material, suggesting periods of reduced oxygenation or deeper water conditions.
### Stratigraphy and Correlation
The Stewartville Formation is part of the Upper Ordovician stratigraphic sequence and is often correlated with other regional formations of similar age. It is typically assigned to the Richmondian stage of the Late Ordovician. The formation lies conformably above the St. Peter Sandstone or the Platteville Formation in some areas and is overlain by Silurian units such as the Niagaran or the Mayville formations.
Stratigraphic studies have utilized the Stewartville Formation as a reference point for regional correlation due to its distinctive lithology and fossil content. Its fossil assemblages help refine biostratigraphic zonations and improve understanding of the temporal and environmental changes during the Late Ordovician.
### Paleontology
Fossils within the Stewartville Formation are diverse and well-preserved, making the formation an important paleontological resource. The marine fauna includes a variety of invertebrates typical of Ordovician shallow seas:
– **Brachiopods:** Numerous species of articulate brachiopods are found, often forming dense assemblages. These organisms were benthic filter feeders and are useful for biostratigraphic dating.
– **Trilobites:** Several trilobite genera have been identified, providing insights into the diversity and evolution of these arthropods during the Ordovician.
– **Bryozoans:** Colonial bryozoans are common, contributing to reef-like structures and carbonate sediment production.
– **Cephalopods:** Nautiloid cephalopods are present, indicating open marine conditions and serving as important index fossils.
– **Other Fossils:** Crinoids, gastropods, and echinoderms are also found, reflecting a complex marine ecosystem.
The fossil assemblages suggest that the Stewartville Formation was deposited in a warm, shallow marine environment with clear waters conducive to carbonate sedimentation and diverse benthic communities.
### Depositional Environment
The sedimentological and paleontological evidence indicates that the Stewartville Formation was deposited in a shallow, epicontinental sea. The presence of carbonate rocks with abundant marine fossils points to a warm, tropical to subtropical setting with relatively low clastic input. Periodic fluctuations in sea level and sediment supply likely influenced the lithological variations observed within the formation.
Features such as cross-bedding and bioturbation suggest dynamic conditions with currents and benthic organism activity. The interbedded shales may represent deeper or quieter water intervals, possibly linked to transgressive-regressive cycles during the Late Ordovician.
### Economic and Scientific Importance
While the Stewartville Formation is not a major source of economically valuable minerals or hydrocarbons, it holds scientific importance for several reasons:
– **Stratigraphic Marker:** Its distinctive lithology and fossil content make it a useful stratigraphic marker for correlating Ordovician rocks across the Midwest.
– **Paleontological Resource:** The formation provides a window into Ordovician marine life, aiding studies of biodiversity, paleoecology, and evolutionary patterns.
– **Sedimentological Studies:** Analysis of its depositional features contributes to understanding sedimentary processes in ancient shallow marine environments.
– **Educational Value:** The formation is often studied in academic settings for its well-preserved fossils and clear stratigraphic relationships.
### Regional Distribution
The Stewartville Formation is primarily exposed in southeastern Minnesota, with subsurface extensions into adjacent states such as Iowa and Wisconsin. Its outcrops are commonly found in river valleys, road cuts, and quarries, where the sedimentary layers are accessible for study.
The formation’s distribution reflects the extent of the Ordovician inland sea and the depositional environments that prevailed during its formation. Variations in thickness and lithology across the region provide clues to paleogeographic and tectonic influences during the Late Ordovician.
### Research History
The Stewartville Formation was first described in the early 20th century during geological surveys of Minnesota. Subsequent research has focused on detailed stratigraphic mapping, fossil identification, and sedimentological analysis. Advances in biostratigraphy and geochronology have refined the age and correlation of the formation within the broader Ordovician framework.
Ongoing studies continue to explore the formation’s paleoenvironmental conditions, diagenetic history, and its role in regional geology. The Stewartville Formation remains a subject of interest for geologists and paleontologists studying the Paleozoic era in North America.
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## See Also
– Ordovician Period
– Paleozoic Sedimentary Rocks
– Brachiopods
– Trilobites
– Stratigraphy of the Midwestern United States
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**Meta Description:**
The Stewartville Formation is an Ordovician sedimentary rock unit in the Midwestern United States, known for its fossil-rich limestone and significance in regional stratigraphy. It provides important insights into ancient marine environments and biotic communities.