BH250-128b
Title
BH250-128b
Subject
Oneota Dolostone
Description
The Prairie du Chien Group is a Lower Ordovician, carbonate-dominated sequence that is widely exposed across the Upper Midwest, including Minnesota, Iowa, Wisconsin, and Illinois. The Oneota Dolostone (e.g., sample BH250-128b) forms the lower unit of this group and is primarily composed of dolostone, with interbedded chert and occasional sandstone or siltstone layers near its base.
The Shakopee Dolostone (also known as the Shakopee Formation) is the upper unit of the Prairie du Chien Group, conformably overlying the Oneota Dolostone. It shares a similar dolomitic composition but is distinguished by the increasing abundance of sandy beds toward its upper section, especially where it approaches the contact with the overlying St. Peter Sandstone. This transition reflects a shift from carbonate to siliciclastic sedimentation.
Layered carbonate structures formed by microbial mats are occasionally found within both units, particularly in Minnesota. In some exposures, cryptozoan-type stromatolites are preserved in carbonate dominated facies.
The Shakopee Dolostone (also known as the Shakopee Formation) is the upper unit of the Prairie du Chien Group, conformably overlying the Oneota Dolostone. It shares a similar dolomitic composition but is distinguished by the increasing abundance of sandy beds toward its upper section, especially where it approaches the contact with the overlying St. Peter Sandstone. This transition reflects a shift from carbonate to siliciclastic sedimentation.
Layered carbonate structures formed by microbial mats are occasionally found within both units, particularly in Minnesota. In some exposures, cryptozoan-type stromatolites are preserved in carbonate dominated facies.
Coverage
Location: Memorial Park, Red Wing, Minnesota, USA
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-128b. Housed at Carleton College in Minnesota.
Type
Hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-128b,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/151.