BH250-128
Title
BH250-128
Subject
Platteville Limestone
Description
Major Minerals: calcite
The Platteville Limestone, deposited during the Middle Ordovician (~461–450 million years ago), formed in a shallow, tropical marine setting along the equatorial margin of ancient North America. Warm, clear waters allowed carbonate mud and abundant shell debris to accumulate on the seafloor, creating a fossil-rich limestone that reflects a vibrant marine ecosystem. Common fossils include brachiopods, bryozoans, corals, crinoids, cystoids, trilobites, gastropods, and trace fossils such as burrows and ostracods. These fossils are often densely packed in shell beds, particularly in intervals rich in echinoderms.
This formation is widespread across the Upper Midwest, particularly in southeastern Minnesota and western Wisconsin. It typically measures 8–9 meters thick in the region and is underlain by the Glenwood Shale and the older St. Peter Sandstone. Despite its age, the Platteville Limestone is resistant to erosion and forms prominent ledges.
Within the formation, thin layers of bentonite altered volcanic ash serve as important stratigraphic markers. These bentonite seams, only 1-5 cm thick, were originally deposited as ash falls from distant Middle Ordovician volcanic eruptions. Over time, the volcanic glass altered to smectite clay minerals, forming soft, plastic bentonites. These beds, visible in places like road cuts (Highway 14, Sogn Valley) and quarries in the Twin Cities, contain trace minerals like zircon and sanidine, which are valuable for radiometric dating. Bentonite layers not only help geologists correlate strata across southeastern Minnesota and beyond, but also offer key insights into Ordovician volcanism and diagenetic processes in marine carbonate settings.
Tephra layers from the Turkana Basin have produced hundreds of tuff beds, many of which remain remarkably pristine, preserving volcanic glass that is extensively used for regional stratigraphic correlation. However, a few tephra units—such as Tuff B-10—are fully altered, with no volcanic glass preserved. In these cases, melt inclusions trapped within phenocrysts have provided a valuable proxy for reconstructing the original geochemical composition of the tuff. The chemistry preserved in these melt inclusions offers critical insight into the pre-alteration nature of Tuff B-10, allowing researchers to infer its volcanic source characteristics despite the complete loss of glass through diagenesis.
The Platteville Limestone, deposited during the Middle Ordovician (~461–450 million years ago), formed in a shallow, tropical marine setting along the equatorial margin of ancient North America. Warm, clear waters allowed carbonate mud and abundant shell debris to accumulate on the seafloor, creating a fossil-rich limestone that reflects a vibrant marine ecosystem. Common fossils include brachiopods, bryozoans, corals, crinoids, cystoids, trilobites, gastropods, and trace fossils such as burrows and ostracods. These fossils are often densely packed in shell beds, particularly in intervals rich in echinoderms.
This formation is widespread across the Upper Midwest, particularly in southeastern Minnesota and western Wisconsin. It typically measures 8–9 meters thick in the region and is underlain by the Glenwood Shale and the older St. Peter Sandstone. Despite its age, the Platteville Limestone is resistant to erosion and forms prominent ledges.
Within the formation, thin layers of bentonite altered volcanic ash serve as important stratigraphic markers. These bentonite seams, only 1-5 cm thick, were originally deposited as ash falls from distant Middle Ordovician volcanic eruptions. Over time, the volcanic glass altered to smectite clay minerals, forming soft, plastic bentonites. These beds, visible in places like road cuts (Highway 14, Sogn Valley) and quarries in the Twin Cities, contain trace minerals like zircon and sanidine, which are valuable for radiometric dating. Bentonite layers not only help geologists correlate strata across southeastern Minnesota and beyond, but also offer key insights into Ordovician volcanism and diagenetic processes in marine carbonate settings.
Tephra layers from the Turkana Basin have produced hundreds of tuff beds, many of which remain remarkably pristine, preserving volcanic glass that is extensively used for regional stratigraphic correlation. However, a few tephra units—such as Tuff B-10—are fully altered, with no volcanic glass preserved. In these cases, melt inclusions trapped within phenocrysts have provided a valuable proxy for reconstructing the original geochemical composition of the tuff. The chemistry preserved in these melt inclusions offers critical insight into the pre-alteration nature of Tuff B-10, allowing researchers to infer its volcanic source characteristics despite the complete loss of glass through diagenesis.
Coverage
Sogn Valley, Minnesota, USA
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-128. Housed at Carleton College in Minnesota.
Type
Hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-128,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/150.