BH250-143
Title
BH250-143
Subject
Franconia Sandstone
Description
Major Mineral: quartz
Minor Mineral: glauconite, plagioclase feldspar and clay minerals
The Franconia Sandstone is a prominent Upper Cambrian sedimentary unit that crops out in southeastern Minnesota and adjacent parts of Wisconsin and Iowa. It is especially well exposed at locations such as Taylors Falls and Red Wing, Minnesota, where it forms dramatic bluffs and riverine cliffs along the St. Croix and Mississippi Rivers.
Stratigraphic Revisions:
In recent years, the stratigraphic nomenclature in Minnesota has shifted. Much of what was formerly designated as the Franconia Formation is now formally recognized as part of the Tunnel City Formation, a revision that aligns regional stratigraphy with that used in Wisconsin and Illinois. These two names refer to overlapping intervals, but the Tunnel City Formation reflects updated lithostratigraphic classification based on improved mapping and subdivision of Cambrian strata.
Cement: Primarily silica (quartz overgrowths); minor iron oxide and clay cement in some layers
Texture: Fine- to medium-grained, well- to moderately sorted; grains are rounded to sub-rounded
Sedimentary Features (notable at Taylors Falls): Planar and trough cross-bedding, ripple marks, bioturbation structures, laminated bedding
Depositional Environment:
The Franconia (Tunnel City) Sandstone was deposited in shallow marine settings on a broad, low-gradient continental shelf. The environment favored slow sediment accumulation, which allowed for the formation of features like glauconite, an authigenic mineral indicative of marine conditions. At Taylors Falls, glauconite is present but less abundant than in the Red Wing area, where glauconitic layers are more pronounced.
For more information see, BH250 – Sandstone Overview with Petrographic Notes
Minor Mineral: glauconite, plagioclase feldspar and clay minerals
The Franconia Sandstone is a prominent Upper Cambrian sedimentary unit that crops out in southeastern Minnesota and adjacent parts of Wisconsin and Iowa. It is especially well exposed at locations such as Taylors Falls and Red Wing, Minnesota, where it forms dramatic bluffs and riverine cliffs along the St. Croix and Mississippi Rivers.
Stratigraphic Revisions:
In recent years, the stratigraphic nomenclature in Minnesota has shifted. Much of what was formerly designated as the Franconia Formation is now formally recognized as part of the Tunnel City Formation, a revision that aligns regional stratigraphy with that used in Wisconsin and Illinois. These two names refer to overlapping intervals, but the Tunnel City Formation reflects updated lithostratigraphic classification based on improved mapping and subdivision of Cambrian strata.
Cement: Primarily silica (quartz overgrowths); minor iron oxide and clay cement in some layers
Texture: Fine- to medium-grained, well- to moderately sorted; grains are rounded to sub-rounded
Sedimentary Features (notable at Taylors Falls): Planar and trough cross-bedding, ripple marks, bioturbation structures, laminated bedding
Depositional Environment:
The Franconia (Tunnel City) Sandstone was deposited in shallow marine settings on a broad, low-gradient continental shelf. The environment favored slow sediment accumulation, which allowed for the formation of features like glauconite, an authigenic mineral indicative of marine conditions. At Taylors Falls, glauconite is present but less abundant than in the Red Wing area, where glauconitic layers are more pronounced.
For more information see, BH250 – Sandstone Overview with Petrographic Notes
Coverage
Location: Taylors Falls, Minnesota, USA
GPS Coordinates: 45°23'41.52"N, 92°40'17.33"W
GPS Coordinates: 45°23'41.52"N, 92°40'17.33"W
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 143. Housed at Carleton College in Minnesota.
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-143,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/169.