BH250-27b
Title
BH250-27b
Subject
Basal Unite of the Baraboo Quartzite
Description
Major Mineral: quartz, orthoclase
Minor Minerals: zircon, iron oxides, muscovite, rock fragments, BIF framgent
BH250-27b Basal Unit of the Baraboo Quartzite
BH250-27b represents the basal portion of the unit and is notably distinct in both texture and composition from the upper quartzite layers. (See BH250-27 for a general overview of the Baraboo Quartzite).
Overview:
The basal Baraboo Quartzite differs from the overlying units. It is coarser-grained and contains abundant lithic fragments. This portion of the formation reflects a less mature sedimentary environment, with evidence of proximal sourcing, limited sedimentary reworking, and higher-energy depositional processes.
Mineralogy:
Quartz dominant (>90%), occurring as coarse, rounded to subrounded grains. Some individual quartz grains reach up to 3 cm in diameter, indicating minimal transport and deposition in a high-energy setting.
Zircon common as rounded and zoned grains, frequently aligned with primary sedimentary structures. The morphology of the zircons suggests derivation from recycled sedimentary or volcanic protoliths.
Iron Oxides, present as both rounded detrital grains of magnetite and hematite, and as disseminated fine-grained oxides throughout the matrix. These contribute to the unit’s characteristic reddish hue.
Sericite and fine grained matrix:
The matrix contains fine-grained quartz, sericite, and iron oxide-rich clasts. Sericite likely represents recrystallized clay minerals from the original cement, indicating a diagenetic and low-grade metamorphic overprint. In hand sample, some clasts resemble fragments of banded iron formation (BIF) including magnetite, hematite, jasper, and chert suggesting derivation from local basement rocks.
Texture:
The unit is texturally immature, with subangular to subrounded grains and poor sorting. Iron oxide cementation, primarily by hematite, imparts a distinct rust-red coloration to the basal strata.
Large format thin sections were prepared to capture the full range of mineralogical and textural features observed in outcrop, including coarse quartz grains, lithic fragments, and matrix components.
Minor Minerals: zircon, iron oxides, muscovite, rock fragments, BIF framgent
BH250-27b Basal Unit of the Baraboo Quartzite
BH250-27b represents the basal portion of the unit and is notably distinct in both texture and composition from the upper quartzite layers. (See BH250-27 for a general overview of the Baraboo Quartzite).
Overview:
The basal Baraboo Quartzite differs from the overlying units. It is coarser-grained and contains abundant lithic fragments. This portion of the formation reflects a less mature sedimentary environment, with evidence of proximal sourcing, limited sedimentary reworking, and higher-energy depositional processes.
Mineralogy:
Quartz dominant (>90%), occurring as coarse, rounded to subrounded grains. Some individual quartz grains reach up to 3 cm in diameter, indicating minimal transport and deposition in a high-energy setting.
Zircon common as rounded and zoned grains, frequently aligned with primary sedimentary structures. The morphology of the zircons suggests derivation from recycled sedimentary or volcanic protoliths.
Iron Oxides, present as both rounded detrital grains of magnetite and hematite, and as disseminated fine-grained oxides throughout the matrix. These contribute to the unit’s characteristic reddish hue.
Sericite and fine grained matrix:
The matrix contains fine-grained quartz, sericite, and iron oxide-rich clasts. Sericite likely represents recrystallized clay minerals from the original cement, indicating a diagenetic and low-grade metamorphic overprint. In hand sample, some clasts resemble fragments of banded iron formation (BIF) including magnetite, hematite, jasper, and chert suggesting derivation from local basement rocks.
Texture:
The unit is texturally immature, with subangular to subrounded grains and poor sorting. Iron oxide cementation, primarily by hematite, imparts a distinct rust-red coloration to the basal strata.
Large format thin sections were prepared to capture the full range of mineralogical and textural features observed in outcrop, including coarse quartz grains, lithic fragments, and matrix components.
Coverage
Location: Baraboo, Wisconsin, USA
GPS Coordinates: 43.491828, -89.640228
GPS Coordinates: 43.491828, -89.640228
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-27b. Housed at Carleton College in Minnesota.
Type
Thin section and hand sample
Large thin sections 2-3 inches
Large thin sections 2-3 inches
Relation
Collection
Citation
Bereket Haileab, “BH250-27b,” BH250 Mineralogy Teaching Collection, accessed April 24, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/319.