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 represents the basal portion of the Baraboo Quartzite and is notably distinct in both texture and composition from the overlying quartzite units (see BH250-27 for a broader overview of the Baraboo Quartzite). This basal interval preserves evidence of a more compositionally and texturally immature sedimentary system compared to the cleaner, more mature quartzites higher in the section.
Overview
The basal Baraboo Quartzite differs significantly from the overlying units. It is generally coarser grained and contains abundant lithic fragments, iron-oxide–rich clasts, and a finer-grained matrix that is largely absent in younger units. These characteristics suggest deposition in a relatively immature sedimentary environment marked by proximal sediment sources, limited sedimentary reworking, and higher-energy depositional processes during the earliest stages of basin development following deposition on older basement rocks.
Mineralogy
Quartz is the dominant mineral (>90%) and occurs as coarse, rounded to subrounded grains. Some individual quartz grains reach up to approximately 3 cm in diameter, suggesting short transport distances and deposition in a relatively high-energy environment. In thin section, quartz commonly displays low first-order interference colors and locally exhibits minor undulose extinction related to later deformation during metamorphism.
Zircon is relatively common and occurs as rounded to zoned detrital grains, in some cases aligned with primary sedimentary structures. Their morphology suggests derivation from recycled sedimentary sources or nearby volcanic protoliths.
Iron oxides are a particularly distinctive component of this basal unit and occur both as rounded detrital grains and as fine-grained disseminated material throughout the matrix and lithic fragments. The most likely iron-bearing phases include hematite, goethite, limonite, magnetite, martite, ferrihydrite, and possible oxidation products derived from pyrite or altered Fe-bearing silicates. The reddish-brown staining, dusty opaque textures, and localized concentrations strongly suggest extensive hematitic alteration.
Bulk geochemical data support the exceptional maturity of the quartz framework while highlighting localized iron enrichment. The rock (BH250-27) contains approximately 98.28 wt.% SiO₂ and only 1.25 wt.% Fe₂O₃, indicating that iron is concentrated in specific lithic fragments rather than uniformly distributed throughout the rock.
Sericite and Fine-Grained Matrix
The matrix contains fine-grained quartz, sericite, and iron oxide–rich clasts. The sericite likely represents recrystallized clay minerals derived from original mud-rich matrix material or early cement. This reflects both diagenetic alteration and later low-grade metamorphic overprinting during quartzite formation.
Some hand samples contain dark clasts that resemble fragments of banded iron formation (BIF) composed of magnetite, hematite, jasper, and chert. If correct, these fragments may indicate erosion of nearby iron-rich basement lithologies and provide important provenance constraints for the earliest Baraboo sediments.
Iron Oxide Alteration
The iron oxide alteration likely records multiple stages of geologic history. Some oxidation may have occurred in the source region prior to transport through weathering of iron-rich source rocks. Additional oxidation likely occurred during early diagenesis as groundwater moved through porous lithic fragments and precipitated iron oxides along fractures, laminations, and pore spaces. Later burial and low-grade metamorphism stabilized hematite, preserving evidence of these earlier oxidation events.
Notably, these iron-rich lithic fragments are largely restricted to the basal member and are uncommon or absent in younger Baraboo units. This likely reflects a transition from an early, locally derived and compositionally immature depositional system to a more mature quartz-dominated system produced by prolonged transport, sediment recycling, and greater compositional maturity.
Texture
The basal unit is texturally immature, characterized by subangular to subrounded grains and relatively poor sorting compared to younger Baraboo quartzites. Iron oxide cementation—primarily hematite—contributes to the distinct rust-red coloration of the basal strata.
Thin Sections
Large-format thin sections were prepared to capture the full range of mineralogical and textural features observed in outcrop, including coarse quartz grains, iron-rich lithic fragments, matrix components, and alteration textures that are difficult to observe in standard thin sections.
Minor Minerals: zircon, iron oxides, muscovite, rock fragments, BIF framgent
BH250-27b represents the basal portion of the Baraboo Quartzite and is notably distinct in both texture and composition from the overlying quartzite units (see BH250-27 for a broader overview of the Baraboo Quartzite). This basal interval preserves evidence of a more compositionally and texturally immature sedimentary system compared to the cleaner, more mature quartzites higher in the section.
Overview
The basal Baraboo Quartzite differs significantly from the overlying units. It is generally coarser grained and contains abundant lithic fragments, iron-oxide–rich clasts, and a finer-grained matrix that is largely absent in younger units. These characteristics suggest deposition in a relatively immature sedimentary environment marked by proximal sediment sources, limited sedimentary reworking, and higher-energy depositional processes during the earliest stages of basin development following deposition on older basement rocks.
Mineralogy
Quartz is the dominant mineral (>90%) and occurs as coarse, rounded to subrounded grains. Some individual quartz grains reach up to approximately 3 cm in diameter, suggesting short transport distances and deposition in a relatively high-energy environment. In thin section, quartz commonly displays low first-order interference colors and locally exhibits minor undulose extinction related to later deformation during metamorphism.
Zircon is relatively common and occurs as rounded to zoned detrital grains, in some cases aligned with primary sedimentary structures. Their morphology suggests derivation from recycled sedimentary sources or nearby volcanic protoliths.
Iron oxides are a particularly distinctive component of this basal unit and occur both as rounded detrital grains and as fine-grained disseminated material throughout the matrix and lithic fragments. The most likely iron-bearing phases include hematite, goethite, limonite, magnetite, martite, ferrihydrite, and possible oxidation products derived from pyrite or altered Fe-bearing silicates. The reddish-brown staining, dusty opaque textures, and localized concentrations strongly suggest extensive hematitic alteration.
Bulk geochemical data support the exceptional maturity of the quartz framework while highlighting localized iron enrichment. The rock (BH250-27) contains approximately 98.28 wt.% SiO₂ and only 1.25 wt.% Fe₂O₃, indicating that iron is concentrated in specific lithic fragments rather than uniformly distributed throughout the rock.
Sericite and Fine-Grained Matrix
The matrix contains fine-grained quartz, sericite, and iron oxide–rich clasts. The sericite likely represents recrystallized clay minerals derived from original mud-rich matrix material or early cement. This reflects both diagenetic alteration and later low-grade metamorphic overprinting during quartzite formation.
Some hand samples contain dark clasts that resemble fragments of banded iron formation (BIF) composed of magnetite, hematite, jasper, and chert. If correct, these fragments may indicate erosion of nearby iron-rich basement lithologies and provide important provenance constraints for the earliest Baraboo sediments.
Iron Oxide Alteration
The iron oxide alteration likely records multiple stages of geologic history. Some oxidation may have occurred in the source region prior to transport through weathering of iron-rich source rocks. Additional oxidation likely occurred during early diagenesis as groundwater moved through porous lithic fragments and precipitated iron oxides along fractures, laminations, and pore spaces. Later burial and low-grade metamorphism stabilized hematite, preserving evidence of these earlier oxidation events.
Notably, these iron-rich lithic fragments are largely restricted to the basal member and are uncommon or absent in younger Baraboo units. This likely reflects a transition from an early, locally derived and compositionally immature depositional system to a more mature quartz-dominated system produced by prolonged transport, sediment recycling, and greater compositional maturity.
Texture
The basal unit is texturally immature, characterized by subangular to subrounded grains and relatively poor sorting compared to younger Baraboo quartzites. Iron oxide cementation—primarily hematite—contributes to the distinct rust-red coloration of the basal strata.
Thin Sections
Large-format thin sections were prepared to capture the full range of mineralogical and textural features observed in outcrop, including coarse quartz grains, iron-rich lithic fragments, matrix components, and alteration textures that are difficult to observe in standard thin sections.
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 August 30, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/319.