BH250-27c
Title
BH250-27c
Subject
Baraboo Quartzite-phylletic Subunit
Description
Major Minerals: quartz, muscovite, iron oxides
Minor Mineral: zircon
BH250-27c represents the phyllitic facies of the Baraboo Quartzite and is distinct from the other units because of its fine-grained, foliated texture and well-developed micaceous bands. Unlike the coarser and more compositionally immature basal unit (BH250-27b), this interval preserves evidence of alternating quartz-rich and fine-grained sediment deposition. Some of the foliation bands likely exploit or parallel original bedding planes, suggesting that primary sedimentary layering was at least partially preserved and later enhanced during low-grade metamorphism.
Mineralogy
Quartz is the dominant mineral and occurs as fine- to medium-grained detrital grains that are now strained and recrystallized. Many quartz grains display undulose extinction under cross-polarized light, reflecting deformation associated with regional metamorphism. In many areas, quartz forms a recrystallized mosaic while still preserving remnants of its original clastic texture.
Muscovite occurs as fine-grained flakes aligned along foliation planes and gives the rock its characteristic silky sheen typical of phyllite. These aligned mica grains define much of the rock’s foliation and reflect metamorphic recrystallization of original clay-rich sediment layers.
Sericite is also abundant and occurs as very fine-grained mica concentrated between quartz grains and along foliation planes. It likely formed through alteration of feldspar and clay minerals and contributes significantly to cleavage development.
Iron oxides, primarily hematite with possible minor magnetite and other opaque phases, occur as disseminated grains, discontinuous streaks, and locally concentrated bands. In thin section, some of these iron-rich layers appear as dark linear seams that may preserve original compositional layering, heavy mineral concentrations, or early diagenetic iron enrichment along bedding planes.
Zircon occurs as small detrital grains and remains stable under metamorphic conditions. These grains are highly resistant to alteration and provide evidence of sediment derivation from older continental source rocks.
Protolith and Depositional Environment
Based on its mineralogy, foliation, and preserved compositional layering, the protolith of BH250-27c was likely a fine-grained feldspathic sandstone, siltstone, or mud-rich sandstone deposited in a fluctuating sedimentary environment. The alternating quartz-rich layers, mica-rich laminae, and iron-rich bands suggest repeated shifts between higher-energy and lower-energy depositional conditions.
The original environment was likely nearshore marine, tidal flat, delta-front, or fluvial-deltaic, where pulses of quartz sand were periodically interrupted by quieter intervals that allowed clay-rich mud and fine iron-bearing sediments to settle. The presence of fine-grained mica-rich layers suggests calm-water deposition during lower-energy periods, while occasional iron-rich bands may reflect heavy mineral sorting, periodic influxes of iron-rich sediment, or localized early diagenetic concentration.
Overall, BH250-27c records a more stratified and environmentally variable depositional system than the basal Baraboo units and likely represents an important transitional interval between the coarse, iron-rich basal deposits and the cleaner, more mature quartzites higher in the Baraboo succession.
Minor Mineral: zircon
BH250-27c represents the phyllitic facies of the Baraboo Quartzite and is distinct from the other units because of its fine-grained, foliated texture and well-developed micaceous bands. Unlike the coarser and more compositionally immature basal unit (BH250-27b), this interval preserves evidence of alternating quartz-rich and fine-grained sediment deposition. Some of the foliation bands likely exploit or parallel original bedding planes, suggesting that primary sedimentary layering was at least partially preserved and later enhanced during low-grade metamorphism.
Mineralogy
Quartz is the dominant mineral and occurs as fine- to medium-grained detrital grains that are now strained and recrystallized. Many quartz grains display undulose extinction under cross-polarized light, reflecting deformation associated with regional metamorphism. In many areas, quartz forms a recrystallized mosaic while still preserving remnants of its original clastic texture.
Muscovite occurs as fine-grained flakes aligned along foliation planes and gives the rock its characteristic silky sheen typical of phyllite. These aligned mica grains define much of the rock’s foliation and reflect metamorphic recrystallization of original clay-rich sediment layers.
Sericite is also abundant and occurs as very fine-grained mica concentrated between quartz grains and along foliation planes. It likely formed through alteration of feldspar and clay minerals and contributes significantly to cleavage development.
Iron oxides, primarily hematite with possible minor magnetite and other opaque phases, occur as disseminated grains, discontinuous streaks, and locally concentrated bands. In thin section, some of these iron-rich layers appear as dark linear seams that may preserve original compositional layering, heavy mineral concentrations, or early diagenetic iron enrichment along bedding planes.
Zircon occurs as small detrital grains and remains stable under metamorphic conditions. These grains are highly resistant to alteration and provide evidence of sediment derivation from older continental source rocks.
Protolith and Depositional Environment
Based on its mineralogy, foliation, and preserved compositional layering, the protolith of BH250-27c was likely a fine-grained feldspathic sandstone, siltstone, or mud-rich sandstone deposited in a fluctuating sedimentary environment. The alternating quartz-rich layers, mica-rich laminae, and iron-rich bands suggest repeated shifts between higher-energy and lower-energy depositional conditions.
The original environment was likely nearshore marine, tidal flat, delta-front, or fluvial-deltaic, where pulses of quartz sand were periodically interrupted by quieter intervals that allowed clay-rich mud and fine iron-bearing sediments to settle. The presence of fine-grained mica-rich layers suggests calm-water deposition during lower-energy periods, while occasional iron-rich bands may reflect heavy mineral sorting, periodic influxes of iron-rich sediment, or localized early diagenetic concentration.
Overall, BH250-27c records a more stratified and environmentally variable depositional system than the basal Baraboo units and likely represents an important transitional interval between the coarse, iron-rich basal deposits and the cleaner, more mature quartzites higher in the Baraboo succession.
Coverage
Location: Baraboo, Wisconsin, USA
GPS Coordinates: 43.435519, -89.773492
GPS Coordinates: 43.435519, -89.773492
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-27c. Housed at Carleton College in Minnesota.
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-27c,” BH250 Mineralogy Teaching Collection, accessed August 30, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/320.