BH250-89b
Title
BH250-89b
Subject
Baxter Hollow Granite: (Probable Source Rock for the Baraboo Quartzite)
Description
Major Minerals: orthoclase, quartz, plagioclase
Minor Minerals: zircon, Fe-Ti oxides, apatite, chlorite, calcite, sericite
BH250-89b represents the Baxter Hollow Granite, one of the most important basement rocks associated with the evolution of the Baraboo Quartzite because it likely served as a major sediment source for the quartz sands that eventually became the Baraboo Quartzite. The Baxter Hollow Granite is part of the crystalline basement exposed in the Baraboo Range and is Paleoproterozoic in age, generally dated to approximately 1,760–1,830 Ma, making it slightly older than deposition of the Baraboo Quartzite, which occurred roughly between 1,750–1,630 Ma. The granite formed during early Proterozoic magmatic events along the southern margin of the Superior Craton prior to deposition of the Baraboo sedimentary sequence. It was later uplifted, exposed at Earth’s surface, and subjected to prolonged weathering and erosion before its sediments were transported into the basin where the Baraboo Quartzite accumulated.
Mineralogically, the Baxter Hollow Granite is a felsic intrusive rock composed primarily of quartz, potassium feldspar, plagioclase feldspar, and biotite, with minor muscovite, zircon, magnetite, and accessory Fe-Ti oxides. Chemically, it is silica-rich and enriched in potassium, sodium, and aluminum, which is typical of evolved granitic magmas. Quartz is particularly important because of its resistance to chemical weathering, whereas feldspars are far less stable under surface conditions and readily alter through hydrolysis. For example, potassium feldspar can weather to clay minerals through reactions such as:
2KAlSi3O8 + 2H+ + 9H2O → Al2Si2O5(OH)4 + 4H4SiO4 + 2K+
Calcium-rich plagioclase may weather through reactions such as:
CaAl2Si2O8 + 2CO2 + 3H2O → Al2Si2O5(OH)4 + Ca2+ + 2HCO3-
These reactions break down feldspars into clay minerals while releasing dissolved ions into groundwater, leaving quartz as the most durable surviving mineral.
Texturally, BH250-89b displays a medium-grained phaneritic texture (figure xx). Quartz typically occurs as glassy anhedral grains, potassium feldspar may show pink to salmon coloration, plagioclase is commonly white to gray, and biotite occurs as dark flaky grains. In outcrop and hand samples, the granite may appear relatively undeformed and may not clearly show the effects of later low-grade metamorphism associated with regional tectonic events. However, thin section observations reveal subtle evidence of this overprinting. Quartz grains commonly display undulose (wavy) extinction (see thin sections of BH250-89b), indicating that the granite experienced mild deformation and recrystallization during later tectonic events, likely related to regional metamorphism associated with the Penokean Orogeny.
The transformation of the Baxter Hollow Granite into the sediment that formed the Baraboo Quartzite required prolonged weathering, erosion, transport, and sedimentary reworking. Once uplifted and exposed at the surface, the granite underwent physical weathering through fracturing, thermal expansion, and mechanical breakdown. Chemical weathering attacked feldspars and mafic minerals, converting them into clays and dissolved ions, while resistant quartz grains remained intact. Rivers transported these quartz grains into shallow marine and coastal environments, where repeated wave action and longshore transport further removed unstable minerals and concentrated nearly pure quartz sand. Over time, this extreme sedimentary maturation transformed granite-derived sediment into one of North America’s most mature quartz arenites.
By the time these sediments were deposited in the basin that became the Baraboo Quartzite, most feldspar, lithic fragments, and unstable minerals had been eliminated, leaving behind the exceptionally pure quartz-rich sandstone seen today. During later burial and deformation associated with the Penokean Orogeny, those sands were metamorphosed into quartzite. BH250-89b therefore preserves the beginning of the Baraboo Quartzite story, the crystalline source rock that had to be weathered, transported, and transformed before one of Wisconsin’s most iconic quartzites could form.
Minor Minerals: zircon, Fe-Ti oxides, apatite, chlorite, calcite, sericite
BH250-89b represents the Baxter Hollow Granite, one of the most important basement rocks associated with the evolution of the Baraboo Quartzite because it likely served as a major sediment source for the quartz sands that eventually became the Baraboo Quartzite. The Baxter Hollow Granite is part of the crystalline basement exposed in the Baraboo Range and is Paleoproterozoic in age, generally dated to approximately 1,760–1,830 Ma, making it slightly older than deposition of the Baraboo Quartzite, which occurred roughly between 1,750–1,630 Ma. The granite formed during early Proterozoic magmatic events along the southern margin of the Superior Craton prior to deposition of the Baraboo sedimentary sequence. It was later uplifted, exposed at Earth’s surface, and subjected to prolonged weathering and erosion before its sediments were transported into the basin where the Baraboo Quartzite accumulated.
Mineralogically, the Baxter Hollow Granite is a felsic intrusive rock composed primarily of quartz, potassium feldspar, plagioclase feldspar, and biotite, with minor muscovite, zircon, magnetite, and accessory Fe-Ti oxides. Chemically, it is silica-rich and enriched in potassium, sodium, and aluminum, which is typical of evolved granitic magmas. Quartz is particularly important because of its resistance to chemical weathering, whereas feldspars are far less stable under surface conditions and readily alter through hydrolysis. For example, potassium feldspar can weather to clay minerals through reactions such as:
2KAlSi3O8 + 2H+ + 9H2O → Al2Si2O5(OH)4 + 4H4SiO4 + 2K+
Calcium-rich plagioclase may weather through reactions such as:
CaAl2Si2O8 + 2CO2 + 3H2O → Al2Si2O5(OH)4 + Ca2+ + 2HCO3-
These reactions break down feldspars into clay minerals while releasing dissolved ions into groundwater, leaving quartz as the most durable surviving mineral.
Texturally, BH250-89b displays a medium-grained phaneritic texture (figure xx). Quartz typically occurs as glassy anhedral grains, potassium feldspar may show pink to salmon coloration, plagioclase is commonly white to gray, and biotite occurs as dark flaky grains. In outcrop and hand samples, the granite may appear relatively undeformed and may not clearly show the effects of later low-grade metamorphism associated with regional tectonic events. However, thin section observations reveal subtle evidence of this overprinting. Quartz grains commonly display undulose (wavy) extinction (see thin sections of BH250-89b), indicating that the granite experienced mild deformation and recrystallization during later tectonic events, likely related to regional metamorphism associated with the Penokean Orogeny.
The transformation of the Baxter Hollow Granite into the sediment that formed the Baraboo Quartzite required prolonged weathering, erosion, transport, and sedimentary reworking. Once uplifted and exposed at the surface, the granite underwent physical weathering through fracturing, thermal expansion, and mechanical breakdown. Chemical weathering attacked feldspars and mafic minerals, converting them into clays and dissolved ions, while resistant quartz grains remained intact. Rivers transported these quartz grains into shallow marine and coastal environments, where repeated wave action and longshore transport further removed unstable minerals and concentrated nearly pure quartz sand. Over time, this extreme sedimentary maturation transformed granite-derived sediment into one of North America’s most mature quartz arenites.
By the time these sediments were deposited in the basin that became the Baraboo Quartzite, most feldspar, lithic fragments, and unstable minerals had been eliminated, leaving behind the exceptionally pure quartz-rich sandstone seen today. During later burial and deformation associated with the Penokean Orogeny, those sands were metamorphosed into quartzite. BH250-89b therefore preserves the beginning of the Baraboo Quartzite story, the crystalline source rock that had to be weathered, transported, and transformed before one of Wisconsin’s most iconic quartzites could form.
Coverage
Location: Baraboo, Wisconsin, USA
GPS Coordinates: 43.385406, -89.798408
GPS Coordinates: 43.385406, -89.798408
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 89b. Housed at Carleton College in Minnesota.
Type
Hand Sample
Relation
Collection
Citation
Bereket Haileab, “BH250-89b,” BH250 Mineralogy Teaching Collection, accessed October 22, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/349.