BH250-175e
Title
BH250-175e
Subject
Bass Limestone
Description
Major Mineral: calcite
Minor Mineral: chert
In thin section, the Bass Limestone contains low-birefringence, low-relief minerals identified as chert (microcrystalline quartz), which formed through diagenetic recrystallization. As the oldest sedimentary unit in the Grand Canyon, the Bass Limestone has experienced low-grade metamorphism, leading to the recrystallization of both calcite and chert.
The calcite grain size in the Bass Limestone is substantially larger than that of younger limestones in the region, such as the Muav Limestone, reflecting the prolonged effects of metamorphism and diagenesis. This contrasts with younger and less-altered limestones like samples BH250-122, 125, 128, 140, 175f, 122wc, and 128c, which retain finer-grained textures.
You will find many calcite grains in Bass Limestone thin sections are cut perpendicular to sub-perpendicular to the c-axis, producing optic axis figures or off-centered uniaxial negative interference figures when viewed under crossed polars.
Minor Mineral: chert
In thin section, the Bass Limestone contains low-birefringence, low-relief minerals identified as chert (microcrystalline quartz), which formed through diagenetic recrystallization. As the oldest sedimentary unit in the Grand Canyon, the Bass Limestone has experienced low-grade metamorphism, leading to the recrystallization of both calcite and chert.
The calcite grain size in the Bass Limestone is substantially larger than that of younger limestones in the region, such as the Muav Limestone, reflecting the prolonged effects of metamorphism and diagenesis. This contrasts with younger and less-altered limestones like samples BH250-122, 125, 128, 140, 175f, 122wc, and 128c, which retain finer-grained textures.
You will find many calcite grains in Bass Limestone thin sections are cut perpendicular to sub-perpendicular to the c-axis, producing optic axis figures or off-centered uniaxial negative interference figures when viewed under crossed polars.
Coverage
Location: Mile 132 Stone Creek, Grand Canyon, Arizona, USA
Nearby Geographic Feature: Grand Canyon
Nearby Geographic Feature: Grand Canyon
Date
August 2021
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-175e. Housed at Carleton College in Minnesota.
Contributor
Bereket Haileab
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-175e,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/231.