BH250-175h
Title
BH250-175h
Subject
Hakatai Shale
Description
Major Minerals: clay minerals and silt-sized particles such as quartz
BH250-175h represents a fine-grained sedimentary rock from the Hakatai Shale, part of the Mesoproterozoic Unkar Group. This unit was deposited approximately 1.25 billion years ago and records a period of basin development associated with crustal extension in the early geologic evolution of the Grand Canyon region. The Hakatai Shale is well known for its distinctive red to maroon coloration in outcrop, although hand samples may range from reddish brown to dark gray depending on weathering state, oxidation, and organic content. This specimen is unusually dark gray to nearly black in hand sample, giving it a more subdued appearance than many typical Hakatai exposures, but it still preserves the fine-grained character and fissility expected of shale.
In hand sample, BH250-175h is extremely fine grained and breaks along smooth, planar surfaces that reflect original bedding and fissility. The rock shows thin laminations and subtle compositional layering visible on freshly broken surfaces. Its fine grain size indicates deposition in a low-energy environment, where clay-sized particles and very fine silt could settle from suspension in quiet water. Such environments may have included tidal flats, restricted marine basins, lagoons, floodplains, or shallow offshore settings where sediment input fluctuated through time.
The thin section reveals beautifully preserved fine lamination. Alternating darker and lighter bands reflect variations in grain size, mineral composition, and possible organic or iron-rich material. The lighter laminae appear to contain silt-sized grains dominated by Quartz and possibly minor feldspar, while the darker layers are composed largely of clay-rich material and opaque phases. These thin, parallel laminations are excellent examples of primary sedimentary layering preserved in fine-grained rocks. The photomicrographs also show thin secondary fractures or veins that have been filled by later minerals, likely Quartz or calcite, demonstrating that the rock experienced later fluid movement after lithification. These cross-cutting veinlets provide a useful teaching example of distinguishing primary depositional features from later diagenetic or tectonic overprints.
The Hakatai Shale plays an important role in reconstructing the early history of the Grand Canyon because it records a quieter interval of sedimentation between more resistant units. It overlies the Bass Formation and underlies the Shinumo Quartzite, preserving evidence of changing depositional environments during development of the Grand Canyon Supergroup basin. This specimen is especially valuable for teaching because it demonstrates fine-grained sedimentary processes, lamination preservation, fissility, vein formation, and the remarkable preservation of Proterozoic sedimentary records over more than a billion years of Earth history.
BH250-175h represents a fine-grained sedimentary rock from the Hakatai Shale, part of the Mesoproterozoic Unkar Group. This unit was deposited approximately 1.25 billion years ago and records a period of basin development associated with crustal extension in the early geologic evolution of the Grand Canyon region. The Hakatai Shale is well known for its distinctive red to maroon coloration in outcrop, although hand samples may range from reddish brown to dark gray depending on weathering state, oxidation, and organic content. This specimen is unusually dark gray to nearly black in hand sample, giving it a more subdued appearance than many typical Hakatai exposures, but it still preserves the fine-grained character and fissility expected of shale.
In hand sample, BH250-175h is extremely fine grained and breaks along smooth, planar surfaces that reflect original bedding and fissility. The rock shows thin laminations and subtle compositional layering visible on freshly broken surfaces. Its fine grain size indicates deposition in a low-energy environment, where clay-sized particles and very fine silt could settle from suspension in quiet water. Such environments may have included tidal flats, restricted marine basins, lagoons, floodplains, or shallow offshore settings where sediment input fluctuated through time.
The thin section reveals beautifully preserved fine lamination. Alternating darker and lighter bands reflect variations in grain size, mineral composition, and possible organic or iron-rich material. The lighter laminae appear to contain silt-sized grains dominated by Quartz and possibly minor feldspar, while the darker layers are composed largely of clay-rich material and opaque phases. These thin, parallel laminations are excellent examples of primary sedimentary layering preserved in fine-grained rocks. The photomicrographs also show thin secondary fractures or veins that have been filled by later minerals, likely Quartz or calcite, demonstrating that the rock experienced later fluid movement after lithification. These cross-cutting veinlets provide a useful teaching example of distinguishing primary depositional features from later diagenetic or tectonic overprints.
The Hakatai Shale plays an important role in reconstructing the early history of the Grand Canyon because it records a quieter interval of sedimentation between more resistant units. It overlies the Bass Formation and underlies the Shinumo Quartzite, preserving evidence of changing depositional environments during development of the Grand Canyon Supergroup basin. This specimen is especially valuable for teaching because it demonstrates fine-grained sedimentary processes, lamination preservation, fissility, vein formation, and the remarkable preservation of Proterozoic sedimentary records over more than a billion years of Earth history.
Coverage
Location: Mile 132 Stone Creek, Grand Canyon, Arizona, USA
Nearby Geographic Feature: Grand Canyon
Nearby Geographic Feature: Grand Canyon
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 175h. Housed at Carleton College in Minnesota.
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-175h,” BH250 Mineralogy Teaching Collection, accessed October 22, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/236.