limestone - yellowish dolomitic limestone from the Lower Permian Kaibab Formation - Unit of 5 student specimens
The Lower Permian Kaibab Formation caps the plateau on the north side of the Grand Canyon. It consists of the lower Fossil Mountain Member and above that, the Harrisburg Member. The Harrisburg Member is overlain by the Moenkopi Formation in an erosional unconformity, though on most of the plateau, the Moenkopi has been removed by erosion. South of the Grand Canyon, the Kaibab overlies the Coconino Sandstone.
In the upper Harrisburg Member, the Kaibab limestone is is widely dolomitized, is yellowish and often sandy, reflecting deposition on a gently sloping continental shelf, with changes in sea level causing shifting of the shallow marine environments where the Kaibab was being deposited. Specimens were collected from the Harrisburg Member at the first roadcut south of Jacob Lake, Arizona, on Arizona Highway 67. The field photo shows the vertical and horizontal fractures and typical blocky structure of this limestone.
Challenge your students. This looks like a limestone but effervesces reluctantly with dilute hydrochloric acid. Tap it with a hammer and then test with a drop of acid. That will generate a weak effervescence. Aha! It's dolomitic!
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