Hance Rapid Afternoon

Hance Rapid Afternoon

This painting of Hance Rapid, River Mile 76.6, shows the supergroup geology looking upstream from the Tonto Trail. The red slope at river right of Hakatai Shale, estuarial mud flats with cross bedding and ripple marks, is part of the Unkar Group which is oceanic and terrestrial sedimentary deposits formed 1.25 b.y.a. during a period of subsidence and deposition of marine sediments averaging about 4km thick. A second period of uplifting began about 750 million years ago and lasting for 200 million years resulted in more fracturing, tilting, and erosion resulting in the complete removal of the Supergroup in most areas in the Grand Canyon. Subsidence and marine deposition again began about 570 million years ago as the Tapeats Sea rolled in over the exposed lowland of schist, granite, and supergroup remnants. The crisp contact line between the Tapeats Sandstone and the underlying rock of greater age is called the Great Unconformity, This line forms the lower limit of the exposed Supergroup in Grand Canyon. The upper limit of the Supergroup is known as the Great Angular Unconformity. The Grand Canyon Supergroup shown in the painting starts with reddish Hakatia Shale, shot thru by an ancient basalt dike. Above rests the Shimumo Quartsite, crossbedded with mudcracks and clay galls. Across the river, the cliffs present Dox Sandstone, soft mudstones and shales from a primordial costal environment. This is overlain by the Gardenas Basalts from about 1 billion years ago. Directly above is the Nankoweep Formation, crossbedded, with Gardenas detritus in lenses throughout. Above, but unseen in the painting, lie the Kwagunt and Galeros formations, capped by the Sixtymile Formation of about 820 million years ago. Above the angular slope of the Supergroup lies the pancake geology of the better known canyon sediments. At the top of the painting Kaibab Limestone forms the Canyon rim. Directly below is the Toroweep Limestone, also whitish grey to tan in color. The white crossbedded Coconino Sandstone sits above the slope forming Hermit Shale with its distinctive red iron staining. Just below is the Supai Group, a Pennsylvanian age group of red and white sandstones and mudstones perched atop Redwall Limestone, which forms the monument in upper left center. Below, but mostly unseen in the painting, lie the Temple Butte Limestone Formation and the Tonto Group, consisting of Muav Limestone, Bright Angel Shale, and Tapeats Sandstone of Cambrian age.

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