Stop 2B - The Bright Angel Shale, Frenchman Mtn.
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The Bright Angel Shale (Cambrian) overlies the Tapeats Sandstone at Frenchman Mtn, just as in the Grand Canyon. As sea level rose during the Cambrian Period, the exposed Precambrian Vishnu Group was first covered with near-shore sand deposits followed by deeper-water muds. We found abundant marine invertebrate trace fossils and shell fragments, including trilobites (extinct, marine arthropods). Deposition of shales is very slow, as demonstrated by the extreme churning of the muds by bottom dwelling organisms (worms and arthropods). |
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As sea level continued to rise during the Cambrian Period, shale deposits in the lower Bright Angel Formation were replaced by limestones. This indicates that sources of clay were limited, perhaps by the distance of the deposits from the shoreline. The calcium carbonate for the limestone was derived from invertebrate shells. Some lime mud may have precipitated directly from the seawater. Imagine how many shelly critters had to live and die to create a layer of limestone over 250 feet thick! The mottled texture is the result of marine animal burrowing in the sediment (bioturbation). |
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This is a field sketch of French Mtn., looking south from Lake Mead Boulevard. It shows the srarigraphic units that have been featured at Stops 2A and 2B. The rocks overlying the Bright Angel Fm. include the Bonanza Springs Formation (Cambrian), and the Sultan Fm. (Devonian). Rocks from the Ordovician and Silurian Systems are absent at both the Grand Canyon and Frenchman Mtn. Thus, another major unconformity marks the passage of millions of years with no stratigraphic record in this region. (Sketch from Stephen M. Rowland, Paleozoic Stratigraphy of Frenchman Mountain, Clark County, Nevada, Geological Society of America Centennial Field Guide 1987 - Cordilleran Section, pp. 53-56.) |
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Carboniferous Strata, Frenchman Mtn. The two peaks of Frenchman Mtn are formed out of Carboniferous carbonate rocks, visible about one mile west of Stop 2B. The peak on the right exposes the Mississippian Monte Cristo Fm. This dolostone unit correlates with the Redwall Fm. in the Grand Canyon. The peak on the left exposes the Pennsylvanian Callville Fm. This unit consists of fossiliferous limestones that pass upward to cross-bedded sandstones. The Callville Fm. is time-equivalent with the Supai Group sandstones and shales in the Grand Canyon. The different sediments at the two localities represent changes in depositional conditions across the region. This is similar to the modern Persian Gulf. Along the coast there are dunes of quartz sand while offshore deposits are predominantly lime sand and mud. |
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Another mile west along Lake Mead Boulevard brings into view the cherty limestones of the Permian Toroweap and Kaibab Fms. The Kaibab Fm. forms the prominent cliff at the top of the Grand Canyon. |
