The Tonto Group is a geological group of three formations in the Grand Canyon representing a marine transgression from the western passive margin[1] onto the North American craton. The three formations include the basal Tapeats Sandstone above the Great Unconformity, the middle Bright Angel Shale, and the upper Mauv Limestone.
The Tapeats Sandstone averages 525 million years old and is made of clean medium- to coarse-grained sand and conglomerate deposited on an ancient shore. Ripple marks are common in the upper members of this dark brown thin-bedded layer. Fossils and imprint trails of trilobites and brachiopods have also been found in the Tapeats. Today it is a cliff-former that is 100 to 325 feet (30 to 100 m) thick.[2] The middle formation, the Bright Angel Shale averages 515 million years old and is made of mudstone-derived shale that is interbeded with small sections of sandstone and shaly limestone with a few thin beds of dolomite.[3] It was mostly deposited as mud just offshore and contains brachiopod, trilobite, and worm fossils (see 3b in figure 1). The color of this formation is mostly various shades of green with some brownish-tan to gray parts. It is a slope-former and is 270 to 450 feet (82 to 137 m) thick.[4] Glauconite is responsible for the green coloration of the Bright Angel.[5] Muav Limestone averages 505 million years old and is made of gray, thin-bedded limestone that was deposited farther offshore from calcium carbonate precipitates (see 3c in figure 1).[3] It is fossil poor yet trilobites and brachiopods have been found in it. The western part of the canyon has a much thicker sequence of Muav than the eastern part.[6] The Muav is a cliff-former, 136 to 827 feet (41 to 252 m) thick.[7]
These three formations were laid down over a period of 30 million years from early-to-middle Cambrian time.[8] Trilobites followed by brachiopods are the most commonly reported fossils in this group but well-preserved fossils are relatively rare.[7] We know that the shoreline was transgressing (advancing onto land) because finer grade material was deposited on top of coarser-grained sediment.[8] Today, the Tonto Group makes up the Tonto Platform seen above and following the Colorado River; the Tapeats Sandstone and Muav Limestone form the platform's cliffs and the Bright Angel Shale forms its slopes.[8] Unlike the Proterozoic units below it, the Tonto Group's beds basically lie in their original horizontal position. The Bright Angel Shale in the group forms an aquiclude (barrier to groundwater seeping down), and thus collects and directs water through the overlying Muav Limestone to feed springs in the Inner Gorge.
- ^ Kiver, Eugene P. (1999). Geology of U.S. Parklands (5th ed.). New York: John Wiley & Sons. ISBN 0-471-33218-6.
{{cite book}}: Unknown parameter|coauthors=ignored (|author=suggested) (help)CS1 maint: ref duplicates default (link) - ^ Beus, Stanely S.; Morales, Michael, eds. (2003). Grand Canyon Geology (2nd ed.). New York, Oxford: Oxford University Press. ISBN 0-19-512299-2.
{{cite book}}: CS1 maint: ref duplicates default (link) - ^ a b Cite error: The named reference
ParkSciencewas invoked but never defined (see the help page). - ^ Beus & Morales 2003, p. 94
- ^ Price 1999, p. 50
- ^ Kaibab.org, "Grand Canyon Rock Layers"
- ^ a b Beus & Morales 2003, p. 96
- ^ a b c Harris 1997, p. 23