View source for Toroweap Formation

{{Short description|Middle Permian geologic unit in the Grand Canyon}}
{{for|the geologic fault of west Grand Canyon with the same name|Toroweap Fault}}

{{Infobox rockunit
| image = Grand Canyon National Park- Toroweap Formation.jpg
| imagesize = 200px
| caption = Toroweap formation in Grand Canyon National Park
| period = Permian
| age = <br />Early [[Permian]], <br />{{fossil range|272|270}}
| type = [[Geological formation]]
| prilithology = [[gypsum]] & [[shale]],<ref name="Lucchitta Hike 3">Lucchitta, 2001. ''Hiking Arizona's Geology'', "Hike 3: [[Tanner Trail]]", pp. 78–82 [78].</ref> also [[sandstone]]
| otherlithology =
| namedfor =
| namedby = <!--Ulrich, 1911-->
| region = (southwest)-[[Colorado Plateau]] <br>Northern Arizona, southeast [[Utah]]
| coordinates = <!--{{Coord|34|00|N|97|00|W|region:US-OK_type:landmark_source:dewiki}}-->
| unitof =
| subunits = <!--Mountain Lake Member, Pooleville Member-->
| overlies = [[Coconino Sandstone]]
| underlies = [[Kaibab Limestone]]-(Permian), Grand Canyon, North Rim, ([[Kaibab Plateau]]), <br />also South Rim, <br /> and elsewhere in Arizona, Utah, Nevada
| thickness = {{convert|200|ft|m|sp=us}} approximate maximum<!--Pooleville Member at type location.<ref name="Fay">Fay, O. and A.A. Graffham. Biostratigraphic and paleontological studies. Echinoderm faunas from the Bromide Formation (Middle Ordovician) of Oklahoma: Lawrence, Kansas, University of Kansas Paleontological Institute Monograph 11 30-33 (1982)</ref>-->
| extent = Grand Canyon, North & South Rims, northeast Arizona, southeast Utah<!--from Blount to Decorah<ref name="Loeblich">Loeblich, A.R. jr. Bryozoa from the Ordovician Bromide Formation Oklahoma. Journal of Paleontology, 16-4 (1942)[http://www.jstor.org/pss/1298838]</ref>-->
| area =
| map =
| map_caption =
}}

The [[Guadalupian|Middle Permian]] '''Toroweap Formation''' is a thin, darker [[geologic unit]], between the brighter colored units of the [[Kaibab Limestone]] above, and [[Coconino Sandstone]] below. It is a prominent unit in [[Grand Canyon]], [[Arizona]], [[Southwestern United States]], found through sections of the South Rim, Grand Canyon, and the North Rim, of the [[Kaibab Plateau]]; also the Kaibab's southeast extension to Cape Royal, the [[Walhalla Plateau]]. The [[Colorado River]] of the Grand Canyon makes its excursion from due-south to due-west around the Walhalla Plateau, as it enters the east end of the Grand Canyon's interior, [[Granite Gorge]]. The formation is also found in southeast [[Utah]].

The Toroweap Formation is a darker unit of [[gypsum]] and [[shale]];<ref name="Lucchitta Hike 3"/> also [[sandstone]]. In photos the ''[[cliff-former|cliff-forming]]'' Kaibab and Coconino units, show the more erodable Toroweap Formation ''[[slope-former|slope-forming]]'' accumulations upon the underlying Coconino Sandstone; likewise below the Coconino, the softer [[Hermit Formation]] is shown forming slopes, above the slope forming 'redbeds' of the [[Supai Group]]. Close-up views of the Toroweap will show the resistant sandstone sub-units as erosion resistant sections.

Cliffs of the Toroweap Formation can be found in [[Walnut Canyon National Monument]], about 70 mi<ref>''Arizona Road & Recreation Atlas'', Benchmark Maps, pp. 28–29, 62–63.</ref> southeast of the Grand Canyon, and east of [[Flagstaff, Arizona]].

[[Marine transgression]], and continental wind-blown sand, laid down the three units of Kaibab, Toroweap, and Coconino. The oceanic sea came from the west, and receded and re-transgressed. The Coconino Sandstone being made of sand dunes, continental, means that near-shore Toroweap deposits, and off-shore Kaibab Limestone, can all have the same actual ages in different regions where they were deposited, (continental, near-shore, open sea). In Arizona, the Toroweap formation is found in two transgressions eastwards,<ref>Chronic, 1983. ''Roadside Geology of Arizona'', Kaibab-Toroweap-Coconino, 2 wedge-(trangressions) graphic (unnumbered, untitled): ''In mid-Permian time, a desert of windblown sand swept northern Arizona, creating the Coconino Sandstone (1). Then shallow Kaibab and Toroweap seas advanced from the west, each depositing a wedge of marine limestone bordered with nearshore [[sandstone]], [[siltstone]], and [[gypsum]] (2). Desert conditions still prevailed on the shores, so dune sandstone there is of the same age as the marine limestones and nearshore deposits.'', p. 207.</ref> with easternmost 'strand-lines' locating the shoreline, Toroweap west, and Coconino sand dunes or beach deposits, east.

==Toroweap Overlook==
{{main|Toroweap Overlook}}
[[File:Toroweap Sunrise.jpg|thumb|246x246px|View from the Toroweap Overlook in the Grand Canyon at sunrise.]]
Halfway through the Grand Canyon (north side of Colorado River), at Toroweap Valley, with Toroweap Lake (a playa lake), an outcrop, or point overlooks the West Grand Canyon section. It is the site of the [[Toroweap Fault]]. Nine miles to the west is the roughly parallel [[Hurricane Fault]] (of [[Hurricane Cliffs]], of Arizona/Utah), the west fault, of the [[Uinkaret Mountains]] ([[Mount Trumbull Wilderness]]). An eastern fault section separates here, and goes north into Utah, passing the west side of the [[Kanab Plateau]].

The [[Toroweap Overlook]] lies above the erosion resistant [[Tapeats Sandstone]], near its west terminus in [[Grand Canyon]]. Southeast, across the river [[The Esplanade (Grand Canyon)|The Esplanade]]-(a regional section) is created, upon the erosion resistant [[Esplanade Sandstone]], a resistant member of the [[Supai Group]], (a unit above the Tapeats, but occurs on both the downside block, and upside block in the south side of the canyon, Colorado River); (The Esplanade is just upstream, with the Toroweap Fault forming its west flank). The Toroweap Overlook was visited in the 1800s by original photographers of the Grand Canyon.

==Geologic sequence==
The Permian geologic sequence common in the Grand Canyon:<ref>Chronic, 1983. ''Roadside Geology of Arizona'', graphic: ''Stratigraphic section of Paleozoic formations of the Colorado Plateau'', p. 179.</ref>
* 4-[[Kaibab Limestone]]
* 3-Toroweap Formation
* 2-[[Coconino Sandstone]]
* 1B-[[Hermit Formation]]
* 1A-[[Esplanade Sandstone]] (also considered upper unit of [[Supai Group]], Pennsylvanian-Permian boundary)

<gallery widths="285px" heights="200px">
File:Grand Canyon National Park-Arizona1575.JPG|North Rim, Grand Canyon: <br>4-Permian formations: <br>-4-[[Kaibab Limestone]]<br>-3-Toroweap Formation<br>-2-[[Coconino Sandstone]]<br>-1-[[Hermit Formation]]<br>-X-(upon [[Supai Group|Supai Group "redbeds"]])
File:WalnutCanyonNorthWall.jpg|Toroweap Formation in Walnut Canyon, east of [[Flagstaff, Arizona]] <br>Darker middle section is Toroweap: above: (3): [[Kaibab Limestone]]; below: (1): [[Coconino Sandstone]] (showing fossil dunes)-(note erosion of dunes compared to horizontal bedding of Kaibab Limestone)
</gallery>

==See also==
* [[Geology of the Grand Canyon area]]
* [[Stratigraphy]]
* [[Marine transgression]]

==References==
=== Citations ===
{{reflist}}

=== General sources ===
* ''Arizona DeLorme Atlas & Gazetteer'', 5th ed., c. 2002, 76 pages, pp.&nbsp;23–24, 31–32. {{ISBN?}}
* ''Arizona Road & Recreation Atlas'', Benchmark Maps, 2nd ed., c. 1998, pp.&nbsp;28–29, 62–63. {{ISBN?}}
* Chronic, Halka. ''Roadside Geology of Arizona'', c. 1983, 23rd printing, Mountain Press Publishing Co. 322 pages. pp.&nbsp;229–232. (''US 89A Marble Canyon – Fredonia''), pp.&nbsp;179–180. (softcover, {{ISBN|978-0878421473}})
* Lucchitta, 2001. ''Hiking Arizona's Geology'', Ivo Lucchitta, c 2001, Mountaineers's Books. (softcover, {{ISBN|0898867304}})

== External links ==
{{commons category multi|Toroweap Formation|Toroweap Formation slopes|Toroweap Formation bedding|Toroweap Point|Toroweap Overlook}}

{{Chronostratigraphy of Nevada}}

[[Category:Shale formations of the United States]]
[[Category:Geologic formations of Arizona]]
[[Category:Geologic formations of Nevada]]
[[Category:Geologic formations of Utah]]
[[Category:Natural history of the Grand Canyon]]
[[Category:Colorado Plateau]]
[[Category:Permian Arizona]]
[[Category:Permian geology of Nevada]]
[[Category:Permian geology of Utah]]
[[Category:Cisuralian Series of North America]]