Shinarump Conglomerate
| Shinarump Conglomerate | |
|---|---|
| Stratigraphic range: early Late Triassic | |
 large Shinarump Conglomerate boulder at start of trail down to White House Ruin, Canyon De Chelly National Monument | |
| Type | Geological formation |
| Unit of | Chinle Formation |
| Underlies | Petrified Forest Member and other sub-units, Chinle Formation |
| Overlies | Moenkopi Formation units-(250-225 Ma), after erosion unconformity Temple Mountain Member-(lowest Chinle Formation), Moenkopi Formation-(Monument Valley, Arizona-Utah), De Chelly Sandstone-(Canyon De Chelly National Park |
| Thickness | 200 feet (61 m) maximum (east of Bidahochi, AZ),[1] variable |
| Lithology | |
| Primary | [[[conglomerate (geology)|conglomerate]], sandstone, siltstone [2] limestone |
| Location | |
| Region | Colorado Plateau |
| Extent | southeast perimeter Black Mesa (Arizona)-NNW; Defiance Plateau-(Uplift)-E & NE; north perimeter foothills White Mountains-S; Painted Desert-NW & W |
The early Late Triassic conglomerate called the Shinarump Conglomerate, formally the Shinarump Member of the Chinle Formation, is a highly resistant course-grained sandstone and pebble conglomerate,[3] sometimes forming a caprock because of its hardness. The Shinarump is found throughout the Colorado Plateau with significant exposures as the canyon rimrock in the vicinity of Canyon De Chelly National Monument, at the north-northeast of the Defiance Plateau/Defiance Uplift. At Canyon De Chelly the Shinarump Conglomerate was laid down upon De Chelly Sandstone, in a region at the west foothill region of the mostly north-south trending Chuska Mountains of northeast Arizona, northwest New Mexico.
Just northwest of Canyon De Chelly, the Shinarump also forms a caprock in Monument Valley across the border region of north Arizona, and south border Utah. The Shinarump Conglomerate there is laid upon units of the Moenkopi Formation, upon vertical cliffs of De Chelly Sandstone, upon the Organ Rock Formation, all units in the Monument Upwarp/Monument Uplift, a system associated with the Black Mesa (Arizona)-Defiance Uplift, all extending southeastwards from Monument Valley, and extending into the western border region of New Mexico, at the east border of the Defiance Plateau, uplift; a steep dip of geology units means younger geologic units are expressed eastwards into New Mexico.
Monument Valley and Canyon De Chelly caprock
In Monument Valley, of Arizona and south Utah, only larger landforms still contain the protective caprock of the Shinarump Conglomerate. The vertical cliffs of highly resistant De Chelly Sandstone protect the highly erodable Organ Rock Formation that forms 'erosion skirts' at the base of the monuments in Monument Valley. For larger mesas, or larger connected land forms, where the more erodable overlying Moenkopi Formation, has not been lost, (with the caprock Shinarump also being undermined), larger sections of Shinarump are thus preserved. Perimeter mesas in the Monument Valley region have the caprock protecting the upper surface, plants, and alluvium from easy erosion of the landform surface.
At Canyon de Chelly, the trail to White House Ruin,[4] descends through the Shinarump Conglomerate, but also has a "slot canyon" type landform with large accumulations of the Shinarump on the wash, or canyon floor. The trail then descends through the cross-bedding (fossil sand dunes) of the De Chelly Sandstone.
See also
References
- ^ [1] Summary at USA, National Geologic Map Database.
- ^ Lucchitta, 2001. Hiking Arizona's Geology, Hike 11, Coliseum Diatreme, pp. 116-119, p. 117.
- ^ Blakey, and Ranney, 2008. Ancient Landscapes of the Colorado Plateau, subsection, Great River Systems of the Moenkopi and Chinle: Triassic Deposition, pp. 58-66, p. 64
- ^ Lucchitta, 2001. Hiking Arizona's Geology, Hike 10, White House Ruin Trail, pp. 11q-113.
- Blakey, and Ranney, 2008. Ancient Landscapes of the Colorado Plateau, Ron Blakey, Wayne Ranney, c 2008, Grand Canyon Association (publisher), 176 pages, with Appendix, Glossary, Index. Contains approximaely 75 shaded topographic maps, for geology, etc, with 54 (23 pairs, (46)) for Colorado Plateau specifically; others are global, or North American.
- Chronic, Halka, 1983. Roadside Geology of Arizona, c. 1983, 23rd printing, Mountain Press Publishing Co. 322 pages. pp. 229–232-(Interstate 40 – Petrified Forest to New Mexico), pp. 210–212; also p. 209 (Winslow to Petr. For. Nat'l Pk.) (softcover, ISBN 978-0-87842-147-3)-->
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