Shinarump Conglomerate

Shinarump Conglomerate Stratigraphic range: early Late Triassic 230–225 Ma PreЄ Є O S D C P T J K Pg N ↓
Massive, fallen Shinarump Conglomerate block (erosion-resistant) at start of trail (viewed up-trail) down to White House Ruin, Canyon De Chelly National Monument (Note: pictured above smooth-sided cliffs of the underlying De Chelly Sandstone)
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-(50 ma erosion unconformity-Canyon De Chelly National Monument)
Thickness	200 feet (61 m) maximum variable
Lithology
Primary	conglomerate, sandstone, siltstone
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 coarse-grained sandstone and pebble conglomerate, sometimes forming a caprock because of its hardness, cementation, and erosion resistance. 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-(280 Ma, an erosion unconformity of 50 my), 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 northern Arizona – and southern Utah. The Shinarump Conglomerate there is laid upon units of the Moenkopi Formation and remain as resistant caprocks preserving the vertical cliffs of De Chelly Sandstone and other units of the Monument Upwarp/Monument Uplift. The associated Black Mesa (Arizona)-Defiance Uplift extends southeast from Monument Valley into the western border region of New Mexico. The relatively steep east dip of geology units results in increasingly younger geologic units eastwards into New Mexico.

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 vertical 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.