Given the question: The answer to the question: What formation was the youngest of the Supai Group? is inside the article: An unconformity of 15 to 20 million years separates the Supai Group from the previously deposited Redwall Formation. Supai Group was deposited in late Mississippian, through the Pennsylvanian and into the early Permian time, some 320 million to 270 million years ago. Both marine and non-marine deposits of mud, silt, sand and calcareous sediments were laid down on a broad coastal plain similar to the Texas Gulf Coast of today.  Around this time, the Ancestral Rocky Mountains rose in Colorado and New Mexico and streams brought eroded sediment from them to the Grand Canyon area.Supai Group formations in the western part of the canyon contain limestone, indicative of a warm, shallow sea, while the eastern part was probably a muddy river delta.  This formation consists of red siltstones and shale capped by tan-colored sandstone beds that together reach a thickness of 600 to 700 ft (around 200 m). Shale in the early Permian formations in this group were oxidized to a bright red color.  Fossils of amphibian footprints, reptiles, and plentiful plant material are found in the eastern part and increasing numbers of marine fossils are found in the western part.Formations of the Supai Group are from oldest to youngest (an unconformity is present at the top of each): Watahomigi (see 5a in figure 1) is a slope-forming gray limestone with some red chert bands, sandstone, and purple siltstone that is 100 to 300 feet (30 to 90 m) thick. Manakacha (see 5b in figure 1) is a cliff- and slope-forming pale red sandstone and red shale that averages 300 feet (90 m) thick in Grand Canyon. Wescogame (see 5c in figure 1) is a ledge- and slope-forming pale red sandstone and siltstone that is 100 to 200 feet (30 to 60 m) thick. Esplanade (see 5d in figure 1) is a ledge- and cliff-forming pale red sandstone and siltstone that is 200 to 800 feet (60 to 200 m) thick. An unconformity marks the top of the Supai Group., can you guess it ?
The answer is:
Esplanade