#nankoweap

The Nankoweap formation.

The Nankoweap formation is the next unit up in the Grand Canyon, found atop the last unit – the Cardenas basalt. The Cardenas can be spotted near the bottom in this wide-angle view of the Canyon. See the black unit that has a dip to it, beneath the flat lying and younger sediments? That’s the Cardenas basalt. The layers on top? That’s the Nankoweap formation.

The Nankoweap formation sits on top of the big thick Cardenas basalt. The boundary between the two units is an unconformity; there are places where the basalt was eroded before sediments started depositing on top. There are also faults which cut through portions of the Cardenas basalt but do not cut the Nankoweap formation, making this unit clearly younger.

The rocks of this formation are sandstones with hematite cement, giving them a reddish color. The lower part of the Nankoweap is fairly finely-bedded while the upper portion has coarser beds.

Paleomagneticists have worked on this unit and found that the lower and upper portions of the Nankoweap record strongly different magnetic directions. This change implies that there is a lot of time missing in the middle of the Nankoweap – a disconformity, an unconformity that sits somewhere but doesn’t have major changes in rock type. The Nankoweap is younger than the Cardenas; zircons present in this unit are as young as 780 million years old, meaning the Nankoweap formation must be younger than that age. Its hard to tell when Nankoweap deposition started but this unit could potentially represent over 100 million years of time, and isotope changes within the unit suggest parts of it were being deposited 750 million years ago.

Like the quartzite below, this rock is composed of sandy sediments derived from erosion of the surrounding areas. There are some pieces of the Cardenas basalt in the Nankoweap, mixed with quartz and feldspar grains. The Nankoweap formation contains a variety of sedimentary structures like mud cracks and ripple marks, indicating that it was deposited in a setting like an arid lake or arid shoreline where the water would appear and disappear as precipitation patterns changed.

-JBB

Image credits: https://upload.wikimedia.org/wikipedia/commons/e/ea/View_from_Lipan_Point.jpg http://en.wikipedia.org/wiki/Cardenas_Basalt#mediaviewer/File:Grand_Canyon_landscape.jpg

Sources: http://bulletin.geoscienceworld.org/content/117/11-12/1573.full https://www.researchgate.net/publication/313843847_Synthesis_of_the_780-740_Ma_Chuar_Uinta_Mountain_and_Pahrump_ChUMP_groups_western_USA_Implications_for_Laurentia-wide_cratonic_marine_basins

Nankoweap Graneries

Grand Canyon National Park