#supai group

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Clouds (No. 132)

Red Rock Scenic Byway

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Clouds (No. 129)

Sedona, AZ (eight pics)

Red Rock Scenic Byway (two pics)

[rabbitcruiser]

Clouds (No. 134)

Red Rock Scenic Byway

[rabbitcruiser]

Clouds (No. 133)

Red Rock Scenic Byway

[earthstory]

The Supai Group

I’m cheating a little bit here. Throughout the rest of the Canyon, I’ve done units at the “formation” level. For the Supai, I’m doing a post at the Group level.

Geologists generally try to divide specific blocks of rock into units, but it’s often the case that divisions between units are fuzzy. One geologist could decide a set of beds that contains limestones and shales is one unit, another might call it 2 units, and another might call it a dozen units. Sometimes there are clear boundaries; an erosional scour and a major change in rock type, but sometimes there just aren’t and so it’s up to scientists to use their best judgment. In fact, the Supai group was originally called the Supai formation, but in the 1970s it was subdivided into 4 other formations.

The Supai Group is composed of 4 formations, all of which tell a similar story. These rocks started being deposited in the Pennsylvanian, around 315 million years ago, and ended around 285 million years ago in the Permian.

The lowest unit, the Watahomigi formation, is a quiet water facies deposit, filled with siltstones mixed with limestones. The next unit up, the Manakacha Formation, still has some siltstones but also mixes in sandstones. The next unit up is the Wescogame formation, which is dominated by cross-bedded sandstones and sandy limestones. Finally, at the top, the Esplanade sandstone is a cliff-forming, cross bedded sandstone that also helps hold up the Esplanade platform, where we see the hiker in that image.

Generally this sequence reflects decreasing water levels. It starts in more quiet waters where shales could be deposited and winds up with a higher-energy sandstone. However, the rocks throughout this sequence waiver back and forth between redbed shales and the more coarse grained limestones, marking the interplay between sea level and sediment supply over millions of years.

The Supai Group’s redbeds are a main source of the red color that stains the redwall limestone. The deep red color is caused by oxidized iron, which coats many of the grains in the unit. The oxidized iron penetrates the whole unit, suggesting that it was a characteristic of the sediment source. No nearby source is known for these hundreds of meters of sediment, but there are rocks of similar age in Texas and Colorado, deposited in arid, near-desert environments where iron could have been oxidized before the sediments flowed to the ocean.

Oh, and my motivation for cheating and calling it the Supai Group instead of doing each formation? Most of the available pictures are just labeled “Supai Group” instead of breaking out the formations so I can’t tell where most of them come from. Even though the geologist labels may change, the behaviors of people labeling photos sometimes don’t.

-JBB

Image credits:  https://www.flickr.com/photos/grand_canyon_nps/7695277618/ https://www.flickr.com/photos/alanenglish/479507364/

Sources used:  https://www.classzone.com/books/earth_science/terc/content/investigations/es2906/es2906page09.cfm http://pubs.usgs.gov/bul/1395j/report.pdf http://3dparks.wr.usgs.gov/coloradoplateau/lexicon/supai_group.htm http://books.google.com/books?id=7vpMW5zALNQC&pg=PA17&lpg=PA17&dq=supai+redbed+color+source&source=bl&ots=F40vyeXERF&sig=ax7_scx18X4cRcKsUTey0BpfT10&hl=en&sa=X&ei=NdbWU8WgB6jKsQT2-YLgDA&ved=0CB0Q6AEwAA#v=onepage&q=supai%20redbed%20color%20source&f=false

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