Geology & Minerals: Coconino NF, Red Rock, Coconino Sandstone

Large-scale cross-bedding in the Coconino sandstoneThe Coconino sandstone (center of the photo) overlies the Schnebly Hill Formation (lower right corner of the photo). Between the two units there was no break in sediment accumulation in the region, but the Coconino sandstone records a shift from coastal dunes to inland dunes.  The inland dunes of the Coconino sandstone contain larger sand grains and are cross-bedded throughout the entire 500 foot thickness of the unit.  The prominent high-angle cross-beds can be seen in the photo.

The Coconino sandstone is gold-colored rather than red because of its larger, more uniform sand grains.  Groundwater is able to travel more freely through uniform sandstones than through rocks containing silt or mud.  The Coconino sandstone probably used to be stained red, but groundwater travelling through the rocks have leached out the iron that produces the red coloration.

Geologists have deduced that the Coconino sandstone was deposited in a terrestrial desert on the basis of several clues: 

  • The individual sand grains that make up the formation have a frosted texture that resulted from collisions with other sand particles as they blew in the wind. 
  • The high-angle cross-beds are also evidence for terrestrial deposition.  On land, if sand is piled up, the slopes of the pile will be stable up to an angle of 33 degrees.  Beyond 33 degrees, the slopes will collapse.  Under water, however, the slopes would collapse at a much shallower angle.  So, high-angle cross-beds are a sure sign of terrestrial deposition.