Sand Dunes to Sandstone

multiple layers of cross-bedding in the Navajo Sandstone

NPS photo/Adrienne Fitzgerald

 
How big were the Navajo desert sand dunes? How did they become sandstone?

The ancient sand dunes were similar to dunes in modern deserts, probably reaching several hundred feet at their greatest height. Only the bottom portion of a dune is preserved in the sandstone, but layer upon layer of dunes built up, until the total thickness of the pile of sand grew to be over 2,000 feet thick (although most of this thickness would have been underground).
 
diagram showing how wind-blown sand produces cross-bedding pattern

Diagram by Geoscientist-In-the Park David Tarailo, sponsored by the Geological Society of America, GeoCorps Program, 2012. 

 
The sand was eventually turned into sandstone due to the pressure from overlying layers, combined with the cementing effects of mineral-laden groundwater. Groundwater containing dissolved minerals –particularly calcium carbonate –percolated through and around the sand grains below the water table, leaving behind calcium carbonate that glued the grains together—turning sand dunes into the Navajo Sandstone.
 
diagram showing loose grain packing, and tight grain packing after compaction
In A, sand is loosely packed after initial deposition. In B, sand is more tightly packed after compaction from overlying sediment, but still has significant pore space for groundwater.

Diagram by Geoscientist-In-the Park David Tarailo, sponsored by the Geological Society of America, GeoCorps Program, 2012. 

 

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Last updated: June 13, 2015

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