Bob Linsell [right], Resource Manager for Granite Construction, took us into their sand and aggregate quarry at their Swan Road facility near the Tucson Airport, to talk about the geology of the materials they mine.
The top 5-6 feet of the Quaternary sediments are caliche-rich which requires that it be bulldozed off. The caliche breaks into blocks that act like cobbles in the aggregate mix but are too weak for the asphalt. Even the crushed caliche is a problem because it absorbs too much moisture.
The underlying sand and gravel layers are the main resource although Bob says they could use more rocks compared to the sand volumes. The rocks are crushed to create rough surfaces for improved adhesion in the asphalt. In the picture at right, Bob is standing at the top of a clay layer that is as much as 17 feet thick in places. The company quarries the sand and rock down to the clay layer but is planning on eventually cutting down through the clay to an underlying set of sand and gravel layers that extend down 65 feet.
There is significant lateral heterogeneity of the deposits, which appear to include a variety of river, fan, flood, and valley deposits. Most of the clasts are sourced from the Santa Rita mountains to the south.