PETROGRAPHIC ANALYSIS OF THE COCONINO SANDSTONE, NORTHERN AND CENTRAL ARIZONA

Little has been published about the mineral composition of the Coconino Sandstone, except that it consists of medium-grained and well-rounded quartz sand, which is expected for an aeolian deposit. The goal of this study is to enhance the understanding and overall nature of this well-known unit. Samples of the Coconino were collected from dozens of sites from a broad range of localities from all over northern and central Arizona. Over four hundred thin sections have been prepared for petrographic analysis. In addition, many samples were analyzed with XRD. Some surprising trends are emerging. The quartz sand is often bimodal. Only the larger quartz grains tend to be rounded or well-rounded. Smaller grains are most often subangular. Feldspars are often a significant component of the mineralogy and are often subangular, like the quartz. Trace minerals appearing are micas, dolomite clasts, dolomite oolites, zircons, chert and one instance of glauconite. Dolomite ooliths have turned up in several different locations. The heavy minerals are often close in size to the quartz sand. Mica grains are often the same size, or larger than the quartz grains. Thin beds of dolomite were found in one location (Andrus Point) within the Coconino sands. We are seeking to understand how these surprising findings can be understood within an aeolian framework. For example, can grains softer than quartz (micas, feldspars, dolomite) survive harsh saltation processes for any length of time? Why are the quartz grains often bimodal? Why are dolomite clasts and dolomite oolites common in some areas, even in the center of this sand sea? As a general rule, shouldn’t the heavy minerals be much smaller than the quartz grains, considering their hydraulic equivalence in air?