WIDESPREAD DOLOMITE IN THE COCONINO SANDSTONE, ARIZONA, USA

In our studies of the Coconino Sandstone from a wide variety of areas from northern and central Arizona we have found that dolomite is present at most locations. Analytical techniques have included thin section and XRD analysis. Most workers have interpreted the Coconino as eolian, which make these findings surprising.

Four different kinds of dolomite have been found in the samples analyzed. 1) Multiple beds of microcrystalline fossiliferous dolomite (up to 8cm thick) have been found at three localities at the extreme northern edge of the Coconino with purities up to 98%. Beds may correlate over a 100 km distance near the base of the formation. 2) Dolomitic ooids (in various stages of development) with quartz and K-feldspar cores occur at nearly every location sampled north of the Grand Canyon. These ooids are only found in the cross-bedded portions of the sandstone. Cross beds containing dolomite in the northern part of the Coconino have no style difference with those in the southern part of the formation, which lack dolomite ooids. 3) Sand-sized dolomite clasts occur at every location sampled in northern Arizona. The clasts are usually similar in size or larger than the quartz grains. 4) Dolomite cements have been found at nearly all localities we have sampled in northern and central Arizona, covering the full extent of the Coconino. Cements are microcrystalline to coarse crystalline. Some of the dolomite cements have been replaced with calcite cements.

Interdunal ponds are insufficient to explain high purity bedded dolomite. The dolomite has no associated evaporate minerals and an extensive range. The ooids and their respective cores are poorly sorted and were buried in poorly sorted cross-bedded sands which probably indicate a marine setting for the cross-bed formation. The dolomite clasts are much softer than quartz and would probably be quickly abraded in an eolian environment. The clasts probably originated from the bedded dolomites in the north; cross-beds in the Coconino indicate a southerly transport of the sand. The cements are surprising because they are regional and all models of dolomite formation require ion specific supersaturated aqueous solutions.

These findings are difficult to explain within current eolian models and may suggest a significant part of the Coconino may have been formed under marine conditions.