Nugget-Navajo-Aztec Sandstone: Interaction of Eolian Sand Sea with Andean-Type Volcanic Arc
John E. Marzolf
The Nugget-Navajo-Aztec sand sea was deposited east of an Andean-type volcanic arc. During the early stage of eolian deposition, fluvially transported sand was concentrated in the marine littoral zone and returned inland by onshore winds from the northwest. With progressive development of the arc, the sea withdrew. Wind direction changed from northwest to northeast. Previously deposited eolian sand was transported southwestward into the volcanic arc.
Proximity of the arc can be detected with great difficulty by examining eolian and underlying red-bed facies. In southern Nevada, the volcanic arc is undetectable in eolian facies, but thin sandstone beds containing volcanic clasts or weathered feldspar in the finer grained red-bed facies indicate arc volcanism; volcanic clasts are distinct in a basal conglomerate. Westward into California, the sub-Aztec Sandstone contains volcanic pebbles. The upper part of the Aztec Sandstone contains a 1 to 2-m thick volcaniclastic siltstone. Farther west, the Aztec Sandstone is interbedded with volcanic flows, ash flows, and flow breccias. These rocks might easily be mistaken for red beds in well cores or cuttings. Sand in sets of large-scale cross-beds remain virtually identical in composition an texture to sand in eolian facies of the Colorado Plateau. Where sets of eolian cross-beds lie on volcanics, the quartzose sandstone contains pebble to cobble-size volcanic clasts. Locally, cross-bed sets of yellowish-white, quartzose sandstone alternate with purplish-gray cross-bed sets containing numerous pebble to cobble-size volcanic clasts. The ability to recognize volcanic indicators within Nugget-Navajo-Aztec eolian facies is important in delineating the western margin of the back-arc eolian basin.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.