Abstract: Navajo Sandstone (Jurassic) of Utah and Northern Arizona--Eolian or Marine in Origin?
M. Dane Picard
Many interpretations of eolian depositional environments recently have come under attack, and alternate environments of deposition have been suggested. These include shallow-marine, littoral, coastal-dune, estuarine, and tidal-current environments.
The depositional environment of the Navajo Sandstone, considered by many to be one of the classic eolian units in North America, especially has been disputed. Horizontal, disturbed, and trough stratification in the Navajo is said to be subaqueous and not eolian. Similarly, shrinkage cracks, some ripple marks, and flute and load casts also are termed subaqueous. Thin beds of dolomite and limestone are said to be aqueous (possibly shallow marine) by the noneolian enthusiasts. Other supposed evidence for subaqueous deposition includes evaporite beds, bioturbated beds, three grains of glauconite, thin shale seams associated with widespread truncation planes, current lineation, the morphologies of log-probability plots of grain-size distributions, and intertonguing of the marine Carmel wit the upper part of the Navajo in southwest Utah.
Although the list of objections is long, the arguments are not convincing because there is little positive evidence of marine deposition in the Navajo. Marine fossils have not been found and the fauna is small, specialized, and partly transient--characteristics typical of continental biotas. The petrography of medium- and large-scale, cross-stratified sandstone in the Navajo reveals many eolian characteristics and no positive petrographic evidence of deposition by marine currents.
The cross-stratification is more similar to that developed in eolian settings, both modern and ancient, than to cross-stratification formed by tidal or longshore currents. Eolian ripple marks, as well as subaqueous types, occur in the Navajo. In addition, across-slope ripples on dune-slip faces are present. Paleocurrent studies using cross-stratification and ripple marks strongly support an eolian interpretation for the medium- and large-scale cross-stratified sandstone.
My interpretation is that most of the medium- and large-scale, cross-stratified sandstone is eolian. The carbonate beds were deposited in small lakes within dune areas. Calcareous siltstone, claystone, and mudstone closely associated with the carbonate rock were deposited in the same setting. Inland sabkhas may have been depositional sites for gypsum, gypsiferous siltstone, and claystone. Minor fluvial beds also have been recognized.
AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado