Synthesis of late Paleozoic and Mesozoic eolian deposits of the ...

patterns and foster the accumulation of dune sand
farther east.
Conclusions
Late Paleozoic and Mesozoic erg deposits of
the Western Interior dominate much of the "great
sand pile", especially on the south-central Col-
orado Plateau. Apparently sand was fed into the
region from the north by northerly winds, westerly
flowing rivers, and southerly flowing coastal cur-
rents. The ergs initiated, expanded, and waned in
response to tectonic, climatic and eustatic events.
In terms of present distribution, maximum extent
was during the Wolfcampian, Early Jurassic and
Middle Jurassic time. With the exception of the
latest Permian, Early, Middle and early Upper
Triassic, eolian deposits occur somewhere across
the region at any given defined interval of time.
Such widespread and continuous deposition of
eolian sediments is unique, in our experience, in
the stratigraphic record.
We hope that this paper provides a service to
both those wanting an introduction to eolian de-
positional systems of the Western Interior and
those familiar with the overall stratigraphic and
depositional framework. We have attempted to
summarize the currently available data for these
units, both published and unpublished. Our con-
clusions and brief discussions are based on great
volumes of data, much of which could not be
presented or illustrated because of space limita-
tions. If one message or thought is left with the
reader, we hope it will be that eolian depositional
systems are very complex and that most are in
need of additional detailed study. The stereotype
of clean, widespread, uniform eolian formations is
unrealistic. Margins range from complex, verti-
caUy stacked facies changes to thinning tongues or
wedges of eolian sandstone. Although a few
margins represent simple onlap-offlap pinch-outs,
most are associated with facies changes to non-
eolian deposits.
Correct reconstruction of facies relations and
geometric configuration of erg intervals is critical
to interpretation of ancient eolian sequences. The
major goal of this paper is to present this informa-
tion so that the depositional history, paleogeogra-
phy and paleotectonics of the sedimentary rocks
of late Paleozoic and Mesozoic age across the
Western Interior can be more fully understood.
Acknowledgments
Many geologists and technical-support people
assisted in the preparation of this paper. Larry
Middleton of Northern Arizona University as-
sisted with the isopach of the Navajo and Aztec
Sandstones. Frank Royse of the Denver office of
Chevron Oil Company provided data for the Nug-
get Sandstone in the thrust belt of Wyoming,
Idaho and Utah. The Bilby Research Center of
Northern Arizona University provided technical
support through the drafting and photography of
Emilee Mead and typing of Louella Holter. David
Best (Northern Arizona University) and J.E.
"Woody" Frezon (USGS, Denver) reviewed the
manuscript and provided helpful suggestions for
improving the paper. We are indebted to the many
tens of field assistants and students who helped
the three of us in the field over the many years.
Jeff Horowitz of the Department of Geology, Uni-
versity of Texas at Austin, and George Garcia and
Carol S. Holtgrewe of the U.S. Geological Survey,
Denver, drafted the maps. Bonnie L. Crysdale of
the U.S. Geological Survey, Denver, helped with
the preparation of the computer-generated base
map.
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