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

34
the southern Monument Uplift and across the
northern Kaibab Uplift. Nothing is known about
the erg to the west and north of its area of outcrop
in southeastern Utah. We offer three untested
hypotheses concerning its margins in these direc-
tions: (1) the eolian deposits thin to an erosional
or depositional edge; (2) they merge with the
Navajo Sandstone because of northwestward
pinch-out of the intervening Kayenta Formation;
and (3) the eolian sandstones undergo facies
change with fluvial and sabkha redbeds as can be
documented along its present southwestern
margin; our discussion of the Navajo-Nugget
Sandstone in the next section favors the third
hypothesis.
The southwestern margin of the Wingate is
exposed along a line of cliffs that trend from near
Holbrook, Arizona to Zion National Park, Utah.
Figure 16 shows restored cross-sections across the
erg margin. Edwards (1985) documented fluvial-
eolian interactions in north-central Arizona and
work in progress by Lars Clemmensen and Henrik
Olsen of the University of Copenhagen in con-
junction with Blakey is studying the erg margin
along the Vermilion Cliffs. These studies docu-
ment an initial erg progradation southwest to a
line from north-central Arizona to south-central
Utah (Fig. 16). Fluvial deposits, chiefly of
ephemeral sheet-flood and stream-flood origin, en-
croached upon and reworked the erg margin. The
fluvial deposits, the Dinosaur Canyon Member of
the Moenave Formation, extended as far north-
eastward as a line from the Defiance Plateau in
Arizona to west of the Circle Cliffs in Utah (Figs.
15 and 16). Most stratigraphic work has placed
the lower southwest extension of the erg in the
Wingate Sandstone and the interval of alternating
fluvial and eolian deposition in the Dinosaur
Canyon Member of the Moenave Formation.
The Wingate Sandstone is in need of additional
sedimentologic and stratigraphic study. Little is
known about regional sedimentologic trends and
history of erg development. The isopachs (Fig. 15)
and cross sections (Fig. 16) show some relations to
regional syn-depositional tectonic patterns
(Blakey, this volume), especially the southern
margin, which closely parallels the Zuni lineament
of Kelley (1955). Apparently the northeast margin
of northwesterly flowing streams in the Moenave
Formation was somehow influenced by this fine.
Lower Jurassic H eolian deposits
Eolian strata of late Early Jurassic age depicted
on the data base and isopach maps (Figs. 17 and
18) probably formed the largest eolian deposits in
North America. These rocks include such well-
known units as the Aztec Sandstone (Nevada and
southeastern California), the Glen Canyon Sand-
stone (northwestern Colorado and northeastern
Utah), the Navajo Sandstone (northern Arizona,
western Colorado, southern and central of Utah),
and the upper member of the Nugget Sandstone
(southeastern Idaho, northern Utah and Wyo-
ming). Eolian strata in southern Arizona that are
thought to correlate at least approximately with
these units are included in several formations,
each of which is restricted to one or a few of the
mountain ranges in the Basin and Range Province.
These includes the Ali Molina, Mount Wrightson,
and Sil Nakya Formations, the Cobre Ridge Tuff,
and the Ox Frame Volcanics (Bilodeau and Keith,
1986). Data used in preparation of the isopach
map are included in Table 5 and the distribution
of data points is shown on Fig. 17. These beds are
here considered late Early Jurassic in age (Pliens-
bachian and Toarcian Ages; Peterson and
Pipiringos, 1979).
Not included in this study are quartzites of
Early Jurassic age in the southern part of western
Nevada that may also be eolian and that may
correlate with late Early Jurassic eolian units
farther east in the Western Interior. Although
highly metamorphosed, cross-bedding in the
quartzites can still be recognized even though finer
details such as grading, ripple cross-laminations,
details of the laminations, and grain shape have
been largely or entirely obliterated, making it dif-
ficult to find conclusive evidence of the mode of
deposition. Proffett and Dilles (1984) considered
Jurassic quartzites (their quartzitic sandstone) of
the Singatsee Range (sect. 27, T13N, R24E, Lyons
County, Nevada) eolian in origin but a field check
revealed only poorly suggestive evidence such as
the moderately large thickness (as much as 1.5 m)
of a small number of sets and the relatively pure