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

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A WYOMING : COLOR, DO A I
SOUTH'CENTRAL WYOMING I
F,~ONT RANGE
SUNDANCE FORMATION " Jurossic Rocks
90 1 500
60 200 0 I0 20 miles
E:
~o ,oo o ,; 2'o,~
0 0
meters feet
Fig. 14. Restored cross-section showing Upper Triassic eolian deposits in Jelm Formation. Location shown on Fig. 12.
Member represents the youngest Triassic deposits
on the Colorado Plateau. It can perhaps be in-
ferred that youngest Rock Point strata are nearly
conformable with overlying Lukachukai units and
that it is no coincidence that this is where some of
the thickest Triassic eolian deposits occur.
The thickest Upper Triassic rocks of the West-
em Interior occur across the northern edge and
along the east flank of the Defiance upwarp. Thus
this long-ranging positive area was a basin during
the Late Triassic. This area lies along the Zuni
trend as defined by Blakey (this volume). It was in
this low area that the only known Triassic eolian
deposits on the Colorado Plateau were deposited.
In southwestern Colorado, these rocks are as-
signed to the upper member of the Dolores For-
mation (Blodgett, 1984). Eolian deposits consist of
cross-stratified sandstone of dune origin and
laminated to hummocky sandstone of sand-sheet
origin, both intercalated with fluvial and lacustrine
deposits (Harshbarger et al., 1957; Stewart et al.,
1972; Blodgett, 1984). We are unable to construct
either isopach maps or percentage-of-eolian-strata
maps from existing literature or unpublished in-
formation known to us. Figure 13 shows a rough
outline of known and inferred eolian strata in
Upper Triassic rocks of the Four Comers region.
Temporal correlation with eolian strata of the
Jelm Formation to the north is possible but un-
proven.
Lower Jurassic I erg deposits
The Lower Jurassic Wingate Sandstone forms a
persistent vertical cliff throughout the canyon-
lands of southeastern Utah and adjacent Arizona
and Colorado. Plagued by a series of miscorrela-
tions, especially in northwestern New Mexico, the
presently accepted eastern margin is shown on
Fig. 15. Only the southwestern and eastern margins
of eolian rocks, where the Wingate intertongnes
with the Dinosaur Canyon Member of the
Moenave Formation (Fig. 16 and Table 4), are
exposed and reasonably well understood. The na-
ture of the formation to the west, northwest, and
northeast is uncertain although we discuss our
inferences later in this paper. As discussed above,
the present terminology of the Wingate Sandstone
is somewhat confusing. North, northwest and west
of Kayenta, Arizona, the Wingate Sandstone un-
conformably overlies the Upper Triassic Chinle
Formation and in this region the formation is
undivided. East, southeast and south of Kayenta,
the formation includes the lower Rock Point
Member which is equivalent to the upper Chinle
Formation (Harshbarger et al., 1957). In this re-
gion the upper part of the formation (the Wingate
Sandstone, undivided to the north) is assigned to
the Lukachukai Member. Our isopach and facies
map (Fig. 15) concerns only the Wingate Sand-
stone (undivided) and Lukachukai Member of the
Wingate Sandstone; the Rock Point Member of
the Wingate Sandstone (Chinle equivalent) is not
included. An eolian origin for most of the Wingate
Sandstone has long been accepted; however, the
details of eolian sedimentation have yet to appear
in the literature. Tabular cosets 2-5 m thick com-
prised of trough sets filled with climbing wind-rip-
ple lamination have been observed at several local-