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

66
misleading. The eolian beds are at or near the base
of the Morrison Formation and generally lie in
depositional contact on the Windy Hill Member,
the youngest unit in the Sundance Formation (Fig.
35A). The J-5 unconformity is at the base of the
Windy Hill and the Windy Hill interfingers with
Morrison strata (Pipiringos, 1968), making that
member more closely related to the Morrison than
to the rest of the Sundance. The relationship of
the eolianites to the Unkpapa Sandstone and re-
lated beds around the Black Hills is not clear
although stratigraphic position low in the Morri-
son Formation and near the J-5 unconformity
demonstrates that they are equivalent or nearly
equivalent in time. The configuration of the central
Wyoming eolianites and the Unkpapa as well as
its Black Hills correlatives might be interpreted as
an indication that a structural downwarp lay be-
tween these two areas. Instead, we suggest that
this reflects a lack of data in the Powder River
basin and may or may not relate to structural
downwarping there.
Bluff Sandstone Member of Morrison Formation
and Junction Creek Sandstone
The Bluff Sandstone Member of the Morrison
Formation along with its correlative--the Junc-
tion Creek Sandstone--is part of a large eolian
sandstone complex on the east side of the Col-
orado Plateau (Figs. 34 and 35B). The name Bluff
Sandstone Member is used for the part of this
complex in southeastern Utah and northeastern-
most Arizona, whereas the name Junction Creek
Sandstone is used for essentially the same strati-
graphic unit in southwestern Colorado and north-
westernmost New Mexico. Equivalency of the two
units and their continuity as a single entity has
been recognized ever since Goldman and Spencer
(1941) established the Junction Creek as a sep-
arate formation. The Bluff is Late Jurassic (late
Oxfordian-Kimmeridgian Ages) because it inter-
tongues with the Tidwell and Salt Wash Members
of the Morrison Formation (also of late Oxfordian
and Kimmeridgian Ages according to Imlay, 1980).
Because of their time-equivalency, the Junction
Creek must be of the same age. Thus, an indica-
tion that the Junction Creek is late Middle Jurassic
in age (late CaUovian Age) by Imlay (1980, p. 75)
is in error. Bluff has priority of nomenclature
(Baker et al., 1936) over Junction Creek (Goldman
and Spencer, 1941) but thus far no one has sug-
gested dropping Junction Creek and applying the
name Bluff to the entire complex.
The Bluff-Junction Creek eolian complex is
highly irregular in plan view and reaches its great-
est thickness of 131 m in southwestern Colorado,
but three other thick areas are on lobes that
extend south, west and northeast from there (Fig.
34).
For the most part, the relationship of the eolian
complex to active structures is indirect as it tends
to lie in intermediate areas between structural
lows and highs. An exception is the northeast
lobe, which tends to be thickest on top of the Late
Jurassic Uncompahgre uplift. The westernmost
part of the eolian complex lies just east of the
southern part of the Monument uplift where stratal
relationships with the Salt Wash Member of the
Morrison suggest that vertical movement on the
uplift played a role in determining the location of
at least the western part of the dune field.
The Salt Wash Member of the Morrison For-
mation was deposited by streams that flowed
northeastward and eastward, entering the Col-
orado Plateau province from the west and
southwest and forming a broad alluvial complex
across the middle of the Colorado Plateau. Lack
of Salt Wash fluvial strata above the thickest part
of the Bluff (O'Sullivan, 1965) indicates that the
streams divided and flowed around the Bluff dune
field, but it seems unlikely that unconsolidated
dune sands could have withstood the erosive power
of Salt Wash streams without some agent other
than the sheer bulk of the dune field to divert the
streams. The Monument uplift was an active
structural element during all of the Jurassic Period
and played a significant role in influencing the
distribution of Morrison sediment (Peterson, 1984,
1986). Positive movement in the Monument region
resulted in a topographic high in the southern part
of the uplift. The high divided the Salt Wash
alluvial complex into two lobes that extended
eastward on either side of the Bluff dune field,
which lay just east of the Monument topographic
high (Peterson and Tyler, 1985). The dune field