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

scribed and discussed ha succeeding paragraphs.
Considerably smaller eolian beds or lenses are
present elsewhere ha southern Utah and south-
western Colorado (Peterson, 1980) but they are
not included ha this study owing to their small
areal extent. According to Imlay's (1980) regional
studies, these beds are Late Jurassic (late
Oxfordian, Kimmeridgian and early Tithonian) in
age. The data for constructing the isopach maps
are given in Table 6; the isopach and facies map is
Fig. 34, and the location of data points is on Fig.
21. Late Jurassic tectonic elements varied some-
what from the Middle Jurassic and are shown on
Fig. 33. Restored cross-sections of this interval are
shown on Fig. 35.
Unkpapa Sandstone
The Unkpapa Sandstone is on the east flank of
the Black Hills in western South Dakota (Fig. 34)
and is a likely correlative to basal Mon-ison sand-
stone beds on the west side of the Black Hills
(Robinson et al., 1964, p. 20). The bedding in this
package of beds has been largely or entirely ob-
literated in most places by unknown causes. De-
spite these problems, field and petrographic stud-
ies by Szigeti and Fox (1981) left the conclusion
that it is predominantly eolian in origin. The
Unkpapa interfingers with the Morrison Forma-
tion and is so closely related to it that Szigeti and
Fox (1981) felt it should be considered a member
of the Morrison.
The Unkpapa Sandstone and presumably corre-
lative sandstone beds at the base of the Morrison
Formation are on the flanks of the Black Hills
uplift ha western South Dakota and eastern
Wyoming (Figs. 34 and 35A). Correlation with
basal Morrison sandstone beds on the northwest
flank of the uplift is not certain but seems rea-
sonable (Robinson et al., 1964). As contoured
from the available surface information, the Unk-
papa and related units are restricted to the struct-
ural high marked by the Black Hills uplift, a
structural feature known to have been active in
Jurassic and Cretaceous time (Robinson et al.,
1964). Studies by McKee et al. (1956) and Szigeti
and Fox (1981) suggest that the sandstone con-
tinues southeast from the Black Hills, but whether
or not it is eolian in that area has not, to our
knowledge, been determined.
The Unkpapa dune field apparently was sur-
rounded by lacustrine environments represented
by mudstone layers in the Morrison Formation
(Szigeti and Fox, 1981). As the region subsided,
the dune field was inundated and eventually
covered by lacustrine deposits. Destruction of
bedding in the formation may have been caused
by chemical reactions between the lacustrine
waters and labile grains or possibly by burrowing
organisms.
Judging from descriptions in the literature, the
percent of eolian sandstone is highest (as much as
35%) in the southern part of the Black Hills (Fig.
34). The thickest part of the Unkpapa (69 m) is in
this general area and the rocks probably lay above
the level of Morrison lake waters for a longer time
than surrounding areas. Thus, if destruction of
some of the sedimentary structures is related to
chemical reactions with lake waters, the relatively
high percentages of cross-bedded sandstone in this
area might reflect nothing more than less time in
contact with the lake waters.
Central Wyoming eolian deposits
Scattered eolian sandstone lenses are present in
the lower part of the Morrison Formation
throughout a large part of central Wyoming. These
units were not recognized until recently and stud-
ies on them are still in progress (D.M. Uhlir, pers.
commun., 1986); hence, an adequate isopach map
could not be constructed. However, a map show-
hag the approximate area in which these eolian
sandstone bodies have been found is given in Fig.
34. The area outlined on this map was found by
preparing an isopach map of the lowest sandstone
unit ha the Morrison Formation from the available
literature (none of which indicated the deposi-
tional environment) and, where necessary, adjust-
ing this to fit the distribution of cross-bedding
dip-vector resultants obtained from the eolian units
and kindly furnished by D.M. Uhlir (pers. com-
mun., 1986) or obtained by one of us (FP; see
Peterson, this volume, for the distribution of the
resultants). The isopach map is not included in
this report because the data are inadequate and
65