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

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during the Jurassic Period (Peterson, 1986). A
reentrant in west-central Colorado and adjacent
parts of Utah reflects the ancestral Uncompahgre
uplift, an important structure that was repeatedly
activated in late Paleozoic, Mesozoic and Ceno-
zoic time. A similar reentrant in central Arizona
may reflect positive movement of the Mogollon
uplift, but lack of data over a broad area makes
the configuration of this feature somewhat specu-
lative.
Differential movement on salt-cored anticlines
in west-central Colorado is reflected in the irregu-
lar configuration of the isopach interval east of
the Monument bench. The salt was deposited dur-
ing Pennsylvanian time but it moved from the
Late Pennsylvanian to at least the end of the
Jurassic to produce several salt-cored anticlines
(Cater, 1970). The anomalously thin area within
the Utah-Idaho trough in west-central Utah is in
an area of considerable structural complexity and
may not be real.
Available data are too sparse to determine the
percent of cross-bedded sandstone in the isopach
interval. Visual observations aided by a few mea-
surements indicate that the isopach interval con-
sists of more than 70% cross-bedded sandstone in
most places in the Western Interior. However, the
eolian sandstone is interbedded with volcanics and
(or) non-eolian, flat-bedded siltstone and sand-
stone in southern Arizona and California where
the percentages are considerably lower. Additional
complications in this area are that the isopach
interval is incomplete due to erosion or faulting,
or reliable criteria to distinguish Middle Jurassic
from Lower Jurassic beds have not been found.
The precise timing of movement on the above-
mentioned structures remains somewhat conjec-
tural because of a lack of detailed facies studies
that might yield an indication of movement during
deposition. Judging from the isopach map alone,
one can only speculate that the movement was
during and (or) immediately after deposition of
the isopach interval. However, judging from the
record of movement on many of these structures
throughout much of the Phanerozoic, it would
seem more likely that the structures moved con-
tinuously during deposition as well as during the
succeeding erosion interval.
Middle Jurassic eolian deposits
Introduction
The base of the Middle Jurassic is either the J-1
or J-2 unconformity (Pipiringos and O'Sullivan,
1978). The unconformity is overlain by a variety
of deposits of continental and marine origin.
Within rocks of Middle Jurassic age we recognize
the following eolian-bearing units: Temple Cap
Sandstone, Page Sandstone, Entrada Sandstone
and Romana Sandstone (Fig. 2).
Middle Jurassic I eolian deposits
The Temple Cap Sandstone of early Middle
Jurassic age (early part of the Bajocian Age
according to Imlay, 1980) is the oldest formation
in the Middle Jurassic San Rafael Group and is
only present in the southwestern part of the Col-
orado Plateau in southwestern Utah (Fig. 2). Based
on its stratigraphic position between the J-1 and
J-2 unconformities, the formation correlates with
the Gypsum Springs Member of the Twin Creek
Limestone in northern Utah, which occupies a
similar stratigraphic position. Information used to
construct the isopach map is given in Table 6 and
the geographic distribution of data points is shown
on Fig. 21. Eolian beds in the formation are of
limited extent but may have once been more
extensive (Chan and Kocurek, this volume).
The Temple Cap consists of two members on
the Colorado Plateau but only the upper member
is eolian. The upper or White Throne Member
consists of crossbedded sandstone deposited in
eolian environments. The underlying Sinawava
Member consists of flat-bedded silty sandstone
and scarce mudstone deposited in sabkha and
possibly hypersaline marine environments. At the
west edge of the Colorado Plateau, the White
Throne grades into red beds that are included in
the Sinawava Member but, in addition to the
lithologies noted above, the westernmost Sinawava
includes thin beds of limestone and gypsum
(Peterson and Pipiringos, 1979).
Temple Cap strata contain the record of crustal
movements coincident with deposition. The for-
mation is wedge-shaped and thickens irregularly