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

een able to document such intertonguing; how-
ever, in many areas of north-central Arizona and
south-central Utah, the Kaibab is very sandy and
the White Rim, Toroweap, and Kaibab are very
difficult to separate on outcrop, much less the
subsurface. Although regional stratigraphic data
may suggest that a White Rim-type sand body lay
to the east of the Kaibab sea, such an occurrence
has not yet been clearly documented.
The White Rim and upper Coconino Sand-
stones were deposited by a coastal sand sea that
bordered the Toroweap sea. Intertonguing with
the Toroweap was caused by changes in sea level
coupled with changes in sand supply. The Sedona
and Monument arches formed an eastern barrier
to eolian deposition and preservation (see Blakey,
this volume). The erg systems were apparently fed
by eolian sand reworked from the north.
The Lyons was deposited between mountains,
alluvial fans, and a coastal plain. The erg ex-
panded and contracted in response to tectonism,
sea-level change and sand supply. The sand may
have been reworked from older ergs to the north
or derived from coeval alluvial fan deposits.
Upper Triassic and Lower Jurassic eolian deposits
Introduction
The latest Permian and most of the Triassic
lack recognized eolian deposits. Eolian deposition
was renewed in the Late Triassic and culminated
with major eolian sedimentation in the Early
Jurassic (Fig. 2). We recognize three erg-bearing
intervals: Jelm Formation, Wingate Sandstone and
Navajo Sandstone and related units, which in-
clude the Nugget and Aztec Sandstones.
Upper Triassic eolian deposits
The Jelm Formation is exposed in south-central
Wyoming and north-central Colorado. The eolian
deposits occur in both members of the Jelm as
recognized by Pipiringos (1972) and Pipiringos
and O'Sullivan (1976). However, neither of the
above papers considered the origin of the Jelm
Formation. An eolian origin for part of the forma-
tion is based on unpublished information gathered
by Peterson. Peterson's field work has confirmed
an eolian origin for the large-scale, cross-stratified
sandstone shown in columns published by
Pipiringos (1972) and Pipiringos and O'Sullivan
(1976). Our maps show the thickness of the Jelm
Formation based on the above sections and the
distribution and percentage of eolian sandstone as
inferred from the sections (Table 3) and Peterson's
preliminary field work. Given the relatively sparse
data, the maps and the following discussion must
be considered preliminary.
Based primarily on regional stratigraphy, the
Jelm Formation is assigned a Late Triassic age
(Pipiringos and O'Sullivan, 1978). No comprehen-
sive discussion of its origin or tectonic setting
presently exists; however, the regional isopach and
facies map (Figs. 12 and 13) provides some back-
ground for preliminary discussion. The erg de-
posits are apparently confined to the southeast
portion of the Jelm Formation (Fig. 14) in south-
central Wyoming. The area roughly coincides with
that of Permian erg deposition (Fig. 8). A large
fluvial plain lay to the west of Jelm erg sedimenta-
tion and possibly served as a partial source of the
sand in the erg. As the south and east margins of
the unit are truncated by the J-2 unconformity, no
information is available concerning its original
extent. Clearly this unit is in need of major re-
gional study.
The Rock Point Member of the Wingate Sand-
stone contains eolian deposits throughout most of
its extent. Although equivalent to the upper part
of the Chinle Formation, Harshbarger et al. (1957)
included the Rock Point in the Wingate Sandstone
because of supposed intertonguing between the
Rock Point and Lukachukai Members. Later work
has failed to confirm intertonguing and instead
has suggested the presence of an unconformity,
the J-0, between the two units (Pipiringos and
O'Sullivan, 1978; Peterson and Pipiringos, 1979).
Nevertheless, the Rock Point and Lukachukai are
still officially considered members of the Wingate
Sandstone, the former Upper Triassic and the
latter, Lower Jurassic. Given that the Rock Point
Member is thickest in the southern Four Corners
region and that the Chinle Formation is beveled to
successively older units to the west by the J-0
unconformity, it seems likely that the Rock Point
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