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

of the Bell Springs Member of the Nugget
(Pipiringos, 1968; Doelger and Steidtmann, 1985)
has not, to our knowledge, been determined but it
is not considered to be appreciable.
The direction of lateral replacement of Navajo
or Aztec tongues by units in the underlying
Kayenta Formation is only documented in a few
areas, but where it has been determined it is
consistent with paleowind patterns determined
from crossbedding dip vector studies. Harshbarger
et al. (1957, p. 22, pl. 2) report southeastward
gradation of Navajo tongues into the Kayenta
Formation in northeastern Arizona, which is
parallel to the southeastward direction of eolian
transport in this area during Early Jurassic time
(Peterson, this volume). It is highly likely that the
Lamb Point and Shurtz Sandstone Tongues of the
Navajo in southwestern Utah (Averitt, 1962; Wil-
son, 1965) are different names in different areas
for the same tongue of the Navajo. If this correla-
tion is correct, the tongue thins southwestward in
the same direction (Fig. 19) that the winds moved
in this area (Peterson, this volume).
During Early Jurassic time an Andean:type
magmatic arc extended northwestward from
southern Arizona into California (Bilodeau and
Keith, 1986). Eolian strata included in the isopach
interval (Fig. 18) lap southward onto older rocks
in this area, apparently because the erg migrated
into highlands produced by uplift in the arc. Inter-
fingering or interbedding of the eolian sand with
arc volcanics was documented by Marzolf (1983a)
and Bilodeau and Keith (1986). In this area, the
upper boundary of the Lower Jurassic Series (and
thus of the isopach interval) is difficult to de-
termine owing to the difficulty of distinguishing
Middle Jurassic volcanics that postdate strata of
the isopach interval from Early Jurassic volcanics
that were coeval with the eolian beds. Clearly, a
study of the isotopic dates of the volcanics in this
area is sorely needed.
In summary, the isopach interval thins east-
ward owing primarily to truncation beneath the
J-2 unconformity, and probably to some as yet
undetermined amount of depositional thinning.
The lower contact of the isopach interval is di-
achronous, becoming younger generally toward
the south owing to interfingering with and lateral
replacement by non-eolian units in the Kayenta
Formation and probably with non-eolian strata in
the lower part of the Nugget and Glen Canyon
Sandstones. In southern Arizona and California,
the eolian unit appears to thin southward by onlap
where it rests on the landward side of the Early
Jurassic Andean-type magmatic arc. Intertonguing
and southward regional replacement of the lower
part of the Aztec and Navajo Sandstones by non-
eolian units should be reflected in southward thin-
ning of the isopach interval, but this is not ap-
parent from the isopach map (Fig. 18). Although
there may be several explanations for this, the
simplest and most appealing is that the inter-
tonguing does not involve an appreciable thick-
ness of beds (100 m or less in most locations).
Also, the zone of intertonguing is at or near the
southern margin of Jurassic outcrops on the Col-
orado Plateau so trends to the south are uncertain.
Several positive and negative structural ele-
ments (Fig. 20) are reflected by the configuration
of the isopach lines. In northeastern Utah and
adjacent areas, a trough is reflected as a west-
southwest-trending area of thick Nugget and Glen
Canyon Sandstones that coincides roughly with
the present-day Uinta Mountains structural ele-
ment. The Uinta Mountains element is known to
have been active, either as a positive or negative
structural feature, since Precambrian time (Bryant,
1985; Hansen, 1986) and movement in the Jurassic
is therefore not unexpected. A parallel trend of
thick Nugget Sandstone just northwest of the
Uinta trough may reflect a separate trough or, by
reconfiguring the isopach lines around the thin
area between the two thick areas, both thick areas
combined may reflect a somewhat complicated
but considerably larger Uinta trough. Another
thick trough-like area is suggested by the con-
figuration of the isopach lines in southern Arizona
but stratigraphic complexities and uncertainties as
well as incomplete sections make such an interpre-
tation speculative.
A broad structural bench or terrace identified
as the Monument bench (Fig. 20) is identifiable in
southeastern Utah in the vicinity of the present-day
Monument upwarp. Isopach and facies studies in
this area indicate that this feature was tectonically
active, either as a structural bench or uplift, all
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