Synthesis of late Paleozoic and Mesozoic eolian deposits of the ...
Synthesis of late Paleozoic and Mesozoic eolian deposits of the ...
Synthesis of late Paleozoic and Mesozoic eolian deposits of the ...
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<strong>of</strong> <strong>the</strong> Bell Springs Member <strong>of</strong> <strong>the</strong> Nugget<br />
(Pipiringos, 1968; Doelger <strong>and</strong> Steidtmann, 1985)<br />
has not, to our knowledge, been determined but it<br />
is not considered to be appreciable.<br />
The direction <strong>of</strong> <strong>late</strong>ral replacement <strong>of</strong> Navajo<br />
or Aztec tongues by units in <strong>the</strong> underlying<br />
Kayenta Formation is only documented in a few<br />
areas, but where it has been determined it is<br />
consistent with paleowind patterns determined<br />
from crossbedding dip vector studies. Harshbarger<br />
et al. (1957, p. 22, pl. 2) report sou<strong>the</strong>astward<br />
gradation <strong>of</strong> Navajo tongues into <strong>the</strong> Kayenta<br />
Formation in nor<strong>the</strong>astern Arizona, which is<br />
parallel to <strong>the</strong> sou<strong>the</strong>astward direction <strong>of</strong> <strong>eolian</strong><br />
transport in this area during Early Jurassic time<br />
(Peterson, this volume). It is highly likely that <strong>the</strong><br />
Lamb Point <strong>and</strong> Shurtz S<strong>and</strong>stone Tongues <strong>of</strong> <strong>the</strong><br />
Navajo in southwestern Utah (Averitt, 1962; Wil-<br />
son, 1965) are different names in different areas<br />
for <strong>the</strong> same tongue <strong>of</strong> <strong>the</strong> Navajo. If this correla-<br />
tion is correct, <strong>the</strong> tongue thins southwestward in<br />
<strong>the</strong> same direction (Fig. 19) that <strong>the</strong> winds moved<br />
in this area (Peterson, this volume).<br />
During Early Jurassic time an Andean:type<br />
magmatic arc extended northwestward from<br />
sou<strong>the</strong>rn Arizona into California (Bilodeau <strong>and</strong><br />
Keith, 1986). Eolian strata included in <strong>the</strong> isopach<br />
interval (Fig. 18) lap southward onto older rocks<br />
in this area, apparently because <strong>the</strong> erg migrated<br />
into highl<strong>and</strong>s produced by uplift in <strong>the</strong> arc. Inter-<br />
fingering or interbedding <strong>of</strong> <strong>the</strong> <strong>eolian</strong> s<strong>and</strong> with<br />
arc volcanics was documented by Marzolf (1983a)<br />
<strong>and</strong> Bilodeau <strong>and</strong> Keith (1986). In this area, <strong>the</strong><br />
upper boundary <strong>of</strong> <strong>the</strong> Lower Jurassic Series (<strong>and</strong><br />
thus <strong>of</strong> <strong>the</strong> isopach interval) is difficult to de-<br />
termine owing to <strong>the</strong> difficulty <strong>of</strong> distinguishing<br />
Middle Jurassic volcanics that postdate strata <strong>of</strong><br />
<strong>the</strong> isopach interval from Early Jurassic volcanics<br />
that were coeval with <strong>the</strong> <strong>eolian</strong> beds. Clearly, a<br />
study <strong>of</strong> <strong>the</strong> isotopic dates <strong>of</strong> <strong>the</strong> volcanics in this<br />
area is sorely needed.<br />
In summary, <strong>the</strong> isopach interval thins east-<br />
ward owing primarily to truncation beneath <strong>the</strong><br />
J-2 unconformity, <strong>and</strong> probably to some as yet<br />
undetermined amount <strong>of</strong> depositional thinning.<br />
The lower contact <strong>of</strong> <strong>the</strong> isopach interval is di-<br />
achronous, becoming younger generally toward<br />
<strong>the</strong> south owing to interfingering with <strong>and</strong> <strong>late</strong>ral<br />
replacement by non-<strong>eolian</strong> units in <strong>the</strong> Kayenta<br />
Formation <strong>and</strong> probably with non-<strong>eolian</strong> strata in<br />
<strong>the</strong> lower part <strong>of</strong> <strong>the</strong> Nugget <strong>and</strong> Glen Canyon<br />
S<strong>and</strong>stones. In sou<strong>the</strong>rn Arizona <strong>and</strong> California,<br />
<strong>the</strong> <strong>eolian</strong> unit appears to thin southward by onlap<br />
where it rests on <strong>the</strong> l<strong>and</strong>ward side <strong>of</strong> <strong>the</strong> Early<br />
Jurassic Andean-type magmatic arc. Intertonguing<br />
<strong>and</strong> southward regional replacement <strong>of</strong> <strong>the</strong> lower<br />
part <strong>of</strong> <strong>the</strong> Aztec <strong>and</strong> Navajo S<strong>and</strong>stones by non-<br />
<strong>eolian</strong> units should be reflected in southward thin-<br />
ning <strong>of</strong> <strong>the</strong> isopach interval, but this is not ap-<br />
parent from <strong>the</strong> isopach map (Fig. 18). Although<br />
<strong>the</strong>re may be several explanations for this, <strong>the</strong><br />
simplest <strong>and</strong> most appealing is that <strong>the</strong> inter-<br />
tonguing does not involve an appreciable thick-<br />
ness <strong>of</strong> beds (100 m or less in most locations).<br />
Also, <strong>the</strong> zone <strong>of</strong> intertonguing is at or near <strong>the</strong><br />
sou<strong>the</strong>rn margin <strong>of</strong> Jurassic outcrops on <strong>the</strong> Col-<br />
orado P<strong>late</strong>au so trends to <strong>the</strong> south are uncertain.<br />
Several positive <strong>and</strong> negative structural ele-<br />
ments (Fig. 20) are reflected by <strong>the</strong> configuration<br />
<strong>of</strong> <strong>the</strong> isopach lines. In nor<strong>the</strong>astern Utah <strong>and</strong><br />
adjacent areas, a trough is reflected as a west-<br />
southwest-trending area <strong>of</strong> thick Nugget <strong>and</strong> Glen<br />
Canyon S<strong>and</strong>stones that coincides roughly with<br />
<strong>the</strong> present-day Uinta Mountains structural ele-<br />
ment. The Uinta Mountains element is known to<br />
have been active, ei<strong>the</strong>r as a positive or negative<br />
structural feature, since Precambrian time (Bryant,<br />
1985; Hansen, 1986) <strong>and</strong> movement in <strong>the</strong> Jurassic<br />
is <strong>the</strong>refore not unexpected. A parallel trend <strong>of</strong><br />
thick Nugget S<strong>and</strong>stone just northwest <strong>of</strong> <strong>the</strong><br />
Uinta trough may reflect a separate trough or, by<br />
reconfiguring <strong>the</strong> isopach lines around <strong>the</strong> thin<br />
area between <strong>the</strong> two thick areas, both thick areas<br />
combined may reflect a somewhat complicated<br />
but considerably larger Uinta trough. Ano<strong>the</strong>r<br />
thick trough-like area is suggested by <strong>the</strong> con-<br />
figuration <strong>of</strong> <strong>the</strong> isopach lines in sou<strong>the</strong>rn Arizona<br />
but stratigraphic complexities <strong>and</strong> uncertainties as<br />
well as incomplete sections make such an interpre-<br />
tation speculative.<br />
A broad structural bench or terrace identified<br />
as <strong>the</strong> Monument bench (Fig. 20) is identifiable in<br />
sou<strong>the</strong>astern Utah in <strong>the</strong> vicinity <strong>of</strong> <strong>the</strong> present-day<br />
Monument upwarp. Isopach <strong>and</strong> facies studies in<br />
this area indicate that this feature was tectonically<br />
active, ei<strong>the</strong>r as a structural bench or uplift, all<br />
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