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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Introduction<br />
The most voluminous expanse <strong>of</strong> <strong>eolian</strong> deposi-<br />
tional systems in <strong>the</strong> geological record occurs<br />
across <strong>the</strong> Western Interior <strong>of</strong> <strong>the</strong> United States.<br />
The area stretches from sou<strong>the</strong>rn Nevada <strong>and</strong><br />
adjacent California across <strong>the</strong> Mogollon Rim <strong>of</strong><br />
Arizona across <strong>the</strong> nor<strong>the</strong>rn half <strong>of</strong> New Mexico<br />
into <strong>the</strong> Texas <strong>and</strong> Oklahoma panh<strong>and</strong>les, north-<br />
ward through eastern Utah <strong>and</strong> western Colorado,<br />
eastward into north-central Colorado, northward<br />
into Wyoming <strong>and</strong> <strong>the</strong> Black Hills <strong>of</strong> South<br />
Dakota, west into southwestern Montana, <strong>and</strong> full<br />
circle south along <strong>the</strong> Cordilleran Hinge Line<br />
through southwest Utah to sou<strong>the</strong>rn Nevada (Fig.<br />
1 * ). Within this vast area, <strong>the</strong> greatest concentra-<br />
tion <strong>of</strong> <strong>eolian</strong>-beating formations occurs on <strong>the</strong><br />
Colorado P<strong>late</strong>au in south-central Utah <strong>and</strong> adjac-<br />
ent Arizona (Blakey, this volume). Eolian deposi-<br />
tion began in <strong>the</strong> Early <strong>and</strong> Middle Pennsylvanian<br />
(Atokan, Desmoinesian), reached an initial climax<br />
during <strong>the</strong> early Permian (Wolfcampian,<br />
Leonardian), waned during <strong>the</strong> Triassic, <strong>and</strong><br />
reached a second climax during <strong>the</strong> Middle<br />
Jurassic; <strong>eolian</strong> deposition declined during <strong>the</strong><br />
Late Jurassic <strong>and</strong> is absent in <strong>the</strong> Cretaceous (Fig.<br />
2). Thus <strong>eolian</strong> <strong>deposits</strong> are distributed throughout<br />
160 m.y. <strong>of</strong> time <strong>and</strong> dominated deposition in<br />
some areas.<br />
Purpose <strong>and</strong> scope<br />
The chief purpose <strong>of</strong> this paper is to describe<br />
<strong>the</strong> geometry <strong>of</strong> <strong>the</strong> <strong>late</strong> <strong>Paleozoic</strong> <strong>and</strong> <strong>Mesozoic</strong><br />
<strong>eolian</strong> <strong>deposits</strong> <strong>of</strong> <strong>the</strong> Western Interior. We are<br />
presenting an atlas that documents geometry,<br />
stratigraphy, <strong>and</strong> facies relations <strong>of</strong> <strong>the</strong> <strong>eolian</strong>-<br />
bearing intervals. The information is presented in<br />
a series <strong>of</strong> maps <strong>and</strong> cross sections, each <strong>of</strong> which<br />
is carefully documented by maps <strong>and</strong> tables to <strong>the</strong><br />
source <strong>of</strong> <strong>the</strong> original data.<br />
Methods<br />
Field-based observations <strong>and</strong> studies <strong>of</strong> <strong>the</strong><br />
authors, both published <strong>and</strong> unpublished, form<br />
* Due to <strong>the</strong>ir large size Figs, 1, 4, 5, 6, 8, 9, 10, 11, 12, 13, 15,<br />
17, 18, 20, 21, 22, 24, 25, 27, 28, 30, 31, 33 <strong>and</strong> 34 are placed<br />
toge<strong>the</strong>r on <strong>the</strong> foldouts in this article.<br />
<strong>the</strong> foundation <strong>of</strong> this paper; however, most <strong>of</strong> <strong>the</strong><br />
basic data has been collected from <strong>the</strong> literature.<br />
The isopachs were constructed from data points<br />
that were carefully plotted by township <strong>and</strong> range,<br />
numbered, listed with author, <strong>the</strong>ir section desig-<br />
nation, interval <strong>of</strong> study, <strong>and</strong> thickness <strong>of</strong> <strong>eolian</strong><br />
interval. Because much <strong>of</strong> <strong>the</strong> information in <strong>the</strong><br />
literature does not specify <strong>the</strong> environment <strong>of</strong><br />
deposition <strong>of</strong> <strong>the</strong> rocks <strong>of</strong> interest or misinterpre-<br />
ted <strong>the</strong> origin <strong>of</strong> <strong>the</strong> rocks, or contains outdated<br />
stratigraphic terminology, we used our collective<br />
experience to define <strong>the</strong> <strong>eolian</strong> intervals within <strong>the</strong><br />
original authors' section. Thus it is possible for<br />
subsequent workers to take our data back to <strong>the</strong><br />
original source <strong>and</strong> see how our interpretations fit<br />
within <strong>the</strong> framework <strong>of</strong> <strong>the</strong> original authors' work.<br />
Space limitations <strong>and</strong> tremendous breadth <strong>of</strong> sub-<br />
ject dem<strong>and</strong>ed that our conclusions be arrived at<br />
directly without much discussion or debate <strong>of</strong><br />
numerous hypo<strong>the</strong>ses.<br />
It should be noted that <strong>the</strong> isopachs show <strong>the</strong><br />
thickness <strong>of</strong> <strong>the</strong> <strong>eolian</strong>-bearing interval <strong>and</strong> not<br />
necessarily <strong>the</strong> thickness <strong>of</strong> a formal rock-strati-<br />
graphic interval or rocks <strong>of</strong> a particular age (Table<br />
1). In general <strong>the</strong> isopachs show <strong>the</strong> thickness <strong>of</strong><br />
erg or dune <strong>deposits</strong> from <strong>the</strong> base <strong>of</strong> <strong>the</strong> oldest<br />
<strong>eolian</strong> s<strong>and</strong>stone to <strong>the</strong> top <strong>of</strong> <strong>the</strong> youngest one. In<br />
some instances <strong>the</strong> thickness <strong>of</strong> simple, thick,<br />
non-<strong>eolian</strong> tongues is subtracted from <strong>the</strong> iso-<br />
pached interval. Because many geologic units con-<br />
tain mixtures <strong>of</strong> <strong>eolian</strong> <strong>and</strong> non-<strong>eolian</strong> rocks, <strong>the</strong><br />
isopachs may vary considerably from previously<br />
published maps <strong>of</strong> a given formation; or where<br />
specified, <strong>the</strong>y may include parts <strong>of</strong> more than one<br />
formation or stratigraphic unit. Most <strong>of</strong> <strong>the</strong> maps<br />
have patterns that show approximate percentages<br />
<strong>of</strong> <strong>eolian</strong> rocks within <strong>the</strong> overall <strong>eolian</strong>-bearing<br />
sequence. If a given <strong>eolian</strong> interval shows an iso-<br />
pach <strong>of</strong> 100 m <strong>and</strong> a percentage <strong>of</strong> 50, this means<br />
that <strong>the</strong> thickness <strong>of</strong> <strong>the</strong> <strong>eolian</strong>-beating rocks <strong>of</strong> a<br />
given interval is 100 m <strong>and</strong> that approximately<br />
half <strong>of</strong> that interval contains <strong>eolian</strong>-deposited<br />
rocks.<br />
We caution <strong>the</strong> reader that because <strong>the</strong> maps<br />
vary slightly in method <strong>of</strong> construction, Table 1<br />
must be consulted when examining our presenta-<br />
tion <strong>of</strong> a given <strong>eolian</strong> sequence; be aware <strong>of</strong> what<br />
<strong>the</strong> data mean! We found it necessary to vary <strong>the</strong>