Document 6531792

GEOLOGY OF WESTERN CONTERMINOUS UNITED STATES
about 3,500 feet below the top of the formation. This
sample contains eight pollen and spore forms regionally
common to Paleocene and Eocene rocks, as well as abundant pollen of Platycarya and Gramineae. Pollen
similar to modern Gramineae (grass) pollen is not yet
known from rocks older than Eocene. Gramineae pollen has been found in Wyoming- in the Green River
formation of early and middle Eocene age, and in rocks
younger than Green River.
A suite of abundantly fossiliferous samples was collected at USGS paleobotanical loc. D1408 (SW % sec.
4, T. 6 N., R. 81 W.) and D1409 (NW % sec. 4, T. 6 N.,
R. 81 W.) from an 800-foot-thick carbonaceous shale
sequence which lies about 2,800 feet stratigraphically
below loc. D1359. Ten samples yielded 38 species of
pollen and spores, of which the dominant f orm in most
of the samples is pollen of Platycarya. Also present
is pollen of Tiliaceae (linden family), here assigned
to Tilia crassipites Wodehouse; Tilia crassipites pollen
is known in Wyoming and Colorado from rocks of
early Eocene through Oligocene age, but is lacking in
rocks of Paleocene age. All but 2 of the 38 species
occur in the Wasatch formation of early Eocene age,
near Sheridan, Wyo. The other two species are found
118.
B261
in the Green River formation of early and middle
Eocene age in southwestern Wyoming.
A local unconformity in the Coalmont formation in
the Pole Mountain-Coalmont area separates the 800foot-thick carbonaceous shale sequence from the lower
part of the formation; in this area the part below the
unconformity has a minimum thickness of about 1,500
feet. Carbonaceous shale samples from several localities in this lower part of the formation yielded only six
pollen species, none of which is an exclusively Eocene
form, but all of which are common in early Tertiary
rocks of the Western United States.
The authors conclude that the lower part of the
Coalmont formation is of Paleocene age, based on the
presence of Paleocene leaves; and that the upper
part of the Coalmont is of Eocene age, perhaps early
Eocene, based on the presence of pollen of Platycarya,
Gramineae, and Tilia crassipites.
REFERENCES
Beekly, A. L., 1915, Geology and coal resources of North Park,
Colorado: U.S. Geol. Survey Bull. 596, 121 p.
Brown, R. W., 1949, Paleocene deposits of the Rocky Mountains
and Plains: U.S. Geol. Survey map.
PRE-CUTLER UNCONFORMITIES AND EARLY GROWTH OF THE PARADOX VALLEY AND GYPSUM VALLEY
SALT ANTICLINES, COLORADO
D. P. ELSTON and E. R. LANDIS, Denver, Colo.
Work done in cooperation icitlt the U.S. Atomic Energy Commission
The salt anticline region of the Colorado Plateau occupies the deep, axial part of the Paradox basin in western Colorado and eastern Utah. The five major northwest-trending salt anticlines (inset, fig. 118.1), which
aro 30 to 70 miles long, have structurally complex central parts 2 to f> miles wide, and salt cores 4,100 to 13,700 feet thick. Southwest of these, the salt-bearing
unit of the Paradox member of the Hermosa formation
(Middle Pennsylvania!!) ranges from 0 to about 3,000
feet in thickness, whereas its original thickness in the
deep part of the basin may have been about 7,000 feet.
Rocks of the Paradox member, consisting of gypsum,
generally fine-grained elastics, and carbonates, crop
out locally in several valleys eroded along the axes of
the salt anticlines, together with some broken beds of
gypsum, that appear to be residual from leached
salt beds. These rocks are about 400 to 1,300 feet thick.
They overlie the salt and are unconformably overlapped by Paleozoic beds that consist of marine limestone
and shale and of marine and continental siltstone, arkosic sandstone, and conglomerate. The aggregate thickness of the younger Paleozoic beds is only a few hundred feet over parts of the salt structures, but is more
than 5,000 feet in areas between the salt structures.
PARADOX VALLEY
Unconformities have been found at several places in
Paradox Valley beneath thinned sequences of the upper member of the Hermosa formation (Middle Pennsylvanian), the Rico formation (Middle and Late
Pennsylvania!! in the Gypsum Valley and Paradox
Valley areas), and the Cutler formation (Permian).
B262
GEOLOGICAL SURVEY RESEARCH 1960—SHORT PAPERS IN THE GEOLOGICAL SCIENCES
The upper member of the Hermosa formation is commonly less than 50 feet thick in scattered outcrops, and
in the northwest part of Paradox Valley it is separated
from the Cutler formation by about 150 feet of interbedded limestone and arkosic sandstone of the Rico formation, both of whose contacts are unconformable.
Both the upper member of the Hermosa. formation and
the Rico formation are about 3,000 feet thick on the
south flank of the Paradox Valley salt anticline.
There is a marked unconformity beneath the Cutler
formation (fig. 118.1). The basal beds (units Pea, Pcb
and Pec) consist of about 100 feet of gray, platybedded to indistinctly bedded, marine (?) sandstone
and conglomerate, which grade upward into fluviatile
red beds typical of the Cutler (unit Pcd). The lowest
unit of the Cutler (Pea), which is about 50 feet thick in
the eastern part of the map area and contains scattered
pebbles derived from underlying rocks, was deposited in
fold troughs on an irregular erosion surface. Although
this unit (Pea) pinches out locally to the west, an outlier rests unconf ormably on the Paradox member of the
Hermosa formation about 900 feet to the south of the
pinch-out. In the western part of the map area, the
next younger unit (Pcb) unconformably overlies the
upper member of the Hermosa formation, which apparently truncates a part of the Paradox member.
GYPSUM VALLEY
Unconformities are seen in Gypsum Valley beneath
the Cutler and Rico formations and beneath the upper
member of the Hermosa formation in the map area of
figure 118.2, and also two unconformities within that
member. The upper member of the Hermosa formation and the Rico formation rest unconformably on several different units of the Paradox member.
The upper member of the Hermosa formation, which
pinches out over the anticline in the central part of
the map area but which is about 100 feet thick on its
flanks, consists chiefly of gray dolomite and limestone.
Its lowermost persistent unit is a bed of resistant dolomite, about 5 feet thick. West of the topographic
saddle near the crest of the anticline, this dolomite overlies black shale of the Paradox member with sharp
angular discordance, and also truncates an isolated dolomite bed of the upper member of the Hermosa that
is sharply folded into the black shale. About 300
feet south of the anticline, the persistent dolomite overlies a gypsum unit of the Paradox member. On the
southwest side of the saddle, about 50 feet of thinbedded dolomite in the upper member of the Hermosa
is truncated in a distance of about 150 feet beneath a
breccia-rubble that contains pebbles, cobbles, and boul-
ders of limestone and sandstone. An overlying dolomite is truncated in turn by the Rico formation.
The Rico formation, which is about 100 feet in maximum thickness but pinches out over the salt structure,
consists of irregularly bedded grayish-red siltstone,
sandstone, limestone, and dolomite. Some of the carbonate beds in the upper half of the formation are
clastic and consist of angular carbonate fragments in
carbonate cement, indicating unsettled conditions of
deposition. In the northeast part of the map area the
Rico formation is unconformably overlain by a thin
wedge of purplish arkosic to conglomeratic sandstone,
typical of the Cutler formation.
CONCLUSIONS
The facts outlined above show that the cores of the
Paradox Valley and Gypsum Valley salt anticlines are
overlain by thin post-Paradox formations of Pennsylvanian and Permian age, pinching out over the anticlines and separated by unconformities. These facts indicate that the growth of the salt cores in both anticlines began in Middle Pennsylvanian time, not later
than sometime during the deposition of the upper member of the Hermosa formation, that the tops of the
salt structures generally stayed near local base level,
and that the unconformities within, and separating, the
thinned late Paleozoic formations record pulses of vertical movement in the salt cores.
Because of the relatively thin cover of rocks above
the salt prior to growth of the cores, it is thought that
growth of the salt core was initiated by tectonic activity. Such activity is recorded at some places by the
arkosic debris, shed from the ancestral Uncompahgre
Range, that is interbedded with evaporite and carbonate rocks in the Hermosa and Rico formations. Repeated tectonic pulses may have caused continual
growth, or at least intermittent growth, in Pennsylvanian time, during which more sedimentation took
place alongside the growing salt structures than on
their tops. After tne uplift of the Uncompahgre
Range, however, the continued growth of the salt cores
during much of Permian time probably resulted from
differential loading.
The widely held concept that, a great thickness of
late Paleozoic beds was pierced by intrusive salt masses
during inception of the salt anticlines is not compatible
with the field evidence. This, however, does not preclude later intrusion into beds of the Cutler formation
that may have covered the salt structures in Permian
time.
BEFEBENCES
Cater, F. W., Jr., 15>55a, The salt anticlines of southwestern
Colorado and southeastern Utah, in Four Corners Geol. Soc.
GEOLOGY OF WESTERN CONTERMINOUS UNITED STATES
B263
_
"
O
o
•o
c.
a
o
I*
• • :-:.W^Wv
B264
GEOLOGICAL SURVEY RESEARCH 1960—SHORT PAPERS IN THE GEOLOGICAL SCIENCES
NVINVAHASNN3d
t??^
•s S3
* £*e~i
fcis-S
ISl-86
|z*-e
l°8s s
hzl'rs
sfth
o>&
O. ¥
\ll&%
3S
W o
">m
^S!
c O> |
°= S
"G
-o
l°|
O T3 I
(U c I
O »l
6
§
I
"3
a3
tn
C.
>4
•s
a
IN
00
O
GEOLOGY OF WESTERN CONTERMINOUS UNITED STATES
Guidebook Field Conf. No. 1, Geology of parts of Paradox,
Black Mesa, and San Juan Basins, 1955: p. 125-131.
— 1955b, Geology of the Davis Mesa quadrangle, Colorado:
U.S. Geol. Survey Geol. Quad. Map GQ-71.
1955c, Geology of the Anderson Mesa quadrangle, Colorado: U.S. Geol. Survey Geol. Quad. Map GQ-77.
Herman, George, and Barkell, C. A., 1957, Paradox salt basin:
Am. Assoc. Petroleum Geologists Bull., v. 41, no. 5, p. 861881.
Jones, R. W., 1959, Origin of salt anticlines of Paradox Basin:
Am. Assoc. Petroleum Geologists Bull., v. 43, no. 8, p. 18691895.
Prommel, H. W. C., and Crum, H. E., 1927, Salt domes of
Permian and Pennsylvanian age in southeastern Utah and
their influence on oil accumulation: Am. Assoc. Petroleum
Geologists Bull., v. 11, no. 4, p. 373-393.
Shoemaker, E. M., 1954, Structural features of southeastern
Utah and adjacent parts of Colorado, New Mexico, and
Arizona, in Utah Geol. Soc., Guidebook to the geology of
Utah, No. 9,1954: p. 48-69.
Shoemaker, E. M., Case, J. E., andoElston, D. P., 1958, Salt
anticlines of the Paradox basin, in Intermountain Assoc.
119.
B265
Petroleum Geologists Guidebook 9th Ann. Field Conf.,
Guidebook to the geology of the Paradox basin, 1958:
p. 39-59.
Stokes, W. L., 1948, Geology of the Utah-Colorado salt dome
region with emphasis on Gyi>sum Valley, Colorado: Utah
Geol. Soc., Guidebook to the geology of Utah, No.'3, 50 p.
1958, Nature and origin of Paradox basin salt structures, in Intermountain Assoc. Petroleum Geologists Guidebook 7th Ann. Field Conf., Geology and economic deposits
of east central Utah, 1950: p. 42-47.
Stokes, W. L., and Phoenix, D. A., 1948, Geology of the EgnarGypsum Valley area, San Miguel and Montrose Counties,
Colorado: U.S. Geol. Survey Oil and Gas Inv. Prelim. Map
93.
Wengerd, S. A., and Matheny, M. L., 1958, Pennsylvanian system
of Four Corners region: Am. Assoc. Petroleum Geologists
Bull., v. 42, no. 9, p. 2048-210G.
Wengerd, S. A., and Strickland, J. W., 1954, Pennsylvanian
stratigraphy of Paradox salt basin, Four Corners region,
Colorado and Utah: Am. Assoc. Petroleum Geologists Bull.,
v. 38, no. 10, p. 2157-2199.
STRUCTURE OF PALEOZOIC AND EARLY MESOZOIC ROCKS IN THE NORTHERN PART OF THE SHOSHONE
RANGE, NEVADA
By JAKES GILLULT, Denver, Colo.
Work done in cooperation with the Nevada Bureau of Mines
Structural analysis of the area has revealed structures that rival those of the Alps in complexity. The
Roberts thrust has moved sheets many thousands of
feet thick, composed of siliceous Ordovician, Silurian,
and Devonian rocks, over carbonate rocks of Cambrian,
Ordovician, Silurian, and Devonian age. Not only is
the Roberts thrust itself folded into a tight overturned
anticline, but the numerous thrust slices composing
its upper plate have been folded into isoclinal folds,
some of them several thousand feet across. Some of
these folds are recumbent, others upright, but all ride
on the Roberts thrust. They arc cut by a vertical fault
about 10 miles long, almost normal to their trend, on
557753 O—00
JIU»'-*£JL!wr.t
18
either side of which very diverse structures have
been developed simultaneously. All these structures
are probably of Early Mississippian age.
Superimposed on, and doubtless to some extent modifying, the Paleozoic structures are thrust sheets involving rocks of Ordovician, Pennsylvanian, Permian, and
probable Triassic age. These sheets, though warped,
are much less complexly folded than those below. Their
transection of the underlying thrust sheets, as well as
their simpler structure and differing facies, prove them
to be younger, but the absence of any dated rocks between Triassic and Miocene in the area makes it impossible to assign a precise date to this orogeny.