DEPOSITIONAL ENVIRONMENT OF THE
VIRGELLE SANDSTONE IN NORTH-CENTRAL WYOMING
A THESIS
SUBMITTED TO THE HONOR'S COLLEGE
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
for the
HONOR'S PROGRAM
by
PATRICK J. CONROY
ADVISER - DR. HARLAN H. ROEPKE
BALL STATE UNIVERSITY
MUNCIE, INDIANA
FEBRUARY, 1982
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DEPOSITIONAL ENVIRONMENT OF THE
VIRGELLE SANDSTONE IN NORTH-CENTRAL WYOMING
Statement of the Problem
The major goal of this paper is to determine the depositional
environment of the lowermost unit of the Eagle Formation (commonly
referred to as the Virgelle Sandstone) in North-Central Wyoming.
Al though this rock unit has been studied in surrounding areas
(particularly Montana, Shelton, 1965) the results of those studies
have not been validated in the vicinity of the Elk Basin oil field.
Thus, further research into the origin of the Virgelle Sandstone is
needed to determine whether or not its depositional environment is
the same throughout the Northern Rockies.
To determine the depositional environment of the Virgelle
Sandstone, an analysis of the lithologies and an interpretation of
the overall stratigraphy are important.
The lithologic characteristics
can indicate the source of the transported sediments, the flow regime
associated with such transportation, and the environment in which the
sediments were deposited (e.g. fluvial plain, point bar deposits).
The overall stratigraphy of the area is important in order to gain a
regional perspective of the paleoecologic conditions of the time,
as well as any geologic disturbances and/or fluctuations that may
have occurred (i.e. transgressions, regressions, local tectonism,
etc •••• ).
This study of the lower unit of the Eagle Formation in
North-Central Wyoming will provide some data and interpretation of
the depositional environment for this upper Cretaceous unit.
History of the Region
According to the Geologic Atlas of the Rocky MOuntain Region
(McCubbin, 1972, pp. 190-250), the Cretaceous climate was basically
warm and humid.
It was similar to the southern Atlantic Coast of
the United States of today.
Extensive seas and seaways were present
in the Northern Rockies at this time and the western shorelines of
these seaways were almost everywhere sandy.
There were many chains
of barrier bars that separated extensive lagoons and estuaries
from the open sea (MCCubbin, 1972, pp. 190-250).
During this time there were variations in the rates of
epeirogenic subsidence as well as fluctuating eustatic sea levels
and these are recorded in a series of major transgressions and
regressions.
There were also variations in the amount of orogenic
activi ty (in eastern Idaho and western MOntana) and the resulting
amount of clastics transported to the basins.
As a result of these turbulent environmental conditions four
major trangressive-regressive cycles are found in upper Cretaceous
sediments (in the Northern Rockies).
They ranged in duration from
18 million years to five million years (McCubbin, 1972, pp. 190-250).
-
It should be evident that tectonism played an important part
in the sedimentary processes that were occurring at this time.
According to Dickinson (1974, pp. 1-27),
Although plate tectonic theory lays primary
emphasis on horozontal movements of the
lithosphere, large vertical movements are also
implied in response to changes in the thickness
of crust, in the thermal condition of lithosphere,
and in the isostatic balance of lithosphere over
asthenosphere. As thick sedimentation requires
either an initial depression or progressive
subsidence to proceed, the auxiliary movements
largely control the evolution of sedimentary
basins. Ancillary geographic changes related
to the governing horozontal movements also
affect patterns of sedimentation strongly.
Of the four major transgressions and regressions that occurred
in the Northern Rockies, the Niobrara and the Claggett are the two
that bracket in time the deposition of the Eagle Sandstone.
During
the period of regression of the Niobrara Seaway (in Santonian and
early Campanian time) there was a series of eastward-prograding
recessive sandstone tongues.
Figure I shows this in the Central
Montana column where the Eagle Sandstone is located.
Figure 2
also shows evidence of this as indicated in the red box.
The lower
red box shows the stratigraphic relationships in the vicinity of the
study area.
Here the Eagle Sandstone also progrades eastward.
Figure 3 shows both the Eagle Sandstone and the Virgelle Sandstone
prograding slightly eastward.
From the distribution of the
continental deposits, regressive prograding shorelines, and the types
of early Campanian shallow water marine sandstone deposits present
in many parts of the seaway, it is apparent that the seaway was
restricted in width in the Montana-Wyoming area at the end of the
Niobraran cycle (McCubbin, 1972, pp. 214-218).
However, there is
no evidence to date of a regional withdrawal of the sea from the
Rocky Mountain region at this time.
Numerous studies (Hancock, 1920; Weimer, 1960) have been done
on the rock units in this area, especially those units comprising
the Cretaceous sequence.
One such study was conducted by John
Shelton (1965) on the gnemis of the lowermost sandstone unit of the
Eagle Formation in Montana.
In south-central MOntana, the Eagle Sandstone is exposed
on the north-plunging Bighorn Uplift.
It is underlain by the
Telegraph Creek Formation (predominantly shale) and overlain by the
Cloggett Shale.
For the most part the basal sandstone of the Eagle
is a massive, cliff-forming unit (Weimer, 1960).
In north-central and
south-central MOntana the name Virgelle Sandstone is given to the
lowermost unit of the Eagle Formation.
However, the basal sandstone
of the Eagle is not everywhere the same lithology (Hancock, 1920).
It changes toward the east from a hard, massive sandstone (as it is
in Shelton's study area) to a soft, sandy shale.
The strike of the Eagle is N 20-25 0 W through Billings,
Montana and the lowermost unit (from here on to be referred to as
the Virgelle Sandstone) is gently convex downward in cross section.
It has a somewhat asymmetric shape in outcrop, and in both outcrop
and the subsurface the boundaries are gradational (Shelton, 1965).
The Virgelle Sandstone shows large-scale, low-angle inclined
bedding and the overall shape of the bed is concave upward.
A
mottled structure is dominant in the lower part of the sandstone
unit and the mottling is characterized by lumps or pockets of
contrasting textures and poorly defined stratification.
There is
also evidence of trails and burrows (Shelton, 1965).
The texture of the unit is geographically variable.
Where
the unit is thick there is an upward increase in grain size from
very fine to fine-grained sand.
The base of the uni t is transitional
from a siltstone up to a very fine-grained sandstone.
The upper
contact (which is sharp) results from the fine-grained sand being
overlain by a bed with an average grain size of silt (Shelton, 1965).
The sandstone is moderately sorted at the base and very well sorted
at the top, and the underlying and overlying sequences are both
moderately sorted.
The Virgelle Sandstone in the Billings area contains abundant
chert and rock fragments, glauconite, muscovite, and calcite
concretions (secondary in origin).
Some swelling clay was also
present in small amounts (Shelton, 1965).
Shelton believes that the unit was deposited as a marine
sand-barrier feature.
According to Shelton,
Characteristics of the unit which are considered
to be diagnostic genetically are: (1) lowangle inclined bedding in the upper part;
(2) mottled structure in the lower part and
along the edges; (3) upward increase in grain
size; and (4) gradational lower and lateral
boundaries of the sandstone body. These
features are characteristic of Galveston
Island, a Recent northwestern Gulf of
MexiCO barrier island.
Shelton draws from his study the following conclusions.
The lowermost sandstone unit was deposited
as a strand-line feature, a barrier island
which is similar to Galveston Island in
the following ways: (1) trend parallel
with the regional depositional strike
(strand-line); (2) growth by beach and
shore-face accretion; (3) repetition of
the vertical sequence laterally in the
direction of accretion-development of
an offlap sequence; (4) burrowing by
organisms in the lower part of the sand
(on the lower shoreface); and (5) large
width-thickness ratio.
The Eagle barrier island differs from the
Recent model in these ways: (1) accretion
toward the mainland (westward); ( 2) greater
distance from the mainland; (3) greater
sand thickness and width; and (4) lack of
eolian deposits - the result of erosion
during the subsequant transgression.
Shelton's findings are indeed supportive of his theory, but
these facts are only documented in the area of his study.
After the regression of the Niobrara, and the subsequent
deposition of the Telegraph Creek Formation and the Eagle Formation,
came the Claggett transgression and regression of the Pierre
Seaway (Figure
4).
Most areas of the regressive stage of the cycle
are characterized by a number of prograding tongues (Figures 1, 2,
and
3).
The regressive phase was also characterized by vertical
uplift, batholith emplacement, and volcanic activity in western
Montana.
This resulted in the development of mountains that
provided an eastward flood of sediment (Figure
5).
This sediment
accumulated in the upper Cretaceous seaway in central Montana and
Wyoming.
Methods of Sample Preparation
The samples were taken from the base of the Virgelle Sandstone
upwards and they were gathered at points where observable variations
were evident in the overall bedding characteristics.
location was a cliff face adjacent to well #87,
NWi
The sampling
Sec 31, T58N,
RlOOW, Park County, Wyoming, in the Elk Basin oil field.
The samples were crushed until most of the grains were
disaggregated and then sieved from 1.5 phi to 4.5 phi using 0.25 phi
intervals.
The individual sieve fractions were then weighed for
each sample and weight corrections for any remaining aggregates
were made.
Data files were then created showing each 0.25 phi
fraction and its corresponding sample weight.
These data files were
used in conjunction with a computer program to obtain graphs of the
grain size distribution of each sample (see Appendix I).
The weight percents were then used to formulate a cumulative
percent curve for each individual sample.
The values for the
5, 16, 25, 50, 75, 84, and 95 percentiles of these cumulative percent
curves were used in a computer program to compute the median, mean,
standard deviation, skewness, and kurtosis values for each sample
(after Folk, 1968).
These values were plotted in two-component
graphs (after Friedman, 1961) to determine sedimentary origin of
the samples (i.e. river sands, dune sands, beach sands) (see
Appendix II).
Interpretation
The lithology of the Virgelle Sandstone is variable in the
section studied.
From the base upward to eight meters the Virgelle
is an interbedded sandstone and shale unit with sandstone depositional
laminae visible in the sandstone above four meters.
Vertical and
U-shaped tubes approximately 1.5 cm in diameter occur at six meters,
and ripple cross-bedding at 6.5-7 meters.
In the eight to 21 meter interval the Virgelle is a continuous
clean sandstone containing some low angle cross-bedding and short,
vertical burrows 6.Omm to 20.0cm in diameter (at 14m).
Large
(O.4m x 5m) limonite concretions above 15m were also present.
From 21 to 23 meters the Virgelle becomes more distinctly
laminated with continued low angle cross bedding.
The sands also
show a slightly coarser texture.
At 27 and 28 meters the Virgelle is a coarse sandstone,
weakly b'onded, and contains a limonite cemented clay chip conglomerate
at 27 m.
All of the samples taken were made up largely of quartz and
chert with other minor constituents (see appendix A, Figure 1).
Glauconite was present in all of the samples and its abundance
decreased up-section.
The glauconite is indicative of a marine
environment and the burrows and coarsening upward sequence (see
Appendix B) suggest a nearshore environment.
The low angle cross-
bedding and the increased laminations upward are typical of foreshore
and beachface environments also.
Comparisons were drawn between cumulative percent curves of
the Virgelle and those presented by Visher (1969) (see Appendix C
and Figures 6 & 7).
are
ve~T
The cumulative percent curves from the Virgelle
similar to those curves by Visher which are examples of
fluvial deposits (Figure
6).
They are similar in that they both lack
the characteristic coarse tail of the curve (evident at
typical of beachface environments.
both have 10% silt and clay.
5%)
that is
They are also similar in that they
Visher's examples of beach foreshore
sands all have a distinctive coarse end of the curve and this is not
evident in the Virgelle samples.
Figure 7 (after Visher, 1969)
shows other depositional environments and it is apparent that these
curves also differ from those of the Virgelle.
Other statistical parameters taken from Friedman (1961) were
used to distinguish between dune, beach, and river sands based on
their textural characteristics (Appendix D, Figures 1, 2 and 3).
When plotting mean grain size vs. skewness all of the samples fell
into the dune sand category.
The same results were obtained from a
comparison of standard deviation vs. mean grain size.
Standard
deviation vs. skewness was also plotted and eleven out of the twelve
samples fell into the river sand category.
From the above data it would appear that the Virgelle was
deposited as a fluvial sand, but this does not explain the presence
of glauconite, low-inclined cross-bedding, and burrowing structures
present in the sandstone.
All of these would indicate a marine
foreshore beach environment, especially the presence of glauconite.
The logical conclusion from the data obtained is that the Virgelle
Sandstone was deposited in a deltaic environment, one that was under
the influence of both shoreline and fluvial processes.
The burrows
and glauconite are reliable indicators of a marine influence and the
similarity between the cumulative percent curves of Visher and those
of the Virgelle Sandstone indicate fluvial processes were at work
shortly before the time of deposition.
However, fluvial channel
sands show fining-upward sequences and the Virgelle has a coarseningupward sequence.
There are several models of both the tidal and deltaic depositional environments that may offer a better insight into the
depositional environment of the Virgelle.
Deltaic systems have been studied by Fisher, Brown, McGowan,
and Scott (1969).
Fisher studied the Gulf Coast Basin Tertiary
Delta Systems and in particular lobate high-constructive deltas.
In
describi.ng distributary mouth bar sands of the delta front, Fisher
states that several depositional units are recognized in the delta
front sand facies which accumulated seaward of the delta plain.
Immediately at the terminus of distributary
channels and forming a prominant sand unit
in the delta from facies is a characteristic
progradational sequence, marked by upward
coarsening and increased sand content. These
units accumulate as distributary mouth bars
(Brown, et. al., 1969, p. 32).
The accumulation rate is the highest of any unit in a delta
front and the sediments associated with distributary mouth bars are
clean sands with abundant, multi-directional cross-bedding of the
trough type.
Some distorted laminations make up a minor but
diagnostic feature of distributary mouth bars (Fishers, 1969).
The Virgelle also has a coarsening upward sequence and the
sand content increases upward also.
results in Appendix A.
60-7~ft
This is evident in the point count
While quartz sands make up approximately
of the lowest sample taken, the highest sample in the exposure
consists of approximately 80-90% quartz sands.
The plot of skewness
vs. standard deviation (see Appendix D. Fig. 1) (after Friedman, 1961)
also indicates the samples are at least partly of river sand origin.
Therefore, a distributary mouth bar deposit could be a possible
explanation for the depositional environment of the Virgelle.
In a study of the North Texas (Eastern Shelf) Pennsylvanian
Delta Systems by Brown, Fisher, et. al., 1969), high-constructive
elongate to lobate delta systems (Cisco Rocks) and their constructive
facies were described.
According to Brown,
delta front facies are normally well-bedded
and well sorted sandstones containing
parallel laminae, some ripple cross laminae,
and symmetrical ripple bed forms on upper
surfaces. Distributary mouth bars are
composed of well sorted, highly contorted
sand. Relict parallel laminae and some
trough cross bedding are normally
preserved.
The Virgelle is well sorted and it does contain some ripple
cross-beds, but there were no symmetrical ripples on the upper surfaces
of the beds in the exposure.
Fisher stated no evidence of symmetrical
ripples in his study of distributary mouth bar sands and so the
presence of such would not seem to be a prerequisite or common feature
of such deposits.
Therefore, the Virgelle is similar to the dis-
tributary mouth bar models based on its coarsening upward sequence,
ripple cross-beds, upward increasing sand content, and well sorted
texture.
Brown and Fisher (1969) have studied the Cretaceous rocks of
the Western Interior and have proposed models for several delta
systems in the region at that time.
The coal-bearing wedges in the Western Interior
Cretaceous all fringed by marginal marine sands,
typified by such units as the Point Lookout,
Emory, Gallup, Eagle, Ferron, Pictured Cliffs,
and Fox Hill s on the wes t side and the Muddy,
Fall River, "D" and "J" sand on the east side
of the basin,
according to Brown and Fisher (1969, p. 69).
Where developed as
regressive sands overlain by delta plain coal-bearing facies, these
marginal sands show features common to delta front deposits, including,
upward-coarsening textural trends, transitional lower boundaries
wi th gradation to underlying marine shales and sharp upper boundaries;
minor amounts of detrital coal and internal sedimentary structures
are additional features.
Brown and Fisher state that basinward,
these maxginal or delta front sands grade to dark, marine shales
like the Mancos and Lewis, similar to modern prodelta muds.
They
also explain that in contrast "transgressive marginal marine sands show
features similar to modern shoreface deposits of delta destructional
sands, strandplain sands, or barrier bar sands" (p. 69).
These sand
units commonly have sharp bases, are well sorted, are commonly
burrowed; they grade seaward to marine muds and landward to lagoonal
muds and coals (see Figures 8-17) (Brown, Fisher, et. al., 1969,
Figs. 121-137).
It is important to note that the Judith River and the
Clagget in Elk Basin may not fit the facies assignments given by
Fisher in Figure 8.
The Claggett shows delta front (lower shale)
and delta plain (Parkman Member) sediments with several thin coals
between Parkman sandstones.
Brown and Fisher describe many characteristics of the Cretaceous
rocks of the Western Interior that are also common to the Virgelle
Sandstone.
Both the units in Brown and Fisher's study and the
Virgelle have coarsening upward sequences, both have transitional
lower boundaries with gradation to underlying marine shales (the
Telegraph Creek Formation with marine fossils is below the Virgelle),
both cOlltain similar sedimentary structures, and the Virgelle is both
well-sorted and burrowed.
These similarities along with the known
existence of deltas in the area during the Cretaceous lend strong
evidence that the Virgelle was deposed as a marginal marine sand.
A study done by Mackenzie (1975) concerning tidal deposits
shows several differences between the characteristics of tidal
environments and the sedimentary characteristics of the Virgelle.
The location of the exposure studied is west of Denver on the east
side of the Dakota hogback, and the Dakota Group in this area is
primarily a shoal-water deltaic assemblage.
The section is made up
of well sorted fine- to very-fine grained quartzose sandstones.
The
lower half consists mostly of a gining-upward sequence of fine
grained, cross-stratified sandstone (fluvial), and the upper half
consists of a very-fine grained tabular bedded sandstone that is
locally incised by sand- or partially mud-filled channels (tidal).
The section contains long-crested, asymmetrical ripples, current
ripples, nested U-shaped burrows (typically
and plant rootlets.
4 c~wide
and 18 cm. deep),
According to Mackenzie, the mud cracks indicate
a sequence of mud deposition (slack water) followed by emergence
(mud cracks), and the overlying rippled sand (submergence) is suggestive
of short period fluctuations in water level.
Mackenzie concludes
that many features of these rocks indicate deposition in a marginal
marine environment in which water level was undergoing short period
fluctuations.
The section of the Virgelle studied contains U-shaped tubes
and some cross-bedding, but there was no evidence of plant rootlets,
asymmetrical ripples or mud cracks.
The Virgelle also has a coarsening
upward sequence, not the fining upward sequence exhibited in the
section studied by Mackenzie.
Therefore, it would seem unlikely
that the Virgelle was deposited solely in a tidal environment subject
to short term sea level fluctuations.
In a study done by Carter (in Gensburg, 1975, pp. 109-116) of the
Cohansey Sand, a Miocene-Pliocene quartzarenite underlying more than
two-thirds of the New Jersey coastal plain, the unit was interpreted
as a barrier island deposit.
The sequence is characterized by
conformable, nonchanneled facies contacts and by laminated sand
facies.
There are large burrows present that Carter says are similar
in size, shape, and orientation to Ophiomorpha.
facies (from bottom to top) is described as:
(2) peat; (3) burrowed, laminated sand;
(5) interbedded sand and grit.
(4)
The sequence of
(1) laminated clay;
laminated sand; and
The interbedded sand and grit is
characterized by multidirectional trough sets; the laminated sand
facies is characterized by gently dipping laminations composed of well
sorted sand; the burrowed, laminated sand facies is characterized by
burrows, remnant stratification, and abundant heavy minerals; the
peat contains many root and leaf fragments; and the laminated clay
facies is multicolored (grey, red, and yellow), thinly laminated,
and contains intercalated silt and sand lenses, peat fragments, and
irregular silt-filled burrows.
Carter interprets this facies as being indicative of a barrier
island deposit.
The interbedded sand and grit facies is interpreted
as a surf zone deposit.
Near the outer portion of the surf zone the
bed form is planar (outer planar facies) but, in the inner portion
of the zone an area of large scale roughness (inner rough facies)
commonly is present.
Structures in the inner rough zone produce
medium scale foresets that mostly dip directly or obliquely seaward,
although landward dipping foresets also occur (Clifton et. al., 1971).
The sand in the inner rough zone generally is relatively coarse and
quite loosely packed, according to Carter.
The laminated sand facies is interpreted as a swash zone
(beach foreshore) deposit.
dipping, planar surface.
This zone is characterized by a seawardThe seaward-dipping laminations are produced
by sheet flow (upper-flow regime) in this zone (Clifton, et. al.,
1971).
The burrowed, laminated sand facies is interpreted as a backshore-lower dune deposit.
The backshore is covered by water only
during very high tides and/or storms, and its stratification consists
mostly of landward-dipping laminations (McKee, 1957, p. 1707).
Moreover, the backshore contains high concentrations of heavy minerals.
The peat facies is interpreted as a salt water marsh deposit.
Carter states that this interpretation is consistent with the analysis
of the peat:
" ••• pollen and spore of this character are found in
lagoonal sequences (back-barrier sequences)".
The laminated clay facies are interpreted as an outer marsh
deposit, according to Carter.
England as an example.
He cites the salt marshes in the Wash,
The marshes in England are characterized by
well laminated silty clays and clayey silts, both containing small
amounts of sand (Evans, 1965).
Although the Virgelle contains laminated sands and some burrowing,
there are only minor amounts of laminated clays and no peat deposits
whatsoever.
There is also no evidence of interbedded sand and grit
in the section studied, but this could be the result of a limited
availabili ty of grain sizes.
There are no high concentrations of
heavy minerals in the Virgelle samples as in the area Carter interprets as being a backshore-lower dune deposit.
However, in the samples
of the Virgelle, when skewness vs. mean grain size and mean grain size
vs. standard deviation were plotted (after Friedman, 1961) (see
Appendix D, Figures 2 and 3, respectively), the points fell into the
dune sand category.
This alone is no evidence that the section of
the Virgelle is characteristic of a backshore-lower dune deposit.
Therefore, the likelyhood that the Virgelle is part of a barrier
island is remote given the current available data.
The standard
deviation vs. skewness plot (see Appendix D, Figure 1) (after
Friedman, 1961) indicates a river sand, as do the cumulative frequency
curves when compared to those by Visher (Figures 6 & 7 and Appendix C).
Thompson (1975) has studied the clastic coastal environments
in the Ordovician MOlasse, Central AppalaChians, and one such study
involved the Lower Bald Eagle Formation of the Central Appalachian
miogeosynclinal sequence.
The facies studied was a fine to medium
grained, clean, well sorted quartzarenite and lithic arenite, with
no interbedded siltstone or mudsonte.
Many of the rocks are thin
bedded to laminated, with primary current lineations and current
crescents on the bedding planes.
Thin bedded zones reach one meter
thick and are erosionally interbedded with cross-bedded units,
according to Thompson.
Thompson interprets this facies to represent traction current
deposition from flow.
Thompson believes that the absence of marine
fossils, evidence of high current velocity (flat bedding and laminations),
unimodal current directions, and absence of herringbone cross-strata
and reactivation surfaces all indicate that current flow was not
reversing, but rather was consistent.
The total absence of mud
from this facies suggests that, although available for deposition in
other environments, mud was either not available for deposition here
owing to absence in the suspension load, or was not deposited because
of consistently high current velocities.
According to Thompson, a
lack of mud in the suspension load is the more reasonable alternative,
because if mud were present at least some would have been deposited
with the sand.
These restrictions suggest that the clean sandstone
represents local distributary channels on the delta front.
The Virgelle contains clean, laminated sands, just as the
facies Thompson studied, but it also contains some low-angle
cross-bedding, ripple cross beds, and some interbedded sandstone and
shale in the lower eight meters of the section.
These features would
seem to indicate that the Virgelle was not deposited in a distributary
channel tY1>e of environment.
Thompson notes a "total absense" of
mud from the facies, but there is a definite clay fraction in the
Virgelle samples.
This can be seen in the grain size distribution
graphs in Appendix B.
Therefore, a distributary channel environment
of high velocity would seem unlikely based on the evidence of
Thompson.
In a study done by Cambell (1978) of the Gallup Sandstone
(upper Cretaceous) of northwestern New Mexico, two distinct facies
of a beach cycle are examined.
According to Cambell, in the foreshore
sandstone the cross laminae in the even, parallel beds dip uniformly
seaward.
shoreline.
As a result, the strike of the laminae is parallel to the
Cambell notes that both wave and current-ripple laminae,
parting lineation, swash and rill marks, and vertical burrows are
all sedimentary structures that were found in the foreshore sandstones
studied.
The shoreface sandstone described by Cambell commonly has a
bioturbated structure where burrowing organisms have churned the
sandstone and destroyed the original laminae.
Where differences in
the composition of the sandstones are slight, the churning structure
is described as mottled or structureless.
The Virgelle has some characteristics of both facies.
It
contains ripple cross-bedding in the 0-8.0 m. interval, short, vertical
burrows and 15-150 cm. partings in the 8.0-21.0 m. interval.
However,
the burrowing is not as intense in the Virgelle as it is in the Gallup
Sandstone.
Although these similarities do not give positive proof
that the Virgelle is a foreshore or shoreface sandstone, they
together with the glauconite pellets do indicate that the environment
of deposition was shallow nearshore marine and more similar to
sediments described by Cambell than to any others previously mentioned.
Conclusion
Based on the data obtained from the study of the Virgelle
Sandstone and on the various models and studies in the literature,
the most logical conclusion as to the depositional environment of the
Virgelle is that of a distributary mouth bar.
The Virgelle exhibits
many characteristics associated with distributary mouth bar deposits
and these include:
(1) clean sands; (2) coarsening upward sequence
(see Appendix B); (3) ripple cross-bedding;
content (see Appendix A);
(5)
(4)
upward increasing sand
well sorted texture; and (6) when
plotting skewness vs. standard deviation (Appendix D, Figure 1)
the results indicated a definate fluvial influence.
These similarities
along with the known existence of deltas in the area during the
Cretaceous constitute sufficient evidence for the distributary mouth
bar conclusion.
The Virgelle is only slightly burrowed and there
is generally good preservation of the depositional laminae; this is
indicative of rapid deposition.
A distributary mouth bar has the
highest accumulation rate of any unit in the delta front (Brown,
Fisher, et. al., 1969) and with a high accumulation rate there is less
opportunity and time for burrows to affect the sands.
A beach type environment would be unlikely because the
Virgelle doesn't exhibit asymmetrical ripples, a fining upward sequence,
or plant rootlets, all of which were associated with beach or tidal
deposits in the models discussed earlier.
The Vargelle is also less
burrowed. than beach-type sands where the burrowing often destroys
most of the depositional laminae because of low sedimentation rates.
The fact that this area was the site of multiple transgressions
and regressions during Cretaceous time and the known presence of
deltas in this region, coupled with the previously reviewed sedimentar,y
data, make the distributar,y mouth bar depositional model the most
applicable for the Virgelle Sandstone in North-Central MOntana.
~<' •
'1999
9999
'90
,.8
.8
.8
..
.
~
~
'0
~
~
J£I(Ylll~l
.g
1
70
. Ci:ORGtA
0:.
'0
~
'0
"'
<J>
SOutH CAROll""
70
M!DDLE FORESHORE
..
50
c
..
30
30
"-
A.
~
.
c
..,
10
a
10
~
Phi Scate·
.25
.12!!
mm.
o.
05
01
01
Phi Scale
0.7
"
2!>
12')
067
mm. Scale
Sca~e
Beach foreshore sands
Beach dune ridge sands
99
~9
r----r---,r---,----;----,---~----r_--_r--_"."
Ft"'tAL SANDS MISSOURIAN AG£,OKLAHOM.
A
ALMOND fORMATION - NORTH Of SUPERIOR. WyOMtNG
B
'f.
...
II
10
30
- W t H U ·. ..Jrt.DSlOff£
1M( ~SlCnQ"
10
Phi Scale
2S
l2!)
mm SClle
Fluvial sandstones
Figure 6.
067
Phi Scale
.~"5
12~
0.7
mm Scale
Deltaic distributary curves
(From Visher, 1969, pp. 1084, 1082, 1095, 1097)
r-__~--,----r---.---;r---r---C----r---,'999
r---,----r---;r---,----r---,----,----r---,9.9.
A
Ha
••
.0
'0
O"SHOlt( "'Altl"[ _ ' 1 1 - -
,.
(40.0' ••.., ........ ,
(.J 'I$lIftOtI HttomCt",Uftt)
'0
50
,0
10
";
01
~
_ _~_ _~_ _ _ _~_~_ _ _ _~_ __L_ _ _ _~_ _~--~OOI
PhI Scale
U~
),m, ,Scak
l~
<'
':;::']]-.;,':< .(
0"
125
mm. Scale
Marine delta area
Turbidity current deposited
sandstone
99.99
r--..,-----,r---~--i---_._-_.--_r_-....,.---,
!t.•
..-,,
,.
.
~
AYON'folCH~
90
NORTH CUOllNA
~
1.
.."
10
I
sO
c'
...i
I
,I
,.
0:
90
,
IIS.0·.It.r,.,,,,.
'"
:-g
••
,
98
,
,"
••••
.0
-
SURF IUCH-----7#-..
"0II1M Col"lINA
112.0' ••
SAVANNAH IIltiU
""
nfln
30
GEO'GIA'
LO.tlt TlOAl FLAT
t., ...... ,
~
c
IWaw.,ittIItN)
10
I~
!
o.
0.'
01
01
!:---.-L.---!!--~_~
Phi Scate
,S
.1
2')
12!1
mm. Sclle
Surf zone sands
Figure
7.
0.,
_____'___~----.L--_7--~0.0 1
Phi Scale
250
12S
mm. Scale
0.'
Wave zone sand distributions
(from Visher, 1969, pp. 1091, 1101, 1086, 108S)
WEST
ALLUVIAL FAN
FLUVIALt
Jndiaoola
Gannett
Price River
Epharim
Judith River
Mesaverde
Frontier
Dakota
Cloverly
EX~LANATIQN,
Fans
Fluvial
•.:
Delta Plain
~; ,'_
:.
•;
-
, ,
;,J,;""
_ ,_:
Delta Front', etc.
Prodelta
Platform carbonates
Figure 8. - Diagrammatic cross section and facies profile, Cretaceous
Basins, Western Interior United States (from Brown, Fisher, et. al., 1969,
Fig. 121).
-- --- -
-- -
--;-- r
--------..-'v'<'
.../
ISOPACH AND SAND· SHALE RATIO
MAP FOR INTERVAL 6ETWEEN TOP
OF MOWRY SHALE ANO TOP OF
FIRST WALL CREEK SANOSTONE
,r
'i,-
\~I
~!
,
I~
FRONTIER FORMATION
OF WYOMING
I
.. tit
!.S/S'" RATIOS
--~oo- ISOPAC.HS,
1
+-~-----'
K:::;::q
I
(LtC HI1 e .l.~
~O,
SSISI'I
i
50' INTERVAL
I
lIRA no
,euT~"O.
0-"'.
H . " ' • • , .. 10
'~"_=1_'
_ _"
I
i
---~-,..~~""'-
i
i
- - - --1- - - - - ~
I
i
.
1
:
I
L
I
I
i
I
\
r----~
:
cuTOf'F
QP(R,liTIONllIi,
i
:1
I'
1
I
I
1
, _ - - _I
I
.
Aflel"''''''''
i
1
L'-- __ _
1
CREtIC 55
fOR '1II0NTIlR
. I
UNIT
1
I·
~_.: ___ ._._-i
Figure 9. - Isopach and sand/shale ratio map, Frontier delta
system (Cretaceous), Wyoming (from Brown, Fisher, et. al.,
1969, Fig. 122, who cite Goodell, 1962, not seen).
--'T"··-------I------------;-I
I
~~A~';~;~~-'~
FRONTIER FORMATION
OF WYOMING
I
1 NUMBER or SANDS AND TOTAL
____ : SAjIII _THICKNESS MAP ro~ THE
\(i~\\~~~~?i
1
~~;,vA;,,:C;W!~~lT~: o~\~E
\. ::r:·ild·~. y:~·r:·i:....
I
fiRST
WAll
'l" . . " ••. e
..
YlU
CREEK
~o.
...u _
..
SANDSTONE
• ...'c.... .,., •
......
.' - - - - -r - - - -'-...,
'I:
I
1
1
'I
1
-
1
r--I
t
I •
'j
1
I
1
I
1
,
- - - _,L,- __ 1
I'
~
UtllT 0#
• .,.. L
f"
"""'a.tty.
C\lTM'
OPIltAflONAL
'.
UNIT.
eMU. ~'I:
I
,Oft ,~ONnul
~-,'_':
I
1
1
I.
1
•
I
I'
1
1 ______
• .,-.. _______ I
___ L
Figure 10. - Number of sands map, Frontier delta system
(Cretaceous), Wyoming, Delta plain facies defined chiefly
by occurance of coals. MOdified and based on data from
Goodell (1962). (From Brown, Fisher, et. al., 1969, Fig. 123).
r--·I
'---,
r·
.--2r:tU
Wyo,
I
,
~--
SANOS;ONE
0< 100' "
0100'-200'
0>200' ,
, l2.8.L.
NEW.
.PROOUCTIONI
_I~
UT~H
,
'-V
,
. /"'. C:;.
~•..s4t'DSrONE
ltL-.'
'--,.---;'._.---.JI.YO
71
CO;
FEheON
•
SA1fpSTONE
1#
i
'
.
}~ •~.It
0
'1/
~. • \..#
\i
/
Figure 11. - Net sand map, Frontier and correlative formations
(Cretaceous), Wyoming and adjacent areas. (From Brown, Fisher,
et. al., 1969, Fig. 124, who cite Barlow and Haun, 1966, not seen),
,
Kf2 OIL
0-50'
o
50'-100'
A - A' CROSS SECTION
.....
~
.. ..
Figure 12. - Production from Ff2 barrier sand with Frontier Delta,
Salt Creek Field area, Wyoming. (From Brown, Fisher, et. al., 1969,
Fig. 12$, who cite Barlow and Haun,1966, not seen).
Figure 13. - Cretaceous deltas of western interior, North America.
Isopach of total interval between top of Mowry Shale and base of
Niobrara Formation. Contour in feet. (From Brown, Fisher, et. al.,
1969, Fig. 126, who cite Barlow and Haun, 1966, not seen).
Figure 14. - Isopach map of Thermopolis Shale and distribution of
Muddy S~dstone and e~uivalents (Cretaceous), Wyoming and adjacent
areas. ,From Brown, Flsher, et. al., 1969, Fig. 127, who cite Haun
and Barlow, 1962, not seen).
il
MILES
"0
100
Figure IS. - Sand distribution of Newcastle delta system (Cretaceous).
(From Brown, Fisher, et. al., 1969, Fig. 128, who cite McGregor and
Biggs, 1968, after Wulf, 1962, not seen).
.
NDT
STUDICD
.
''''V------------=.--~,~. ,·:---i~---I-a-.·' '•. ,
A,';,;.'.l,----
----"T--,:------.-' '.-' --"i-------,,---:',~'~,~--~--------- ~.,-------~~----~~r--,•
a~'-----~""C-....~-'., ---II~,'---=-:-----~--.:......,~
..
'
'~~~.-'~'--~ --------------~~~
r
~
,~-------•••'.CI 'Ie.... ____-:-__
. . . \i
-~
......
DIAGRAMMATIC
PALEOGEOGRAPHY
AT CLOSE OF
SKULL CREEK TIME
~o
10 0
,
,
I
I
..
-.--=1~~:\I:
.i
_
I
'CALEI MILES
,
'.CI
. . . " ••
0
DILTAIC
CJ
IP,NIIttTIC
~
l ....ltAlilltlTlC
-1
II . .IIItED OILTA
DI'",.UTU ••
tAo 0L0t.1T,
_or,
~
~-
"
-- \ .
..
-- ...-- .
l a ' ,.
0
"'"
-'
Figure 16. - Regional setting of Newcastle delta system (Cretaceous).
(From Brown, Fisher, et. al., 1969, Fig. 129, who cite Wulf, 1962,
not seen).
,-----_._-----
-------
o
, .. . . . . . . . . 111 ...
".n.............
[:·.··;·1
CJ
[.:.:':.:.:.:';\
[§]
--
.................
.......... ...
~
~
J .~.
Walhakie
.:
I
Balin
-----::::~:
h________
."
!
i : :: ""'i._
I ..
.1!
.
.•
~- -:---~-r
•. "
--~.....
~
.. ~
.
I'
.
' ..
~.,
Sand
Woah
BOlin
..
. . ill
'IQ",,.( )
OUTO~O'
AHO ""'I..£OG£OGIIA""G MAP
tHOWIIit.
'ACtl. ""[IIIN' OUItING D(fIOIITION
01 .otlC. ,HI""
'OfI"TION AND I:QU' .... Lf""
.... ~.l.'U' T'M' - ...T, IIIIA.OII ' " ' . , '
IEIW!J~
Il.....
• u··. -..
~
l·~"""
I ........ •. .
'f
l~
.... , . . . . .....
Figure 17. - Late Cretaceous deltas, Wyoming. (From Brown, Fisher,
et. al. t 1969, Fig. 130, who cite Hale, 1961, not seen).
....
LtIII:t!lI!:!ft ....
APPENDEX A
POINT COUNT RESULTS
#
6
8
10
12
1
2
3
4
Quartz
50
59
44
67
61
60
44
78
Chert
20
24
35
17
16
15
7
15
9
6
10
7
7
2
2
1
10
3
2
0
0
0
0
0
Plagioclase
3
2
2
2
3
1
2
1
Biotite
4
2
3
0
0
1
2
0
Rock Fragments
0
2
1
4
4
6
6
3
Carbonate
0
0
0
3
9
15
37
2
Leucoxene
4
2
3
0
0
0
0
0
SAMPLE
Glauconite
K-Spar
Figure 1.
5
7
9
11
SEDIMENTARY ANALYSIS
SAMPLE#
l(lm) 2(4m) 3(7m) 4(9m) 5(10.5m) 6(14m) 7(17m) 8(2Om) 9(22.5m) 10(25m) 11(27m) 12(28m)
Median(PHI)
2.75 2.90 2.90 2.90
2.90
2.95
2.85
2.85
2.65
2.15
2.25
2.20
Mean(PHI)
2.88 3.00 3.05 2.87
3.03
3.00
2.92
2.92
2.80
2.43
2.53
2·32
Standard
Deviation
0.55 0.52 0.70 0.47
0.54
0.48
0.58
0.49
0.76
0.84
0.85
0.55
Skewness
0.47 0.40 0.34 0.07
0.45
0.29
0.30
0.34
0.35
0.56
0.53
0.59
Kurtosis
1. 76 1. 72 1.02 1.43
1.68
1. 78
2.00
1. 78
1.28
1.37
1.49
2.19
Figure 2.
APPENDIX B
GRAIN SIZE DISTRIBUTION OF VIRGELLE SANDSTONE
ELK BASIN. WYOMING
SAMPLE #1
~ ~------r-~~~=-----'Ir------'I------'I-------'I--~---'I-------'I------~I------~I
j
-
.502)
2.500
3000
3.5~~
PHI INTERVALS
4.0~0
4.50~
5.000
5.500
6.000
GRAIN SIZE DISTRIBUTION OF VIRGELLE SANDSTONE
ELK BASIN. WYOMING
SAMPLE *2
~
.~
1-------r-~~~~~--_,------,_------,_----_.--~--·'I-------rI------~I------~1
1. 000
1.S0~
2.500
3.0~0
3.S~~
PHI INTERVALS
4.000
4.S0~
5.~00
5.S00
6.00~
GRAIN SIZE DISTRIBUTION OF VIRGELLE SANDSTONE
ELK BASIN. WYOMING
1.00121
1.5121121
2.121121121
1________________ ... ____ .. __ . ___ .
SAMPLE
2.5121121
#]
3.121121121
3.t;12I121
PHI INTERVALS
-.
GRAIN SIZE DISTRIBUTION OF VIRGELLE SANDSTONE
ELK BASIN. WYOMING
SAMPLE *4
~4-------r-·~~~~~--'I-------rI------'I-------;1
======~I~----'Ir------'I------'I
1.000
2.000
2.500
3.000
3.500
4.000
4.500
5.000
5.500
6.000
1.5~0
PHI INTERVALS
GRAIN SIZE DISTRIBUTION OF VIRGELLE SANDSTONE
ELK BASIN. WYOMING
1.000
2.12J012J
SAMPLE *5
2.512J12J
3.12J12J12J
3512J12J
PHI INTERVALS
4.12J12J0
4.512J0
5.0"'12J
5.512J12J
6.000
,--,-----_._---------------------------------_.-
GRAIN SIZE DISTRIBUTION OF VIRGELLE SANDSTONE
ELK BASIN, WYOMING
SAMPLE *6
~ 1----.-~~~~~-.----._--_.----_.-==~-TI-----r1----~I---~I
1.000
1 _5QJQJ
2.~~~
2.5~~
3_~~~
35QJ~
PHI INTERVALS
4_QJ~~
4.5~~
5.~0~
5.5~~
6.~
GRAIN SIZE DISTRIBUTION OF VIRGELLE SANDSTONE
ELK BASIN, WYOMING
SAMPLE #7
~4-------'-~~~~----'-------~·-----'f-------'f~~~,I-------,r-----~I----~I
1.000
1.£e0
2.000
2.£00
3.000
3.£00
4.000
4.£00
5.000
5.£02
6.000
PHI INTERVALS
GRAIN SIZE DISTRIBUTION OF VIRGELLE SANDSTONE
ELK BASIN. WYOMING
SAMPLE *8
-
~ ~------'-'~~~~~---'------'-------'I-------'I------~I-------rl------~I------~I
, .00l2I
.-
2.!1J00
2.500
3.000
3.500
PHI INTERVALS
4.000
4.500
5.000
5.500
6.000
-
GRAIN SIZE DISTRIBUTION OF VIRGELLE SANDSTONE
ELK BASIN. WYOMING
SAMPLE #9
-
~ ~-----'-------r------r------,------.------.------.-------.-----~----~
1.000
2.QJQJQJ
2.5QJQJ
3.QJQJQJ
PHI
-
3.SQJQJ
INTERVAL~
4.QJI2JQJ
4.5QJQJ
5.QJI2JQJ
5.SI2JI2J
6.QJQJ2
~~------------------------------------------------------------------------------ ..
~
~
~
GRAIN SIZE DISTRIBUTION OF VIRGELLE SANDSTONE
ELK BASIN, WYOMING
SAMPLE *10
--,r-------r.------,r------,r-------,r---=---'r------~r------~r---~
2.000
2.500
3.000
3.£00
PHI INTERVALS
4.000
4.500
5.000
5.500
6.000
GRAIN SIZE DISTRIBUTION OF VIRGELLE SANDSTONE
ELK BASIN. WYOMING
SAMPLE *11
~
GRAIN SIZE DISTRIBUTION OF VIRGELLE SANDSTONE
~
ELK BASIN. WYOMING
SAMPLE #12
-
1.000
1 . 512!'~
2.512!12!
3.12!00
3.500
PHI INTERVALS
-
I
4.512!12!
I
5.512!12!
I
6.000
-
APPENDIX C
-
l~l~rl_l!1
i
I
,
I
1-:
~ ~
,
_
I
t
'.r'
C
~
0
Ul
~
H
H
H
--
(:-
~
§
re
~
0
Il::l
~
:>-t
0
~
~
~
.2
2 . .5
3
3. .5
PHI SCALE
4
4. .5
i
~ __
~
0
Ul
~
H
H
H
-,
~
§
s::
~
0
rx:
re
:>-t
0
~
~
iii
2
2.5
3
3.5
PHI SCALE
4
4.5
APPENDIX D
(Figure 1)
SKEWNESS VS. STANDARD DEVIATION
+ 3.00
+2.00
RIVER
SAND
+1.00
•
••
••
•
•
•
-1.00
BEACH
SAND
-2.00
-3.00
0.10
0.30
0.50
0.70
0.90
1.10
STANDARD DEVIATION (BOUNDARIES AFTER FRIEDMAN, 1961)
(Figure 2)
SKEWNESS VS. MEAN GRAIN SIZE
-
+3.00
+2.00
DUNE
SAND
+1.00
•
•
•
•
•
••••
•
-1.00
BEACH
SAND
-2.00
-3.00
1.40
1.80
2.20
2.60
3.00
MEAN (BOUNDARIES AFTER FRIEDMAN, 1961)
3.40
(Figure 3)
MEAN GRAIN SIZE VS. STANDARD DEVIATION
,,-
3.00
• •
•••
DUNE SAND
••
2.80
RIVER
SAND
• •
2.60
•
2.40
•
2.20
2.00
0.10
0.30
0.50
0.70
0.80
STANDARD DEVIATION (BOUNDARIES AFTER FRIEDMAN, 1961)
1.10
References
Barlow, J. A., Jr., and Haun, J. D., 1966, Regional Stratigraphy of
Frontier Formation and Relation to Salt Creek Field, Wyoming,
Amer. Assoc. Petrol. Geol. Bull., v. 50, pp. 2185-2196, not seen.
Brown, L. F., Cleaves, A. W., Erxlaben, A. W., 1973, Pennsylvanian
Depositional Systems in North-Central Texas, Bureau of Economic
Geology, Univ. of Texas at Austin, Guidebook No. 14, 120 pp.
Brown, L. F., Fisher, W. L., McGowan, J. H., Scott, A. J., 1969,
Delta Systems in the Exporation of Oil and Gas, Bureau of
Economic Geology, Univ. of Texas at Austin.
Brown, L. F., Wermund, E. G., 1969, Late Pennsylvanian Shelf Sediments,
North-Central Texas, Dallas Geological Society, 68 pp.
Cambell, C. V., 1978, Model for Beach Shoreline in Gallup Sandstone
(Upper Cretaceous) of Northwestern New Mexico, New Mexico
Bureau of Mines and Mineral Resources, Circular 164, 29 pp.
Clifton, H. E., Hunter, R. E., and Phillips, R. L., 1971, Depositional
Structures and Processes in the Non-Barred High Energy Nearshore,
Journ. Sed. Petrol., 41, 651-670.
Cobbon, W. A., and Reeside, J. B., Jr., 1952, Correlation of the
Cretaceous Formations of the Western Interior of the United
States, Geol. Society of America Bulletin, Vol. 63, No. 10,
pp. 1011-1044.
Dickinson, W. R., Tectonics and Sedimentation, SOCiety of Econ.
Paleon. and Mineral., Special Publication No. 22, Nov. 1974,
pp. 1-27.
Evans, G., 1965, Intertidal Flat Sediments and Their Environments of
Deposition in the Wash, Geol. Assoc. (London) Quart. Journal 121,
pp. 209-245.
Folk, R. L., 1968, Petrology of Sedimentary Rocks, Univ. of Texas,
170 pp.
Friedman, G. M., 1961, Distinction Between Dune, Beach, and River
Sands from Their Textural Characteristics, Jour. Sed. Petrol. 1:
514-529.
Gill, J. R. and Cobbon, W. A., 1966a, The Red Bird Section of the
Upper Cretaceous Pierre Shale in Wyoming, U. S. Geol. Survey
Prof. Paper 393-A, 73 p.
Ginsburg, Robert N., 1975, Tidal Deposits, A Casebook of Recent
Examples and Fossil Counterparts, Springer-Verlag, pp. 117-127.
Goodell, H. G., 1962, The Stratigraphy and Petrology of the Frontier
Formation of Wyoming, in Symposium on Early Cretaceous Rocks
of Wyoming and adjacent areas --- Wyoming Geol. Assoc.,
17th Field Conf., 1962: Casper, Wyo., Petroleum Inf., pp. 173210, not seen.
Hale, L. A., 1961, Late Cretaceous (Montanan) stratigraphy, eastern
Washakie Basin, Carbon County, Wyoming, in Symposium on Late
Cretaceous rocks, Wyoming and adjacent areas, Wyo. Geol. Assoc.,
16th Ann. Field Conf., 1961: Casper, Wyo., Petroleum Inf.,
pp. 129-137, not seen.
Hancock, E. T., 1920, Geology and Oil and Gas Prospects of the Huntley
Field, Montana, U.S. Geol~ Survey Bull. No. 711, pp. 105-148.
Haun, J. D., and Barlow, J. A., Jr., 1962, Lower Cretaceous Stratigraphy
of Wyoming, in Symposium on Early Cretaceous rocks of Wyoming
and adjacent areas --- Wyoming Geol. Assoc., 17th Field Conf.,
1962: Casper, Wyo., Petroleum Inf., pp. 15-22, not seen.
Klein, G., 1970, Deposition and Dispersal Dynamics of Intertidal
Sand Bars, Jour. Sed. Petrol., 40(4), 1095-1127.
-------,
Co.
1977, Clastic Tidal Facies, Oontinuing Education Publication
Mackenzie, D. B., 1975, Tidal Sand Flat Deposits in Lower Cretaceous
Dakota Group Near Denver, Colorado. Tidal Deposits, A Casebook of Recent Examples and Fossil Counterparts, R. Ginsburg,
Editor, Springer-Verlag, pp. 117-127.
McCave, I. N., 1970, Deposition of Fine Grained Sediments From Tidal
Currents, Jour. of Geophysical Research, 75(21), 4151-4159.
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not seen.