for the - Ball State University

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 20-250 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. The sampling

location was a cliff face adjacent to well #87, NWi 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). The cumulative percent curves from the Virgelle

are ve~T 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 5%) that is

typical of beachface environments. They are also similar in that they

both have 10% silt and clay. 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 coarsening-

upward sequence.

There are several models of both the tidal and deltaic de-

positional 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. This is evident in the point count

results in Appendix A. While quartz sands make up approximately

60-7~ft 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 4 c~wide and 18 cm. deep),

and plant rootlets. 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. The sequence of

facies (from bottom to top) is described as: (1) laminated clay;

(2) peat; (3) burrowed, laminated sand; (4) laminated sand; and

(5) interbedded sand and grit. 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. This zone is characterized by a seaward

dipping, planar surface. The 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 back

shore-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. He cites the salt marshes in the Wash,

England as an example. 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 inter

prets 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. As a result, the strike of the laminae is parallel to the

shoreline. 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; (4) upward increasing sand

content (see Appendix A); (5) 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.

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1 SOutH CAROll"" M!DDLE FORESHORE 70

50 c .. .. 30 "-~

c .. , a 10 .. ~

05

01

Phi Scale " 2!> 12') 067

mm. Scale

Beach foreshore sands

r----r---,r---,----;----,---~----r_--_r--_"."

ALMOND fORMATION - NORTH Of SUPERIOR. WyOMtNG B

Phi Scale

-WtHU· .. .Jrt.DSlOff£ 1M( ~SlCnQ"

.~"5 12~ 0.7

mm Scale

Deltaic distributary curves

. .. II

10

30

10

Figure 6. (From Visher, 1969, pp. 1084, 1082, 1095, 1097)

r-__ ~--,----r---.---;r---r---C----r---,'999

U~

),m, ,Scak <' ':;::']]-.;,':< .(

Turbidity current deposited sandstone

. ~ '" ~ : -g 0:

c'

i ... ~ .. c . " !

Phi Scate .1 ,S 2') 12!1 0.,

mm. Sclle

Surf zone sands

Ha

••

'0

,.

99.99

!t .•

98

90

,. sO

1.

I~

0.'

01

r---,----r---;r---,----r---,----,----r---,9.9.

O"SHOlt( "'Altl"[ _ '11-(40.0' •• .., ........ , (.J 'I$lIftOtI HttomCt",Uftt)

A

.0

'0

50

,0

10

";

01

~ __ ~ __ ~ ____ ~_~ ____ ~ __ _L ____ ~ __ ~--~OOI

PhI Scale l~ 125 0"

mm. Scale

Marine delta area

r--..,-----,r---~--i---_._-_.--_r_-....,.---, ••••

..-, , ,. , AYON'folCH~ NORTH CUOllNA , IIS.0·.It.r,.,,,,. I

,I

I ," , - SAVANNAH IIltiU nfln

SURF IUCH-----7#-.. "" GEO'GIA' "0II1M Col"lINA LO.tlt TlOAl FLAT 112.0' •• t., ...... , IWaw.,ittIItN)

••

90

10

.0

30

10

o.

01

!:---.-L.---!!--~_~ ____ _'_ __ ~----.L--_7--~0.0 1

Phi Scale 250 12S 0.'

mm. Scale

Wave zone sand distributions

Figure 7. (from Visher, 1969, pp. 1091, 1101, 1086, 108S)

WEST

ALLUVIAL FAN

Jndiaoola Gannett Price River Epharim Judith River

EX~LANATIQN,

Fans

Fluvial

Delta Plain

FLUVIALt

Mesaverde Frontier Dakota Cloverly

-•. : ~; ,'_ :. • ; , , ;,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).

-- --- - -- - --------..-'v'<' --;-- r FRONTIER FORMATION

'i ,-,r \~I

I ~! , I ~ 1

+-~-----' i I i i i I

... /

CREtIC 55 Aflel"'''''''' cuTOf'F fOR '1II0NTIlR

QP(R,liTIONllIi, UNIT

OF WYOMING ISOPACH AND SAND· SHALE RATIO MAP FOR INTERVAL 6ETWEEN TOP OF MOWRY SHALE ANO TOP OF

FIRST WALL CREEK SANOSTONE .. tit !.S/S'" RATIOS

--~oo- ISOPAC.HS, 50' INTERVAL

SSISI'I K:::;::q I I lIRA no (LtC HI1 e .l.~ ~O, ,euT~"O. 0-"'.

H." ' •• , .. 10 '~"_=1_' __ "

---~-,..~~""'- - - - --1- - - - - ~ I . i 1 : i I \ L

I I

r----~

: i 1 i : I 1 ' 1 I I 1

,_ - - _I I . L'-- __ _

1

. I I· 1

~_.: ___ ._._-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 FRONTIER FORMATION ~~A~';~;~~-'~ I OF WYOMING

UtllT 0# f" • ., .. L eMU. ~'I: I """'a.tty. C\lTM' ,Oft ,~ONnul

OPIltAflONAL UNIT. ~-,'_': I '. 1

I 1 NUMBER or SANDS AND TOTAL

____ : SAjIII _THICKNESS MAP ro~ THE

\(i~\\~~~~?i 1 ~~;,vA;,,:C;W!~~lT~: o~\~E I fiRST WAll CREEK SANDSTONE \. ::r:·ild·~ .. y:~·r:·i: ....

'l" .... " ••. e YlU ~o. • ... 'c .... .,., • ... u _ ...... .. ..

. ' - - - - -r - - - -'-..., I 'I 1 : 1 'I 1 -

1

r---I t I •

'j

1 I 1

I 1 ,--- _, 1 I' L,- __ ~

I. 1 1 • I I' 1 1 • ___ L ______ .,-.. _______ I

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).

I r--·-r· '---, SANOS;ONE

0< 100' " 0100'-200'

0>200' ,

.PROOUCTIONI

_I~ UT~H ,

'-V ,

.--2r:tU Wyo, I

~-- ,

, l2.8.L. NEW.

.. /"'. C:;. ~ • ..s4t'DSrONE ltL-.' '--, .---;'._.---.JI.YO

FEheON • ' 71 CO; . ~ '1/ }~ • .It ~ .• \..#

SA1fpSTONE 0

1# i / \ 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).

NDT . STUDICD

il 100 MILES

"0

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).

''''V------------=.--~,~. ,·:---i~---I-a-.·' ' •. , A,';,;.'.l,----. ' ----"T--,:------.-' '.-' --"i-------,,---:-',~'~,~--~---------.. ~.,-------~~----~~r---

, • a~' -----~""C-.... ~-'., ---II~,'---=-:-----~--.:......,~ '~~~.-'~'--~ --------------~~~

r -.--=1~~:\I: i ,~--------

.. _ . • •• '.CI 'Ie.... ____ -:-__ ~-

~ ...... -~ \i ...... '.CI ... " •• " la' ,. 0 "'" -- \ .

DIAGRAMMATIC PALEOGEOGRAPHY

AT CLOSE OF SKULL CREEK TIME

10 0 ~o , , I I I ,

'CALEI MILES

0 CJ ~ -1 _or ~ ,

. . -- .--DILTAIC - .. IP,NIIttTIC - .

-' l .... ltAlilltlTlC

II .. IIItED OIL TA DI'",.UTU •• tAo 0L0t.1T,

Figure 16. - Regional setting of Newcastle delta system (Cretaceous). (From Brown, Fisher, et. al., 1969, Fig. 129, who cite Wulf, 1962, not seen).

,-----_._----- -------

o [:·.··;·1

CJ [.:.:':.:.:.:';\

[§]

................. --.......... ~ ...

,. ......... 111 ...

".n .............

Walhakie

Balin

~ .: I ." ~

J .~. ! .1! . .• ii: : I

: ""'i._ .. -----::::~: h________ ~- -:---~-r

•. " .. ~ . I' .

' ..

~~., -..... Sand Woah

BOlin

Figure 17. - Late Cretaceous deltas, Wyoming. et. al. t 1969, Fig. 130, who cite Hale, 1961,

.. . . 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""

(From Brown, not seen).

.... ~.l.'U' T'M' - ... T, IIIIA.OII '"'.,' Il ..... IEIW!J~

~ • u·· .. -.. l·~""" I ........ • .. l~ ....

'f .... ,.... ..... LtIII:t!lI!:!ft ....

Fisher,

APPENDEX A

POINT COUNT RESULTS

SAMPLE # 1 2 3 4 5 6 7 8 9 10 11 12

Quartz 50 59 44 67 61 60 44 78

Chert 20 24 35 17 16 15 7 15

Glauconite 9 6 10 7 7 2 2 1

K-Spar 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

Figure 1.

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

2.500 3000 3.5~~ 4.0~0 4.50~ 5.000 5.500 6.000 PHI INTERVALS

j .502)

-


ELK BASIN. WYOMING SAMPLE *2

~ .~ 1-------r-~~~~~--_,------,_------,_----_.--~--·'I-------rI------~I------~1 1. 000 1.S0~ 2.500 3.0~0 3.S~~ 4.000 4.S0~ 5.~00 5.S00 6.00~

PHI INTERVALS


ELK BASIN. WYOMING SAMPLE #]

1.00121 1.5121121 2.121121121 2.5121121 3.121121121 3.t;12I121 1 ________________ ... ____ .. __ . ___ . PHI INTERVALS

-.



~4-------r-·~~~~~--'I-------rI------'I-------;1 ======~I~----'Ir------'I------'I 1.000 1.5~0 2.000 2.500 3.000 3.500 4.000 4.500 5.000 5.500 6.000

PHI INTERVALS



1.000 2.12J012J 2.512J12J 3.12J12J12J 3512J12J 4.12J12J0 PHI INTERVALS

4.512J0 5.0"'12J 5.512J12J 6.000

,--,-----_._---------------------------------_.-


ELK BASIN, WYOMING SAMPLE *6

~ 1----.-~~~~~-.----._--_.----_.-==~-TI-----r1 ----~I---~I 4.5~~ 5.~0~ 5.5~~ 6.~ 1.000 1 _ 5QJQJ 2.~~~ 2.5~~ 3_~~~ 35QJ~ 4_QJ~~

PHI INTERVALS


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



-

~ ~------'-'~~~~~---'------'-------'I-------'I------~I-------rl ------~I------~I

2.500 3.000 3.500 4.000 4.500 5.000 5.500 6.000 , .00l2I 2.!1J00 PHI INTERVALS

.-

-



-

~ ~-----'-------r------r------,------.------.------.-------.-----~----~ 1.000 2.QJQJQJ 2.5QJQJ 3.QJQJQJ 3.SQJQJ

PHI INTERVAL~ 4.QJI2JQJ 4.5QJQJ 5.QJI2JQJ 5.SI2JI2J 6.QJQJ2

-

~~------------------------------------------------------------------------------ .. ~ ~ ~


ELK BASIN, WYOMING SAMPLE *10

--,r-------r.------,r------,r-------,r---=---'r------~r ------~r---~ 2.000 2.500 3.000 3.£00 4.000 4.500 5.000 5.500 6.000

PHI INTERVALS



-

-

~ ~ GRAIN SIZE DISTRIBUTION OF VIRGELLE SANDSTONE


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 1-: I i , I ~ ~ _ t ~ __ ,

'.r'

C

~ 0 Ul

~ H H H (:-

~ §

-- re

~ 0 Il::l ~ :>-t 0

~ ~ ~

.-

2 2 . .5 3 3 . .5 4 4 . .5 PHI SCALE

~ 0 Ul

~ H H H

~

-, § s::

~ 0 rx: re :>-t 0

~ ~ iii

2 2.5 3 3.5 4 4.5

PHI SCALE

APPENDIX D

+ 3.00

+2.00

+1.00

-1.00

-2.00

-3.00

-

BEACH SAND

SKEWNESS VS. STANDARD DEVIATION

•

• ••

• • • •

RIVER SAND

0.10 0.30 0.50 0.70 0.90 1.10 STANDARD DEVIATION (BOUNDARIES AFTER FRIEDMAN, 1961)

(Figure 1)

-

+3.00

+2.00

+1.00

-1.00

-2.00

-3.00

1.40

BEACH SAND

SKEWNESS VS. MEAN GRAIN SIZE

• •

DUNE SAND

• • • • • • • •

1.80 2.20 2.60 3.00 MEAN (BOUNDARIES AFTER FRIEDMAN, 1961)

(Figure 2)

3.40

MEAN GRAIN SIZE VS. STANDARD DEVIATION (Figure 3)

,,-

• • 3.00 •••

DUNE SAND

• • RIVER 2.80 • • SAND

2.60

•

2.40 •

2.20

2.00

-

0.10 0.30 0.50 0.70 0.80 1.10 STANDARD DEVIATION (BOUNDARIES AFTER FRIEDMAN, 1961)

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. 173-210, 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.

McCUbbin, D. G., 1972, Cretaceous System, pp. 190-250, in Geol. Atlas Rocky Mtn. Region, W. W. Mallory Editor-in-Chief, Rocky Mtn. Assoc. Geologists, Denver.

McGregor, A. A., and Biggs, C. A., 1968, Bell Creek Field, Montana: A rich stratigraphic trap, Amer. Assoc. Petrol. Geol. Bull., v. 52, pp. 1869-1887, not seen.

McKee, E. D., 1957, Primary Structures in Some Recent Sediments, Amer. Assoc. Petrol. Geol., Bull. 41, pp. 1704-1747.

Shelton, J. W., 1965, Trend and Genesis of Lowermost Sandstone Unit of Eagle Sandstone at Billings, Montana, Amer. Assoc. Petrol. Geol., Vol. 49, No.9, pp. 1385-1397.

Thompson, Allan, M., 1975, Clastic Coastal Environments in Ordovician MOlasse, Central Appalachians. Tidal Deposits, A Casebook of Recent Examples and Fossil Counterparts, R. Ginsburg, Editor, Springer-Verlag, pp. 135-143.

Visher, G. S., 1969, Grain Size Distributions and Depositional Processes, Jour. Sed. Petrol., 39: 1074-1106.

Weimer, R. J., 1960, Upper Cretaceous Stratigraphy, Rocky Mtn. Area, Amer. Assoc. of Petrol. Geol. Bull., Vol. 44, No.1, pp. 1-21.

Wulf, G. R., 1962, Lower Cretaceous Albian rocks in northern Great Plains, Amer. Assoc. Petrol. Geol. Bull., v. 46, pp. 1371-1415, not seen.

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