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Graduate Student Theses, Dissertations, & Professional Papers Graduate School

1966

Silurian system in eastern Montana

Frank Kendall Gibbs The University of Montana

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Recommended Citation Gibbs, Frank Kendall, "Silurian system in eastern Montana" (1966). Graduate Student Theses, Dissertations, & Professional Papers. 9265. https://scholarworks.umt.edu/etd/9265

This Thesis is brought to you for free and open access by the Graduate School at ScholarWorks at University of Montana. It has been accepted for inclusion in Graduate Student Theses, Dissertations, & Professional Papers by an authorized administrator of ScholarWorks at University of Montana. For more information, please contact [email protected]. THE SILURIAN SYSTEM IN EASTERN MONTANA

by

Frank Kendall Gibbs

Geological Bhgineer Colorado School Of Mines, 1955

Presented in partial fulfillment of the requirements for the degree of

Master of Science U niversity of Montana

1966

Approved by:

1 Chairman, Board of Examiners

Deary" Graduate School

J A r r c '^’^ -7______Date

( i ) UMI Number: EP40067

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ProOuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106 - 1346 table of contents

Page

Introduction ------— — 1

General geology 3

Tectonic setting 3

Methods and procedures 5

Previous woric ——------—————™™— .——— 7

Surface stratigraphie definition in Manitoba — — — — 7

Subsurface stratigraphie definition — ------— — 9

Summary of nomenclature development — — — — I 3

Stratigraphy ——------— —— 14

Interlake Formation ™—™—. 14

Subdivision and nomenclature 16

A ge ------—— ------17

Lithology of the In terlak e Formation in Montana — ---- 18

Lower beds — 19

Porosity development in the Interlake Formation —— 20

Photographic illustrations (Plates I, II, III, IF, and V) 21

(11) Page

Relation of the Interlake Formation to adjacent strata — ------28

Lower c o n ta c t — ------—------28

Upper contact ------— ------29

D istrib u tio n of the In terlak e Formation in Montana ——- —— 30

Modem ideas concerning carbonate deposits and their classification ————— ——-— ------—- — — 32

Dolomite and dolomitization — —— — — — — 35

A process for dolomitization —— ------— — —— 36

A classification of dolomites 36

Conditions o f deposition of the In terlak e Formation in Montana — 37

Bibliography —- —- — — —------—------41

Appendix A (Compilation of data used in mapping) — •— •— ------46

( i i i ) ILLUSTRATIONS

Figure Page

1 — Index map of thesis area —— ————————————— vii

2 — Gamma ray neutron definition of the Interlake Formation in eastern Montana ——— — — —— I 5

3 — Sample depths o f rocks used in photographic illustrations — — 22

4 — Lithology» lower and middle beds» Interlake Formation (from cores» Murphy» et al.» East Popular # 1)—In Pocket

5 — Cross-section A—A', gamma ray log correlations of the Interlake Formation in eastern Montana —— In Pocket

6 — Cross-section B—B* » gamma ray log correlations of the Interlake Formation» North Dakota to eastern Montana —— — — —— ——— In Pocket

7 — Isopachous map of the lower Interlake beds —————In Pocket

8 — Isopachous map of the middle Interlake beds — ——In Pocket

9 — Paleogeologic map» post-Silurian pre-Mlddle Devonian erosion surface and isopachous map of the upper In terlak e beds, eastern Montana ------— In Pocket

10 — Structure contour map, top Stony Mountain Formation (base Interlake Formation) ———In Pocket

Plate I — Finely micrograined dolomite; thin section photograph — — — ------23

Plate II - Relict clastic texture in dolomite; thin section

Plate IH- Finely micrograined dolomite with typical laminated structure; thin section photograph — — — 25

Plate IV - Dolomite (dolomitized algal and roicritic limestone); thin Section photograph ——™ 26

Plate V — Dolomite, dolomite breccia; thin section photograph - 27

(iv ) ABSTRACT

The Interlake Fonnation, which constitutes the Silurian System throughout the TAlliston Basin was found to be present in the sub­ surface of about the eastern one third of Montana* Subsurface boundaries and informal subdivisions were established at 100 well locations in eastern Montana by carrying gamma ray log correlations into the area from the central part of the Williston Basin in North

Dakota. Descriptions of well cores and cuttings were used in conjunction with the gamma ray logs in making subsurface correlations.

The Tnterlake cmnfapmahly ovexlies the Stony Mountain Fonaation in the northeastern part of Montana» but to the west and southwest an unconformity possibly exists between the two formations. The upper contact of the Interlake is an unconformity» and the overlying Ashem

Formation of Middle Devonian age rests on eroded and truncated rocks of the Silurian System.

The Interlake has been reduced from group to formation rank» and divided informally into lower» middle and upper beds. Beds formerly regarded as the Stonewall Formation are included in the lower unit of the Interlake Formation. The unit previously called Stonewall

Formation does not qualify for formation rank in the subsurface.

The rocks of the lower and middle beds are mainly gray to light brownish gray to near white» medium to very fine grained» calcareous dolomite» and finely laminated» gray to near white» micrograined dolomite. To the northeast the middle beds become mostly dolomitised fragmental limestones with interbedded thin» anhydritic» argillaceous micrograined dolomite* The upper beds consist of dolomitized fragmental limestones and interbedded thin, micrograined dolomite and argillaceous, anhydritic, micrograined dolomite.

Most of the sediments of the Interlake Fomation are classified as "dolomites in evaporitic sequences", but the general lack of high rank salts indicates the environment of deposition was penesaline in the lagoons of the shelf areas.

Isopachous maps show that some areas of the shelf were more positive than others during deposition of the Interlake. These more positive areas in certain instances were sites of patch reef and algal growth.

Where these rocks have retained favorable reservoir characteristics they may now contain producible o il.

' v i . ) J SASKATCHEWAN 1 MaHjTOBA ONTARIO \T//fl/,^ '^~rr/'~n— •— -—^------^ \

north DAKOTA', \ ^ ’////////// a ^ 2

atG S NOT r vrj

ttOV»

SOUTH DAKO

- ' 4

NEBRASKA — ^

s MOWING

THESIS AREA

WILLIS TON BASIN INDEX

MAP ?IG . 1 WESTERN INTRODUCTION

In this thesis the Interlake Fonaation« which constitutes the

Silurian System in the Williston Basin, is studied and its limits defined in th e subsurface of eastern Montana, Eastern Montana forms the western flank of the large cratonic Williston Basin, The basin is located to the west and southwest of the Canadian Shield and extends through southwestern Manitoba, southern and central

Saskatchewan, most of North and South Dakota, and eastern Montana

(See Fig, 1),

Rocks of the Silurian System crop out along the Canadian Shield in

Manitoba and in the mountain ranges of northern Utah and southeastern

Idaho east and north of Logan, Utah, but are not known to be present on the surface any^ere in Montana, Thus, the rocks of the Interlake

Fomation which underlie most of the Williston Basin (Porter and

I\ille r , 1959) must be studied and their limits fixed using subsurface geologic methods and infomation obtained from deep bore holes drilled in exploring for oil.

The objectives of this thesis are: (1) to determine the distribution of the Interlake Formation Wiich makes up the Silurian System in eastern Montana, (2) to determine the relationship between the

Interlake Formation and the underlying and overlying formations, and

(3) to interpret the environment of deposition of the Interlake

Formation in eastern Montana,

(1) ACKNOWLEDGEMENTS

For support of my graduais work a t the U niversity Of Montana I am indebted to the National Defense Graduate Fellowship Program of the

United States Department Of Health, Education, and Welfare, Office of

Education, and to the Geology Department of the University Of Montana.

The completion of this thesis would have been impossible without the guidance of the Staff of the Geology Department of the University

Of Montana. Particular thanks are extended to my thesis advisor.

Dr. Robert M. Wéidman, and to myGraduate School advisor Dr. Fred

Honkala. Dr. James A. Peterson of my thesis committee was very helpfbl in evaluating and editing the manuscript.

Mr. Andrew F. Bateman, Jr., of the United States Geological Survey in Great F a lls , Montana helped by pointing out some o f the regional aspects of the thesis subject, and also provided me with several iiqportant references.

Special thanks are extended to my wife, Mrs. Margaret Jean Gibbs, for her help in calculating and compiling the data used in the sub. surface mapping.

Acknowledgements and thanks for services and materials necessary for preparation of the thesis are made as follows ;

(1) to the University Of Montana, Geology Department for purchase

of the mechanical logs needed for subsurface correlations;

(2) to Mr. Fred McCotter and American Stratigraphie Company,

B illin g s, Montana fo r supplying lith o lo g ie logs of th e

(2) eastern Montana subsurface;

(3) to the Montana O il and Gas Conservation Commission,

B illin g s, Montana fo r providing working space in th e ir

Billings office, access to cores and samples taken from

wells drilled in Montana, and furnishing a base map of

Montana from which sepias were made for subsurface mapping

bases;

(4) to Phillips Petroleum Company, Bartlesville, Oklahoma for

reproducing all of the maps, cross-sections, and illustrations

for the thesis, and allowing time from my regular work to

arrange for these services.

GENERAL GEOLOGY

TECTONIC SETTING

In broad perspective the VUliston Basin is one of several similar depositional and structural basins which ring the Canadian Shield

(Eardley, I 963). The Canadian Shield has been the eastern lim it of the Williston Basin, and the north-trending Alberta Shelf in north- central Montana and in Alberta, Canada has been the western lim it.

In south-central Montana and north-central Vÿoming the Wyoming

Shelf (Eardley, I 963) has separated the Vdllliston Basin from the

CordiUeran miogeosyndine periodically since Cambrian time. During

(3) the Ordovician Period a connection existed between the two tectonic features through southern Montana and northern Wyoming (Alpha# 1958* p# 12# 13)* Also# the rate of internal thinning of rock units within the Silurian System# and the nature of the sediments within these units indicates that the lAlliston Basin may have been connected with the

Cordilleran miogeosyndine through southern Montana and northern looming during Middle Silurian time#

The Williston Basin# a large and persistent structural feature# has been a center of sedimentary deposition periodically since Cambrian time (Alpha# 1958$ P* 13)• Also# according to Baillie (1955)$ the general region was subsiding almost continuously during Paleozoic time# However# the centers of subsidence for each period were not s%q>erlmposed# but varied in position during the Paleozoic Era#

During the Silurian Period the center of the Williston Basin was in northwestern North Dakota about 50 miles east of Williston# North

Dakota (P o rter and Fuller# 1959# and Carlson and Eastwood# I 962)#

Characteristically# very broad shelf areas existed between the basin center and the margins# and eastern Montana# the area of this study# fonaed the broad western shelf of the basin# This shelf was for the most part a relatively stable area where depositional environments varied between shallow open-marine and penesaline# Isopachous maps prepared in this thesis show that some areas of the shelf were persistently more stable than others# and that locally# hinge lines developed on the shelf# Except possibly for the basin center the

Williston Basin was the site of widespread post-Silurian pre-Mlddle

Devonian erosion (Baillie# 1951 and 1955) suggesting that the shelf

W regions became more unstable toward the end of Silurian deposition*

During this erosional inteival sediments of the Silurian System were completely removed from areas of the outer margins of the basin, and it is likely that even in the basin center a considerable hiatus exists

(Porter and Fuller, 1959) • Inasmuch as the VdUiston Basin has been a persistent feature since

Cambrian time the thickness of the sedimentary section found on the basin margins today is considerably less than that found in the basin center. This is due to both internal thinning of the rock units in the shelf areas and post-depositional erosion. In the case of the

Silurian System a section several hundred feet thick that is present in the basin center is absent in the outcrop area and also in the sub­ surface on the flanks of the basin. Therefore, nomenclature developed in these outlying areas must be modified to include the sediments found in the central basin.

METHODS AND PROCEDURES

Information for this thesis was obtained from cores, cuttings and gamma ray neutron logs from bore holes drilled in petroleum exploration in eastern Montana and North Dakota. Gore samples were examined a t the

Montana Oil and Gas Conservation Commission o ffic e in B illin g s,

Montana, and also cores and cuttings from the collection of the Univer­ s ity Of Montana in Missoula, Montana were examined using the binocular

(5) miorosoope. Descriptions of cores and cuttings from wells drilled in eastern Montana and North Dakota were provided by the Billings*

Montana office of American Stratigraphie Company* and these were used in conjunction with the gamma ray neutron logs for subsurface cor­

relation work*

Subsurface boundaries for the Interlake Fomation and its informal divisions were established at one hundred well locations in eastern

Montana by carrying correlations into the area from the Amerada

Petroleum Company H* Bakken No. 1* Sec. 12* T. 157 N.* R. 95 W.*

Williams County* North Dakota in the central part of the Williston

Basin. The boundaries established by Porter and Fuller (1959#

Fig. 7) in this well for the Interlake are accepted with the

exception of the lower one which is placed about 50 feet lower (see

Fig. 6* Cross-section ^-B* ). Cross-sections A—A* and B—B*

(Figures 5 and 6 in pocket) were made the base lines of control to which correlations were tied. The correlations are considered reliable

except near the erosional edges of the units. Depths and elevations of boundaries used in mapping are cong)iled in Appendix A. The Inter­ lake Fomation was divided informally into lower* middle* and upper beds using the nomenclature introduced by Porter and Riller (1959).

Two regional cross-sections* three isopachous maps* a subsurface structure map* and a paleogeologic map were prepared to show the distribution and attitude of the Interlake Formation of the Silurian

System in eastern Montana.

Thirteen thin sections were made from core chips from the Shell Oil

Company-Northern Pacific Railroad Richey No. 1* Sec. 19* T. 23 N.* (6) R* 50 B.# Dawson County, Montana. Photographs showing low magnifi­ cation of the thin sections were taken and included in the thesis to show textural varations occurring in the rocks of the Interlake

Fonaation,

Identification of clay minerals present in one of the shale beds found in the above well was made using X-ray diffraction equipment available in the Geology Department at the University Of Montana,

PREVIOUS WORK

SURFACE 8TRATIGRAPHIC DEFINITION IN MANITOBA

The outcrops of Silurian strata nearest to the area of study are four to five hundred miles to the northeast in Manitoba, Canada along the southwestern edge of the Canadian Shield, Tyrrell (1892) wrote mainly of the Silurian strata in compiling observations of the outcrops in Manitoba, Further work with the surface sections by

Kindle (191^) resulted in his proposing the name Stonewall Formation for all of the rooks of the Silurian System exposed in Manitoba,

Inqportant definitive work was accomplished by Baillie (1951) who did detailed mapping and made faunal studies of the Silurian section in the Interlake area of Manitoba, He proposed the Name Interlake

Group for the Silurian System of rocks exposed in the area, sub­ divided the Group into units A, B, C, D and B in ascending order,

(7) and except for unit **1", deferred assigning formation names to the units until they could be established in the subsurface. The name

"Stonewall Fomation" was restricted to unit "A", the lowest of his

Interlake Group. In defining the Stonewall Formation on the outcrop

Baillie proposed that the formation should include "those strata between the underlying Stony Mountain Formation of Ordovician age and the overlying Yireiana decussata-bearine dolostone of unit "B".

This definition placed the base of the Interlake at the base of a l6-foot thick sequence of interbedded red argillaceous dolomites, siltstones, and arenaceous dolomites (Baillie, 1951* p. 15)* How­ ever, in a later work Baillie (1952) decided that the lower boundary should be placed at the greatest lithologie break, that is, at the ton of the red clastic-containing zone. This change in the boundary was necessary %d%en additional work by Baillie (1952) indicated that the red elastics formerly included in his Stonewall Formation were tran­ sitional with the underlying beds of the Stony Mountain Formation.

Baillie (1951* PP* 51# 52) observed that the Stonewall Formation con­ tained fossils representative of both Ordovician and Silurian times, and suggested that the Stonewall might represent a transition from

Upper Ordovician to Lower Silurian. The contact at the top of the red dolomite, siltstone, and arenaceous dolomite sequence had been used by Okulitch (19^3) ih separating the Silurian and Ordovician Systems in his work with the Stony Mountain Formation along the Manitoba out­ crops. He too noted the affinity of the fauna of the Stonewall part of the section to that of the underlying formations of Ordovician age.

After studying the Stonewall Fomation as defined by Baillie, and

(8) reddscrlbing the fauna» Steam (1956» p. 8) proposed a Late Ordovician age for the Stonewall Formation and removed it from the Interlake Group*

Also» on the basis of a minimum of evidence (as pointed out by Porter and FùHer» 1959» PP« 178-180) Steam contended that a hiatus repre­ senting all of Barly Silurian time separated the Stonewall Formation from the overlying Interlake» This contention was firmly and effec­ tively rejected by Porter and Fuller (1959» PP* 178-180) who did how­ ever» still accept the Stonewall as a formation of Ordovician age. It must be noted that Porter and Fuller based their opinions on the sub­ surface stratigraphie relations basinward from the outcrops studied by Steam» and a h iatu s such as th a t advanced by Steam (1956» p. 8) could possibly be present in the outcrop area and not be represented in the subsurface toward the center of the Williston Basin»

The upper lim it of the Silurian System has been much easier to define than the lower» The overlying Ashem Formation» defined and described by Saillie (1950» pp» 10-12)» rests disconformably on the older rocks of the Paleozoic Era throughout the entire northern Great Plains region» The contact is described by Baillie as "one of the greatest unconfonnities in the Paleozoic succession» and represents Late

Silurian and Early Devonian time"»

SUBSURFACE STRATIGRAPHIC DEFINITION

The first published account of subsurface work in the central and westem portions of the Williston Basin dealing with sediments of the

Silurian System was by Rader (1952) who described the general lithol-

(9) ogy of the Interlake from cores and cuttings available at that time.

He observed that the units of Baillie>s Interlake Group differed very little lithologically, and made no atten^t to carry them in the sub­ surface. Be included the Gunton Member of the Stony Mountain Formation of Ordovician age with the Interlake* and thus his isopachous maps do not delimit rocks of the Interlake Fonnation alone. Rader (1953a ) believed that most of the sediments of the Interlake Formation were originally fossiliferous-fragmental, bio stromal and normal marine limestone deposits which were dolomitized almost conqpletely after deposition.

Stanton (1953# P# 60) discussed th e Ordovician and S ilu rian Systems as found in the subsurface of southeast Saskatchewan. He considered the Interlake as undividable in the subsurface and treated it as one u n it.

During the early nineteen fifties the North Dakota Geological

Survey established general descriptions of the rocks of Upper

Ordovician and Silurian age occurring in the subsurface of North

Dakota, and in 1962 published Bulletin 38 by Carlson and Eastwood which described and discussed the rocks of Ordovician and Silurian age found in the subsurface of North Dakota.

Andrichuk (1959# P* 2361), in his detailed subsurface study of the

Ordovician and Silurian stratigraphy and sedimentation in southern

Manitoba, rejected Steam's removal of the rocks of the Stonewall

Formation from the Interlake Group, noting that the main sedimentary break in the subsurface occurs at the base of the Interlake as defined by Baillie (1952). Andrichuk accepted B aillie's Group designation for

(10 ) the Interlake» but vas unable to define the units of the Group or the Stonewall Formation In the subsurface• Thus, he used an Informal terminology of lower and upper Interlake, and divided the two units on the basis of the texture of the dolomite observed In the cores and cuttings* Generally the finer textured dolomite made up his lower

Interlake, and coarser textured dolomite made up his upper Interlake*

In the area covered by Andrichuk *s study the upper Interlake beds of

Porter and Fuller (1959) and of this thesis are not present due to

Early Devonian erosion* Andrichuk* s upper Interlake Is about equiva­ lent to the middle Interlake beds of Porter and Fuller (1959)# and as defined In this thesis (Figure 2)*

In 1959 Pbrter and Fuller published a paper In which the formation boundaries of the Lower Paleozoic formations of the lAlllston Basin were projected Into the subsurface from the outcrops and then cor­ related throughout the northern and central parts of the basin* Their woxic, combined w ith the Ideas developed by previous workers, showed the general distribution of the rocks of Lower Paleozoic age through­ out much of the Williston Basin* Information obtained In deep drilling

In the central part of the basin along the Besson anticline was uti­ lized, and It was demonstrated that In the basin center an anomalously thick section of rocks Is present above what had been considered the youngest Silurian rocks* This section of rocks, found mostly In northwestern North Dakota, Is Included In the Silurian System by

Porter and Fuller (1959# p. 165 and p* 179)* They suggest that the rocks represent deposition during Late (?) Silurian time* If this

Interpretation Is correct the hiatus In the basin center between the

(11) Silurian and Devonian is significantly less than across the shelf regions and along the basin margins«

Porter and Fuller (1959» P* 134) attempted to carry the Stonewall

Fomation into the subsurface. Without mention of Baillie's definition of the upper boundary of the Stonewall on the basis of fauna they used an argillaceous break indicated on gamma ray neutron logs as the fomation top. Admitting their correlation into the subsurface from the outcrop to be unsatisfactory, they fomalized the subsurface unit defined on the gamma ray logs with the name "Stonewall Fomation".

The unit should probably not have been given formation status in the subsurface on the basis of such uncertain definition.

F u ller ( 1961, p . 1336) shows a map generally outlining the lim its of the lower Paleozoic strata in the Williston Basin and adjacent areas, but his gamma ray log markers and the subsurface correlations estab­ lishing the boundaries of the strata are not illustrated. For this reason his stratigraphie boundaries in southeastern Montana cannot be detemined. An examination of the continuous core from the Shell Oil

Company Little Beaver No. 1, Sec. 13» T. 4 N., R. 61 £., Fallon

County, Montana f a i l s to show rocks which would c o rre la te w ith those shown as Stonewall Formation in the nearest control published by

Porter and PXiller (1959» p. 139) for the Shell Oil Company Pine Unit

No. 1, Sec. 30» T. 12 N., R. 57 S ., Wibaux County, Montana some 50 miles to the north.

the only published faunal study which deals in part with the Silurian-

Ordovician boundary in the subsurface of eastern Montana i s by Ross

(1957 )# His conclusions are based on occurrences of Late Ordovician

(12) brachipods and corals* Cores from five wells in eastern Montana were examined and the limited fossil content described* Fossil preservation was generally poor, bat Ross was able to establish limits for the range of rocks deposited during Late Ordovician time* Along the Cedar

Greek anticline, where several of the wells used by Ross are located, the Interlake Fomation as defined in this thesis falls within the

S ilu rian System*

SUMMARY OF NOMENCLATURE DEVELOPMENT

The initial geologic work with the Silurian System in the Williston

Basin was on the outcrops in Manitoba, Canada* The Devonian-Silurian boundary, which is also the top of the Interlake Formation, is represented by a major unconfomity and is readily agreed upon by all workers on the outcrop* This boundaiy is also one usually agreed upon by subsurface geologists* The lower boundary of the Interlake as established by Baillie (195^) is also readily identified both on the outcrop and in the subsurface* Definition has been carried west­ ward and southwestward from the Manitoba outcrop area, and since the subsurface correlations were accomplished mainly by using gamma ray logs idiose response usually varies characteristically with lithology, the lower boundary of the Silurian System used in the subsurface of the VUliston Basin corresponds to that of Baillie (1 9 5 2 ), Okulitch

(19^3)» and Schultz (195B)* This boundary also corresponds to the base of the Interlake Fomation established in this thesis*

In this thesis the Stonewall Fomation is not recognized in the

(13) subsurface, and it is held that the unit called Stonewall Formation by Porter and Fuller (1959» P* 131) and by Fuller (1961, p* 139) does not qualify for the rank of formation* Also, inasmuch as the informal unit subdivisions of the Interlake Group established at surface exposures in the Interlake area of Manitoba by Baillie (1951»

1952) and the formations described by Steam (1956$ P« 8) have not been recognized in the subsurface of the VUliston Basin, and considering that the Stonewall does not qualify as a formation, the

Interlake is herein reduced to formation status* This is in accord­ ance w ith guidelines published by the American Commission On S tra ti­ graphie Nomenclature (1961» p* 626» A rticles 3 & 6; p* 62?» Article 9; p* 629» Article 13; p. 630» Article 16)*

STRATIQRAPHI

INTERLAKE FORMATION

The sub-units of the In terlak e Group as defined by B a illie (1951$

1952) and by Steam (1956) on the outcrops of Manitoba have not been identified in the subsurface of the VUliston Basin* Porter and Fuller

(1959) attempted to carry B aillie's unit A (the Stonewall Formation) into the subsurface and throughout the Williston Basin, but it is believed their definition is inadequate for the reasons presented in the previous section* Therefore, the rocks found in the subusrface of

(14) MOBIL NO. F-33.23-P.DAMM S E C .2 3 . T.29N..R.54E. ROOSEVELT CO. MOWT. p o rter & FULLER THIS th esis 1958,1959 UPPER UPPER 1964

INTERLAKE BEDS in t e r l a k e b e d s

MIDDLE M ID O LE N NTERLAKE INTERLAKE CO BEDS BEDS or =D

C/) LOWER LOWER Ü M INTERLAKE INTERLAKE A

■ ' i — BEDS BEDS N

STONEWALL < FORMATION STONY 6 o GUNTON JO o > STONY GUNTON MTN. o o MEMBER < o MTN. Û: MEMBER Q FM. STONY MTN. STONY MTN. FM. > o SHALE MEMBER RED RIVER FORMATION AT

FIGURE 2 fig u r e 2 NOTE THAT IN THIS THESIS THE INTBRLAKE GatnmO'Ray Definition of the IS REDUCED TO FORMATION RANK, AND THE Inierioke Formation STONEWALL FORMATION NOT RECOGNIZED. Eastern Montano -J the Vdlliston Basin in eastern Montana between the Stony Mountain

Formation of Upper Ordovician age (Ross, 1957) suid the Ashem

Fonaation of Middle Devonian age (Baillie, 1955) herein desig­ nated as the Interlake Fomation. The fomation boundaries are defined in Figures 2 and 3 end on cross-sections A—A’ and B—B*

(Figures 5 and 6 in pocket).

Subdivision And Nomenclature

The Interlake Formation is divided infomally into lower, middle and upper beds using the nomenclature introduced by Porter and Fuller

(1959# p. 141). The upper and lower boundaries of the Interlake

Formation shown in Figures 2, 3# 4, 5 and 6 may be recognized in saoqples and cores by lithologie changes and on gamma ray logs made by measuring the natural radioactivity of the rocks in the formation.

The subdivision of the Interlake Formation is accon^lished mainly on the basis of the characteristic gamma ray response obtained from the thin beds of gray, green, red and maroon shale, and very argillaceous dolomite that occurs interspersed within the major rock types. These beds are remarkably widespread and persistent in composition. The source of the argillaceous material in the thin but widespread beds is not certain, but in one instance a volcanic origin is suggested. An

X-ray analysis of shale cored at a depth of 9585 feet in the Shell Oil

Company-Northern Pacific Railroad Richey No. 1, Sec. 19# T. 23 N., R.

50 S ., Dawson County, Montana shows th is approximately 10-foot thick bed to consist of nearly pure illite . Hower (1964) suggests that this

(16) oecurrence of Illite possibly represents diagenetically altered volcanic ash* The most tenable explanation for the extraordinarily widespread thin beds which contain relatively high concentrations of

radioactive material seems to be that volcanic ash distributed through

the atmosphere by winds settled into the seas and altered to illite with an accony>anying concentration of potassium a fra c tio n of which is

radioactive. Thus, the lateral continuity of these beds may be

ejqplained by allowing distribution of the original material through

the atmosphere and synchronous deposition of the material on the sea bottom over large areas. These shale beds and argillaceous dolomite beds may provide the nearest approach to iso time lines that can be

obtained in regional geologic woiic in the Williston Basin, In some

areas such as on the Cedar Creek anticline where continuous cores provide the infonaation, the lower and middle beds may be defined on the basis of lithology. In most instances however, the definition provided by mechanical logs is required for correlations between wells.

Age

Although the Interlake Fonnation as found in the subsurface of eastern Montana contains but few f o s s ils , i t is agreed by a l l w riters who have studied the fauna that the formation represents a part of the Silurian System, Porter and Fuller (1959) suggest that the

Interlake represents deposition during at least Early and Middle

Silurian, and that in the central part of the Williston Basin

(northwestern North Dakota) where the upper beds of the Interlake are

(17) present they suggest a Late (?) Silurian age for these beds (Porter and Fuller# 1959» p. 179)# Paleontological woric by Brindle (1960# p. 22) and by Boss (1957) tends to substantiate the Early and Middle

Silurian ages* As yet no faunal study of the upper beds has been published# and the suggested Late (?) Silurian age remains in doubt.

Continuous marine deposition in the basin center throughout most of the Ordovician and Silurian Periods is indicated but not proven.

Steam* s work (1956) with the fauna from the outcrops in Manitoba raises the question as to whether the lower portion of the Interlake

Fondation is all Silurian in age or is in part Ordovician in age.

Lithology Of The Interlake Fomation In Montana

The classification of dolomites advanced by Leighton and Pendexter

(1962# pp. 53-58) is used in describing the rocks of the Interlake

Formation. This system with its basis of conqpositional groups modified by textural terms seems best suited to describing the generally non-fossiliferous # but textured dolomite rocks of the

Interlake in Montana. Carbonate rock having 50 to 90 per cent dolomite content is called calcareous dolomite# and if the rock is over 90 per cent dolomite it is temed dolomite. In actual practice this division is approximated by observing the amount and rate of effervescence of rock samples in dilute hydrochloric acid.

(18) Lover Beds

The predominant rock types making up the lower beds of the Inter- lake Formation in Montana are gray to light-brownish gray to near white* medium- to very fine-grained* calcareous dolomite* and finely

laminated* gray to near white* finely micro grained dolomite (see Plate

I and Figure 4). Occasional thin beds of anhydrite* and dolomitized

fragmental limestone occur (Plate II) mainly in about the north­

eastern quarter of the thesis area. The finely laminated or banded

dolomites are possibly of algal origin. Similar appearing rocks are

described by Baillie (1951» P* 20) as "stromatolitic" rocks commonly

found in mounds and small reefs at outcrop in Manitoba.

)&ddle Beds

The rocks of the middle beds of the Interlake Formation are similar

to those of the lower beds in the southern part of the area* for

example in the Shell Oil Company L ittle Beaver Bo. 1* Sec. 13» T. 4 B.*

R. 61 S.* Fallon County* Montana. To the north however* in the Shell

Oil Company-Borthem P acific Railroad Bo. 1* Sec. 19» T. 23 B.* R. 50

S.* Dawson County* Montana the dolomite of the middle beds is predomi­ nantly anhydritic* dolomitised fragmental limestone containing traces

of skeletal debris (Plate IF). Also present are thin beds of micro­ grained dolomite (Plate III).

(19) Upper Beds

In Montana the upper beds of the Interlake Formation are found only in the northeastern part of the state just west of the Montana-North

Dakota border* These dolomite and calcareous dolomite beds are mainly dolomitized fragmental limestones with interbedded thin micrograined dolomite and argillaceous micrograined dolomite* Porter and Fuller

(1959$ P# 165) report that in the Besson anticline area of the central WLUiston Basin in North Dakota the upper beds contain lenses of

coarse, frosted sand grains but that mainly they are made up of highly dolomitized vuggy fragmental limestones alternating with dense, porcellaneous, highly anhydritic dolomites*

Porosity Development In The Interlake Formation

Throughout the Interlake Formation occasional porous zones occur*

In the Carter Oil Company Margaret Nelson No* 1, well drilled in Sec*

4, T* 37 N*, R* 53 E*, Sheridan County, Montana vugs and cracks in a gray to brownish gray, fine-grained calcareous dolomite may be observed near the top of the middle beds* Dolomite crystals up to a millimeter in length have grown in vugs and fractures in the fine-grained rock*

Also, many of the vugs are partially filled with authigenic crystals of anhydrite. Porous zones such as this are believed to be due to dissolution of calcium sulfate and possibly sodium chloride during pre-

MLddle Devonian weathering of the Interlake Formation* Undoubtedly fracturing has played an important part in forming porous zones in (20) the rocks particularly in areas where structural adjustments have taken place such as along the Cedar Creek Anticline. It is certain however, that the rocks which formed from organic skeletal debris on

and around reefs andL mounds have been the most susceptible to dis.

solution and now make up the main porous bodies occurring in the

Interlake Formation. Thin beds of anhydrite occur locally, but more

commonly anhydrite is found as isolated crystals within the micro,

grained dolomite, as cavity-filling masses and as interstitial material

in the dolomites of coarser texture. Almost without fail the coarser-

textured dolomites found in the Interlake Formation in Montana are

anhydritic to some extent and porosity development seems to be a

function of how much of the anhydrite has been removed by solutions.

Interbedded occasionally with the thicker dolomite beds are thin

(less than ten feet thick) beds of very argillaceous dolomite, and

gray, green, brown and maroon shale. There is probably very little permeability to fluids through these shales except where they have been extensively fractured, and even then in many cases observed in

this study the fractures are filled with anhydrite.

Photographic Illustrations

The photographia of thin sections which follow (Plates I through V) show the textural variations which are found in the dolomites of the

Interlake Formation in Montana. These photographs were made with unpolarized light, and were enlarged from 4-inch by 5-inch negatives.

Figure 3 indicates the well and depths of samples used for thin sections.

(2 1 ) SHELL OIL CO.-N. P. R. R. RICHEY NO. 1 SE NW NW SEC. 19, T.%)N., R.50E. DAWSON CO., MONTANA

?ate

P late 17

Plate I:

Plate H

fig u re 3 SHOW INC SAMPLE DEPTHS VOR PHOTOGRAPHIC ILLUSTRATIONS

(22) t-I-

PLATE I (Thin section photography 17*7)

Finely micrograined dolomite* Common in the lower beds of the Interlake Formation* Richland County» Montana (See Figure 3 for well and depth locations)

(23) PUTS H (Thin section photograph* I8«9)

Ddonite (dolonltitad fragmental and pelletoidal limestone with relict original texture). White patches are anhydrite* and anhydrite surrounds the grains. Prom an unusual ocourrenoe In the lower beds of the Interlake fbxmatlon, Hlohland Gountj* Montana. (See Figure 3 for well and depth locations) (24) PUTE m (Thin section photograph» X8.4)

Finely micrograined dolomite with typical laminated structure. Common in the lower beds of the Interlake Foraation» Richland County» Montana. (See Figure 3 for well and depth locations)

(25) X

. / V ;

W m È f ‘ ï ;'>

mctÈiy (Thin section photograph» X9«6)

Dolomite (doXomitlaed algal and m ioritlc limestone) White patches are anhydrite» and anhydrite surrounds most grains» Gommon in the middle beds of the In terla k e Foxmation» Richland County» Montana» (See Figure 3 for well and depth locations) (26) i

*4 >:T TW. ' - '-

FUTB_? (%in section photograph» X7«7)

Finely micrograined, slightly argillaceous dolomite and dolomite breccia# White veins and patches are of anhydrite# From the lowermost part of the Devonian section just above the Silurian-Devonian unconfonmity, Richland County, Montana (See Figure 3 for well and depth locations)

(27) RELATION OF THE INTERLAKE FORMATION TO ADJACENT STRATA

Lower Contact

Carlson and Eastwood (1962, p. I 3) describe the lower contact of the

Interlake Formation in the subsurface of North Dakota as conformable and gradational w ith the underlying formation* However, in eastern

Montana this relationship does not seem to be the case* A sharp lithologie break similar to that described by Baillie (195^) for the base of the Interlake in Manitoba is present along the southwestern basin margin in Montana* This contact is very well shown in cores from the Shell Oil Company L ittle Beaver No* 1, Sec* I 3, T* 4 N*, R*

61 E*, Fallon County, Montana* Here the light gray to nearly white, fine-grained, calcareous dolomite of the Interlake Formation contrasts shaiply with the underlying maroon and greenish-gray, argillaceous dolomite of the upper part of the underlying Stony Mountain Foi;mation*

This contrast in lithology exists along the Cedar Creek anticline and to the west of it, but is much less noticeable in the northeastern part of the state* Rader (1953a) notes that in the southernmost occurrence of rocks of the Silurian System the dolomites are quite sandy with the clastic ratio increasing southward* The relationship seems to be that the Interlake Formation conformably overlies the

Stony Mountain Fomtation in the central part of the Williston Basin, but to the west and southwest along the Cedar Creek anticline and to the west of it an unconformity develops between the two formations*

(28) upper Contact

The upper contact of the Interlake Formation Is an unconformity which is present throughout the VUliston Basin. Baillie (1955. P*

588 ) describes this post-Silurian pre-Middle Devonian unconformity as

*one of the greatest unconfonoities in the Paleozoic succession*.

The eroded surface of the Interlake Fomation is covered by the

Ashem Formation (Baillie, 1950, pp. 10-12) of Middle Devonian age.

Along the Cedar Creek anticline and along the southwest margin of the

W illiston Basin in Montana the Ashem Formation co n sists of pale green,

red and maroon, dolomitic shale and claystone containing an estimated

five per cent or less of medium- to coarse-grained, rounded to sub­

rounded quarts grains. Pieces of the underlying light gray, fine­

grained dolomite of the Interlake Formation are enclosed in a very

pale green, dolomitic shale matrix in the lower part of the Ashem in

many of the cores examined* In some cores of the Interlake Formation

evidence of erosion and collapse may be seen in the upper part of the

formation. In the Lion Oil Company Knight Bo. 1, Sec. 29, T. 14 N.,

R. 60 S., TAbaux County, Montana the American Stratigraphie Company

reports the top 60 feet of the Interlake contains considerable chert,

secondary quartz, and a few coarse, rounded quartz grains. Similar

rounded quartz grains and angular grains of chert are found in the

overlying Ashem Formation. Because of their relatively high re­

sistance to mechanical and chemical weathering the grains became

concentrated on the upper surface of the Interlake Formation during

the long erosional period prior to deposition of the Ashem, and were

(29) then included in the Ashem Formation* The common occurrence of the rounded quartz grains is limited to the southwestern part of the mapped area along and adjacent to the Cedar Creek anticline* The distribution of these quartz grains suggests their source lay to the south and southwest in the Black Hills region of South Dakota and

Wyoming where basal sands of the Whitewood Dolomite and Big Horn

Dolomite of Ordovician age, and older formations were being eroded*

In the northeastern p a rt of Montana the colors of the Ashem become much darker and only in thin sections and on the thin edges of rock chips is the reddish coloration observed* The dolomite fragments of the underlying Interlake Formation may almost always be observed in cores, but the rounded quartz grains found to the south are not present*

DISTRIBUTION OF THE INTERLAKE FORMATION IN M)NTANA

Rocks of the Interlake Formation of the Silurian System occur in

Montana only in the subsurface of eastern Montana* From about the southeast com er o f Montana the buried erosional edge of the In te r­ lake Formation strikes northwesterly across the state to intersect the Canadian border about one third of the way across the state from east to west* The triangular-shaped area (about 40,000 square miles) formed by th is lin e and the north and ea st borders o f Montana makes up most of the western flank of the WLUiston Basin.

The lower, middle and upper beds of the Interlake Formation in

Montana are isopached on Figures 7, 8 and 9 respectively. These informal units thin westward form the Dakotas due to both deposi-

(30) tional thinning and post-Silurian pre-Middle Devonian erosion. The erosional edge of the next higher unit is shown on each isopachous map so that areas where the units have not been thinned by erosion may be defined. Figure 9» is also a paleogeologic map of the post-

Silurian pre-Middle Devonian surface* and shows the present distri­ bution of the lower* middle and upper beds of the Interlake Form­ ation in the subsurface of eastern Montana.

The lower beds of the Interlake Formation isopached on Figure 7 are more widespread than the middle or upper beds since they were least

affected by erosion. The lower beds attain a thickness of about 200

feet in northeastern Montana, and thin depositionally to about 100

feet some 120 miles to the west where pre-Middle Devonian erosion becomes a factor in thinning the unit from the top down.

In most of eastern Montana the top of the Interlake Formation is

found to be coincident with the top of the middle beds. The middle beds (Figure 8) gradually thin to the west from a thickness of about

350 feet in the northeastern co m e r of the state. Irregularities in

the isopachs are believed to represent both depositional thinning due

%o persistent structural highs* and subsequent deeper erosion across

the highs. Comparing the isopachous maps with the present subsurface

structure shows coincidence of unit thinning with structurally higher

areas in most instances. Where this occurs it suggests the structures may have been synchronous with deposition. The shale bed marking the boundary between the middle and lower beds may have been deposited as a volcanic ash fall covering most of the V&lliston Basin* and thus may

represent an isotime line. In Montana the upper beds occur in only a

(31) relatively small area in the northeastern comer of the state

(Figure 9)* The beds attain a thickness of around 200 feet in easternmost Montana* but in the basin center in North Dakota are about 570 fe e t th ick (Carlson and Eastwood* 1962* p« 12)«

Figure 10 is a structure contour map on the base of the Interlake

Formation (top of the Stony Mountain Formation). An increase in the rate of dip toward the basin center shown on this map indicates a possible hinge line that coincides approximately with the western erosional edge of the upper beds* Referring to the isopachous map of the middle beds (Figure 8) the increase in the rate of deposi tional thinning of th is u n it occurring in the northeast com er of Montana in central Sheridan County tends to confina that a hinge line was active here at least during d^ositioi| of this part of the Interlake Formation*

The location of this hinge line is important in guiding petroleum exploration in northeastern Montana* for it is along such features that organic carbonate buildups may occur and act as reservoirs for petroleum* Hinge lines are persistent structural features and this one could also be important in the overlying Devonian and Mississippian sediments for forming a trend of potential petroleum reservoir rocks*

MODERN IDEAS CONCERNING CARBONATE DEPOSITS AND THEIR CLASSIFICATION

Before an interpretation may be made as to the depositional conditions of the Interlake Formation ideas concerning the mechanisms

( 32) by which carbonate rocks attain such a wide range of texture must be considered* Also* since the Interlake consists almost entirely of dol­ omite and calcareous dolomite# modem thinking about the formation of dolomite must be examined and considered*

Bam and Pray (1962# pp* 2-19) review modem ideas concerning both descriptive and genetic classifications of carbonate rocks* They point out that most carbonate sediment is of intrabasinal origin# and has been transported only short distances* In contrast# particles making up sandstones and shales have usually been transported into the depositional basin* Thus# the nature of carbonate deposits is com­ pletely dependent upon processes within the basin itself* Not only the structural configuration of the basin# but also the amount of organic activity the waters will support are significant in determining the nature of the sediments which will be formed* The obvious organic

contributions to caiiwnate sediments are becoming better known and understood# but the contributions to sediment volume by the burrowing and sediment-ingesting organisms are not so easily evaluated* Ham and Pray (1962# pp. 4-^) point out the following concerning detemiining the origin and environment of deposition of carbonate rocks:

(1) An interpretation based on texture as to the origin and

environment of deposition of carbonate rocks# including

carbonate elastics conç>osed of transported material# is much

more difficult than one for sandstones and shales;

(2) Degrees of sorting are not always analogous to sandstones for

the equal-size clasts of a well-sorted carbonate rock may be

reflecting mainly the growth size of the source organisms and

(33) be unrelated to sorting due to wave and current action;

(3) Even the formation of oSlites may depend upon some little

understood organic operation;

(4) Perhaps the most significant barrier to genetic interpretation

of carbonate textures is the marked susceptibility of carbonate

rocks to post-depositional changes through solution and re-

crystallization ;

(5) In many instances the origin of the finer grained limestones

may be impossible to deteimine.

Modem classifications of carbonate rocks (Ham* et al* 1962) attach significance to the presence or absence of lime mud or micrite (Folk*

1959) in interpreting the wave and current energy present during de­ position of the sediment* The absence of micrite in rocks which have retained their depositional texture is considered as signifying turbu­ lent conditions of deposition* The application of the concept that carbonate clasts are subject to the same mechanics of deposition as siliceous particles to the classification and interpretation of carbon­ ates has been made by Ham* et al* (1962), and particularly by Folk

(1962)* Powers ( 1962)* and previously by Bramkamp and Powers (1956)» and Folk (1959)* However, Powers (1962* p. 1?0) s ta te s th a t a d ire c t relation between textural rock types and water depths at the place of deposition does not exist* but that the energy available to move the sedimentary material is the significant factor and this energy level is not always dependent upon water depth* Examples of mud deposition along a shore line* and current-rippled calcareous sand accumulating on the ocean bottom in 6OOO fe e t of w ater are cited by Powers*

(34) DOLOMITE AMD DOLOMITIZATXON

A finding of Powers (1962, p. 189) pertinent to considerations of the Interlake Formation of this thesis is that lime muds (micrites) are much more susceptible to dolomitisation than the coarser-grained

rocks* It is also significant that in some instances Powers (1962, p.

141) was able to demonstrate that the formation of authigenic anhydrite had proceeded dolomitisation*

Cloud and Barnes (1957# pp. 182-186) summarise the ideas on dolomite formation, and reiterate the points which they believe to be especially important* Pertinent here are the following :

(1) The sea appears to be the only adequate source for the mag­

nesium in laterally extensive dolomite beds;

(2) ExaRq>les of direct precipitation of dolomite are unknown in

recent sediments, but direct precipitation is theoretically

possible;

(3) Due to its metastable form aragonite may be more easily

dolomitised than calcite;

(4) E^qperimental evidence indicates that dolomitisation takes

place most readily under conditions of elevated temperatures*

Thus, a shallow-water origin for dolomite is indicated, and

shoaling and restriction of marine waters favors dolomitisation *

doud and Barnes (1957# P. 184) "favor the deduction that penecon- temporaneous or later diagenetic alteration of calcium carbonate sed­ iments is a more probable dolomitisation process than is primary precipitation"* (35) A Process For Dolomitlzatlon

Adams and Rhodes (1960$ p . 1912) provide a workable th e o re tic a l b asis for applying the idea that ^restricted lagoon-open ocean couples are essential for the formation of extensive dolomites** # They demonstrate with examples from the Pendan Basin of Texas and New Mexico that lime­

stones which were closely associated with evaporite lagoons at the time of their deposition have usually been dolomitized, but those lacking the evaporitic association usually have not. Adams and Rhodes (I960, p . 1919) conclude that dolomitization of limestone deposits apparently occurs as a nomal process on barred shelves in close association with evaporite deposition. They believe that heavy hypersaline brines formed in the restricted lagoonal areas seeped downward through the unconsolidated carbonate particles of the lagoon floor bringing about dolomitization by replacement of calcium by magnesium in the carbonate structure. The magnesium-calcium ratio in the heavy brine is reasoned to have been considerably greater than in normal sea water.

A CLASSIFICATION OF DOLOMITES

Leighton and Pendexter (1962, p. 57) have set forth criteria for defining two distinct groups of dolomitic rocks based on textural character and rock associations. These groups are: (1) **dolomites in ev ap o ritic sequences**, and (2) those th a t are c le a rly the **product of dolomitization of limestones**. Characteristics which they ascribe to dolomites in evaporitic sequences include the following:

(36) (1) Generally finely micrograined;

(2) Associated with anhydrite# chert and micritic limes;

(3) May be laminated or brecciated;

(4) No retention of relict limestone texture.

The characteristics which they have found to be common in dolomitized limestones are;

(1) Coarseness of texture;

(2) Generally associated with limestone sequences;

(3) Fossil molds may be present;

(4) Relict limestone textures are present and gradational

sequences showing increased development of dolomite may

be observed* Both groups of dolomite rocks are present in the Interlake Formation in eastern Montana# but those having the characteristics of group one

(above) are the most common*

CONDITIONS OF DEPOSITION OF THE INTERLAKE FORMATION IN MONTANA

Keeping in mind the ideas which have been discussed concerning carbonate deposition and dolomitization# and considering also the interpretations of Baillie (1951)» Rader (1953a» 1953b)» and

Andrichuk (1959)» the depositional environment of the Interlake Formation in Montana may be described as follow s;

During deposition o f the In terlak e Formation eastern Montana formed

(37) the extensive western shelf of the slowly subsiding Williston Basin*

Shallow* warm* marine and restricted marine waters covered most of the eastern half of Montana* Land areas which were only slightly elevated above sea level were far removed from the present occurrence of the

Interlake Formation* The amount of terrigenous material being trans­ ported into this portion of the basin was extremely small. Carbonates were the major sediments being deposited* and deposition is thought to have been continuous throughout the region where the Interlake is present in Montana* The same narrow range of depositional environments existed for hundreds of miles from any given locality in the sea*

Throughout much of eastern Montana a very shallow water* highly oxidizing environment of deposition is suggested by the low content of organic material in the rocks of the Interlake Formation*

Organic buildups of carbonate material occurred commonly across the entire shelf forming a complex of patch reefs and algal mounds* Algal mounds* patch reefs and underwater dunes so restricted circulation of sea water that only surge channels were kept clear of lime mud deposits*

During much of the year winds were probably very light because the seas were so extensive that only small contrasts in air-mass temperatures existed* During the long calms mounds and reefs choked on their own debris*

It is reasoned that large vertical buildups of organic material did not develop due to the extremely shallow-water conditions and the slow subsidence of the shelf region* Rather* as suggested by Baillie (1951» p. 56) it is likely that lateral growth of both algal and coral reefs was the case* Apparently the lagoonal areas of restricted circulation

(38) were of larg e area l extent however, fo r in Montana most of the sediments of the Interlake Formation must be classified as "dolomites in evapo­ ritic sequences" using the classification of Leighton and Pendexter

(1962, p. 57)* It is believed that these rocks fomed by the penecon- temporaneous dolomitization of the widespread lime mud deposits of the lagoons, Dolomitization by seepage refluxion as advanced by Adams and

Rhodes (I960, pp* 1912-1920), and King (19^7, pp. 470-47?) i s accepted as a reasonable mechanism to account for the almost complete dolomit­ ization of the rocks* The general lack of high rank salts in the rocks of the Interlake Foimation indicates the environment of deposition was penesaline in the lagoons, and probably only reached concentrations of a saline environment (above 353 parts per thousand) in local highly restricted branches of the lagoons.

The lower beds of the In terlak e Formation in Montana have mainly the characteristics of "dolomites in evaporitic sequences", and were de­ posited almost entirely in a penesaline environment. The abundant micrograined dolomite found in the lower beds is inteipreted as representing original lime mud deposits which accumulated in the low energy environment of an extensive, barred, shallow-water shelf region.

The dolomites of the middle beds are generally coarser-grained but they too appear to be mostly related to penesaline conditions of de­ p o sitio n , Adams and Rhodes ( i 960, p, 1915* Fig* 3) define a penesaline environment as one having salinity concentrations between 72 and 353 parts per thousand. This compares with open ocean salinities of around

35 parts per thousand. Thus, even though the middle beds change in character from south to north and from west to east in Montana, be-

(39) coming coarser-grained and grading toward rocks having the character­ istics of dolomitized limestone* they still fall mostly in the range of the penesaline environment*

In Montana the rocks of the upper beds of the Interlake Formation are found only in the northeastern part of the state. They have many of the characteristics of dolomitized limestone or the Qroup 2 dolomites of Leighton and Pendexter (1962* p. 57)* and are reported to be pelletoidal and fragmental in nature by Carlson and Eastwood

(1962* p . 15) where the beds reach their maximum thickness in the basin center in North Dakota. The American Stratigraphie Company describes fossil bioclastic debris in the rocks of the upper beds in

Montana. Probably due partly to the coarseness of their pelletoidal and fragmental grains they have not been as completely dolomitized as the lower and middle beds* but possibly of more inportance is the fact that the upper beds were deposited in a considerably less restricted environment than the middle and lower beds. The erosional remnant of upper beds that we find today was probably not deposited in the restricted part of the shelf* but was in and near a true reef- barrier environment.

(40) BIBLIOGRAPHY

Abrassart^ C* P ., e t a l . , 1958, Montana o il and gas f ie ld s , a symposium: Billings Geol. Soc«, Billings, Montana, 24? p.

Adams, John A. S. and Weaver, Charles E«, 1958, Thorium—to —Uranium ratios as indicators of sedimentary processes: example of concept of geochemical facies: Am. Assoc. Petroleum Geologists Bull., V. 42, pp. 387-^30.

Adams, J. E., and Rhodes, M. L., i 960, Dolomitization by seepage refluxion : Am. Assoc. Petroleum Geologists Bull., v. 44, pp. 1912- 1920.

Alpha, Andrew G., 1958, Tectonic h isto ry o f Montana in Montana o il and gas fie ld s , a symposium: B illin g s Geol. Soc., B illin g s, Montana, pp. 10- 31.

American Commission on S tratig rap h ie Nomenclature, I 96I , Code of stratigraphie nomenclature: Am. Assoc. Petroleum Geologists Bull., V. 45, pp# 645-665. Andrichuk, J. M., 1959, Ordovician and Silurian stratigraphy and sedimentation in southern Manitoba, Canada: Am. Assoc. Petroleum Geologists Bull., v. 43, pp. 2333-2398.

Baillie, A. D., 1950, Devonian geology of Lake Manitoba—Lake Winnipegosis area: Manitoba Dept, of Mines and National Resources, Manitoba Mines Branch, Pub. 49-2.

Baillie, A. D., 1951, Silurian geology of the Interlake area, Manitoba; Manitoba Dept, o f Mines and National Resources, Manitoba Mines Branch, Pub. 50-1, 82 p.

Baillie, A. D., 1952, Ordovician geology of Lake Winnipeg and adjacent areas, Manitoba: Manitoba Dept, of Mines and National Resources, Manitoba Mines Branch, Pub. 51-6.

Baillie, A. D., 1955, Devonian system of Williston Basin: Am. Assoc., Petroleum Geologists Bull., v. 39, pp. 575-629.

Bishop, Margaret S., I960, Subsurface mapping: John Wiley and Sons, Inc., New York, I 98 p.

Borden, R. L., 1955, Ordovician and Silurian stratigraphy of the southern part of the Prairie Provinces, Canada: Jour. Alberta Soc. Petroleum Geologists, V. 3, no. 10, pp. 169-177.

(41) Bramkamp » R* A», and Pawers, R. W#, 1956» d a s s i f l cation of Arabian carbanate rackss Goal* Soc* America Bull*» v. 69» PP* 1305-1316*

B rindle 9 J* E*, I960» The faunas a f the Lawer Paleozoic carbonate racks In the subsurface of Saskatchewan: Saskatchewan Dept, af Mineral Resources» Report Mo. 52» p* 45*

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daxk» Thomas A.» and Steam» Colin W.» I96O» The geological evolution of North America: The Ronald Press Company» New York» 434 p.

Cloud» Preston E.» Jr.» and Barnes» Virgil E.» 1957» Early Ordovician sea In central Texas» In Treatise on Marine Ecology and Paleocology: v« 2» Geol* Soc. o f America Memoir No. 67 » pp* 163-214. Dallmus» K* F. » 1956» Mechanics of basin evolution and Its relation to the habitat of oil In the basin» In Habitat of Oil: Am. Assoc. Petroleum Geologists» Tulsa» Oklahoma» pp. 883-931*

Dunbar» Carl 0.» and Rodgers» John» 1957» P rinciples of stratig rap h y : John Vfiley and Sons» Inc.» New York» 356 p.

Eardley» A* J.» I 963» Structural geology of North America: Harper & Brothers» New York» 624 p.

Folk» Robert L.» 1959» Practical pétrographie classification of lime­ stones» Am. Assoc. Petroleum Geologists Bull.» v. 43» pp. 1-36

Folk» Robert L.» 1962» Spectral subdivision of limestone types» In c la s s ific a tio n of carbonate rocks» a symposium: Memoir 1» Am. Assoc. Petroleum Geologists» Tulsa» Oklahoma» pp. 62-84.

Fuller» J. G. C. M.» 1961» Ordovician and contiguous formations In North Dakota» South Dakota» Montana» and adjoining areas of Canada and United States: Am. Assoc. Petroleum Geologists Bull.» V* 45» pp. 1334-1363* GlHuly» James» 1949» Distribution of mountain building In geologic time: Geol* Soc. of America Bull.» v* 60» pp. 561-590.

Halner» J. L.» 1956» The geology of North Dakota: Noz*th Dakota Geological Survey Bull. No. 31» 46 p.

Ham» W. E.» and Pray» L. C.» 1962» Modem concepts and c la s s ific a tio n s of carbonate rocks» In Classification of Carbonate Rocks, a sym­ posium: Memoir 1» Am. Assoc. Petroleum Geologists» Tulsa» Oklahoma» pp. 2-19*

(42) Ham, W, £•, et al., 1962, dassification of carbonate rocks, a symposium* Memoir 1, Am» Assoc. Petroleum Geologists, Tulsa, Oklahoma, 279 P*

Hower, John, 1964, Personal communication, discussion at University of Montana, Missoula, Montana.

Kindle, E. M., 1914, The Silurian and Devonian section of western Manitoba: Canadian Geological Survey Summary Report No. 1912, pp. 247-261.

King, R. H,, 1947 , Sedimentation in Permian Castile Sea: Am. Assoc. Petroleum Geologists Bull., v. 31, pp. 470-477*

Krumbein, W« C., and Sloss, L. L., 196], Stratigraphy and sedimentation: Second Edition, W. H. Freeman and Company, San Francisco, California, 660 p .

Kupsch, W. 0., 1953* Ordovician and Silurian stratigraphy of east-central Saskatchewan: Saskatchewan Geol. Survey Rept. No. 10.

Ladd, Harry S, et al., 1957* Treatise on marine ecology and paleocology: V. 2, Geol. Soc. America Memoir No. 67 .

Leighton, M. W., and Pendexter, C. I 962, Carbonate rock types, in c la s s ific a tio n of carbonate rocks, a symposium: Memoir 1, Am. Assoc. Petroleum G eologists, Tulsa, Oklahoma, pp. 33-61.

McCabe, William S., 1954, Vdlliston Basin Paleozoic unconformities: Am. Assoc. Petroleum Geologists Bull., v. 38 , p. 1997.

North Dakota Geological Society, Saskatchewan Geological Society, 1956, WLUiston Basin Symposium: BismaxSc, North Dakota.

(Nculitch, V. J., 1943, The Stony Mountain Foimation of Manitoba: Trans. Roy. Soc. Canada, 3rd ser., v. 37* sec. 4, pp. 59-74.

Patterson, J. R., 1961, Ordovician stratigraphy and correlations: Am. Assoc. Petroleum Geologists Bull., v, 45, pp. 1364-1373#

P o rter, J . W., and F u lle r, J . G. C. M., 1958, Lower Paleozoic rocks of northern WiUiston Basin and adjacent areas; Read before the Second International W lliston Basin Symposium, Regina, Saskatchewan, Canada, April 23, 1958.

Porter, J. W., and Fuller, J. 0. C. M., 1959* Lower Paleozoic rocks of northern WiUiston Basin and adjacent areas: Am. Assoc. Petroleum Geologists Bull., v. 43, no. 1, pp. 124-189.

(43) Powers, R. W#, 1962, Arabian Upper Jurassic carbonate rocks, in classification of carbonate rocks, a symposium: Memoir 1, Am. Assoc. Petroleum G eologists, Tul% , Oklahoma, pp. 122-192.

Bader, M. T., Jr., 1952# Ordovician and Silurian carbonates of the central VUliston Basin: Billings Geol. Soc. Guidebook, 3rd Annual Field Conference, pp. 48 -55*

Rader, H* T., Jr., 1953a# Silurian stratigraphy of the WiUiston Basin, Abstract: Geol. Soc. of America Bull., v. 64, p. 1552.

Rader, M. T., Jr., 1953b# Ordovician, Silurian, and Devonian stratigraphy of a portion of northern Montana; Billings Geol. Soc. Guidebook, 4th Annual Field Conference, pp. 64-66.

Rich, J. L., 1951# Three critical environments of deposition and criteria for recognition of rocks deposited in each of them: Geol. Soc. America Bull., v. 62, pp. 1-20.

Ross, R* J., 1957 # Ordovician fossils from wells in the WiUiston Basin, eastern Montana: U. S. Geol. Survey Bull. 1021-M, pp. 439- 510. Sandbery, C. A., and Hammond, C. R., 1958# Devonian ^stem in VUliston Basin and central Montana: Am. Assoc. Petroleum Geologists Bull., v. 42, pp. 2293-2334.

Schlumberger Well Surveying Corporation, 1958, Introduction to Schlufflberger well logging: Document 8, Houston, Texas, 1?6 p.

Schultz, £« H., 19589 The Ordovician-Silurian contact in the VUliston Basin: Billings Geol. Soc. Guidebook, 9th Annual Field Conference, pp. 44-47*

Sloss, L. L., Dapples, E. C., and Krumbein, W. C., I960, Lithofacies Maps, an atlas of the United States and southern Canada: John VUey and Sons, Inc., New lozic, 108 p.

Sonnenberg, Frank P ., 1956# Tectonic p attern s o f cen tral Montana : Billings Geol. Soc. Guidebook, 7th Annual Field Conference, pp. 73^1* Stanton, M. S., 1953* Ordovician# Silurian# and Devonian stratigraphy of western Saskatchewan: Billings Geol. Soc. Guidebook, 4th Annual Field Conference, pp. 59-63*

Steam, C. W., 1956, Stratigraphy and paleontology of the Interlake Group and Stonewall Formation of southern Manitoba: Geol. Survey of Canada, Memoir 281, 162 p.

(44) Swartz, C. K*, et al*, 1942, Correlation of the Silurian formations of North America, Geol. Soc. America Bull., v. 53# PP* 533-538*

Thom, W. T., Jr., 1923» %e relation of deep-seated faults to stru c tu ra l featu res of c e n tral Montana : Am. Assoc. Petroleum Geologists Bull., v. 7» PP, 1-13# Twenhofel, W. H., et al., 1954, Correlation of Ordovician formations of North America: Geol. Soc. America Bull., v. 65» PP* 247-298.

Tÿrrell, J. B., 1892, Summary report of a geological examination of the Lake Winnipeg region, Manitoba: Canadian Geological Survey Summary Report No. 1891 (Annual Report No. 5» PP* 19-25).

Webb, J . B ., 1954» Geological history of the plains of western Canada, in western Canada sedimentary basin: Am. Assoc. Petroleum G eologists, Sÿnqposium, pp. 3-28.

Hyllie, H. R. J., 1957» The fundamentals of electric log interpretation: Academic Press In c ., New York.

(45) APPEHDIX A

Compilation Of The Subsurface Data Used In

Mapping The In terlak e Formation

Frank K* Gibbs

1954

(46) STONY KTN. NAME AND LOCATION ELEV. INTERLAKE FCRMATIÛN FŒÎMATION OF WELL K. B. UPPER HPns MIDDLE BEDS LOWER HEDg

DEPTH THICKNESS DEPTH THICKNESS DEPTH thickness DEPTH SU^SEA TOP TOP TOP TOP EI£V.

Sun No. 1 C KE NE Sec, 10, T. 37 N,, R. 57 E. Sheridan C o., Mont. 2275 9450 96 9546 296 9832 196 16,018 -7743 (2 ) Carter Oil-Kelson #1 NE NW Sec. 4 , T, 37 N R. 53 E ., Sheridan Co., Mont. 2504 M issing 9173 299 9472 194 9,666 -7162 (3 ) Ajnerada-Grinkier #1 NE Sec. 20, T.37N. R. 52 E ., Sheridan C o., Mont. 2477 M issing 9112 240 9352 184 9,536 -7059 (4 ) Richfield-Kadoc

C m SW Sec. 31, T, 37 N ., R, 49 E ., D aniels Co., Mont, 2551 M issing 8653 211 8864 Not pens trated (5 ) Gulf Govt. #1, SW SW SW Sec. 31, T. 37 N ., R. 34 E ., P h illip s Co., Mont. 2384 M issing M issing 6180 70 6,250 -3866 STONY KTN. NAME AND LOCATION ELEV. INTERLAKE FORMATION FORMATION OF WELL K.B. UPPHi BF-fK MIDDLE RFTK LOWER

DEPTH THICKNESS DEPTH THICKNESS DEPTH thickness DEPTH SU>SSA TOP TOP TCP TOP £ 1 £ V .

(6) United Western 0 4 G 16-1 State, C SE SE Sec. 16, T.37 N., R.30 E.,Phillips ''Co Montana 2798 M issii g M issl ig 6115 ? 55 7 6170 ? -3370 ? (7) Amerada #1 Johnson

SW W Sec. 33, T.36 K R.53 R., Sheridan Co. Montana 2325 Misai: g 9206 232 9438 180 9618 -7293 (8) Gulf Paulson C SW SW Sec. 15,T.36N R.53 E., Sheridan Co. Montana 2300 Missir 9236 264 9500 182 9682 -7382 (9 ) Amerada #1 Emilson C KE SE Sec. 12,T.36N^ R.53 E., Sheridan Co.j Montana 2371 Mis sir g 9310 290 9600 184 9784 -7413 (10) Amerida #1 I^ucks C ME SW Sec.35,T.36 N. R,52 E,,Sheridan Co., Montana 2U 2 M issir g 9228 222 9450 175 9625 -7183 STONY KTN. NAME AND LOCATION ELEV. INTERLAKE FORMATION FORMATION o r WELL E.B. UPPIS BF-fK MIDDLE HF.TK LOWER HF-fK DEPTH TKDCKNESS lEPTH THICKNESS DEPTH THICKNESS DEPTH SU&iSSA TOP TOP TOP TOP EIEV. (n) Ajrerada #1 Ruegsegger C NE KE Sec.2A,T.36N. R.52 E., Sheridan Co. Montana 2A01 M issing 9084 214 9298 197 9495 •7094 (12) AjTierada #1 Grant T\E NW See.2 ,T.36 N., R .51 E ., Sheridan Co Montana 2362 M issing 8893 224 9117 190 9307 -6945 (13) Carter #1 Danielson C NE SW Sec.l2^T.36Na R,A7 E ., D aniels Co,, Montana 2407 Missing 8390 198 8588 167 8755 —6348 (14) Z, Brooks #1 State C SB SE SeCol8,T,36N. R.4A £ • , D aniels C o., Montana 2996 K issing 8217 153 83 70 130 8500 -5504 (15)

Gulf § \ Gray C SE NW Sec.26,T.35N. R .51 E ., Sheridan Co. Montana 2485 M issing 9284 200 9484 189 9673 •7188 STONY KTN. NAME AND LOCATION ELEV. DTTERLAKE FORMATION FORMATION or HELL E.B. UPPER HRns MIDDLE BEDS LC^'KR BEDS

DEPTH THICKNESS DEPTH THICKNESS DEPTH thickness DEPTH SUB-SEA TOP TOP TOP TOP £I£V.

(16) Union #1 Kuhring C KW NE Sec.32,T.35N, R.A7 E ., Daniela Co., Montana 2666 Kissini 8593 99 8692 148 8840 -6174 (17) Texas N.F.G. (NOT-?) ¥1, SE SE Sec.l9,T3UJ R.55 E., Sheridan Co., Montana 2352 9643 43 9686 176 9862 173 10035 -7683 (18) Texas #1 Marsh SE NW Sec.U,T.3L N., R.5L E., Sheridan Co., Montana 2592 10176 54 10230 250 10478 198 10676 -8084 (19) Hunt #1 Hagen C KE NW Sec.7,T.3L N., R.52 E., Sheridan Co., Montana 2530 M issing 9530 180 9710 186 9896 -7366 (20) Texas Whitewater #1 C SE SE U,T.3L N., R.31 E., Phillips Co., Montana 2317 M issing 5390 ? 23 ? 5413 ? 177 7 5590 ? -3273 STONY MTN. NAME AND LOCATION ELEV. INTERLAKE FORMATION FORMATION OF WELL K.B. UPPm BEDS MIDDLE BEDS LOWER BEDS DEPTH THICKNESS DEPTH THICKNESS DEPTH THICKNESS DEPTH SUB-SEA TOP TOP TOP TOP EIEV,

(21) Texas et al #1 Brekke C NW rJE Sec,5,T,33 H, 56 Ec, Sheridan Co, Montana 22U6 10260 92 10352 273 10625 192 10817 •8571 (22) Pan American ^iGodden C SE SE Sec,5^T.33 N. R,L9 E,, Daniels Co,, Montana 2473 M issing 8760 210 8970 170 9140 -6667 (23) S o c o n y F-11-6 Rhodes C K^W ICW Sec,6,T,33 N., R,L7 E.y Daniels Co., Montana 2698 M issing 8528 ? 162 ? 8690 ? 180 8870 -6172 (.2U) Carter #1 Sioux Tribal C KE SE Sec.8,1.32 N., R.52 E., Roosevelt Co Montana 2530 M i s s i n g 9855 191 10046 185 10231 -7701 (25) Texas #1 KcSowan Ni KE NW Sec.2,T.32 N. R.50 E., Roosevelt Co Montana 2680 M issing 9344 191 9535 191 9726 -7046 STONY mu. NAME AHD LOCATION ELEV, INTERLAKE FORMATION FCftMATlON OF WELL K.B. UPPHi beds MIDDLE HEnfi LOWER BEDS DEPTH THICKNESS DEPTH THICKNESS DEPTH THICKNESS DEPTH SUd-SEA TOP TOP TOP TOP ELEV.

(26) Calc# #1 Grimm C NE SE Sec.l3,T.32 N R,i+9 E., Roosevelt Co Montana 2L37 Missing 9015 137 9152 176 9328 -6891 (27) Gulf et al ^1 Lindsey C NE KE Sec.9, T.32 N R.42 E.., Valley Co., Montana 2956 Missing 7775 70 7815 118 7993 -5037 (28) Seaboard #1 Hickter, C SW KE S e c .1 8 ,T .32 N R.3? E., Valley Co., Montana 2785 Missing 6673 ? 37 ? 6710 120 6830 —1015 (29) Suoerior Oil #4427 C SE SE Sec.27,T.31 N R .58 E ., Sheridan Co. Montana 2356 1U 57 ILO 11597 293 11890 190 12080 -9721 (30) Juniper Oil #1 Masters C SV» KW Sec.l9,T .31 N R.5L E ., R oosevelt Co. Montana 2222 10218 91 10312 192 10531 180 10711 -3192 STONY KTN, NAME AND LOCATION ELEV. INTERLAKE FORMATION FORMATION OF WELL K.B* UPP31 RRrA middle heps LOWER BEDS DEPTH THICKNESS DEPTH THICKNESS DEPTH THICKNESS DEPTH SU&-SEA TOP TOP TOP TOP EIEV.

(31)

P h illip s ë l Tule Cree> SE SE Sec.20,T.30 N, R.19 E,, Roosevelt Co, Montana 2618 M issir g 8665 99 8761 112 8906 <*6288 (32) Hu2ible et a l , #lMcKee C NVJ SE 5ec.l3,T.30 N. R.17 E., Roosevelt Co Montana 2701 M issir g 8655 133 8788 160 8918 »62ll (33) P h illip s # 1-Jl Shultz C SE NE Sec.2L,T.30 N. R.15 E., Valley Co., Montana 2761 M issir g 8275 113 8U8 169 8587 5826 (34) Murphy ^1 Firmoon C SE SE Sec.12,T.30 N E .ll E., Valley Co., Montana 2711 Mdssln I 7192 78 7570 150 7720 -1976 (35) Seaboard #1 Tampico C SE NW Sec.5,T.30 N. R.39 E,, Valley Co., Montana 2512 M issii g 6628 92 6720 110 6930 -1318 STONY KTN. NAME AND LOCATION ELEV. INTERLAKE FORMATION FORMATION OF WELL K.B. UPPER beds MIDDLE HEPS LCWER BEDS

DEPTH THICKNESS DEPTH THICKNESS DEPTH thickness DEPTH SU^SEA TOP TOP TOP TOP EIEV, (36) Gulf #1 Cornwell C SW KE Scc ,U,T. 30N. R.38 E*, V alley Co.^ Montana 23U M issing 6U3 ? 26? 6449 126 6575 *4231 (37) Seaboard #1 Loberg C SE NE Sec.20,T.30N. R.36 E., Valley Co., Montana 2480 M issing 6096 ? 59? 6155 115 6270 -3790 (38) Mobile # F-33-23-P C NW SE Sec.23,T29K., R.5L £,, Roosevelt Co Montana 2210 10,890 92 10982 216 11198 199 11397 -9187 (39) Murphy et al #lPopul SW KE See.2,T.28 N., R.51 E., Roosevelt Co Montana 2111 K issing 8180 200 8380 180 8560 —6449 (40) P h illip s 1-A Twin But es C SW KW Sec.3fT.28 N H.23 E., Blaine Co.,

Montana 2916 K issing M issirg M issin I M issing ???? STONY KTN. NAME AND LOCATION ELEV. INTERLAKE FCRMATIÛN FORMATION OF WELL E.B. UPPHi RF.ns MIDDLE BEDS LCMER BEDS DEPTH THICKNESS DEPTH THICKNESS DEPTH THICKNESS DEPTH SU&iSEA TOP TOP TOP TCP £1£V. (a ) Brown et a l #1 Harmon C m SW Sec.29,T.27 N. H.58 Eo,Roosevelt Ce., Montana 2062 11590 160 11750 288 12038 202 12240 -10178 ( 4 2 ) Bert Fields #lFigmaka .V NW NE Sec.l4,T.27N. ^.39 E., Valley Co., Montana 2357 Kissing 6910 32 6942 138 7080 - 4 7 2 3 (43) S h e ll (('22-29 E tzel S-^SEM Sec. 29,1.26 K. R.51 E.^ Richland Co., Montana 2328 K issing 9110 170 9280 164 9444 - 7 1 1 6 (44) Delhi etal #1 Good KE KE SE Sec.21,T,26K, R.49 E«, McCone Co., Montana 22L8 M issinj 8795 95 8890 166 9056 o^d08 (45) Calco #1 Govt#,C SW SW Sec.33,T.26 N., R.34 E., Valley Co. Montana 2551 K issing M issin 5 ? 6200 ? 130 6330 -3779 STONY KTN. NAME AMD LOCATION ELEV. INTERLAKE FORMATION FORMATION or WELL K.B. UPP£R RFFA MIDDLE BEDS LOWER BEDS DEPTH THICKNESS DEPTH THICKNESS DEPTH THICKNESS DEPTH SUB^SEA TOP TOP TOP TOP EIZV. (46) McAlestcr Fuel #1-B C N£ NE SEC.35,T.25N R .54 E*| Richland Co. Montana 2397 Miasin ^ 10862 178 11140 190 11330 -8933 (47) Harrison #1-0C Govt. C NW NW Sec.19,T.2% H.35 2,, Valley Co., Montana 2B70 M issin 5 Missing Missing? 6740 ? 130 ? 6870 w»ooo (48) Sun e t a l ^IDynneson

C SW SE Scc.30,T,24 % R.58 E., Richland Co Montana 2504 11690 132 01822 261 12083 224 12307 ‘9803 (49) Pure #1 Leuenberger C NW NE Sec.30,T.24N. R.49 E ., McCone Co., Montana 2307 M issing 8935 88 9023 187 9210 -6903 (50) Shell et al # 23-9 C NE SW Sec.9 ,T.24 N. R.48 E ., McCone Co., Montana 2414 M issing 8833 95 8928 162 9090 -6676 STONY KTN. NAME AND LOCATION ELEV. INTERLAKE FORMATION FORMATION OP WELL K.B. UPPai REDS MIDDLE BEDS LOktR BEDS DEPTH THICKNESS DEPTH THICKNESS DEPTH THICKNESS depth Sl»SEA TOP TOP TOP TOP EJJEV.

(51) Sun O il #1 Beagle rJE SW SW Sec. 1 7 ,T .23 N.,R,59 E.,Richland C o., Montana 2164 11578 177 11755 285 12040 203 12243 -10079 (52) Eraniont et al #1 N.P.

C 5 ; \ : r ! Sec.21,T.23N. R. 5A. E.,Richland Co.,

Montana 2671 M issin I 10874 233 11107 177 11284 —8613 (53) Shell #1 Richey SE NV nv Sec49,T.23 N ., R*50 E ., Richland

C o., Montana 2524 M issin I 9465 165 9630 162 9792 -7268 (5L)

S h e ll § 11-21 E C KW N'ri Sec.21,T.23N. R.19 E ., McCone Co.,

Montana 2490 M issin I 9267 113 9380 154 9534 —7044 (55) T. F. Hodge if2Eegbrec it C KE SW S e c .3 ,T .23 N R,/+9 E ,, McCone C o., ? 7 Montana M issin I 9255 160 9415 155 9570 STONY KTN. NAME AND LOCATION ELEV. INTERLAKE FORMATION FORMATION OF WELL E.B. UPPJEE RF.fK MIDDLE HF.ns LOWER BF-ns DEPTH THICKNESS DEPTH THICKNESS DEPTH THICKNESS DEPTH SU>S£A TOP TCP TCP TCP ELEV.

(56) DeKalb et al #1 Gov't, C SE SE Sec.2 ,T.23 N. R. 31 E*,Phillips Co* Montana 2563 Mi s s iig M issir 6090 ? 47 ? 6137 •3574 (57) Conoco Zortman SE SE W Sec.,29, T.23 K..R.26 E., Phillips Co*,Montana 3070 M isslig M issii g M issing 6036 -2966 (58) Shell ^ 32-33-B Riche ^ C SW NE Sec.33,T.22N, R.18 E,, McCone Co., Montana 2A99 M issl ig 9264 ? 48 ? 9312 168 9480 «6981 (59) Shell # 22-25 B E^ SE NW S e c .2 5 ,T .22^ R.48 E., McCone Co., Montana 2561 M issiig 9400 ? 63 7 9463 157 9620 ►7059 (60) Shell # 22-35 A C SE NW Sec.35,T.22N. R.47 E,, McCone Co., Montana 2487 M issiig 9240 ? 70 ? 9310 140 9450 -6963 STONÏ KTN. NAME AND LOCATION ELEV. INTERLAKE FCEKATION FORMATION o r HELL E.B. UPPER BF.fK MIDDLE BEDS LOWER REns DEPTH THKKNESS, DEPTH THICKNESS DEPTH TKIBKNESS DEPTH SU&aSEA TOP TOP TOP TCP ELEV.

(61) Amerada #1 G ov't, SE SE SW S e c .10, T.22 K.,R.U E., McCone C o,, Montana 2778 M issing 8378 ? 82 ? 8460 140 8620 .5842 (62) S h e ll # 12-5 Paxton C S« '5J Sec.5,T.21N., R.18 E«, McCone Co., Montana 2166 Misslng 9336 ? 40 '? 9376 164 9540 -7374 (63) Pegasus et al #F-11-

2 0 ? , C NW KW S e c .20, T.21 K., R.46 Ec, McCone Co., Montana 2755 M isslng 9124 91 9215 171 9386 -6631 (U)

Stanolind ë 1-E C KE SE Sec.21,T.20Nc R.52 E.,Dawson Co., Montana 2893 M issing 10400 200 10600 180 10780 -7887 (65)

Amerada et a l ë \ NPRR C SE SE Sec.5,T.20 N. R.i^5 E ., McCone C o., Montana M issing 8550 70 8620 172 8792 STONÏ KTN. KAME AND LOCATION ELEV. lOTERLAKE FCRKATION FORMATION OP WELL K.B. UPPSi RF.nR MIDDLE HPTK LOWER RPris DEPTH THICKNESS DEPTH THICKNESS DEPTH THICKNESS DEPTH SUd>Sfii. TOP TOP TOP TCa> ELEV. (66) J. W. Brown #1 KW KW SE Sec.21,T.20 !<’.,R.39 E.,Garfield C o., Montana Miss -ng M issiig 7 7693 123 ? 7816 (67) Sack Brooks #ICoweLl C Œ KE Sec.l7,T.20N. R.25 E,, Fergus Co., Montana 2936 Miss -ng M isslig Missin, j K issing ? (68) Shell # 24-17 C SE SW Sec.l7,T.19N. R.53 E., Dawson Co., Montana 2718 Miss: Jig 10223 227 10450 184 10634 7916 (69) Ar.erada et a l #1NP-P SE SE Sec.31,T.19 N., R .51 E ., Dawson Co., Montana 3009 Miss: ng 10210 n o 10320 190 10510 -7501 (70) Superior Oil #22—10 Goff, SE NW Sec.10, T.19 K., R.U E., McCone Co., Montana 2715 Kiss: ng 9070 30 9100 ? 180 9280 -6565 STONY MTN. NAME AND LOCATION ELEV. INTERLAKE FCEMATIQN FORMATION OF WELL K.B. UPPfR RKDR MIDDLE HROR LOkER BEDS DEPTH THICKNESS DEPTH THICKNESS DEPTH THICKNESS DEPTH SUB-SBA TOP TOP TOP TOP EIEV.

(71)

S h e ll n Kubesh 41-26 C NE NE Sec.26,T.18N, R .53 E ., Dawson C o., Montana 2665 Miss: ng 9970 200 10170 198 10368 -7703 (72) Stanolind #1 NP-F

C \ : v ; TIE Sec.29,T.18N. R.43 E.> Garfield Co. Montana 2613 Kiss: ng 8938 52 8990 133 9123 -6510 (73) Richfield ^INewton C SE SE Sec ?,T.17 N. R .53 E.^ Dawson Co., Montana 2577 M iss:ng 9800 145 9945 185 10130 -7553 (7 :) Gulf #1 Schwartz C SE NE Sec.3,T.17 N. R .52 E ., Dawson Co., Montana 2626 Miss: ng 9862 103 9965 195 10160 -7534 (75) Mobil Producing Co. ^F44-3LP Hodges C SE SE Sec.3L,T.l6N. Dawson Co., Montana 2600 Miss ng 10000 240 10240 180 10420 -7820 STONY KTN. NAME AND L X A T IO N ELEV. INTERLAKE FCRKATIÛN FORMATION OF WELL K.B. UPPER RRDR MIDDLE HFTK LOWER BEDS DEPTH THKKNESS DEPTH THKKNESS DEPTH THKKNESS DEPTH SU»>SEA TCP TCP TCP TOP ELEV.

Texaco Cedar C: k. C KV Sec.21,1,16 N R.54 E,,Dawson Co., Montana 2390 M issini 9056 152 9208 187 9395 -7005 (77) Texaco #1 Kauski C KW SW Sec.23,T.l6N . R.50 E.,Prairie Co., Montana 3078 Missinj : 9820 • 125 9945 143 10088 -7010 (78) S h e ll #114-34 Slcunk Cr Note :C( rrelate t le gamma r iv-neutron log from h is w ell with the log SW SW SW Sec.34,T.l6N fi'om the St m olind # KPl S e c .5 ^ T.20 N ., R.45 Ec(w ill No.64 R*42 E., Garfield Co. Montana 2740 Missinj M issing 8778 82 8860 -6 1 2 0 (79) Gen. Pet. 5‘-25P,Freedoi Dome, KW bfW SW S e c .25 T. 16N. ,R . 38E. ,Garf ie l( Co., Montana 3203 Missing Missing Kissing; 7605 —4402 (80) R. Low. #1 S n g st. C NE SW Sec.28,T.16N. NoteiTop Red River Formation at 7800 fj; R.36E,,Garfield Co., Top Precambri1 a n at 8840 f t . Montana 3119 Missing M issing Missin; ; M issing STONY MTN. NAME AND LOCATION ELEV, INTERLAKE FCRKATION FORMATION OP WELL E.B. UPPER BEDS KIDDLE BEDS LOWER BEOS DEPTH THICKNESS DEPTH THICKNESS DEPTH THKKNESS DEPTH S U » ^ TOP TOP TOP TCP EIEV.

(81) Shell Gov't. 3L-26 NW SW SE Sec.26,T.15 N*, R* 54 E*, Dawson C#$. Montana 2393 Mis îing M issir g 6560 152 8'712 -6319 (82) Lion #1 Wibaux C rCE SW S e c .2 9 ,T .U K .,R*60E., Wibaux Co, Montana 2823 Mis sing 10475 175 10650 165 10815 -7992 (83) Texaco #1 Î^PG Cedar Creek, C SE SE Sec* 19,T,UK*,R.55 Eo, Dawson Co*, Montana 2352 Mis sing 9685 175 9660 175 10035 -7683 (84) D elta #1 SW NE NW Sec.6,T.14N. R.55Eo, Dawson C e,, Montana 2169 Mis îing 8405 147 8552 190 8742 -6573 (85) Amerada et a l #1NPRR C KW NE Sec.32,T.12N. R.53E., Prairie Co., Montana 2657 Mis sing 9270 100 9370 166 9536 -6879 STONY KTN. NAME AND LOCATION ELEV, INTERLAKE FORMATION FORMATION o r WELL K.B. UPPER RF.ns MIDDLE BEDS LOWER BEDS DEPTH THICKNESS DEPTH THICKNESS DEPTH THICKNESS DEPTH SU^^SEA TOP TOP TOP TOP ELEV.

(86) Ohio Oil Gov't. Sheep Mt*, C SW SW SeCo33,Ta2N.,RoA9E*, P r a ir ie Co©,Montana 2522 Miss Lng 8546 29 8575 120 8695 -6173 V o ( / Pure #1 NPRH

C KW Sec.25,T.12N. 9 R.LlE., Rosebud Co., Montana 3094 Miss Lng Kissing K issing 8470 -5376 (88) Shell Pine #1 SW SW KE Sec*30, T.12N.,R.57E., Wibaux Coo,Montana 2733 K iss Ing 8560 177 8737 151 8888 -6155 (99) Davis Oil #1 Lutts C W KW Seco23,T.llN* ► R*6C3£©, Wibaux Co,, Montana 3175 MissLng 9970 120 10090 165 10255 -7080 (90) Shell #22.-21 Roberta

C SE SV; Sec.21,T.UN. > R,&3Eo, Rosebud Co©, Montana 3024 Miss Lng Missing M issing 8378 -5356 STONY MTN. NAME AND LOCATION ELEV, INTERLAKE FORMATION FORMATION OP WELL K.B. UPPER MIDDLE BEDS I C M m BEDS

DEPTH THICKNESS DEPTH THICKNESS DEPTH thickness DEPTH SUd^SEA TOP TOP TOP TCP EIEVo

(91) S h e ll 21-6 Cabin Cree : C NE Sec.6,T.10N., R,58Eo,Falcon Co*, Montana 2643 M issing 8400 203 8603 141 8744 -6101 (92) C a lifo rn ia Cocf^lPenne C SE 15,T,ION., R*56E .,Fallon Co., Montana 2894 M issi] ig 9570 126 9696 194 9890 -6996 (93) Pure #1A Chase C SE Sec.IL,T,*^,, R.L3E., Rosebud Co., Montana 2302 Missii ig Missing Missing 8230 «5428 (94) Anschutz #1 Blum NE ÎJW NV Scc.21,T .7N . R.4SE,, Custer Co., Montana 2713 M issii g 8300 50 8350 123 8473 •5760 (95) Shell #1 Beaver 23-13 C KE Srf Sec.l3,T .yJ,, R.61E.,Fallon Co., Montana 3102 M issii g 7845 158 8003 130 8133 -5031 STONY MTN. NAME AND LOCATION ELEV. INTERLAKE FORMATION FORMATION OF WELL K.B. UPPfR RRrK MIDDLE HEPS LOk'ER Rgg?

DEPTH THICKNESS DEPTH THICKNESS DEPTH THICKNESS depth SUBmSEA TOP TOP TOP TOP EIEV.

(96) Carter #1 Trawek C KE KE Sec.6,T.2N., R.61E., Carter Co., Montana Miss: ng 97 160 -5694 (97) Kurphy #1 McIntosh SE Sa KE 5ec.21,T.Ul., R.50E.,Custer Co., Montana K iss;ng M issing 108 -5549 (98) Carter #1 Liscomb C SW SW Sec.l5,T .lN ., R.45E., Custer Co., Montana Miss: ng M issi ig 43 -5653 (99) Amerada #1 Washburn C NW NE Sec,31,T .5S., R.57E., Carter Co., Montana ( 1 0 0 ) Kerr-McCee—Argo O il Johnson Estate Ko.l, C KE NW Sec.3L,T*162N R.lOrW^, D ivide Co., 10950 -8690 North Dakota !260 10253 159 10412 341 10753 197 STONY MTN, NAMB AND LOCATION ELEV. INTERLAKE FORMATION fcrm I tion or WELL K.B. UPPHi BEDS MIDDLE PF.ns LOh'ER BEDS DEPTH THICKNESS lEPTH THICKNESS DEPTH THICKNESS DEPTH SU^SEA TOP TCP TOP TCP ELEV.

( 101) Carter #1 D Moore Wi KW KE Sec.7,T.163 N.,R.102W.,Divide Co North Dakota 2205 9610 110 9720 333 10053 187 L0240 ‘8035 ( 102) Amerada #3 C. Hanson C SW NW Sec.18,T.158 N., R.94W,,Mountrail Co.,North Dakota 2339 11335 474 11809 339 12148 219 12367 .10028 (103) Amerada #1 Bakkcn C SW W Secel2,T.157 K ., R .95W*^Williams Co*, North Dakota 2458 11520 500 12020 360 12380 240 12620 -10162 (lOL) Skelly et al #1 Hoehn C KE SE Sec.13,T.152 N., R.102W.,HcKcnzie Co., North Dakota 2278 12490 294 12784 344 13128 222 13350 .11072 (105) Amerada Binhomer Risser #1,C SW SB Seco 12, TU9N.,R«96W* McKenzie Co., North Dakota 2438 12585 590 13175 345 13520 250 13770 •U 3 3 2 STONY WN. NAKE AND LOCATION ELEV. INTERLAKE FORMATION FORMATION OF WELL K.B. UPPSl BEDS MIDDLE BEDS LOWER BEDS DEPTH THICKNESS DEPTH THICKNESS DEPTH THICKNESS DEPTH SUBkSEA

TOP TOP TOP TOP £I£V.

(106) Gulf #lBennie Pierr* C KW SW Sec.28,T.lL8

N« ,R10Z*W,, McKenzie Co 9 North Dakota 2338 11840 270 12110 283 12393 187 12580 -10242 (107) Shell-NPRR #32-16-1 C Sk' KE Sec.16,1.115 N, ^RplOlW,^McKenzie Co., Nbrth Dakota 2463 12020 250 12270 372 12642 200 12823 -10360 (108) Gulf et al #1 Gov't. C NE Sec.24,T.143 N. ,R.103W.,Gol(jen Valley Co.,N.Dakota 2515 11580 '^5 11655 390 12045 175 12220 - 9705

(109) Stanolind #1 C SE SE See.17,T.143 N. ,R,IOOa .,Billings Co.,North Dakota 2815 12150 ? 12725 198 12923 -1 0 1 0 8