INTRODUCTION

This article is primarily a progress report concerned with the regional geology of North Dakota and South Dakota. However, the writer found that in order to understand the geology of this region, it is necessary to study, compare, and correlate the formations in the adjoining states on the east, south, and west. During the course of the investigation, outcrops were examined along the Laramie Range, on the Hartville uplift, and in the Black Hills. Drillers' logs of approximately 300 tests were plotted and well cuttings from 75 deep holes were examined.

GENERAL FEATURES

Figure 1 is a map showing the areal geology of the northern Great Plains. It has been adapted from the areal map of the United States prepared by the United States Geological Survey. The areal geology shows the chief structural features of the region: (1) on the east side, the west-dipping high pre-Cambrian area of the Archean shield and the Sioux uplift; (2) the Black Hills, and the Cedar Creek or Baker-Glendive anticline which forms the west flank of the (3)

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Fig. 1. Areal geology of Northern Great Plains.

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Williston basin; (4) the Laramie Range with its northeast projection known as the Hartville uplift; (5) the Big Horn Mountains which trend northwest from the Laramie Range; and (6) the Powder River Basin which lies between the Big Horn Mountains and the Black Hills.

The sedimentary rocks in this area range in age from Cambrian to Recent and are approximately 16,000 feet thick in the center of the Powder River Basin according to John G. Bartram.(FOOTNOTE 3)

Although there was regional tilting in various directions during the early Paleozoic, the Black Hills were first anticlinally folded in post-Mississippian, pre-Pennsylvanian time with the main orogeny occurring at the end of Upper Cretaceous or in early Eocene time. In the region covered by this article, the last period of folding of which we have any record occurred after the White River beds had been deposited for these strata are gently folded in southwestern South Dakota.

STRATIGRAPHY

QUATERNARY SYSTEM

PLEISTOCENE OR RECENT SERIES

In North Dakota and South Dakota, alluvial deposits of reworked shales, gravels, and sandstones from Pierre, Fox Hills, Lance, and Fort Union beds are present in all of the larger stream valleys and are common in many of the smaller ones. From the evidence observed, the writer is inclined to consider all or at least part of these deposits as the western and southern equivalents of the northern boulder clays of glacial origin. The thickness of the deposits varies from a feather edge to 10 feet. In eastern North Dakota in the valley of the Red River, glacial Lake Agassiz deposited silt which reaches a maximum thickness of 100 feet.

PLEISTOCENE SERIES

Glacial deposits extend southward from the Canadian border into Nebraska and westward from Minnesota to a north-south line slightly west of the Missouri River Valley. The glacial drift contains gray to buff boulder clays, gravels, and coarse-grained unconsolidated sands, which thin west and south from a maximum of 500 feet near the North Dakota-Manitoba border.

TERTIARY SYSTEM

OLIGOCENE SERIES

WHITE RIVER GROUP

The White River group, exposed in the badlands of southwestern South Dakota, is composed of volcanic ash, a pinkish gray, sandy, massive clay, intercalated with thin beds of gray to pink, fine to coarse, unconsolidated, calcareous sands. Toward the base of the group the amount of sand gradually increases until the clays disappear. At most localities there is a basal conglomerate. Rapid lateral

FOOTNOTE 3. Guide Book, 14th Annual Field Conference, Kansas Geol. Soc. (1940), p. 113.

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gradations take place throughout the beds of the White River group. The thickness of this group varies form a few inches to approximately 400 feet.

EOCENE SERIES

Fort Union formation:
The Fort Union crops out in South Dakota only as isolated exposures near the northwest boundary of the state. In North Dakota this formation occupies most of the western half of the state. Here it may be divided into upper and lower members. The former contains 450 feet of gray sandy clay and gray to buff, lignitic, fine- to medium-grained sand which is finer-grained, more lignitic, and less cross-bedded than the Lance sandstone. The lower member of the Fort Union contains more gray, sandy clay than the upper member. The Fort Union varies from a feather edge in South Dakota and central North Dakota to more than 1,200 feet in western North Dakota. It is probably conformable with the underlying Lance in the Williston basin.

PALEOCENE SERIES

Lance formation:
The eastern edge of the Lance formation is found in Kidder, Logan, and McIntosh counties, North Dakota. Southwest from these counties it extends as a broad band 50-100 miles wide into southwestern Harding County, South Dakota. The Lance also may be divided into upper and lower members. The upper has two phases, the marine or Cannonball phase and the Ludlow or lignitic phase. The Cannonball contains gray, massive, poorly consolidated sandstone with intercalated gray sandy shales. The Ludlow, which is very similar to the Hell Creek or lower Lance, contains gray to buff, very lignitic, unconsolidated sand and gray to brown, lignitic sandy shales. The shales contain many beds of brown, slightly weathered limonite pebbles. These pebbles are not nearly so weathered as those contained in the un erlying Fox Hills. At the base of the Lance there is commonly buff, friable, fine- to coarse-grained, cross-bedded sandstone which locally contains small pebbles from the underlying Fox Hills. There is some evidence that the basal Lance sand lies on successively older Fox Hills in an easterly direction. The Cannonball-Ludlow varies from a few inches to 300 feet and the Hell Creek attains a maximum thickness of 600 feet.

CRETACEOUS SYSTEM

Fox Hills formation:
Southwest from Kidder County, North Dakota, the Fox Hills crops out as a band 10-40 miles wide, crossing the state line into north-central Corson County, South Dakota, and extending westward into Harding County. It is believed that strata of the Fox Hills will be found under cover in most of the west half of North Dakota. The upper half of the Fox Hills contains shaly, friable, gray sandstone, which grades downward into gray sandy shales containing several zones of ironstone concretions, one of which is very fossiliferous. At the top of a complete section of Fox Hills is gray to buff, concretionary sandstone containing many shells of Ostrea glabra. The contact between the Fox

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Hills and the underlying Pierre is gradational. In the Dakotas, a complete section of Fox Hills contains approximately 350 feet.

Pierre shale:
The Pierre shale forms the surface rock over most of the eastern half of North Dakota and South Dakota but its outcrop narrows toward the Black Hills due to the Tertiary overlap. Downdip from the outcrop it is everywhere found in the subsurface of both the Dakotas. The Pierre is dark to gray, uniform gumbo shale with several calcareous zones in the basal beds. There are also irregular occurrences of dark reddish gray, manganiferous, ironstone concretions which vary from a few inches to more than 2 feet in diameter. The contact with the underlying Niobrara is gradational. The Pierre thickens from 1,200 feet near the Black Hills to more than 2,000 feet in western North Dakota. The Shannon sandstone, which produces oil in eastern Wyoming, occurs in the Pierre. The Judith River and Eagle gas producing sands on the Baker-Glendive anticline of southeastern Montana, occur in a western equivalent of the Pierre shale.

Niobrara formation:
The Niobrara crops out as a narrow band around the Black Hills and the Sioux uplift but most of its outcrop in eastern North Dakota is obscured by the overlap of the Quaternary lake silt. Downdip from the outcrop it is everywhere present in the subsurface. The Niobrara contains 200-325 feet of impure chalk which grades into dark gray shale speckled with white chalk inclusions. The Niobrara is conformable with the adjacent formations.

BENTON GROUP

In descending order, the Benton group is made up of the Carlile shale, Greenhorn limestone, and Graneros shale. The strata of the Benton group crop out on all flanks of the Black Hills and at scattered points on the south and west sides of the Sioux uplift but are found only in the subsurface elsewhere in the Dakotas. The Carlile is gray, fine-grained shale which locally contains 5-15 feet of shaly sandstone at the top. It is not as dark colored as the overlying Niobrara or the underlying Graneros. The thickness of the Carlile varies from 750 feet in the Black Hills to 100 feet in eastern South Dakota. The Greenhorn contains alternating beds of dark gray, calcareous shale and thin beds of aragonitic, fossiliferous limestone. The average thickness is 60 feet. The Graneros is dark gray, bentonitic clay shale which locally contains gray, fine-grained shaly sandstone in the lower part. This sand which is not developed very far east of the Wyoming line is the Muddy or Newcastle sandstone. The thickness of the Graneros varies from 1,100 feet in the Black Hills to 90 feet in Clay County in the southeast corner of South Dakota. This thinning of the Graneros is probably due to the Dakota sandstone transgressing upward across the basal shale beds of the Graneros in an easterly direction, thereby making the top beds of the Dakota younger on the east flank of the basin than they are farther west. The Benton group is conformable with the underlying Dakota group. The Wall Creek and Newcastle or Muddy sands, which are oil-bearing in eastern Wyoming, are found in the Benton group.

DAKOTA GROUP (FOOTNOTE 4)

The strata of the Dakota group crop out as hogbacks completely surrounding the Black Hills and are found throughout the Dakotas in the subsurface. The upper member of this group is gray, fine- to coarse-grained, angular, well cemented to porous sandstone with thin interbedded shale streaks. The middle or Fuson shale member is gray, greenish, or maroon clay shale, locally containing coaly beds and a thin limestone near the base. The lower or Lakota sandstone member is gray to buff, coarse-grained to conglomeratic, cross-bedded sandstone containing thin streaks of shale similar to that of the Fuson. Although the Dakota is Upper Cretaceous and the Fuson-Lakota beds are Lower Cretaceous in age there is no evidence of great unconformity between the two. On the other hand there is generally a conglomerate marking the unconformity at the base of the Lakota. The thickness of the Dakota group varies from 725 feet in Fuson Canyon in the Black Hills to a few inches in parts of eastern South Dakota near the Sioux uplift. Sands of the Dakota group produce gas in parts of North Dakota and South Dakota and oil in eastern Wyoming.

JURASSIC SYSTEM

The Canadian facies of the Jurassic is found in the subsurface of northeastern North Dakota. It contains gray marine shales as well as red to maroon shale intercalated in a few places with streaks of anhydrite and sandy shale. These shales, approximately 200 feet thick, overlap beds ranging in age down to and including the Devonian. Although the relation between these beds and the Black Hills Jurassic is not clear, the meager evidence available suggests that they are the eastern equivalents of the Sundance strata.

Morrison shale:
The Morrison crops out high on the flanks of the Black Hills and probably exists in the subsurface in the western third of the Dakotas. It contains greenish gray, pure to sandy, clay shales which locally contain maroon beds. In places, in the lower part of the Morrison, there are thin beds of light gray lithographic limestone. On the south, southeast, and southwest sides of the Black Hills, the lower half of these sandy clays grades laterally into massive, white, fine-grained sandstone which Darton called Unkpapa. The thicknesses of the Morrison and Unkpapa are complementary. Nevertheless, the thickness is very erratic because of the overlying unconformity. Westward from the east side of the Williston basin, the Morrison thickens from a few inches to 220 feet near Piedmont in the Black H lls.

Sundance formation:
The Sundance crops out on the inner margin of the Morrison in the Black Hills. The writer thinks that the Sundance sands and shales will probably be found under the western third of South Dakota and under

FOOTNOTE 4. Willis T. Lee in U. S. Geol. Survey Prof. Paper 149 considers the Muddy sand of the Big Horn Basin, Wyoming, and its equivalents in the other parts of Wyoming and Colorado, to be the "Upper sand" member of the Dakota group rather than a sand member in the lower Colorado shale. The Muddy sand has graded laterally into shale over much of North and South Dakota. Since it is impossible to pick the top of the Muddy sand, the writer has restricted the Dakota group to the Dakota sandstone, Fuson shale and Lakota sandstone.

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at least the western third of North Dakota. The Sundance shales are very similar to those of the Morrison from which they differ in being darker green. The Sundance contains very fine-grained, glauconitic, angular, porous to well cemented, gray to pink sandstone which becomes more shaly northeast. There is 150-200 feet of brownish red shale containing two or three light gray to pink, finely crystalline limestones in the basin north of the Black Hills. These beds are possibly correlative with the Twin Creek of Wyoming. The Jurassic, probably Sundance, lies on the beveled edges of strata ranging from Triassic to Devonian in age in the east half of North Dakota. The thickness varies from a few inches along the eastern margin of this overlap to 735 feet in the basin north of the Black Hil s. The Sundance sands produce oil in the Lance Creek field of eastern Wyoming.

TRIASSIC SYSTEM

Spearfish formation:
The outcrop of the Spearfish beds forms the Red Valley which completely surrounds the Black Hills. These strata probably underlie the western third of North and South Dakota. The Spearfish contains rusty to red, fine-grained, argillaceous sands and pure to sandy shales which south and east of the Black Hills are intercalated with two or three beds of anhydrite. The number and thicknesses of the sands increase northwest of the Black Hills. In the lower part of the Spearfish is a red, very fine-grained, shaly sandstone or sandy shale which some geologists place in the Permian rather than the Triassic. The Spearfish is conformable with the underlying Minnekahta limestone but thins eastward due to pre-Sundance erosion. The maximum thickness known to the writer occurs in the north part of the Black Hills where 830 feet of these strata were penetrated in the Wytex Oil Company's Hamilton No. 1, Sec. 32, T. 8 N., R. 1 E., Butte County, South Dakota.

PERMIAN SYSTEM

Minnekahta limestone:
The outcrop of the Minnekahta limestone rims the Black Hills and the writer believes that it occurs beneath the surface in the west half of North Dakota and probably the west third of South Dakota. The Minnekahta contains 40-60 feet of purplish gray, very finely crystalline limestone or dolomite which in many places gives off a bituminous odor when struck with a hammer.

Opeche formation:
In the Black Hills, the Opeche crops out in the shale valley beneath the Minnekahta hogback. In the subsurface it has been found as far east as Oliver County, North Dakota, and Stanley County, South Dakota. This formation contains 60-160 feet of predominantly dark purplish red shale with minor anhydrite and red, shaly sandstone, or siltstone. The Opeche-Minnelusa contact appears to be gradational.

PERMO-PENNSYLVANIAN SYSTEM

Minnelusa formation:
The Minnelusa crops out on the outer margin of the high Mississippian limestone plateau of the Black Hills. It is the writer's opinion

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that the stratigraphic test of the Carter Oil Company in Sec. 34, T. 118 N., R. 78 W., Potter County, South Dakota, penetrated porous limestone and dolomite of the Minnelusa from 2,227 to 2,322 feet. From this depth to 2,373 feet, the formation contained red or green sandy shale and thin sandstones. The test being drilled by the same company in Sec. 18, T. 141 N., R. 8 W., Oliver County, North Dakota, penetrated approximately 625 feet of Minnelusa, of which 180 feet was porous, medium to coarse sand. So variable is the Minnelusa that bed-for-bed correlations can not be made in an area as restricted as the Lance Creek oil field. The Minnelusa contains gray to red, fine to coarse sandstone; gray to pink, crystalline to dense, slightly cherty limestone and dolomite; anhydrite; and black oaly to red shale. The upper part contains more sand and the lower two-thirds is predominantly dolomite and anhydrite with a basal series of red shale and sand. The Minnelusa contains the greatest percentage of sandstone southwest of the Black Hills. These sandstones seem to grade northward into red sandy shales and anhydrite so that in The California Company test in Sec. 3, T. 154 N., R. 96 W., practically all of the Minnelusa is represented by non-porous red sandy shale and evaporites. There is a widespread unconformity at the base of the Minnelusa. The average thickness of this formation in the Black Hills is 450 feet. The maximum thickness known to the writer is 1,335 feet penetrated in the Ohio Oil Company's Jones No. 1, southwest of the Black Hills on the Hat Creek anticline, Niobr ra County, Wyoming. Several sands in the Minnelusa are the chief deep oil-producing zones in the Lance Creek field.

MISSISSIPPIAN SYSTEM

BIG SNOWY GROUP

The Big Snowy beds, probably Chester in age, were named by Harold W. Scott as a result of his work in the Big Snowy Mountains of Montana.(FOOTNOTE 5) So far as the writer knows, these beds of gray, finely crystalline to dense, oolitic limestone, containing minor anhydrite, red to black clay shales, and gray, fine-grained sandstone, are not exposed in the Black Hills or on the Hartville uplift. Due to pre-Pennsylvanian erosion, variable thicknesses of the Big Snowy strata are found in the subsurface north of the Black Hills. In the deeper parts of the Williston basin, it is probable that the Big Snowy is conformable with the underlying Pahasapa or Madison limestone. Approximately 1,450 feet of beds in The California Company's Kamp No. 1, Williams County, and 500-600 feet in the Prairie Oil and Gas Company's Armstrong No. 1, Kidder County, North Dakota, are believed to belong in the Big Snowy group. A comparison between the Kidder County test and those drilled on the Baker-Glendive anticline suggests that some anhydrite was being deposited in the deeper parts of the basin at the same time that limestone and dolomite were being laid down in eastern North Dakota. The 6,700-foot oil zone on the Baker-Glendive anticline probably belongs in the Big Snowy group.

FOOTNOTE 5. Harold W. Scott, "Some Carboniferous Stratigraphy in Montana and Northwestern Wyoming," Jour. Geol., Vol. 43, No. 8, Pt. 2 (November-December, 1935), pp. 1011-32.

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LOWER MISSISSIPPIAN

Pahasapa-Englewood limestones:
The outcrop of the Pahasapa and the underlying Englewood forms the high limestone plateau near the crest of the Black Hills whence it probably extends eastward over the western half of South Dakota and the western two-thirds of North Dakota. The Pahasapa is light gray, very finely crystalline, massive, calcitic limestone containing small amounts of chert, oolites, and dolomite. In the Black Hills, the Englewood contains 30-60 feet of purple to gray, finely crystalline, argillaceous dolomitic limestone or dolomite. The vertical change of the Pahasapa into the Big Snowy above and the Englewood below is so gradual that so far the writer has not been able to recognize definite limits for the Pahasapa. In the region of the Baker-Glendive anticline, the basal 175-200 feet of probable Pahasapa ontains many streaks of light gray coarsely crystalline limestone which here and there contains pink spots. Underlying this phase in The California Company's Kamp test, are approximately 850 feet of calcareous beds containing large amounts of gray to dark gray, finely crystalline to dense, argillaceous limestone. The lower 300 feet of the Mississippian limestone in the Carter Oil Company's Semling No. 1, Oliver County, North Dakota, also showed this shaly phase which is probably older than any Pahasapa-Englewood exposures in the Black Hills.

In The California Company's Kamp No. 1, Williams County, North Dakota, black greasy shale contains Kinderhookian conodonts.(FOOTNOTE 6) It is the writer's opinion that Kinderhook shale is present in tests drilled by the Carter Oil Company, in Sec. 18, T. 141 N., R. 81 W., Oliver County, North Dakota, and in Sec. 34, T. 118 N., R. 78 W., Potter County, South Dakota. Exposures near Wind Cave in the Black Hills show no discordance of dip between the Englewood and the underlying Deadwood although this unconformity represents all of Devonian, Silurian, and Ordovician time.

DEVONIAN-SILURIAN SYSTEMS

The Devonian-Silurian strata of southeastern Manitoba are the only exposures of these rocks in or near the area under discussion. In the subsurface these beds occur in the basin north and east of the Black Hills and south and probably southwest of the Sioux uplift. In Manitoba these beds contain anhydrite, gypsum, and white to red, pure to shaly, finely crystalline to crystalline limestone and dolomite with here and there a sandy or cherty zone. The intercalated shales are generally dark red and calcareous. In southeastern South Dakota, the Siluro-Devonian beds contain white to gray, crystalline to lithographic dolomite with thin streaks of leached green shale and sandy dolomite. It is believed that approximately 625 feet of Siluro-Devonian beds occur in The California Company's Kamp o. I, Sec. 3, T. 154 N., R. 96 W., Williams County, North Dakota. This is the maximum thickness known to occur in the Williston basin. In the east half of South Dakota there is an unconformity at the

FOOTNOTE 6. Personal communication, Charles Ryniker.

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base of the Devonian because it overlaps the Ordovician dolomite. In this area the dolomite at the base of the Devonian contains scattered grains of coarse, rounded, frosted sand.

ORDOVICIAN SYSTEM

Whitewood limestone:
The Whitewood is light gray, lithographic to crystalline, granular dolomite or dolomitic limestone which thickens from a feather edge in the south-central part to 60 feet in the north part of the Black Hills. Locally it contains dark pinkish spots. Beneath the limestone there is 70 feet of dark green, sandy clay shale containing phosphate nodules. In an easterly direction a zone of white to gray, dark-spotted, fresh to weathered chert occurs approximately 150 feet above the top of the green Decorah shale. This chert is similar to a chert zone that occurs in the Galena dolomite of Iowa and Nebraska. In the Black Hills the Whitewood lies on the green shales and sandstones of the Cambrian without any discordance of dip. The Whitewood of Furnish, Barragy, and Miller is probably Richmond, Tre ton, and Black River in age.(FOOTNOTE 7)

In the Carter Oil Company test on the Baker-Glendive anticline, Fallon County, Montana, 815 feet of beds have been assigned to the Whitewood. Below the limestone member of the Whitewood there is dark green fossiliferous clay shale which is probably a western equivalent of the Decorah shale. The thickness of this middle Ordovician shale varies from a feather edge to 100 feet. Below this shale the drill penetrated approximately 120 feet of white, fine to coarsely crystalline, mottled limestone containing two thin beds of dark green, sandy, clay shales. This limestone is possibly the equivalent of the Platteville limestone of Iowa. West and northwest of the Sioux uplift these lower sandy limestone beds seem to be absent, for the Decorah shale is in contact with the pre-Cambrian in the st atigraphic test drilled by the Carter Oil Company in Sec. 12, T. 9 N., R. 27 E., Stanley County, South Dakota. However, in the same company's test in Sec. 34, T. 118 N., R. 78 W., there is 15-20 feet of gray, subangular to rounded, coarse sand which may be the western equivalent of the St. Peter sandstone.

LOWER ORDOVICIAN ?

Underlying the Whitewood in the Carter Oil Company test on the Baker-Glendive anticline is 5-10 feet of white St. Peter-like sandstone. Beneath this sandstone is 90 feet of light gray, fine to coarsely crystalline, sandy limestone with a few streaks of gray, fine to coarse, porous, frosted, rounded sand.

CAMBRIAN SYSTEM

Deadwood formation:
Outcrops which encircle the granite and schist in the Black Hills show that the Deadwood contains hard, brown to gray, glauconitic,

FOOTNOTE 7. W. M. Furnish, E. J. Barragy, and A. K. Miller, "Ordovician Fossils from Upper Part of Type Section of Deadwood Formation, South Dakota," Bull. Amer. Assoc. Petrol. Geol., Vol. 20, No. 10 (October, 1936), pp. 1329-41.

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fucoid-bearing sandstone, dark green, sandy, clay shales with thin streaks of gray, crystalline dolomitic limestone and a thin basal conglomerate. The Carter Oil Company test previously mentioned penetrated approximately 500 feet of gray, glauconitic, sandy limestones, green sandy shales, and gray, fine to coarse, glauconitic sandstones which probably belong in the Deadwood formation. The maximum thickness of the Deadwood in the Black Hills is found in the north part where it contains 380 feet of rocks. It thins at the rate of 8 feet per mile toward the south and is overlapped by the lower Mississippian limestone in a test drilled in Sec. 3, T. 10 S., R. 2 E., Fall River County, South Dakota. It is the writer's opinion that red micaceous shales and conglomerate at the base of the Miss ssippian limestone in the Hartville uplift should be placed in the Mississippian rather than in the Deadwood.

Eastward from the Black Hills, the Cambrian grades into light gray, sandy dolomite intercalated with a few glauconitic sandstones. A well in Sec. 18, T. 90 N., R. 48 W., South Dakota, penetrated 233 feet of Cambrian sandy dolomite containing 55 feet of sandstone near the middle and a basal conglomerate. The Deadwood unconformably overlies the pre-Cambrian.

PRE-CAMBRIAN ROCKS

In the area under discussion, the pre-Cambrian crops out only in the Black Hills and the Sioux uplift. In the latter area it is represented by the purplish red Sioux quartzite. In the Black Hills the pre-Cambrian contains schists, granite, pegmatite, and various other types of igneous and metamorphic rocks. The pre-Cambrian is found beneath the glacial drift in a large area of western Minnesota.

STRUCTURAL GEOLOGY

Figure 2 is a structure map which is contoured on the top of the Dakota sandstone. It shows a large area of rocks with low dip in eastern North and South Dakota. This relatively flat area forms the east flank of the Williston basin, which extends from the northeast corner of Montana into north-central Nebraska. The Cambridge arch with its superimposed Chadron dome, the Black Hills, and the Baker-Glendive anticline form the west flank of this basin. In southeast Wyoming, the north part of the Denver basin is shown. The east dip from the Laramie Range to the bottom of this basin is very steep. There is a fold extending northeast from the Laramie Mountains toward the Black Hills. The Hartville uplift, the Hat Creek anticline, and the Old Woman anticline are surface expressions on this br ad arch. From the crest of the Black Hills, the rocks dip west at a fairly uniform rate almost to the foothills of the Big Horn Mountains, thereby making the deepest part of the Powder River Basin a short distance east of the axis of the mountain range. Many of the men mapping structure in the surface rocks of the Dakotas find that there are well developed trends. The local structures, covering 6-20 square miles, with closures of 30-60 feet, occur on these trends many of which can be traced 30 miles or more. In the east-central Dakotas the surface

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Fig. 2. Structure of Northern Great Plains region, contoured on top of Dakota sandstone.

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Fig. 3. South-north cross section from northwest Nebraska to southeast Montana.

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structures trend northeast and southwest, but the structures in the west third of both North Dakota and South Dakota are reported to trend west of north.

Darton (FOOTNOTE 8) mentions faulting near Jewel Cave, on Whitewood Creek below Deadwood, in Deadman Gulch, north of Roubaix, around Bear Butte, in Nevada Gulch, and northwest of Lead on Bogus Jim Creek. So far as is known to the writer, there is no evidence of faulting in any other part of the area under discussion.

Figure 3 is a cross section which extends north from the Union Oil Company's Agate No. 1, Sioux County, Nebraska, across the west side of the Black Hills to the Carter Oil Company's Northern Pacific Railroad No. 1, in Fallon County, Montana. The first Paleozoic period of tilting to affect the Dakota area occurred in post-Deadwood, pre-Decorah time as shown by a north-thickening wedge of pre-Decorah Ordovician limestones. There are approximately 220 feet of these limestones and thin sandstones in the Carter Oil Company test on the Baker-Glendive anticline that are not present in exposures in the Black Hills. In the Black Hills area there may have been post-Richmond, pre-Silurian tilting, for the Whitewood thickens to 815 feet in the northernmost well in the section. However, until more evidence is available, the writer considers this is the normal regional thickening of the Whitewood. This section shows a wedge of probable Siluro-Devonian dolomites pinching out on the north flank of the Black Hills. There was northward tilting in pre-Mississippian time, for in a southerly direction the lower Mississippian limestone rests on the beds ranging from Devonian to pre-Cambrian in age. There was northerly tilting in pre-Pennsylvanian time because the Minnelusa sandstone rests on successively older Mississippian rocks near the Black Hills.

The upper and middle Mississippian rocks, which are not exposed in the Black Hills, attain a thickness of 1,450 feet in The California Company's Kamp No. 1, Williams County, North Dakota. The Kamp well is about 250 miles north of the Black Hills. The lower Mississippian beds thicken northward approximately 750 feet between the Carter test on the Baker-Glendive anticline and the Kamp test. The Jurassic and Triassic beds thin slightly toward the north, but the thickness of the Permian and Pennsylvanian is fairly uniform in that direction. In the Cretaceous strata, the thickness of the Niobrara remains constant north of the Black Hills. The Benton thins slightly in the same direction but the Dakota thickness increases. All post-Permian beds become thinner between the Black Hills and the outhern limits of the section. The Permian reverses this tendency and thickens approximately 1½ feet per mile. This southward thickening is undoubtedly due to the addition of basal beds. Although there is very little evidence derived from well study, it is probable that the Pennsylvanian thickens in the area covered by the south half of the section.

In the Black Hills Folio, Darton states that the basal Sundance sand rests on

FOOTNOTE 8. N. H. Darton and Sidney Paige, "Central Black Hills, South Dakota," U. S. Geol. Survey Geol. Atlas Folio 219 (1925).

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the eroded surface of the Spearfish in Hell Canyon and Centennial Prairie. Since the Twin Creek, lower Sundance, seems to be present only in the basin north and west of the Hills, it is probable that the eastern Sundance represents only the upper part of the formation as it is developed in the west part of the Williston basin. Although both the Sundance and the underlying Spearfish thin toward the east, it is the writer's opinion that the northwest tilting and consequent erosion of the Spearfish occurred in post-Triassic, pre-Sundance time and that as erosion lowered the high area at the east, the Sundance sea encroached farther and farther upon it.

Figure 4 is a section from Saunders County, Nebraska, north to the Sioux uplift in southeast South Dakota. One should notice that the top of the Dakota sandstone dips north from Saunders County to the very edge of the Sioux uplift. This north dip can be verified by outcrops. On the north flank of the Sioux uplift, Niobrara chalk overlaps the pre-Cambrian Sioux quartzite. Although exposures of the contact with the quartzite are lacking on the south side of this uplift, it is assumed a condition similar to that on the north side exists; that is, beds ranging from the Niobrara to the Dakota are in contact with the quartzite. South of the uplift, the Dakota rests successively on Cambrian, Ordovician, Devonian, Mississippian, and Pennsylvanian. The absence of the Maquoketa shales as far so th as Saunders County, Nebraska, suggests that there was tilting during or after Ordovician time because the Silurian dolomite rests on eroded Galena dolomite,

Fig. 4. North-south cross section from Saunders County, Nebraska, to Minnehaha County, South Dakota.

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the normally intervening Maquoketa shale being absent. The fact that Devonian dolomite lies on eroded Ordovician Galena dolomite in a test drilled by the Sioux Valley Oil and Gas Company, Sec. 18, T. 90 N., R. 48 W., Union County, South Dakota, proves that there was pre-Devonian southerly tilting and erosion in the area of the Sioux uplift. There was southward tilting preceding the St. Peter sea invasion of the north part of the central United States because the St. Peter sandstone rests on successively older Ordovician, Cambrian, and possibly pre-Cambrian, as one approaches the Sioux uplift. This same overlap occurs from the Ozark Mountains on the south to Wisconsin and Minnesota on the north.

Figure 5 is a section extending from northwestern to eastern South Dakota. It shows the eastward thinning of all Cretaceous strata younger than the Dakota. The eastward thickening of the Dakota was probably caused by the deposition of sand near the Sioux uplift at the time that the Graneros shale was being laid down in the deeper parts of the basin. Wells in Manitoba and North Dakota show that the Jurassic overlaps beds as old as the Devonian. Although pre-Cretaceous erosion of the Jurassic in this area has possibly removed all evidence that the Jurassic ever overlapped beds lower than the Devonian, it is the writer's opinion that at one time Jurassic was in contact with beds as old as pre-Cambrian in eastern North Dakota. There is a thin section of Triassic in the Gulf Oil Corporatio 's Hunter No. 1, Sec. 28, T. 3 N., R. 16 E., Pennington County, and it is absent in the test drilled in Sec. 12, T. 9 N., R. 27 E., Stanley County, South Dakota. Samples from the Carter Oil Company's Semling No. 1, Sec. 18, T. 141 N., R. 81 W., Oliver County, North Dakota, show that the Sundance has overlapped the Spearfish-Minnekahta limestone and part of the Opeche formation at that point. These data suggest that the Triassic Spearfish formation is confined to the west half of North and South Dakota. The Pennsylvanian rests on successively older Mississippian beds in an easterly direction. Since there is no evidence known to the writer, suggesting that the Pennsylvanian strata overlap the beveled edges of the Devonian, Silurian, Ordovician, and older beds, it is shown as overlapping on y the upper, middle, and part of the lower Mississippian on the west flank of the Sioux uplift. Two tests drilled for stratigraphic information by the Carter Oil Company in Sec. 12, T. 9 N., R. 27 E., and Sec. 34, T. 118 N., R. 78 W., respectively, show that the Decorah shale, Black River in age, or possibly St. Peter sandstone, lower Ordovician, is in contact with pre-Cambrian rocks. In Manitoba, shales at the top of the Winnipeg sandstone, which also overlies the pre-Cambrian, contain fossils that indicate Black River age.(FOOTNOTE 9) This would make the Winnipeg shale equivalent to at least a part of the Minnesota Decorah shale. These data indicate that the Cambrian and pre-St. Peter Ordovician dolomites and sandstones, which are present on the south flank of the Sioux uplift, are abs nt in Manitoba, eastern North Dakota, and as far south as Stanley County, South Dakota.

FOOTNOTE 9. G. S. Hume, "Oil and Gas in Western Canada" (2d ed.), Geol. Survey Canada Econ. Geol. Ser., No. 5 (1933), p. 257.

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Fig. 5. Northwest-southeast cross section of South Dakota.

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Fig. 6. Isopach map of Minnelusa formation. Contour interval, 100 feet. x, location of surface section. Well symbol shows location of well data (751-S, sample data). Scale: 1.375 inches = 25 miles.

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Fig. 7. Isopach map of combined Pennsylvanian Minnelusa formation and Mississippian Pahasapa and Englewood formations. Contour interval, 100 feet. x, location of surface section. Well symbol shows location of well data (751-S, sample data). Scale: 1.375 inches = 25 miles.

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Figures 6 and 7 show that there was local folding in the Black Hills area in post-Mississippian, pre-Pennsylvanian time. A small x has been used to show the locations of measured sections, which were compiled from published information. Well locations are shown by the conventional symbol. The letter S placed on the right side of the numerical thickness denotes that the data were obtained from microscopic examination of well cuttings. Figure 6, an isopach map of the Permo-Pennsylvanian Minnelusa formation, shows that the thinnest sections of the Minnelusa are found on the north flank of the Black Hills where as little as 405 feet is exposed. Samples from a well drilled at Rapid City, on the steep east flank of the Hills, show that approximately 700 feet of Minnelusa was penetrated ther . On the south flank 1,170 feet of these beds were drilled, and on the west side 848 feet were penetrated. Figure 7 shows the combined thicknesses of the Minnelusa and the Mississippian Big Snowy, Pahasapa, and Englewood limestones. Here also, the thinnest section is found on the north flank of the Hills where 795 feet of Minnelusa-Englewood strata were measured. Samples from the Bull Creek well of the Union Oil Company of California, Sec. 3, T. 57 N., R. 62 W., Crook County, Wyoming, show that 1,071 feet of Minnelusa-Englewood strata are present down the north flank of the Black Hills. At Rapid City, 995 feet of these beds were penetrated. On the south flank of the Hills, 1,337 feet of Minnelusa-Englewood beds were drilled, and on the west flank 1,248 feet were penetrated without reachi g pre-Mississippian strata. These data show that there was between 200 and 300 feet of local thinning in the Minnelusa-Englewood rocks in the Black Hills area. Consequently, the writer believes that in pre-Pennsylvanian time the Black Hills was an anticline with 200-300 feet of local closure as opposed to the prevailing idea that anticlinal folding of the Black Hills did not take place until post-Cretaceous time. The Cambridge arch was folded in post-Mississippian, pre-Pennsylvanian time for the Minnelusa is in contact with pre-Cambrian a short distance south of the South Dakota-Nebraska line. The presence of provable pre-Pennsylvanian folding on the arch less than 75 miles southeast of the south part of the Black Hills suggests within itself that there should be pre-Pennsylvanian foldin in the Black Hills area. Furthermore, in a personal communication, E. C. Reed of the Nebraska Geological Survey reports that there is approximately 100 feet more of Mississippian on the flanks of the Hartville uplift than is present near the crest. This fact suggests that there was local folding in pre-Pennsylvanian time on this uplift also.

There was tilting toward the south and west in pre-Dakota time because that formation successively overlaps older rocks in northerly and easterly directions. There is some evidence that the Lance rests on successively older Fox Hills in an easterly direction in South Dakota. Tertiary igneous intrusions into the sedimentary rocks in the north part of the Black Hills formed many steep-sided domes. There was structural deformation after the Fort Union was deposited, for these beds are folded on the Baker-Glendive anticline of eastern Montana. In a personal communication, W. L. Russell states that Oligocene White River

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beds in South Dakota have been gently folded. This is the last known period of deformation to affect the area.

SUMMARY OF TILTING AND UPLIFT

Post-Oligocene
Post-Fort Union
Possibly post-Fox Hills, pre-Lance
Post-Morrison, pre-Lakota
Post-Spearfish, pre-Sundance
Post-Chester, pre-Pennsylvanian
Post-Devonian, pre-Mississippian, shown by absence of Chattanooga shale near Sioux uplift
Post-Silurian, pre-Devonian, in area of Sioux uplift
Post-Galena, pre-Silurian, in area of Sioux uplift
Post-Beekmantown, pre-Chazy, in area of Sioux uplift
Post-Deadwood, pre-Decorah
Pre-Cambrian folding

PRE-PENNSYLVANIAN PALEOGEOLOGIC MAP

Figure 8 represents the areal geology of the north-central Great Plains at the time that the Pennsylvanian seas advanced over the eroded land surface. This pre-Pennsylvanian map was prepared by plotting with different symbols the age of the different rocks that are in known contact with the basal Pennsylvanian beds. Where the data were obtained from outcrops, the edge of the Pennsylvanian is shown by a shaded line, the downdip side of which contains a heavily stippled border. Where subsurface control was available, the writer differentiated between the data obtained by microscopic examination of formation cuttings and that secured from drillers' logs by using a larger black dot to denote the location of the test supplying the sample data. In this manner, one can see the degree of cont ol on which the period boundaries are based. This map shows a large high pre-Cambrian area extending northward from Colorado to the longitude of Wheatland, Wyoming, about 80 miles north of the Colorado line. Although there is very little control in the intervening space, the writer joins this high pre-Cambrian area with that of a high granite area extending northwestward from the Central Kansas uplift. It is improbable that the Siluro-Devonian rocks will be found in northwestern Nebraska and in the shallow syncline separating the Black Hills from the Chadron dome. The absence of any rocks of this age in the Black Hills, on the Hartville uplift southwest of the Hills, and in the intervening area, as shown by deep wells drilled to date, leads one to the foregoing conclusion.

Beds of Cambro-Ordovician age are overlapped by the lower Mississippian limestone in the Black Hills. On the east flank of the Central Kansas uplift, the Pennsylvanian overlaps beds down to and including the pre-Cambrian. The Pennsylvanian is also in contact with the Cambro-Ordovician rocks along the east flank of the Cambridge arch which is a northwest extension of the Central Kansas uplift. Bredthauser et al. Bookwalter No. 1, Sec. 27 ? T. 18 N., R. 13 W., Valley County, Nebraska, penetrated approximately 400 feet of Cambro-Ordovician rocks beneath the Pennsylvanian. The thin pre-Pennsylvanian section in this well, which is located far east of the axis of the Cambridge arch, suggests the probability that there is a high pre-Cambrian area extending northeast from the

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Fig. 8. Pre-Pennsylvanian areal geology.

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Fig. 9. Pre-Cretaceous areal geology.

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arch to the Sioux uplift. The Devonian overlaps the Silurian on the south flank of the Sioux uplift and on the southeast flank of the Cambridge arch.

The lower Mississippian seas covered the Black Hills as well as the Laramie Mountains and the Big Horn Mountains, depositing a thick series of limestones. It is possible or even probable that the upper and middle Mississippian rocks were deposited over much of the area under discussion, for they are found in wells only 50 miles north of the Black Hills. Subsequent folding and pre-Pennsylvanian erosion was not of sufficient magnitude to remove all of the lower Mississippian rocks in the Black Hills, the Hartville uplift, and the Laramie and Big Horn mountains.

The Cambridge arch was again folded in post-Triassic time because in a southeasterly direction, the Sundance rests on successively older Triassic. The Morrison overlaps the Sundance and the Spearfish beds and it is found in contact with the Permian in Grant County, Nebraska. As the magnitude of the post-Cretaceous or early Eocene folding was greater in the area of the Black Hills than it was in the Cambridge arch area, the former stands out as the more prominent feature on the basis of present areal geology.

So far as is known to the writer, there is very little evidence available bearing on the possibility of pre-Pennsylvanian folding in the region of the Sioux uplift. There is a short section of approximately 100 feet of lower Mississippian limestone underlying the Pennsylvanian in a well near Fremont, Nebraska, 75 miles south of the uplift. Since the Fremont well is much closer to the axis of the Granite Ridge than to the Sioux uplift, it is the writer's opinion that the structural influence exerted by the Granite Ridge folding is responsible for the large amount of pre-Pennsylvanian erosion there. A well at Dennison, Crawford County, Iowa, is approximately 20 miles closer to the Sioux uplift than the Fremont well. The Dennison test reached the Devonian strata after penetrating 339 fee of lower Mississippian limestone beneath the Pennsylvanian. This thickness of lower Mississippian limestone seems to be very near normal for this part of Iowa.

PRE-CRETACEOUS PALEOGEOLOGIC MAP

Figure 9 is a pre-Cretaceous paleogeologic map representing the areal geology of the unconformity surface upon which the Dakota group of sediments were laid down. It was prepared by the same method that was used in the compilation of the one for the pre-Pennsylvanian. It shows that in an easterly direction the Jurassic overlaps the Triassic, Permian, Pennsylvanian, Mississippian, and part of the Devonian in North Dakota. The Jurassic is in turn overlapped by the Dakota which finally rests on the pre-Cambrian Sioux quartzite. Shale cuttings containing Jurassic fossils were found in a test drilled by the Commonwealth Oil and Gas Company in Sec. 26, T. 2 N., R. 9 W., first principal meridian of Manitoba. These Jurassic shales unconformably overlie Devonian strata according to the Canadia Geological Survey. According to the writer's interpretation,

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basal Sundance rocks are in contact with Permian Opeche shale, in the test drilled by the Carter Oil Company in Sec. 18, T. 141 N., R. 81 W. It is a matter of opinion whether in eastern South Dakota the Jurassic overlaps the older rocks as shown in North Dakota or whether the Permian to Ordovician strata occur as narrow bands between the Jurassic and the west edge of the pre-Cambrian quartzites, schists, and granites. In both the Carter Oil Company stratigraphic tests in Sec. 12, T. 9 N., R. 27 E., Stanley County, and Sec. 34, T. 118 N., R. 78 W., Potter County, South Dakota, sandstones of the Dakota group are in contact with Minnelusa rocks. This suggests that the North Dakota Jurassic overlap may not be present in South Dakota. On the other hand, the presence of large numbers of the cigar-shaped Belemnites densus, a Sundance index fossil, associated with many Sioux quartzite pebbles in the glacial drift of Cherokee County, northwestern Iowa, suggests that this drift came from the northwest where the Jurassic was closely associated with the Sioux quartzite. Correlation of subsurface information obtained from many wells is the only method by which this problem can be solved.

POSSIBLE PRODUCING ZONES

Table

The possible oil- and gas-producing zones in the Dakotas are discussed, beginning with the youngest and continuing to the oldest. About 25 miles west of Westhope, gas was produced from a pool near Mohall, Renville County, North Dakota. This gas occurred at a depth of approximately 280 feet in beds of Upper Cretaceous age, probably upper Fox Hills. Nine and one-half miles south of Westhope, Bottineau County, North Dakota, gas wells with an average open flow of 2½ million cubic feet were completed at a depth of 150-200 feet. This gas, which was piped into Westhope, was produced from a sandy lense at the base of the glacial drift or in the top of the underlying Pierre shale. Another small Pierre gas field was developed near Lansford in the same county.

On the Baker-Glendive anticline, gas has been produced for a long time from the Judith River and Eagle, sands found in the western equivalent of the Pierre

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shale.(FOOTNOTE 10) This sandstone extends into western North Dakota but apparently it is absent in most of South Dakota.

The Wall Creek and Newcastle sands which occur in the Benton group, both produce oil and gas in eastern Wyoming. It is possible that the Newcastle sand will be found some distance east of the Dakota-Montana boundary. A well in Lawrence County, South Dakota, is reported to have had a showing of oil in the Newcastle sand.

A wide area on both sides of the Missouri River has produced gas with the Dakota artesian water. This gas has been used locally for a number of years. Pierre, South Dakota, and Edgeley, North Dakota, have marketed gas developed from this source. The writer has no record of any live oil being found in the rocks of the Dakota group in North Dakota or South Dakota, but Noel Evans (FOOTNOTE 11) reports a seep in these rocks on Pine Ridge, west of Sundance, Wyoming.

The Sundance sand, which produces oil in several eastern Wyoming fields, is present in a large part of the Dakotas. The Embar, which is the western equivalent of the Minnekahta limestone, produces oil in Wyoming. In several places in South Dakota, the Minnekahta gives off a petroliferous odor when it is struck by the hammer. The Minnelusa formation contains several rich pay zones in the Lance Creek field. This formation produced oil in two wells in Sec. 34, T. 6 S., R. 2 E., Custer County, South Dakota. For several days, a well in Red Canyon, Sec. 17, T. 8 S., R. 3 E., Fall River County, South Dakota, produced 2-5 barrels of 41° gravity oil from the Minnelusa. Three wells produced oil from the 6,700-foot dolomite, probably Big Snowy in age, on the Baker-Glendive anticline, Fallon County, Montana. Several tests in North Dakota and South Dakota have reported showings of oil in various zones in the Big Snowy strata as well as in the underlying Pahasapa limestone. The writer believes that the deep oil on the Baker-Glendive structure is coming from beds of Devonian or Silurian age. Other tests in North Dakota and South Dakota have recorded showings of oil from similar rocks. Several showings of oil in beds of Cambrian or Ordovician age have been reported in the State Royalty Petroleum Company test, Sec. 35, T. 18 N., R. 1 E., Harding County, South Dakota.

If any oil can be found in the wedge belts, the Dakotas are destined to become a major producing area on account of the wide extent of the wedges of porous beds.

North Dakota and South Dakota contain strata in the Cretaceous, Jurassic, Permian, Pennsylvanian, Mississippian, and Siluro-Devonian that have produced oil in near-by regions. The well on the Camp Crook anticline, Sec. 35, T. 18 N., R. 1 E., South Dakota, had a good showing of live oil in strata of Cambrian or Ordovician age according to scout reports. The North Dakota Williston basin continues northwestward into northern Alberta and northeastern British Columbia.

FOOTNOTE 10. John G. Bartram and Charles E. Erdmann, "Natural Gas in Montana," Geology of Natural Gas (Amer. Assoc. Petrol. Geol., 1935), pp. 245-76.

FOOTNOTE 11. Consulting geologist, Houston, Texas.

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Table TENTATIVE CORRELATION TABLE

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Oil and gas have been produced on the southwest side of this basin at Turner Valley, Bow Island, Medicine Hat, and Foremost. On the north flank of the Canadian extension of the basin, oil and gas are produced on the Athabaska River, Peace River, Battle River, and at Viking. Oil and gas occur at Turner Valley in Cretaceous sandstone and Paleozoic limestone and at Bow Island, Battle River, Viking, Medicine Hat, Foremost, Athabaska, and Peace River in Cretaceous sandstones in the Montana and Colorado groups. Since there is commercial production on the north and west sides of the Canadian part of the Williston basin, it is logical to assume that under proper structural conditions oil and gas will be found in similar rocks in the Dakota part of this basin.

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