Geology of the Souris River Area North Dakota
Lemke, Richard W.
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The Souris River area comprises about 5,500 square miles in north-central North Dakota. It is bounded roughly by the east loop of the Souris River on the east, by the Max moraine on the southwest, and by the International Boundary on the north. Much of the total area is a ground-moraine plain. The Souris River and its tributary, the Des Lacs River, are the main streams. Most of the area, however, is without integrated drainage. The highest point is on the Max moraine at an altitude of about 2,500 feet. The lowest point is in the Souris River valley at the International Boundary at an altitude of 1,410 feet. The northeast-facing escarpment of the Max moraine forms the boundary between the Great Plains physiographic province and the Central Lowlands province. The part of the Great Plains province that is within the mapped area includes the Max moraine and is known as the Coteau du Missouri.A large part of the proposed Missouri-Sour is irrigation project in North Dakota is within the mapped area. Extensive exploration for oil within and adjacent to the area has resulted from discovery of large oil reserves elsewhere in the Williston basin. Farming, however, remains the chief industry, Minot, with a population of 22,032 in 1950, is the largest city. Most of the area is mantled with surficial deposits, chiefly of Pleistocene age. Rocks of Late Cretaceous and Tertiary age underlie the surficial deposits and crop out locally. Data on subsurface formations are based on logs of several wells drilled for oil. One well in the northeast part of the area penetrated Precambrian rocks at a depth of 8,262 feet. In addition to Tertiary and Cretaceous sediments, Jurassic, Triassic, Mississippian, Devonian, Silurian, and Ordovician rocks have been recognized in the drill holes. The Pierre shale, Fox Hills sandstone, and possibly the Hell Creek formation of Late Cretaceous age directly underlie surficial deposits along the east and northeast margins of the mapped area. Elsewhere, Tertiary rocks underlie the surficial deposits. The Pierre shale is nowhere exposed in the area, but well data and exposures nearby show that it consists largely of grayish blue, poorly indurated shale, which has pronounced rectangular partings. Several species of Foraminifera from the formation have been identified. There is only one exposure of possible Fox Hills sandstone. It consists chiefly of poorly consolidated medium-grained orange-yellow sandstone that contains concretions. Although the Hell Creek formation under- lies Tertiary rocks in the western part of the area, it is believed to be nowhere exposed and may not persist far enough east to come in direct contact with the surficial deposits.The Tertiary rocks consist of the Cannonball and Tongue River members of the Fort Union formation of Paleocene age. The marine Cannonball member is the uppermost bedrock unit in most of the eastern and southeastern parts of the area; a few exposures along the Souris River valley and to the south-east indicate a maximum thickness of about 40 feet. The member consists chiefly of thinly bedded fossiliferous brown sand and sandy shale. It contains numerous species of Foraminifera, some newly described. The continental Tongue River member overlies the Cannonball member and is the uppermost bedrock unit of most of the area. Under the Max moraine, it may have a thickness greater than 900 feet. The Tongue River member consists of poorly consolidated sandstone, sand, silt-stone, shaly clay, and lignite. A few vertebrate and invertebrate fossils have been found. The lignite beds, some of which are 10-15 feet thick, are the chief aquifers of the area. Other beds range widely in permeability. Slope stability also varies markedly. The Pleistocene deposits are Wisconsin in age, and, except for a few deposits along the valley of the Des Lacs River, all are believed to be of the Mankato substage of glaciation. The deposits have been mapped as the following geologic units: Max moraine, ground moraine, overridden ice-contact deposits, linear-ridge deposits, diversion-channel deposits, river-terrace deposits, kames and eskers, end moraines in the Souris loop area, ice-marginal outwash-channel deposits, glaciofluvial deposits undifferentiated, and deposits of glacial Lake Souris. The Max moraine covers the southwest part of the area. Its surface is characterized by innumerable knobs, ridges, and kettles and other undrained depressions. The moraine consists mostly of stony clay till, generally 100-200 feet thick; locally, where it fills preglacial valleys, it may be much thicker. The Max moraine forms the surface of the Coteau du Missouri and owes much of its prominence to a buried bedrock platform. When dry, the till of the Max moraine stands in nearly vertical slopes, but when wet, it tends to slump; it is nearly impermeable. Ground moraine is the most widespread map unit. It forms a relatively fiat plain which slopes gently northeastward from the Max moraine to the area covered by deposits of glacial Lake Souris. In most places, its thickness is 50-200 feet. It consists essentially of stony clayey till. Pebbles and larger size stones are mostly carbonate rocks, followed by granitic and gneissic rocks. The till of the ground moraine, like that of the Max moraine, is very stable when dry and is nearly impermeable.The overridden ice-contact deposits are exposed locally along the Souris River from Minot downstream for several miles. They are both overlain and underlain by till and consist mostly of fine sand to coarse gravel a few inches to about 30 feet thick. In the large-size fraction, carbonate rocks predominate; granitic and gneissic rocks are next most abundant. Many of the beds are characterized by small, moderate- to high-angle faults, which suggests collapse and slump. In the southeast part of the area, many straight parallel ridges trend southeast. These ridges are interpreted to be a special type of drumlinoidal feature, but, because definite proof is lacking as to their origin, they have been mapped as "linear ridge deposits." Shallow parallel grooves in the intervening ground moraine and arcuate transverse ridges of till, interpreted to be recessional moraines are associated with the ridges. The linear ridges are generally 1-3 miles long, 5-15 feet high, and have even crestlines. The largest ridge, however, is 13 1/2 miles long and 15-30 feet high. Larger ridges consist mostly of stratified sand and incorporated bodies of till. Ridges less than 5 feet high consist almost entirely of till. Glacial inelt water that was diverted from the Souris River valley by ice has cut several large diversion channels. The main diversions were about 25 miles downstream from Minot, where shallow channels, mostly 1/2 - 2 miles wide, were cut in ground moraine. Laid down in these channels were deposits of gravel, sand, and silt, generally 5-15 feet thick. Terrace remnants, possibly kame terraces, are fairly numerous along the valley walls of the Souris and Des Lacs Rivers. Most are less than half a mile long and a few hundred feet wide. The terrace deposits range in thickness from a few inches to more than 50 feet; they range in composition from silt to boulders, but sand and gravel predominate. Carbonate rocks are most abundant; granite and gneissic rocks are next most abundant. Sorting is poor to moderate. Permeability and porosity are high. Most of the coarser fraction is satisfactory for construction purposes. The abundant kames and eskers are characterized by variations in size, shape, lithology, and structure. They have the common features of (a) consisting predominately of poorly sorted gravel, sand, and silt, and of minor amounts of till, (b) possessing collapsed bedding, and (c) having specific dimensions and landforms different from other glaciofluVial deposits. Five huinmocky areas surrounded by ground moraine have been mapped as end moraines in the Souris loop area. They are not typical of other end moraines in the region because they contain large amounts of stratified ice-contact deposits incorporated in the otherwise stony clay till. During melting of the last glacier ice covering the region, glacial melt water cut a conspicuous and unusual pattern of long outwash channels in the ground moraine. These channels are believed to have formed mostly along successive positions marginal to the southwest side of the last ice lobe to cover the area. Outwash that consists of gravel, sand, silt, and clay underlies the bottom and, in a few places, the walls of the channels. This ranges in thickness from a thin skin to 20 feet or more. Permeability of the sand and gravel is high, and special problems of leakage will probably arise if canals or dams are built in this material. Many stratified glacial deposits, of uncertain origin, are scattered throughout the mapped area; they are described under the heading Glaciofluvial deposits, undifferentiated as to origin." Deposits of glacial Lake Souris cover most of the eastern part of the area. Their surface is conspicuously fiat. The deposits range from a known thickness of 73 feet to a feather- edge. They consist mostly of sand and silt and in most places are underlain by till. Recent deposits are landslides, dune sand, and alluvium. However, all three probably began to form while the last ice sheet was melting from the area and hence are partly con- temporaneous with some of the latest Pleistocene deposits. Large and impressive landslides are abundant along the val- ley walls and tributaries of the Des Lacs River and along the valley walls of the upper reaches of the Souris River. In the Des Lacs River area, long parallel blocks of Fort Union strata, overlain by a thick section of till, have broken away from steep valley-wall slopes and have rotated, producing a series of suc- cessively lower ridges parallel to the valley wall. Little mate- rial is sliding at present. In the Souris River valley area, only till and stratified glacial deposits appear to have slid. The type of slide in the Souris River valley forms crescent-shaped areas of low, arcuate, parallel ridges and undrained depressions on the upslope side. There is no evidence of recent sliding and most of the material probably moved soon after the glacial ice melted from the valley. Dune sand is confined mostly to the glacial Lake Souris area. The dunes consist of parallel to subparallel ridges, as much as 50 feet high, whose long axes generally trend northwest. Dune groups formed by several closely spaced individual dunes also trend northwest. Most dunes are partly stabilized by shrubs and small trees but some are active. Permeability is high. Alluvium consists of clay, silt, sand, and fine gravel deposited chiefly along the valley floors of the Souris and Des Lacs Rivers, and their main tributaries. It is probably less than 30 feet thick under the valley floors of the two main valleys and is underlain by glacial outwash of similar composition. Permeability is generally less than in other stratified surficial deposits and subsurface drainage is generally poor. Slope stability generally is low to moderate. Most of the area is believed to lie within a broad, shallow syncline that trends northwest into Canada. A zone of folds and faults may extend many miles parallel to the escarpment of the Coteau du Missouri along a bedrock bench just west of the mapped area. The escarpment itself may be a fault scarp. If so, the Coteau du Missouri may be high structurally as well as topographically.The preglacial drainage pattern differed from the present pattern. The ancestrial Knife River crossed the Coteau du Missouri and may have flowed northward through the eastern part of the mapped area. Parts of the valleys of the Souris and Des Lacs River probably were cut in Pleistocene time, prior to the last glaciation. North Dakota was glaciated at least three times. Tills of the lowan, Tazewell, and Mankato substages of the Wisconsin stage have been recognized south of the mapped area. However, except for a few exposures along the valley of the Des Lacs River that might be of Tazewell or lowan substage, only deposits of Mankato substage have been recognized in the Souris River area. During the later part of the Mankato substage, a long, narrow lobe of ice advanced southeastward across the area between the escarpment of the Coteau du Missouri and the Turtle Mountains. This lobe, here named the Souris River lobe, extended a short distance southeast of the mapped area. At approximately the same time that the Souris River lobe covered much of the Souris River area, a second lobe of ice advanced around the east flank of the Turtle Mountains and covered the area east of the Souris River area. This lobe is named the Leeds lobe. During deglaciation, the sides and the terminus of the Souris River lobe melted simultaneously, and southeast-trending ice-marginal channels formed along the southwest side of the lobe. The southern part of glacial Lake Souris came into existence as the Souris River lobe melted back from low ground southeast of the mapped area; further melting expanded the lake to the northwest. The lake first overflowed southeastward through spillways; finally, when the Souris River valley became free of ice, the lake drained northward into Canada. Mineral resources include lignite, petroleum, gas, construction materials, sodium sulfate (Glauber salt), and clay. The last two have little or no present economic significance. North Dakota produced about 3,260,973 tons of lignite in 1950, of which 577,515 came from the Souris River area. The Souris River area lies mostly in the eastern part of the North Dakota lignite fields, although its eastern boundary is beyond the lignite-'bearing strata. Natural exposures of lignite are scarce and present-day mining is confined almost exclusively to a few strip mines south of Velva. Reserves in the Souris River area are estimated at 9,762.5 million short tons. Oil was first discovered in the State in 1951, about 75 miles west of Minot. By 1952, twelve dry holes had been drilled in the Souris River area. In 1953 oil reportedly was found in the northeast part of the area. Some kinds of construction materials are abundant in the Souris River area but others are scarce. Sand and gravel are plentiful and nearly all the deposits are suitable for road metal. Most deposits can also be used for concrete aggregate and other purposes. On the other hand, quarry rock suitable for building purposes is almost totally lacking. Surface geology and agricultural development are directly related. Some characteristics of the geologic units favor agricultural development; others are detrimental. Poor surface drainage and low permeability of the ground moraine severely limit irrigability of much of the area. On the other hand, the ground moraine is well suited for dry farming. Poor surface drainage prevents rapid runoff and the nearly impermeable till holds moisture in the upper part of the soil profile. The till decomposes rather quickly into a fertile soil. A direct relation also exists between geology and construction. At present, the major construction is road building. Sand and gravel are plentiful for bituminous mix, as well as for road metal. Till is the most common subgrade material, forming a stable roadbed. It compacts well, is little susceptible to frost heaving, and is fairly stable in road cuts. Roads built across stratified glacial deposits are generally stable. Several types of problems, however, develop in building roads across deposits of glacial Lake Souris. Roads built in sand are subject to wind erosion of grade, filling of ditches, gullying, and lateral displacement of grade, owing to low cohesion of the sand grains. Roads built in silt deposits are susceptible to frost heaving. Drainage is a problem in building roads across valley alluvium. Lignite beds should be avoided in constructing roads because they are poor fill material and are the chief aquifers. Landslides should be avoided because of the danger of reactivating the slides. Leakage is probable in canals constructed in some outwash deposits.