Dakota Formation

The Dakota is a sedimentary geologic unit name of formation and group rank in Midwestern North America. The Dakota units are generally composed of sandstones, mudstones, clays, and shales deposited in the Mid-Cretaceous opening of the Western Interior Seaway. The usage of the name Dakota for this particular Albian-Cenomanian strata is exceptionally widespread; from British Columbia and Alberta to Montana and Wisconsin to Colorado and Kansas to Utah and Arizona. It is famous for producing massive colorful rock formations in the Rocky Mountains and the Great Plains of the United States, and for preserving both dinosaur footprints and early deciduous tree leaves.
Owing to extensive weathering of older rocks during the Jurassic and Triassic, the Dakota strata lie unconformably atop many different formations ranging in age from Precambrian to Early Cretaceous. With a few local exceptions, it is the oldest Cretaceous unit exposed in the northern Great Plains, including Kansas, Nebraska, Iowa, Minnesota, and Wisconsin, as well as the Desert Southwest. It generally consists of sandy, shallow marine or beach deposits with marine-influenced mudflat sediments, and occasional stream deposits.

Naming and rank

and F.V. Hayden first used Dakota in 1862 to name the distinctive red sandstone exposures along the Missouri River near Dakota City, Nebraska. But, with this name, they applied the term "group", which at that time had the meaning of formation rank, as presently used. Dakota Formation is the unit's primary name and rank in the Great Plains. Formation rank is also applied in western extents as the unit thins and exhibits formational characteristics, the marine shales are absent, and fossil pollen species correlate with those found in the unit on the Missouri River. In the San Juan Basin and other intermontane basins and plateaus of the Southwest, Dakota Sandstone is the formal name for the oldest Cretaceous sandstone as well as tongues of that terrestrial sandstone extending into the dark marine shales of the Mancos. However, Dakota Sandstone is everywhere a common informal name for the unit, especially for the sandstone beds.
The Dakota Group rank is employed along the territory of the Dakota Hogback in Colorado and Wyoming, the Colorado Plateau, the Dry Cimarron River, and the Denver Basin. This group ranking recognizes sequences and members that exhibit local formational characteristics, especially marine shales that are less developed or absent further from the center of the seaway. As these locations were at the time of formation the earliest and deepest areas of the seaway, these groups can include older ages of rocks than are usually included elsewhere under the Dakota name. The names of the member formations of the Dakota Group vary between these regions as the geology there is studied further; but, the earliest unit included in Dakota Groups, excluded elsewhere, is the terrestrial Lytle Formation, which is older than any other Cretaceous rock in Colorado or Kansas. The Skull Creek Shale and Plainview Sandstone of Colorado are also included in the Dakota Group, as is the Cedar Mountain Formation of Utah and its Buckhorn Conglomerate member. However, these units represent a separate seaway sequence in the time between the Lytle and "upper Dakota" sequences, and in the plains to the east, the same units are named Kiowa Shale and Cheyenne Sandstone, which are considered separate from the Dakota Formation as defined in Kansas and, moreover, do not appear at the type location in Nebraska. However, subsequent sequence stratigraphic and palynostratigraphic research has demonstrated that the Dakota Formation at the type location includes sand and mudstones covering the same ages and sequences as the "marine shale facies of the Graneros" and "early Late Albian Kiowa-Skull Creek".
In certain places, the classification is undivided; for example, in far southwest Utah, the strata is designated the Dakota Conglomerate without further division.
When nearer the surface, the sandstone beds of the Dakota Formation form various Dakota Aquifers, important water sources in some areas of the Great Plains and the Southwest, far greater in extent than the High Plains Aquifer, and famous for its artesian properties.
Elsewhere, when deeper, especially in the Denver Basin, these same sands have been sought after for hydrocarbon reserves. "Drillers", who navigate deep strata by monitoring material brought to the surface in the drilling mud, alphabetically designated the hydrocarbon "producing sands" of the Dakota in the Denver Basin as "D", "J", "M", and "O". Colorado's "D" and "J" of the driller's Denver Basin have been particularly important to Nebraska, Kansas, and Oklahoma in their efforts of correlating their eastern outcrops with the Dakota units in the Denver Basin and the western units in general.

Geological history

Deposition of the sediments that would become the Dakota Formation began during the late Early Cretaceous. This deposition marked a reversal from over 100 million years of erosion. This reversal was due to rising of the mouth of the rivers, called a rise in base level, as the Cretaceous Seaway formed. This rise lowered the gradient of the rivers causing them to deposit sediment inland because their velocity could no longer sustain high volumes of sediment.
Measurements show that the rivers flowed westward and southwestward towards the encroaching sea from source areas near the present-day Great Lakes. The point of deposition slowly moved eastward as the seaway rose. This change is seen by a gradual shift in the composition of sandstones from having a lot of Paleozoic-age rock detritus in Kansas to sandstones having all Precambrian crystalline rock debris in Iowa.
This shift means that the rivers had completely eroded away the Paleozoic rocks in the river source area by the time the Seaway rose high enough for the rivers to deposit sediments in Iowa. The very top of the Dakota Formation was deposited along the coast as indicated by some fossil marine invertebrates. Fossil plants, coal deposits and kaolinite clays show that the climate was warm and wet during deposition of the Dakota Formation. Some of the ancient preserved soils show that an extensive flood plain forest was present.

Western Interior Seaway sequences

This Cretaceous seaway experienced a number of geological sequences, which, during particular lowstands, temporarily reestablished a land connection between the east and west continent at the ancestral Transcontinental Arch. Each sequence represents a cycle of major progression of the seaway into the western interior of North America followed by retreat. The sequences of the seaway typically express facies sequences of, first, a low-stand erosional surface discontinuity, then a transgressive pattern of terrestrial sand and mud followed by near shore marine sediments, a high stand pattern that may establish far-shore marine shale and limestone, a regressive pattern of a return to near shore marine sediments to terrestrial mud and sand, and a final low-stand erosional surface. Five of the first sequences of the Western Interior Seaway are relevant to the Dakota classifications.
The first sequence, typified by the Lytle Formation, did not complete the linkage of the north and south embayments before retreating. The second sequence is typified by the corresponding Skull Creek and Kiowa formations. These first two sequences are not present at the type area along the Missouri River. The sediments broadly considered as Dakota then record the Mowry sequence with the Muddy, J, or Lower Dakota sandstones and the D or Upper Dakota sandstones forming at the discontinuities at the beginning and end of that cycle. In the east the limited marine shales of the Mowry sequence are assigned to the Dakota Formation, while in the center the mudstones and marine shales are commonly assigned a separate unit between upper and lower sandstone units, and in the Southwest, the much thicker marine shales are assigned to tongues of the Lower Mancos. The Greenhorn Cycle is the final relevant sequence as it overlays all Dakota classifications, with the exception of certain sandstones of Graneros age, such as classified in Wisconsin and Iowa.

Lithology

Over the range of the usage of the Dakota name, the unit is primarily known for its massive beds of sandstone, which commonly shows shades of red, but also gray, yellow, or white. The sand was carried and deposited by rivers or accumulated in dunes or shoreline strands, and later cemented by red iron oxide or white calcite, depending on the local groundwater conditions that followed the sedimentation. The degree of cementation can range from softly crumbling to resistant to hammering. The sandstone beds can have local conglomerations of gravel. The composition of the sand and gravel varies depending on the sources of the rivers that made each deposit.
The amount of sandstone, averaging 25-50%, can very greatly over short distances between extremes of 5% to 80%. The general remainder of the unit, however, generally in complementary 80% to 5% proportions, is layered mudstone and clay deposited on floodplains, swamps, and estuaries. Similar to the sand, the soil-forming mud was modified by groundwater conditions to accumulate iron oxide or calcite. Coloration can be dark to light red, grey, yellow, and white. Iron oxide accumulation can approach the hardness and luster of hematite.
In evidence of the general low-lying nature of the Dakota's lush, hot-house Earth environment, lignite and coal have formed in various areas.
Marine shale is also a part of the Dakota sequences. Less common in the remote extents, particularly in the east, the shale is more representative of the deeper portions of the inland seaway. Moreover, shales on top of the upper Dakota on the plains of the east are usually assigned to the Granola or equivalent units, while in the west the thickest interbedding "tongues" of shale are generally assigned to the lower Marcos. Nevertheless, near the western limits, where the Dakota "pinches out" between the Morrison and the Mowry, the unit returns to the totally terrestrial sand-mud-sand pattern and fossil pollens of the Nebraska type location.
These characteristics of chaotic, land-formed sandbanks and mudflats lying above flinty, whiter marine megacyclic Permian limestones and below grey, rhythmic, chalky shales, persist for thousands of miles, locally variable as they may be, causing common use of the name Dakota in spite of many efforts to apply localized names.