Stratigraphic relationships of Devonian and Mississippian rocks in the western Mid-Continent region provide overwhelming evidence that mid-Paleozoic seas were transgressive and widespread throughout the area. The present distribution of these marine strata is largely the result of erosion along the Transcontinental arch and adjacent uplifts.

Strata of Early Devonian age are known only in areas that were included in the Cordilleran, Ouachita, and Arbuckle geosynclines and the Tobosa, Illinois, and North Kansas basins. These rocks were deposited on a highly eroded terrane. In medial Devonian time the seas transgressed southward from Alberta, Canada, into the North Dakota part of the Williston basin; westward and southward from Iowa into the Kansas-Nebraska part of the North Kansas basin; and eastward from the Cordilleran geosyncline into western Colorado. Middle Devonian strata are absent from Oklahoma and the Transcontinental arch, but are present farther south in the Tobosa basin and the Ouachita geosyncline. If Middle Devonian strata were deposited in Oklahoma and South Dakota, they have been completely removed by pre-La e Devonian erosion which was widespread throughout the eastern half of the western Mid-Continent. Late Devonian rocks indicate a resumption of transgression, and they may have covered the entire region although present distribution of these rocks is the result of Early Mississippian and later periods of emergence along the Transcontinental and associated arches.

Mississippian rocks are conveniently divided into Lower Mississippian Series (Waverlyan) which comprises the Kinderhookian Stage and the Osagian Stage, and the Upper Mississippian Series (Tennesseean) which is made up of the Meramecian and Chesteran strata. The dark-shale units of western Montana and northwestern Wyoming, the shale and siltstone units of the Williston basin, and the black shale units which surround the Ozark uplift seemingly contain the Devonian-Mississippian systemic boundary. Elsewhere, basal Mississippian clastic and carbonate rocks are found unconformably not only on Devonian but also on Silurian, Ordovician, Cambrian, and Precambrian rocks. Local facies changes of Mississippian rocks in the area of the Ozark uplift indicate that this region was emergent during th early part of Mississippian time.

Mississippian rocks of the western Mid-Continent are dominantly marine, shallow-water deposits. Seaways must have been very extensive in Mississippian time and the Kinderhookian transgressing sea probably came from both the north and south and was deposited on a peneplain-like surface. In western Kansas, late Kinderhookian rocks overlie Early and medial Ordovician rocks and are overlain by late Osagian rocks. Conodonts recovered from cores of western Kansas indicate that Upper Devonian rocks once were present west of the Central Kansas uplift.

Mississippian rocks probably covered the entire Mid-Continent and Rocky Mountain regions but were most thin over the Transcontinental arch and the incipient Ancestral Rockies. Locally in south-central Kansas, rocks of Osagian age rest directly on pre-Mississippian rocks and show either (a) that the sea covered territory not inundated by the first transgression, or (b) that rocks of the Kinderhookian Stage were entirely eroded before deposition of Osagian rocks. Some workers believe that a major unconformity is present between Lower and Upper Mississippian rocks on the basis of megafossil studies recovered from outcrops peripheral to the Ozark uplift. However, study of microfossils from these rocks fails to support this hypothesis.

Late Meramecian carbonate rocks in western Kansas

End_Page 1901------------------------------

contain abundant quartzose clastics thought to be derived from the Central Kansas uplift and Las Animas arch.

In the Williston basin, Mississippian rocks are dominantly carbonates but in Late Mississippian time thick evaporites were deposited. Late Mississippian or Chesteran deposits are not common in the Rocky Mountain region and may have been removed by pre-Pennsylvanian erosion. Chesteran time was characterized by advance and retreat of the shallow seas in the Mid-Continent area and strata reflect rhythmic displacement of the strandlines into basinal areas. The Chesteran seas were less widespread than Meramecian seas.

Present distribution of Mississippian rocks outlines Late Mississippian-Early Pennsylvanian tectonic features, but the dominant tectonic feature throughout Devonian-Mississippian time was the Transcontinental arch.

End_of_Article - Last_Page 1902------------