Lower Pennsylvanian deposits of the midcontinent region are distinguished by paleontological and lithological characters and by widespread interruption of sedimentation associated with local crustal disturbances which set these rocks apart from younger formations. The Lower Pennsylvanian comprises strata which are assigned to the Springeran and Morrowan Series.
The magnitude of the hiatus at the base of Springeran deposits in the type area--the Ardmore Basin of southern Oklahoma--is not known. The type Morrowan section of northwestern Arkansas overlies the Pitkin limestone (Upper Mississippian), rocks of Springeran age being absent. Morrowan rocks (Kearny) unconformably overlie Mississippian limestone in western Kansas (Thompson, 1944; Maher, 1947) and in various parts of Colorado, Morrowan deposits unconformably overlie Leadville limestone (Lower Mississippian), erosional remnants of Upper Mississippian limestone, or Mississippian (?) black shales (Thompson, 1945).
One of the greatest diastrophic disturbances of Pennsylvanian time affected parts of North America near the close of Morrowan time. Deformation occurred in the Arbuckle area (Criner Hills) and the ancestral Rocky Mountains were uplifted. Isolated remnants of marine Morrowan rocks which remain after pre-Atokan or early Atokan erosion include deposits called Kearny formation in western Kansas, Glen Eyrie shale in Colorado, and Belden formation in western Colorado and Utah. Eroded Kearny and Glen Eyrie beds in western Kansas and eastern Colorado, respectively, are unconformably overlain by rocks of Atokan or Desmoinesian age (Cherokee shale in Kansas, Fountain formation of eastern Colorado). The type Morrowan rocks of Arkansas are overlain unconformably by Winslow sandstone (late Atokan), and the Wapanucka limestone (Morrowan) in eastern Oklahoma is overlain unconformably by middle Atokan shales. The Sloan limestone (Morrowan) of the Llano uplift occurs as erosional remnants unconformably below the Big Saline limestone (Atokan) with coarse conglomerate (Gibbon) at the base, on the east and the west sides of the uplift (Plummer, 1947; Thompson, 1947).
Few fossils are known from Springeran deposits but the Morrowan Series contains a rich marine fauna which resembles that of the upper part of the Chesteran (Late Mississippian) succession. Several groups of Mississippian fossils, particularly blastoids and certain bryozoans and brachiopods, are very similar to Morrowan forms. This invertebrate fauna contains several distinctive Pennsylvanian elements, however. Fusulinids are very abundant in early Pennsylvanian rocks of many areas, but all observed fossils of this group belong to Millerella. This genus occurs in rocks considered to be late Chesteran (upper Kinkaid formation) in Illinois (Cooper, 1947) and Mississippi (Mellen, 1947). Also, Millerella occurs in post-Morrowan rocks. It is abundant in Atokan strata of many areas, less abundant in Desmoinesian rocks, and it occurs sparsely throughout higher Pennsylvanian formations. Since Millerella is the only known genus of fusulinids in pre-Atokan Pennsylvanian rocks, this part of the system is designated as the Zone of Millerella (Fig. 5). Deposits belonging in this part of the section can be recognized by common occurrence of Millerella and absence of other fusulinids (Moore & Thompson, 1949).
Figure 5. Fusulinid zones and ranges of fusulinid genera in Pennsylvanian rocks of Kansas and adjacent areas. (Modified from Moore & Thompson, Am. Assoc. Petroleum Geologists.)
Springeran rocks are not known to be represented in Kansas. The time of Springeran sedimentation in other areas, during which thousands of feet of deposits were made in the southern Oklahoma and Arkansas geosyncline, was an age of weathering and erosion of exposed Mississippian and older rocks in the Kansas portion of the midcontinent region. If there was sedimentation in parts of Kansas during this earliest part of the Pennsylvanian Period, such deposits have not been discovered.
Deposits of Morrowan age are exposed in northeastern Oklahoma and are recognized under the name of Kearny formation in the subsurface of western Kansas (Thompson, 1944; Maher, 1947). The Morrowan deposits belong in the Zone of Millerella (Fig. 5). Morrowan rocks are well exposed in northeastern Oklahoma but outcrops have not been discovered in other parts of the northern midcontinent region. This division of the Pennsylvanian is recognized in agreed classification as applicable to Kansas and Oklahoma. Probably throughout the northern midcontinent area, including Pennsylvanian deposits north of the latitude of Tulsa, the lower boundary of the Morrowan Series, where rocks of this age are present, coincides with the major unconformity that separates Pennsylvanian from older systems. The upper boundary is a well-marked disconformity in most places, but in parts of the subsurface there is indication that Morrowan strata are limited above by a disconformity or nonconformity. The superjacent rocks seem definitely assignable to the Desmoinesian Series in parts of the region, but in other parts post-Morrowan-pre-Desmoinesian beds have been differentiated.
Rocks of Morrowan age which have been identified from well samples from southwestern Kansas are classed as belonging to the Kearny formation (Thompson, 1944). The type section (C, Fig. 6) is located in northwestern Kearny County, Kansas, and comprises 127 feet of alternating greenish gray finely crystalline limestone beds and dark greenish to black shale. Fossils are found in layers distributed throughout most of the section. Among them are fusulinids belonging to the genus Millerella (M. marblensis, M. pressa, M. pinguis, M.? advena, and M.? advena ampla), which are not associated with more advanced genera of fusulinids (Thompson, 1944, p. 415). The Kearny beds lie unconformably on Upper Mississippian (Ste. Genevieve) limestone. They are overlain unconformably by Desmoinesian deposits in the type section and presumably throughout the eastern border area of pre-Desmoinesian Pennsylvanian deposits in southwestern Kansas (Fig. 6), but westward the Morrowan rocks are separated from Desmoinesian strata by a varying thickness of Atokan deposits. The Atokan beds are judged to rest disconformably on the Kearny formation,
Figure 6. Sections of Morrowan and Atokan rocks encountered in wells in southwestern Kansas. These strata are not exposed anywhere within the borders of Kansas but are represented by outcrops in Oklahoma.
The thickness of Kearny deposits ranges from a featheredge along its eastern border to a maximum reported thickness of about 440 feet (A, Fig. 6) in southern Morton County, near the southwestern corner of Kansas (Edson, 1947, Eason Oil Company's No. 1A Carver well in SW sec. 32, T. 34 S., R. 42 W.). At this place the top of the Morrowan rocks was penetrated at a depth of 4,260 feet. Thickness of Kearny beds in southern Hamilton County, Kansas (B, Fig. 6), is about 215 feet (Maher, 1947) and in a well just west of the Kansas-Colorado boundary near the southwestern corner of Greeley County, 230 feet of rocks are assigned to the Morrowan (Collins, 1947).
Kansas Geological Survey, Geology
Placed on web Jan. 26, 2009; originally published Nov. 1949.
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