Kansas Geological Survey, Subsurface Geology 12, p. 79-81
Darwin R. Boardman II and James E. Barrick
Texas Tech University
The glacial-eustatic control mechanism for Late Pennsylvanian cyclothems in the North American midcontinent has been postulated for some time beginning with Wanless and Shepard (1936), and more recently further developed by Shenck (1967), Evans (1967), Heckel (1977, 1980, 1989), Boardman et al. (1984), Boardman and Heckel (1989), and Boardman and Malinky (1985). However, the effects of the glacial-eustatic sea-level fluctuations on the distributions of invertebrate faunas have yet to be adequately investigated.
Preliminary analysis of ammonoids, conodonts, and fusulinids from Late Pennsylvanian strata in both the northern and southern midcontinent (north-central Texas) show profound patterns of faunal distribution that can be best attributed to the repetitive changes in sea-level. Five faunal intervals have been identified from latest Desmoinesian to early Virgilian strata (fig. 1). These faunal intervals are bounded either by major unconformities (Type 1), or major transgressions.
Figure 1--Midcontinent sea-level fluctuation curve for lower Upper Pennsylvanian showing maximum transgressive Kansas units on top and the north-central Texas units on the bottom; KAV-1 to KAV-15 represents Kawvian Epoch transgressive event numbers; faunal intervals with appropriate faunal characterization on the right.
The Desmoinesian-Missourian boundary, corresponding to the boundary between the Latest Desmoinesian and the Early Missourian Faunal Interval, is characterized by several major extinction events including the conodonts Neognathodus, Idiognathodus delicatus, I. concinnus, and I. antiquus, the ammonoid Gonioglyphioceras, the fusulinid Beedeina, and the chonetid brachiopod Mesolobus. This extinction event separating these two faunal intervals corresponds to a major lowering of sea level (Type 1 unconformity) that also marks the Middle-Upper Pennsylvanian (Oklanian-Kawvian) Series boundary. The initial Missourian transgression (KAV-1) is of an intermediate magnitude and is characterized by relatively few new occurrences. However, among the first occurrences at this boundary include the appearances of the nodose Idiognathodus species group including I. lobatus, I. eccentricus, I. clavulatus, I. sagittalis. Additionally, strata deposited during this transgression is marked by the appearances of the ammonoid Pennoceras, and the gastropod Plocezyga (Plocezyga) costata. Within the Early Missourian Faunal Interval, the largest transgression KAV-3 is marked by the appearances of the conodonts Gondolella sublanceolata, G. denuda, and the first species of Streptognathodus sensu stricto (S. cancellosus). Additionally, the appearance of the fusulinid Eowaeringella and the ammonoid Schistoceras is noted.
The Early Missourian-Middle Missourian Faunal Interval boundary is marked by a Type 1 unconformity characterized by the extinctions of the fusulinid Eowaeringella, the conodonts I. eccentricus, I. clavulatus, I. sagittalis, Streptognathodus cancellosus along with the ammonoids Pennoceras, Eoschistoceras, Bisatoceras, and Maximites. The base of the Middle Missourian Faunal Interval corresponding to the KAV-4 transgression is characterized by the appearance of the fusulinid Triticites, conodonts Streptognathodus confragus, Idiognathodus magnificus, and I. toretzianus as well as the ammonoid Somoholites kansasensis. Additionally, this transgression marks a major ecologic shift in the Idiognathodus-Streptognathodus plexus. After the KAV-4 transgression, Streptognathodus occupies the more nearshore carbonate and terrigenous clastic facies while Idiognathodus is more common in the more offshore facies previously characterized by Neognathodus. Ammonoid assemblages after the KAV-4 transgression also demonstrate a changeover in generic composition. For the first time, Schistoceras, Gonioloboceras, Subkargalites, and Prouddenites are dominant components of the faunal association. Within the Middle Missourian Faunal Interval, several faunal changes of lesser magnitude are also noted. Strata deposited during the second major transgression (KAV-5) are characterized by the appearance of the Streptognathodus elegantulus-S. gracilis-S. excelsus species group of conodonts, and the first occurrences of the ammonoids Preshumardites, Paraschumardites, and Uddenoceras. The third major transgression (KAV-6) produced a minor change in conodont and ammonoid faunas, characterized by the striking acme of large lobed Idiognathodus magnificus, the appearance of Streptognathodus oppletus, and the first occurrences of the ammonoids Gleboceras and Aristoceras.
The boundary separating the Middle-Upper Missourian Faunal Interval is not characterized by an identifiable Type 1 unconformity and accompanying major extinction horizon. However, this boundary corresponds to a major transgressive event (KAV-10) which produced a plethora of first occurrences including the conodonts Idiognathodus simulator, Streptognathodus firmus, and S. alekseevi, the first thickly fusiform fusulinid Triticites primarius, the ammonoids Neoglaphyrites, Cardiella, and ?Aktubites n.sp. The regression following the KAV-10 transgression, however, is responsible for the extinction of the Streptognathodus elegantulus-S. gracilis-S. excelsus species group.
The Upper Missourian-Early Virgilian Faunal Interval is typified by a Type 1 unconformity. A major changeover in ammonoid faunas characterizes this boundary, including the appearances of ?Aktubites stainbrooki, Vidrioceras, Marathonites, Neopronorites, Emilites incertus, and Uddenoceras harlani. Additionally, the appearance of the conodont Streptognathodus zenthus, and the gastropod Plocezyga (Plocezyga) obscura is prominent.
Late Pennsylvanian faunal intervals, as defined herein, are usually marked by Type 1 unconformities, which, although short-lived, are responsible for relatively major and perhaps periodic extinctions. When these extinction events were followed by major transgressions, the faunal-interval boundaries are also marked by extensive speciation events. When these faunal-interval boundaries were followed by minor transgressive intervals, the boundaries are primarily recognized by extinction events only. Faunal-interval boundaries marked by major transgressions without accompanying Type 1 unconformities are recognized primarily by speciation events without major extinctions. Previous interpretations showing Type 1 unconformities at all cyclothemic boundaries are not supported by extensive field evidence nor by faunal data. The use of faunal-extinction data to crosscheck interpretations for magnitudes of sea-level drops is a potentially important tool for sequence stratigraphic modeling.
All of the above-described faunal intervals are present in both the northern as well as the southern midcontinent. Additionally, preliminary analysis suggests that they can be recognized in the Illinois basin, Appalachian basin, Marathon uplift, and the southern Urals, suggesting that these faunal intervals are indeed controlled by major glacial-eustatic fluctuations. The potential exists for developing a system of biostratigraphic zones using a combination of ammonoids, conodonts, and fusulinids within a cyclothemic framework that can be employed to effect correlations on a global scale.
Boardman, D. R., and Heckel, P. H., 1989, Glacial-eustatic sea-level curve for early Upper Pennsylvanian sequence in north-central Texas and biostratigraphic correlation with curve for midcontinent North America: Geology, v. 17, p. 802-805
Boardman, D. R., and Malinky, J. M., 1985, Glacial-eustatic control of Virgilian cyclothems in north-central Texas: American Association of Petroleum Geologists, Transactions of the Southwest Section, p. 13-23
Boardman, D. R., Mapes, R. H., Yancey, T. E., and Malinky, J. M., 1984, A new model for the depth-related allogenic community succession within North American Pennsylvanian cyclothems and implications on the black shale problem; in, N. Hyne (ed.), Limestones of the Midcontinent: Tulsa Geological Society, Special Publication 2, p. 141-182
Evans, J. K., 1967, Depositional environment of a Pennsylvanian black shale (Heebner) in Kansas and adjacent states: Ph.D. dissertation, Rice University, Houston, Texas, 162 p.
Heckel, P. H., 1977, Origin of phosphatic black shale facies in Pennsylvanian cyclothems of midcontinent North America: American Association of Petroleum Geologists, Bulletin, v. 6 1, p. 1,045-1,068
Heckel, P. H., 1980, Paleogeography of eustatic model for deposition of midcontinent upper Paleozoic cyclothems; in, Paleozoic Paleogeography of West-central United States, T. Fouch and E. Magathan (eds.): Rocky Mountain Section, Society of Economic Paleontologists and Mineralogists, Symposium 1, p. 197-215
Heckel, P. H., 1989, Current view of midcontinent Pennsylvanian cyclothems; in, Middle and Late Pennsylvanian Chronostratigraphic Boundaries in North-central Texas--Glacial-eustatic Events, Biostratigraphy, and Paleoecology; a guidebook with contributed papers, part H--contributed papers, D. Boardman, J. E. Barrick, J. M. Cocke, and M. K. Nestell (eds.): Texas Tech University, Studies in Geology 2, p. 17-34
Schenk, P. E., 1967, Facies and phases of the Altamont Limestone and megacyclothem (Pennsylvanian), Iowa to Oklahoma: Geological Society of America, Bulletin, v. 78, p. 1,369-1,384
Wanless, H. R., and Shepard, F. P., 1936, Sea-level and climatic changes related to late Paleozoic cycles: Geological Society of America, Bulletin, v. 47, p. 1, 177-1,206
Kansas Geological Survey
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Web version May 12, 2010. Original publication date 1989.