Kansas Geological Survey, Subsurface Geology 12, p. 77-78
by
Thomas E. Yancey
Texas A&M University
In Pennsylvanian strata of the midcontinent, cyclic strata consist of mixed carbonate-siliciclastic sediments that were deposited in a single depositional system, which is the basis for a complex and integrated mixed carbonate-siliciclastic depositional model. Pennsylvanian cyclic sediments were deposited primarily on sloping shelf surfaces (carbonate modelers refer to these as distally steepened ramps), on which carbonate sedimentation occurred preferentially on mid- to inner-shelf positions, in moderate to shallow water depths. Carbonate deposition was roughly depth-controlled. Shoreward and outward of the carbonate zone, siliciclastic sediments accumulated, characteristically as clay muds on the outer side and as quartzose sands and clay muds on the shoreward side. Siliciclastic sediment was deposited over the entire shelf, but within the carbonate zone enough carbonate sediment was produced to dilute (but not exclude) siliciclastics and generate carbonate deposits within the otherwise pervasive blanket of siliciclastics. Migration of these bands during changing sea levels over wide continental shelves (up to 100 km [60 mi]) produced the characteristic alternating siliciclastic and carbonate layers of cyclothem sequences.
The depositional relationships of carbonate and siliciclastic sediments on Pennsylvanian shelves is determinable from characteristic lithologic succession in cyclothem deposits. Cyclothems were deposited during times of regularly fluctuating sea levels, and the deepest water deposits occur in the middle of the cyclothem. This deep-water layer is siliciclastic. Both transgressive and regressive hemicycles typically contain a lithologic layer between the deep-water siliciclastics and the shorezone siliciclastics (Heckel, 1984). Consequently, carbonate deposition is more related to water depths on the shelf than to distance from the shoreline, a contradiction of ideas that the outer shelf is the primary area of carbonate deposition. Carbonate layers result from migration of carbonate-generating windows across the shelf.
In the Kansas-Oklahoma and north Texas areas, the depositional surface is known to be a shelf with highland or exposed areas on one side, and shelf-slope break to a basin on the other (Bennison, 1984; Brown et al., 1973). A sloping character for this surface can be inferred from considerations of the best geometry for cyclothem accumulation, and from comparisons with modern continental shelves. Nearly all modern continental shelves slope basinward, on shelves with mixed carbonate-siliciclastic sedimentation as well as siliciclastic sedimentation (Ginsburg and James, 1974). Higher rates of sediment accumulation nearshore result in a gradient on the shelf. Shelf gradients can be sustained during times of fluctuating sea levels (such as the Quaternary and Carboniferous), because migrating shorelines move zones of higher and lower sedimentation back and forth across the shelf. On the eastern shelf of Texas, similar inclination of depositional surfaces occurs at many levels despite the sporadic development of local carbonate buildups (Brown et al., 1973). Wide modern shelves along the Gulf of Mexico have gradients ranging from 0.4 to 0.7 m/km (1.3-2.3 ft/mi) over widths of 100-200 km (60-120 mi; for siliciclastic western Gulf and carbonate eastern Gulf), which is a reasonable figure for Pennsylvanian shelves. Such a shelf is fundamentally a ramp surface and may be called a distally steepened ramp. Confusion on terms arises from the practice of carbonate modelers to use shelf to mean a platform surface and ramp to include all graded surfaces, while noncarbonate surfaces with the same graded condition as a ramp (including the shelves that are present on modern continental margins) continue to be called shelves.
Siliciclastic-mud accumulation in deeper waters and depth control on carbonate deposition indicate that large amounts of clay mud moved across the shelf, since the central shale is commonly 5-10 m (17-33 ft) thick in Texas cyclothems. This siliciclastic mud could have bypassed the carbonate zone by transport along specific routes such as areas of deltaic deposition, or by widespread suspension transport over the carbonate sediments during storm episodes. The latter mechanism is more compatible with development of the cyclothem sequence and is a means of continually passing clay and silt across the entire carbonate belt. Many fine- and coarse-grained limestones in cyclothems contain a 20-40% noncarbonate component, and they commonly contain thin shale-parting layers, which represent short-term suppression of carbonate-sediment production. They probably record events when large amounts of siliciclastic sediment moved onto and across the carbonate zone. Boundaries between carbonates and siliciclastics should be gradational, and this is commonly seen, especially in the thicker regressive sequences of cyclothems. Gradations may be confined to 20-30-cm (8-12-inch) intervals (in vertical section) but show the expected lithologic gradient in which die margins are more argillaceous and shell-rich than the platy algal-rich beds within the central parts of the carbonate interval. Within a carbonate-deposition area, siliciclastic input is diluted but not excluded. Coated-grain and ooid-bearing deposits that occur at the tops of regressive carbonate sequences indicate conditions of very low clay-mud input into the ocean, conditions which developed only during times of ocean withdrawal from the shelf surface.
Figure 1--Controls on carbonate-sediment production on an inclined shelf.
Bennison, A.P., 1984, Shelf to trough correlations of late Desmoinesian and early Missourian carbonate banks and related strata, northeast Oklahoma: Tulsa Geological Society, Special Publication 2, p. 93-126
Brown, L. F., Cleaves, A. W., and Erxleben, A. W., 1973, Pennsylvanian depositional systems in north-central Texas: Texas Bureau of Economic Geology, Guidebook 14, 122 p.
Ginsburg, R. N., and James, N. P., 1974, Holocene carbonate sediments of continental shelves; in, The Geology of Continental Margins, C. Burk and C. Drake (eds.): Springer-Verlag, New York, p. 137-155
Heckel, P. H., 1984, Factors in midcontinent Pennsylvanian limestone deposition: Tulsa Geological Society, Special Publication 2, p. 25-50
Kansas Geological Survey
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Web version May 12, 2010. Original publication date 1989.
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