Kansas Geological Survey, Open-file Report 2002-61
Evan K. Franseen1, Howard R. Feldman2, R.M. Joeckel3, and Philip H. Heckel4
1Kansas Geological Survey
2Exxon-Mobile Upstream Research
3Conservation and Survey Division, University of Nebraska-Lincoln
4Department of Geoscience, University of Iowa
KGS Open-file Report 2002-61
Late Paleozoic strata, consisting of repetitive alternations of carbonate and siliciclastic rocks (cyclothems; Wanless and Weller, 1932) that occur throughout North America, have long been the focus of studies attempting to isolate primary controls on facies patterns and stratal geometries (e.g. Wanless and Weller, 1932; Moore, 1936, and see Ross, 1991 for a review). Most studies have emphasized a glacioeustatic and/or tectonic control for facies patterns. Especially in the Midcontinent, glacioeustacy has been invoked as the major control on the formation of Pennsylvanian cyclothems (e.g. Wanless and Shepard, 1936; Heckel, 1986; 1994). The correlation of marine units overlying subaerial exposure surfaces over subcontinental distances has further enforced a dominant eustatic control on facies architecture and sediment distribution patterns. As an example, major Missourian to Virgillan flooding surfaces in Kansas can be correlated to the Texas succession (Boardman and Heckel, 1989), and with the Appalachian basin (Heckel et al., 1998). With the recognition of glacioeustacy as the dominant control on Midcontinent stratal patterns, more recent studies have interpreted cyclothems within a sequence stratigraphic context (Heckel, 1994; Heckel et al., 1998; Watney et at., 1995).
Other recent studies have emphasized the impact of climate on facies patterns in cyclothemic strata. Several studies have documented long-term climate shifts during the Late Paleozoic based on changes in the proportion of climate-sensitive rock types (Cecil, 1990; West et al., 1997). Other studies have focused on shifting climate patterns tied to sea-level position at the scale of individual cyclothems in Pennsylvanian and Permian strata of North America (Tandon and Gibling, 1994; Miller et al., 1996; West et al., 1997; Rankey, 1997).
This study builds on these previous contributions to the analysis of Late Pennsylvanian strata in the Midcontinent. Based on extensive outcrop and subsurface data, and utilizing a sequence-stratigraphic approach, we have identified eight successive sequences in Missourian to Virgilian strata that can be traced regionally in the Midcontinent from Iowa/Nebraska to SE Kansas, near the Oklahoma border (Figs. 1, 2, 3). Detailed descriptions of the sequences can be found in Feldman et al. (1998). Within each sequence, systems tracts are defined (following terminology of Van Wagoner, 1996), which allows us to place facies patterns and geometries of strata from proximal to more basinward locations within the context of relative sea-level position. This in turn allows us to evaluate the influence of other controlling factors, including climate. Our results indicate that: 1) the most robust indicators of climate are obtained from sequence boundaries and lowstand deposits (incised valley fills), 2) there is a climate cycle with a duration of several sequences in the study interval, 3) this climate cycle is intermediate in scale and duration between long duration climate shift throughout the Pennsylvanian and into the Permian, and short term variability observed within individual sequences, and 4) this intermediate scale climate cycle had a dramatic impact on facies distribution and stratal geometries, being most profound in the expression of incised valley fills of the lowstand systems tract, and on the highstand systems tract, but had little impact on transgressive systems tracts.
Figure 1--Generalized stratigraphy in northeastern Kansas of strata referred to in text. Numbers refer to sequences mentioned in text, and sequence boundaries are names for the overlying sequence. Sequence boundary 3 is basinward of the lowstand shoreline for this area. Sequences 1, 2, 7 and 8 contain small incised valleys, whereas sequences 4, 5, and 6 contains large, sand-filled incised valleys
The outcrop belt from which our data were obtained extends from southeastern Kansas north to Iowa and Nebraska (Fig. 2). This line of section is generally along depositional dip during Missourian and Virgilian deposition, although the axis of siliciclastic transport and deposition was somewhat oblique to this section, coming more from the northeast. This area was a broad, stable, subsiding shelf with increasing subsidence basinward to the south (Watney et al., 1995). Tectonic elements such as the Nemaha uplift controlled accommodation space and impacted fluvial incision patterns.
Figure 2--Map of the study area showing locations of measured sections (dots) and line of the cross section shown in Figure 3.
Figure 3--Cross section of upper Missourian to lower Virgilian rocks exposed along outcrop belt (see Fig. 2), but also including shallow subsurface data. Section is slightly oblique to depositional dip. Small-scale variations in thickness have been smoothed to increase readability. The thickness of thin units, such as black shales and coals, has been exaggerated. Sequence boundaries are shown as solid lines only in areas where they can be confidently correlated. Basinward extent of sb 4 is based on the extension of the large incised valley basinward in the subsurface; no interfluve sb has been recognized for this sequence. Cross section based on Bowsher and Jewett (1943), and supplemented with our own observations (Archer and Feldman, 1995; Archer et al., 1994; Cunningham and Franseen, 1992; Joeckel 1989, 1994, 1995; Feldman et al. 1993, 1995; Lanier et al., 1993; and unpublished data). Additional data from Ball (1964), Goebel et al. (1989), Kansas City Power and Light (1975), and Troell (1969).
The shelf within the study area was alternatively exposed and submerged during high-frequency sea-level cycles, although the position of the both the highstand and lowstand shorelines varied from cycle to cycle. The length of time represented by these Pennsylvanian sea-level cycles is difficult to establish precisely. All estimated cycle durations have been based on dividing the number of cycles into estimated duration of periods or stages. Common estimates range from 230,000 to 400,000 years per cycle (e.g. Heckel, 1986; Goldhammer et al., 1991; Connolly and Stanton, 1992). The most recent, and probably most accurate, estimation of cycle duration is based on radiometric ages of Late Pennsylvanian caliches, which yielded an average cycle duration of 163,000 years (Rasbury et al., 1997, 1998).
Climate indicators are best preserved at sequence boundaries (paleosols) and in lowstand systems tracts (LST's), typically expressed as incised valleys and their fills. One group of sequence boundaries is characterized by high chroma paleosols with slickensides and common pedogenic carbonate. The incised valleys associated with these sequence boundaries are small (generally less than 1 km wide and less than 20 m deep), and are filled mostly with locally derived limestone conglomerate and shale deposited in fluvial to estuarine environments. The plant assemblages are dominated by gymnosperms, mostly xerophytic walchian conifers. These sequence boundaries are interpreted to have formed in relatively dry seasonal climates with well-drained interfluves and small valley networks. The other group of sequence boundaries is characterized by large incised valleys (over 300 m wide and over 20 m deep) that are filled with mature sandstone and mudstone deposited in fluvial to estuarine environments. Paleosols are not commonly preserved on interfluves, but where the are present range from low to high chroma. Coal occurs both on interfluvial paleosols and within valley fills. The plant assemblage from the incised valley fills is dominated by fern foliage, seed ferns, and sphenopsids. These sequence boundaries reflect high water tables and large, extensive river systems indicating wetter seasonal climates.
When arranged in stratigraphic order, from basal sequence to upper sequence, the data indicate that the lower two sequences developed under relatively dry seasonal conditions (Fig. 4), sequence 3 may reflect a climate transition, sequences 4,5 and the sequence boundary and LST of sequence 6 formed under relatively wet seasonal conditions, and sequences 7 and 8 reflect a return to relatively dry seasonal conditions.
Figure 4--Comparison of sea-level history with climate indicators. The sea-level history for this interval is interpreted based on the types and landward extent of marine facies and from the maximum basinward extent of exposure indicators (Heckel, 1986).
Transgressive systems tracts (TST's) in all eight sequences generally are characterized by thin, extensive limestones and mostly by thin shales, suggesting a minor climate impact probably because rising seas sequestered siliciclastic sediment in updip positions. The highstand system tracts (HST's) show a variable range of expression (extent and abundance of siliciclastic facies versus limestones) consistent with the overall dry-wet-dry climate pattern through the sequences similar to the climate shifts from the sequence boundary and LST data. HST's that consist of a core shale and extensive, thick limestone with little overlying shale are associated with dry-type paleosols and LST'S. HST's dominated by thick marine shales are associated with wet-type paleosols and lowstands.
Other factors affected sequence development. Broad patterns of uplift and subsidence determined where accommodation space could be generated and controlled fluvial drainage patterns. Carbonate-dominated facies in shelfward areas during wet-type sequences reflect development away from siliciclastic source areas and depocenters. A test of the climate model is the relationship of sequence climatic expression to magnitude of sealevel change of individual sequences. Although there is a general decrease in magnitude of sea-level cycles in the wetter versus drier climate cycles (based on landward extent of the most offshore marine facies in the condensed section), one of the one of the largest magnitude sea level changes occurs within the middle of the wetter climate interval. Thus, climate plays a role distinct from that of sea-level change, and our results indicate that modest climate shifts over the scale of several sequences have a clear impact on sediment dispersal patterns and sequence architecture.
A study of eight Missourian to Virgillan (Pennsylvanian) carbonate and siliciclastic sequences in the Midcontinent documents a cyclicity of climate ranging from wet seasonal to dry seasonal with a duration of several sequences which is intermediate, both in magnitude and duration, to previously documented longer term shifts to drier climates over much of the Pennsylvanian and shorter-term shifts described within individual sequences. Although the climate shifts were only modest (wet seasonal to dry seasonal), this intermediate scale of climate change had a profound impact on sequence architecture and sediment dispersal patterns.
Paleosols at sequence boundaries and lowstand incised valley fills (LST's) contain the diagnostic evidence of the climate shifts between wet seasonal and dry seasonal. Wet seasonal climate indicators include low chroma paleosols, presence of coals, plant fossil assemblages dominated by fern foliage, seed ferns, and calamities (swamp dwellers), and large incised valleys dominated by large volumes of sediment from distant sources. Dry seasonal climate indicators include paleosols with large slickenside sets, pedogenic carbonate and high chroma values, absence of coals, plant fossil assemblages dominated by walchian conifers (well-drained landscapes) and abundant fusain (occasional forest fires), and small incised valleys dominated by locally derived sediment.
TST's in all eight sequences generally are characterized by thin limestones and thin shales suggesting a minor climate impact. TST character may be more indicative of rapid rates of relative sea-level rises and relative sediment starvation due to rising sea level.
In general, HST's of the eight sequences match with the evidence at sequence boundaries and in LST's for an overall dry-wet-dry climate shift throughout the succession, based on abundance of shale versus limestones and thickness of shale. Two HST's that do not show as clear a relationship appear to coincide with times of climate transition.
Other factors affected sequence development as well. Broad patterns of uplift and subsidence determined where accommodation space could be generated and controlled fluvial drainage patterns. Carbonate-dominated facies in shelfward areas during wet-type sequences reflect development adjacent to siliciclastic source areas and depocenters. Sealevel position determined the locations of lowstand shorelines, timing of the fluvial transport and subsequent deposition of sand and mud. However, climate determined the extent of development of fluvial drainage networks and the supply of siliciclastic sediment available for deposition.
There may be a link between the intermediate-scale climate shifts and a similar scale sea-level cyclicity. There is a general pattern of wetter climate cycles being associated with low-magnitude sea-level oscillations, and drier climate cycles with higher magnitude sea level fluctuations. In addition, wetter climates, as determined from lowstand evidence, correspond to lower magnitude flooding of the shelf during highstand deposition.
Our results, indicating that even relatively subtle intermediate-scale climate shifts can have a direct impact on sediment dispersal patterns and sequence architecture, can help develop predictive models that include variations in sediment. Sequence boundaries and LST's are likely to be the best locations to find diagnostic evidence of climate. Climate indicators from these locations may be useful in predicting facies patterns in HST'S. An increased understanding of climatic conditions aids in evaluating the relative importance of climate, sea level and other factors as dominant controls on sequence and sequence-set development.
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Kansas Geological Survey, Geology
Placed online Aug. 30, 2007; OFR from 2002.
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