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Ichnology of a Pennsylvanian Equatorial Tidal Flat

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Sedimentary Environment

Paleocurrent Analysis

Paleocurrent data have been collected as part of the facies analysis. Integration of paleocurrent information and facies is essential, because paleocurrent patterns are controlled environmentally, with coastal settings displaying complex patterns resulting from the interaction of waves, tidal currents, and fluvial input (Klein, 1977). Paleocurrent orientations were determined from the azimuths of the axes of relict troughs in unit A1, the azimuths of the axes of gutter casts in unit A1, the azimuths of the ripple trains in unit B1, and the dip direction of cross-lamination in unit D2 (fig. 71). Additionally, the azimuth of the channel axis of unit D3 also was recorded.

Figure 71--Equal-area paleocurrent rose diagrams in the Stull Shale Member at Waverly trace fossil site. A. Axis of gutter casts in unit A1. B. Ripple trains in unit B1. C. Axis of relict troughs in unit A1. D. Dip direction of cross-lamination in unit D2.

Data from relict troughs were collected from two separate bedding planes. Orientation means from both surfaces do not show significant variations, ranging from 104° (or 284°) to 126° (or 306°). Paleocurrent data from ripple trains were measured from five different bedding planes. Orientation means range from 102° (or 282°) to 127° (or 307°). Orientations of ripple trains and relict troughs are therefore consistent and suggest that waves approached from a southwest-northeast direction. Assuming a northwest-southeast-oriented shoreline, as suggested by paleogeographic reconstructions, waves were approaching perpendicular to the intertidal zone. Although in situ gutter casts are relatively rare, available data indicate a north-northwest-south-southeast orientation with an orientation mean of 154° (or 334°). Additionally, gutter casts found as loose blocks commonly display ripple trains at the top oriented perpendicular to the axes of gutter casts. Trends in gutter casts suggest orientations oblique to perpendicular to the shoreline, indicating roughly downslope flows.

Data also were collected from cross-laminations in two intertidal runoff channels. One of these channels shows bipolar-bimodal distributions, indicating roughly north-south flood and ebb flows. The other channel displays unidirectional paleocurrent patterns, with a vector mean of 38°. Finally, the fluvial channel axis is oriented 10° (or 190°), suggesting a trend from perpendicular to oblique to the shoreline.

Klein (1967, 1977) noted that tidal-flat environments exhibit a quadrimodal paleocurrent pattern, with upper-intertidal zones displaying both landward- and seaward-oriented tidal flows and lower-intertidal zones having bipolar tidal currents flowing parallel to the shoreline. Data from the upper-intertidal facies at Waverly indicates tidal flow perpendicular to the coastline. However, rippletrain orientation in the lower-intertidal facies indicates flow perpendicular to the shore rather than parallel to depositional strike as suggested by the quadrimodal model. The latter probably reflects the direction of wave propagation rather than tidal flow.

Depositional Model

The facies model for siliciclastic tidal flats proposed by Klein (1977) was based on studies of modern tidal flats (e.g., Hantzschel, 1939; Van Straaten, 1952, 1954, 1961; Reineck, 1963, 1967, 1972). Examples in the fossil record are quite common (e.g., Klein, 1971; Sellwood, 1975; Carter, 1975; Ovenshine, 1975; Tankard and Hobday, 1977). Klein's facies model includes the upper-, middle- and lower-intertidal zones. Because tidal energy increases seaward, tidal flats in general are landward-fining, in contrast to wave-dominated shorelines. Therefore, a typical tidal-flat profile in a landward direction consists of a lower-intertidal sand flat, a middle-intertidal mixed (sand and mud) flat, and an upper-intertidal mud flat. Landward of the mud flat, supratidal salt marshes are present; the subtidal zone occurs seaward of the sand flat. Upper tidal flats are dominated by deposition of fine-grained suspended particles; lower tidal flats are characterized by bedload transport of sand-sized sediment. Middle-intertidal areas are typified by alternation of traction and fallout from suspension. Klein (1971) listed 10 phases involved in tidal sedimentation and subsequent modification of the deposits: (1) tidal-current bedload transport with bipolar-bimodal reversals of flow direction, (2) time-velocity asymmetry of tidal-current bedload transport, (3) late-stage ebb outflow and emergence with sudden changes in flow directions, (4) alternation of tidal-current bedload transport with suspension settlement, (5) tidal slack-water mud deposition, (6) tidal scour, (7) exposure and evaporation, (8) burrowing and organic diagenesis, (9) differential compaction, loading, and hydroplastic readjustment, and (10) high rates of sedimentation combined with regressive sedimentation.

Most of the phases of tidal transport mentioned above are recognized at the Waverly locality. Evidence of tidal-current bedload transport coupled with flow reversals is recorded by current ripples exhibiting bimodal-bipolar paleocurrent patterns in intertidal runoff channels of unit D2. Evidence of time-velocity asymmetry is typical of high-energy intertidal sand bodies and subtidal sand waves and is lacking in the lower-energy tidal-flat facies of Waverly. Late-stage emergence and sudden changes in flow directions are evidenced by interference ripples, flat-topped ripples, water-falling marks, and washout structures in units A1 and B1. Alternation of tidal-current bedload transport with suspension fallout is recorded by heterolithic bedding (flaser, wavy, and lenticular) and mud drapes in units A1, B1, D1, and D2. Evidence of tidal slack-water suspension fallout is represented by mud drapes and flaser bedding in units A1, B1, D1, and D2. Flute marks and gutter casts in units A1 and B1 record tidal scour. Exposure is indicated by wrinkle marks and mudcracks in units A1 and B1. Evidence of burrowing is extensive, particularly in unit B1 and, to a lesser degree, in unit A1. Load casts and sand volcanoes in unit B1 record soft-sediment deformation. Finally, high rates of sedimentation coupled with regression are clearly evidenced by the four separate progradational parasequences, A, B, C, and D.

Six major subenvironments have been identified in the lower interval of the Waverly section (fig. 72): (1) sand flat, (2) mixed flat, (3) mud flat, (4) supratidal paleosols, (5) intertidal runoff channels, and (6) fluvial channel. The sand flat is represented by unit B1. Current-bedload transport was the dominant sedimentary process, but mud deposition during slack-water periods also occurred. Biogenic activity reached a maximum in this subenvironment. The mixed flat is represented, at least partially, by units A1, C1, and D1. Mixed-flat deposits originally may have been present in unit B2, but if so, they were obliterated subsequently by pedogenic processes. Tractional sand deposition and mud settlement were equally important. Biogenic activity was relatively restricted. The mud flat is recorded, in part, by units A1 and D1. Mud-flat sediments also originally may have been present in units B2 and C2, but any evidence was completely destroyed by pedogenic processes. Deposition of mud from suspension was the dominant depositional process. Biogenic activity was remarkably scarce. Supratidal paleosols are represented in units B2 and C2. These paleosols developed in marshes landward of the intertidal zone, but as a result of the progradation, pedogenic processes affected underlying mud- to mixed-flat deposits. Intertidal runoff channels occur in unit D2, where they dissect mud- to mixed-flat deposits. There is no evidence of biogenic activity in these channels. The fluvial channel is represented by unit D3. There is no evidence of tidal influence in this channel and biogenic activity is restricted to abandoned channels.

Figure 72--Depositional model of the Stull Shale Member at Waverly.

Tidal flats may develop in a number of depositional environments within the coastal setting, including fluvioestuarine transitions, estuary bays, and open-marine shorelines (Buatois, Mángano, et al., 1997a; Mángano and Buatois, 1997). High diversity of trace fossils suggests that the tidal flat at Waverly was formed on the open coast under normal-marine conditions.

Paleotidal Range

Estimation of paleotidal range from tidal-flat sequences remains problematic. Klein (1971) provided a discussion of methods for determining paleotidal range, and he advanced his own method, which is based on measurement of the preserved thickness of the prograding tidal-flat succession. Terwindt (1988) discussed the shortcomings of the available methods and suggested some general criteria for estimating tidal ranges from ancient deposits. Simpson (1991) also discussed the associated problems and attempted to estimate the tidal range of a Cambrian tidal flat, taking into account paleorelief, thickness of tidal-flat deposits, trace fossils, and bedform amplitude.

Paleotopographic information is not available for the preserved deposits at Waverly. According to Klein (1971), the thickness of the sediment interval from low tidal-flat facies to high tidal-flat deposits is equivalent to the mean tidal range. Simpson (1991) noted that this method assumed constant sea level, lack of subsidence, insignificant sediment compaction, and a conformable succession.

The maximum recorded thickness of a tidal-flat progradational succession at Waverly is 1.10 m in parasequence D. This value, however, includes essentially upper-intertidal, mud-flat deposits. Although no detailed analysis of subsidence rates is available for the Waverly locality, isopach maps suggest renewed subsidence during deposition of the Shawnee Group (Lee, 1943). No evidence of soft-sediment deformation was observed in parasequence D, but in other parasequences (e.g., parasequences A and B) compaction due to sediment loading was relatively significant as evidenced by the presence of synsedimentary deformation structures. These factors undoubtedly influenced the resulting thickness preserved in the stratigraphic record. Although subsidence may lead to an overestimation of the tidal range, compaction may result in an underestimation of the actual range. Therefore, estimation of the paleotidal range based on thickness of the Waverly tidal-flat sequences is quite risky.

Simpson (1991) attempted to use trace fossils as a tool for defining tidal range, but he recognized that they are of limited value in constraining high- and low-tide lines. Actually, trace fossils may be only indirectly used to estimate tidal range because ichnologic information is basically a tool to refine facies interpretations based primarily on physical sedimentary structures. At Waverly, the highest diversity of biogenic structures is found in the sand-flat deposits, probably close to the low-tide line. Low ichnodiversity in the mixed- to mud-flat facies is probably related to a stressful regime associated with rigorous conditions.

Bedform amplitude was one of the criteria listed by Terwindt (1988) as useful in distinguishing between macro-, meso-, and microtidal settings. He suggested that thickness of crossbedded sets tend to be greater where tidal ranges are greater. Based on this criterion, the low amplitude of bedforms at Waverly suggests low tidal ranges. Simpson (1991) noted that bedform amplitude is problematic, because it also may be controlled by velocity and water depth.

Terwindt (1988) also pointed out that the number and dimensions of intertidal drainage channels may yield insights into tidal range, with channels being small, widely spaced, or even absent under lower-tidal regimes. Intertidal channels at Waverly are rather small and are suggestive of lower paleotidal ranges.

In high-tidal ranges, tides influence fluvial systems; consequently diagnostic sedimentary structures and bedding types are present tens of kilometers landward from river mouths (e.g., Smith, 1988; Gastaldo et al., 1995). Fluvial-channelized deposits of unit D3 lack physical structures indicative of tidal action, suggesting low tidal ranges by default. Although most, if not all, criteria currently used to estimate paleotidal ranges are somewhat problematic, all the available evidence points to a microtidal regime for the Waverly tidal flat.

Effects of Climate on Deposition

Paleogeographic reconstructions suggest an equatorial position for Kansas during the Late Carboniferous (Scotese and McKerrow, 1990). Kansas underwent a northward migration from approximately 200S in the Early Carboniferous to 15°N during the Late Permian (West et al., 1997) (fig. 73). West et al. (1997) analyzed the importance of climate to explain differences among the five different types of cyclothems recognized in the Permian-Carboniferous of Kansas. Study of lithofacies within the different cyclothem types (Cherokee, Kansas, Shawnee, Wabaunsee, and Permian) indicates a long-term Pennsylvanian-Permian drying trend. Virgilian Shawnee cyclothems are characterized by an abundance of fossiliferous mudrocks and limestones, with variegated mudrocks also present. In contrast to the underlying Kansas-type cyclothems, black mudrocks are rare. Coals are very thin or absent. Sedimentologic features of the Shawnee cyclothems suggest a transition from tropical-rainy and wet-seasonal climates to drier-seasonal climates (West et al., 1997).

Figure 73--Variation of paleolatitudinal position of Kansas during the late Paleozoic. Note equatorial position for Waverly during the Virgilian. Based on West et al. (1987).

Analysis of tidal-flat deposits at Waverly is compatible with the paleoclimatic situation for the Shawnee cyclothems suggested by West et al. (1997). Absence of evaporite deposits, little or no plant debris, high density of trace fossils in a zone between the high- and low-water levels with few biogenic structures at the high-water level, and moderate mud accumulation suggest climatic conditions that were intermediate between the humid regime of the earliest Virgilian and the more arid conditions that characterize the lower Permian.

Sequence Stratigraphy

Utility of Trace Fossils in Sequence Stratigraphy

The use of trace fossils in sequence-stratigraphic interpretations is a very promising field in ichnology (see reviews by Pemberton, MacEachern, et al., 1992, and Savrda, 1995). Trace fossils aid in sequence stratigraphy by allowing recognition of allostratigraphic surfaces (e.g., MacEachern et al., 1992), identification of system tracts (e.g., Pemberton, MacEachern, et al., 1992), and characterization of parasequences (e.g., Pemberton, Van Wagoner, et al., 1992; Martin and Pollard, 1996).

At Waverly, ichnofossils are valuable tools for understanding the depositional evolution in relationship to sea-level history. In this section, we discuss the Stull Shale Member and related units within a sequence-stratigraphic framework. Trace-fossil information is integrated with sedimentologic and stratigraphic data to obtain a more accurate picture of the depositional history.

Parasequence Stacking Patterns

The sedimentary units of the lower part of the Waverly succession form four parasequences separated by successive flooding surfaces. A fifth parasequence is represented in the upper half of the Waverly succession, including the Spring Branch Limestone Member. Each of the parasequences records a regressive event of tidal-flat progradation. Typical tidal-flat successions constitute regressive fining-upward packages (e.g., Klein, 1971; Terwindt, 1988), and coarsening-upward successions are unusual, even in overall transgressive settings where fining-upward packages occur. This type of situation reflects local progradation of tidal flats (Sellwood, 1975; Dalrymple, 1992). A rare example of a coarsening-upward transgressive tidal-flat succession was described by Reineck (1972). A somewhat similar situation to that of Reineck (1972) was recorded by Kvale and Archer (1990) from the Pennsylvanian Brazil Formation, Indiana, where a coarsening-upward tidal sequence indicates transgression over a coastal peat swamp. This package is replaced upward by a fining-upward succession, which indicates a subsequent regression in the area (Kvale and Archer, 1990).

The four parasequences in the lower part of the Waverly succession form a parasequence set. Parasequence stacking patterns reflect the ratio between rate of deposition and rate of accommodation space (Van Wagoner et al., 1988). Progradational parasequences are formed when the rate of deposition exceeds the rate of accommodation. Facies and trace-fossil analyses suggest a shallowing trend from parasequence A to D, indicating that the lower parasequence set is progradational. For example, lower-intertidal deposits are only present in the lower parasequences, and the fluvial facies is restricted to the top of parasequence D. Vertical distribution of trace fossils from highly diverse marine associations near the base (unit B1) to a monospecific suite of terrestrial trackways (Unit D3) also supports an overall regressive event. Facies and stratigraphic analyses indicate that the lower part of the Stull Shale Member represents part of a highstand systems tract.

A major transgressive surface separates the fluvial facies at the top of the lower parasequence set from the overlying subtidal orthomyalinid bivalve packs tones and wackestones. This surface marks the base of the second parasequence set. In the Stull Shale Member, this set is represented by the orthomyalinid packstone and wackestone parasequence. Although this set is only partially exposed at Waverly and individual parasequences cannot be distinguished, a retrogradational pattern within an overall transgressive trend is evident. The transgressive Orthomyalina beds extend over an area of more than 2,000 km2 in east and southeast Kansas (West et al., 1996). The transgressive aspect of the upper parasequence set also is evidenced by deposition of open-marine carbonate facies recorded by the Spring Branch Limestone Member. This upper part of the Waverly succession represents a transgressive systems tract.

The sequence-stratigraphic significance of the fluvial deposits is problematic. Two alternative hypotheses are suggested. The first is that establishment of the fluvial system may record maximum progradation during stillstands, representing the top of the highstand systems tract. In this interpretation the basal erosive surface resulted from autocyclic processes. The second alternative is that the fluvial body resulted from channel incision associated with a lowering of sea level. In this case, the base of the fluvial channel should be considered a sequence boundary and the fluvial deposits part of the lowstand systems tract. Available information suggests that the first hypothesis is the most reasonable. Examination of coeval successions fails to reveal that this surface has regional, wide lateral extent. Additionally, the presence of fluvial deposits at the tops of regressive successions is more easily explained as the result of simple progradation without invoking any discontinuity. Therefore, we infer that lowstand deposits were absent from this area. A sequence boundary is located at the inversion from the progradational to the retrogradational para sequence stacking patterns (Jackson et al., 1990). Accordingly, the base of the transgressive orthomyalinid packstones and wackestones is considered a co-planar surface, indicating amalgamation of lowstand and transgressive erosion.

Sequence-stratigraphic Significance of Outside Shales and Paleosols

Outside shales exhibit significant variability and include deltaic, paralic, and fluvial facies (Heckel, 1985, 1990, 1994; Watney et al., 1989). Although typically mudrock units, locally some outside shales may be sandstone dominated (Heckel, 1994). There is a general agreement that accumulation of cyclothems was controlled by sea-level changes, and that glacial eustasy related to the advance and retreat of Gondwanan ice sheets was the responsible mechanism (Heckel, 1994).

Heckel (1994) stated that outside shales were formed during low stands of sea level. However, detailed analysis of vertical-facies changes and bounding surfaces suggest a different scenario for the Stull Shale Member at Waverly. At Waverly, progradational stacking patterns of tidal-flat strata and associated facies provide evidence that the lower part of the Stull Shale Member represents a highstand systems tract. Wackestones and packs tones of the upper part of the Waverly section belong to a transgressive systems tract. No significant deposition occurred during the lowstand. Clearly, sea-level controls involved in the deposition of outside shales are more complex than envisaged by traditional cyclothem models.

Development of paleosols on mixed- to mud-flat facies overlying sand-flat deposits indicates progradation. Paleosols have been used as evidence for subaerial exposure in late Paleozoic cyclothems of the North American midcontinent (e.g., Heckel, 1986; Watney et al., 1989). Additionally, paleosols typically are considered to be part of lowstand systems tracts (e.g., Heckel, 1986). However, paleosols may occur in two different stratigraphic settings with contrasting implications in terms of sea-level position. When paleosols sharply overlie open-marine deposits, they most likely develop as a result of sea-level lowering (e.g., unconformity-based paleosols in valley interfluves). In contrast, when paleosols occur at the tops of regressive packages, where evidence of progradation from open-marine to coastal, deltaic, and even fluvial facies occurs, they probably record a regressive peak during the maximum highstand/stillstand. The latter situation seems to be very common within outside shales (Heckel, 1994, p. 69), and it suggests that part of what historically have been considered as lowstands actually represent highstand systems tracts. In the present case, the stratigraphic position of the paleosol suggests that pedogenic processes occurred during a regressive maximum and that the paleosol is part of a highstand systems tract.

Ichnology of Key Stratal Surfaces

Trace fossils may help in the recognition of key stratal surfaces. Substrate-controlled ichnofacies are instrumental in the recognition of erosional discontinuities in the stratigraphic record (MacEachern et al., 1992; Pemberton, MacEachern, et al., 1992). Several such surfaces exist in the Waverly succession and two of them contain trace fossils.

The transgressive surface of erosion that separates parasequence A from parasequence B (TS1) marked the flooding of mud- and mixed-flat deposits (unit A1) and an increase in water depth with subsequent deposition of lower-intertidal sand-flat facies (unit B1). Mud- and mixed-flat deposits contain unlined burrows (Palaeophycus tubularis) passively filled by sand from the overlying sand-flat facies. This suite is considered an example of a poorly developed Glossifungites ichnofacies.

The Glossifungites ichnofacies is characterized by sharp-walled, unlined, passively filled, dwelling burrows of suspension feeders (MacEachern et al., 1992; Pemberton, MacEachern, at al., 1992). It develops in stable and cohesive substrates. Ravinement erosion associated with the transgression led to the exhumation of compacted and dewatered sediments, making a firm substrate available for colonization of the Glossifungites tracemakers. This surface may represent a high-energy parasequence boundary (MacEachern et al., 1992; Pemberton, MacEachern, at al., 1992; Pemberton and MacEachern, 1995). Similar surfaces in modern intertidal deposits were recognized by Pemberton and Frey (1985) in the Georgia coast, where they formed due to transgressive retreat of the beach. Firmground burrowers include various bivalves, crustaceans, nemerteans, and polychaetes. Burrows of the polychaetes Nereis succinea and Drilonereis longa tend to remain open and fill passively and, according to Pemberton and Frey (1985), might be preserved as Palaeophycus in the fossil record. These polychaete burrows are potential modern analogues of Palaeophycus tubularis in the Waverly section.

A more complicated situation occurs at the top of unit B1. This surface delineates the top of the sand-flat deposits, separating this facies from the overlying paleosol (unit B2). The surface is iron-stained, probably due to infiltration from the paleosol interval. Basal terminations of U-shaped Diplocraterion isp. are abundant on this surface. These burrows have unlined walls and commonly crosscut other traces. The sharpness of burrow margins gives these structures a boring-like appearance and may be confidently called pseudoborings. U-shaped burrows at the top of unit B1 probably record animals burrowing in dewatered muds, reaching a level slightly below the sand/mud interface. Subsequent erosion of the muds removed most of the U traces leaving only the basal terminations of the burrows at the top of the underlying sandstone.

This surface is interpreted as a firmground recording an example of a composite ichnofabric (cf. Bromley and Ekdale, 1986). Explanation of the events that led to the formation of this firmground is difficult, because it involves erosional exhumation of a firm substrate and subsequent erosion of the burrowed sediments (i.e., two successive erosional events separated by one burrowing event). A further complication arises because the top of unit B1 does not seem to represent any discontinuity in the stratigraphic column, but only a boundary between genetically related facies within a prograding tidal-flat sequence. However, unequivocally gradational deposits between the sand-flat facies and the paleosol have not been observed. To explain this, it has been assumed that transitional mixed- to mud-flat sediments were deposited, but their primary features were obliterated by pedogenic processes.

Although the presence of substrate-controlled ichnofacies, such as the Glossifungites ichnofacies, is considered suggestive of an allogenic origin (i.e., erosional exhumation due to a sea-level change), autocyclic processes also may exhume firm substrates. Pemberton and Frey (1985) noted that some of the Glossifungites surfaces in the Petit Chou Island were formed by tidal-stream erosion. Interestingly, similar small, vertical, U-shaped burrows have been documented from these firmground intertidal deposits of the Georgia coast (Pemberton and Frey, 1985). In modern semiconsolidated muds of Petit Chou Island, these structures are produced by the polychaete Polydora ?websteri. The complex origin of this firmground surface is consistent with evidence of palimpsest, time-averaged surfaces in the Waverly succession, discussed above.

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Kansas Geological Survey, Geology
Placed on web May 21, 2015; originally published 2002.
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