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Introduction
Tidal flats are complex depositional environments that are highly sensitive to physical processes of sedimentation, sea-level changes, biogenic activity, climate, and tectonism. Late Paleozoic eustatic sea-level rises caused the development of extensive epicontinental seas over the cratonic USA midcontinent (Moore, 1964; Heckel, 1977; Ross and Ross, 1987; Watney et al., 1989). Extensive Carboniferous-Permian coastlines were influenced by tides, allowing the formation of tidal-flat areas that are preserved within carbonate/siliciclastic cyclothems in the midcontinent. Paleogeographic reconstructions (Scotese and McKerrow, 1990) indicate that these cyclothems accumulated in an equatorial position.
At the Waverly trace-fossil locality, in eastern Kansas (fig. 1), tidal-flat deposits occur within the Stull Shale Member of the Kanwaka Shale, Shawnee Group (Virgilian, Upper Pennsylvanian) (fig. 2). These deposits display a suite of associated physical and biogenic sedimentary structures that provide valuable insights into the paleoecological and depositional dynamics of this ancient equatorial tidal flat. Trace fossils are exceptionally diverse, abundant, and well preserved in this facies, comprising 41 different ichnotaxa. Sandstones exposed in the lower part of the succession display bedding planes densely covered with trace fossils. Associated physical sedimentary structures (e.g., wave ripples, wrinkle marks, flat-top ripples, flaser and wavy bedding) indicate deposition in a very shallow, tidal flat. Higher in the exposure, biogenic structures are represented only by large arthropod trackways preserved at the top of a channel-fill sandstone body.
Figure 1--Distribution of outcrops of the Shawnee Group, showing location of the Waverly trace-fossil site.
Figure 2--Stratigraphy of the Shawnee Group.
Numerous studies have focused on the animal-sediment interactions in modern tidal flats (e.g., Schäfer, 1952; Van Straaten, 1952; Reineck, 1967; Howard and Dörjes, 1972; Howard and Frey, 1973, 1975; Swinbanks and Murray, 1981; Ghare and Badve, 1984; Swinbanks and Luternauer, 1987; Frey, Hong, et al., 1987; Frey, Howard, and Hong, 1987; Frey et al., 1989; Raffaelli and Hawkins, 1996; Hild and Günther, 1999; Dittman, 1999; Dittmann et al., 1999; Bertness, 1999; Little, 2000). However, relatively few authors have attempted to apply actualistic observations and models of animal-sediment interactions to the study of ancient tidal flats (e.g., Goodwin and Anderson, 1974; Miller and Knox, 1985; Wescott and Utgaard, 1987; Simpson, 1991). The presence of trace fossils and body fossils associated with physical sedimentary structures in the Waverly succession provides an ideal opportunity to analyze the ethology of the benthic biota, to reconstruct the paleoecology of an ancient equatorial tidal-flat ecosystem, and to integrate both biogenic and sedimentologic evidence to obtain a more realistic picture of this ancient tidal shoreline. Although high-resolution data on community structure and biotic interactions are limited in the fossil record, the potential of an integrated approach in exceptionally preserved trace fossil faunas has not yet been fully explored.
Therefore, the aims of this paper are to: (1) describe and interpret the tidal-flat facies and associated deposits from the Waverly section; (2) describe the Waverly trace fossils; (3) analyze the paleoecologic and ethologic significance of this ichnofauna; (4) discuss the depositional dynamics, sea-level controls, paleotidal range, and climatic framework of this Pennsylvanian tidal coastline; (5) provide a case study of an integrated biogenic, sedimentologic, and stratigraphic approach to the analysis of coastal siliciclastic depositional systems; (6) analyze the environmental significance of the Waverly ichnofauna in the regional context of the Stull Shale Member; (7) discuss the implications of this tidal-flat assemblage for ichnofacies models of shallow-marine siliciclastic successions; and (8) analyze the importance of tidal-flat ichnofaunas from an evolutionary paleoecology perspective.
Previous Work on Pennsylvanian Ichnology in Kansas
The trace-fossil content of some Pennsylvanian units in Kansas has received considerable attention during the last three decades. One of the first series of studies was performed by Bandel (1967a,b). Bandel (1967a) analyzed the ichnofauna from two different Upper Pennsylvanian (Missourian and Virgilian) sandstones in northeast Kansas: the Rock Lake Shale Member (Stanton Limestone, Lansing Group) and the Vinland Shale Member (Stranger Formation, Douglas Group). He emphasized the different taxonomic composition of both shallow-marine assemblages and related this discrepancy to local paleoenvironmental conditions. Bandel (1967b) described and interpreted, in detail, arthropod locomotion traces from the Upper Pennsylvanian (Virgilian) Tonganoxie Sandstone Member (Stranger Formation, Douglas Group) at the Buildex quarry. He concluded that isopods and limulids made the traces, and suggested their presence close to the mouth of a fluvial valley, pioneering the idea of estuarine valley systems, subsequently developed by sedimentologists and sequence stratigraphers in the last decade.
Maerz et al. (1976) described and interpreted traces attributed to the activity of bivalves and ophiuroids from the Upper Pennsylvanian Rock Bluff Limestone Member (Deer Creek Limestone, Shawnee Group). This is one of the few publications on Pennsylvanian trace fossils from a carbonate unit in Kansas.
Hakes (1976) monographed the ichnology of four Upper Pennsylvanian (Missourian and Virgilian) units of eastern Kansas: Rock Lake Shale Member, South Bend Limestone (Stanton Limestone, Lansing Group), Stull Shale Member (Kanwaka Shale Formation, Shawnee Group), and Tecumseh Shale (Shawnee Group). He described and illustrated 41 ichnospecies and provided comments on their distribution and environmental significance. Subsequently, Hakes (1977) analyzed the trace-fossil content of the Upper Pennsylvanian (Virgilian) Lawrence Shale (Douglas Group) of eastern Kansas. Based on ichnologic evidence, he demonstrated that strata previously considered continental were actually marginal marine. His observations on marginal-marine ichnofaunas were later summarized in a paper dealing with the diagnostic features of brackish-water assemblages (Hakes, 1985). In this paper, he also added new information on the Timberhill Siltstone Member (Stanton Limestone, Lansing Group).
More recently, Mángano et al. (1997) and Buatois, Mángano, et al. (1997, 1998a,b,c) analyzed the trace fossils of the Tonganoxie Sandstone Member at Buildex quarry. Buatois, Mángano, et al. (1997) suggested that the Buildex ichnofauna represents the activity of a freshwater biota that inhabited a tidal-influenced, fluvio-estuarine transitional environment, emphasizing the importance of ichnologic evidence in reconstructing paleosalinities in estuarine valleys. Buatois et al. (1998a) provided a systematic treatment of the Buildex traces and reanalyzed the ichnofossils originally described by Bandel (1967b), reinterpreting the supposed isopod trails as myriapod trackways. Buatois et al. (1998b) emphasized the sequence-stratigraphic significance of the Buildex ichnofauna, and Mángano et al. (1997) described superbly preserved resting and feeding traces of monuran insects and formally defined the new ichnogenus Tonganoxichnus for these structures. Based on ichnologic evidence, these authors reconstructed the ethology and paleobiology of these ancient wingless insects. Buatois et al. (1998c) described myriapod trackways and trails and reviewed the taxonomic status of some arthropod ichnotaxa.
Mángano and Buatois (1997) briefly discussed the variability of tidal-flat ichnofaunas formed along salinity gradients, using examples from different Upper Pennsylvanian units, including the Stull Shale Member and the Tonganoxie Sandstone Member. Selected ichnotaxa from the Stull Shale Member at Waverly also have been analyzed recently by Mángano et al. (1998), who discussed the ethologic implications of contrasting feeding strategies based on bivalve traces. These authors emphasized the importance of detailed analysis of bivalve traces as a tool to reconstruct substrate conditions and depositional history of tidal-flat strata. Mángano et al. (1999) analyzed the paleoecologic significance of the trace fossil Asteriacites in the Pennsylvanian of eastern Kansas and western Missouri, on the basis of specimens collected in several units, including the Stull Shale Member at Waverly.
In recent years, ichnologic studies in Kansas have expanded to include subsurface data. Buatois and Mángano (1997) and Buatois et al. (1999) analyzed trace fossils from cores of the Lower Pennsylvanian Morrow Sandstone in the subsurface of southwest Kansas. These authors emphasized the importance of detailed ichnologic analysis as a tool in refining the characterization of petroleum reservoirs. More recently, Buatois et al. (in press) discussed the importance of Morrow ichnofaunas in sequence-stratigraphic analysis.
Geologic Setting
Regional and Stratigraphic Framework
During the the mid-Pennsylvanian (late Morrowan-Desmoinesian), the collision of Laurasia and Gondwana lead to the formation of the Ouachita Mountains and the Arkoma basin as part of the tectonic events of the Wichita Orogeny (Rascoe and Adler, 1983). The Ouachita Mountains were the thrust belt, and the Arkoma basin represents the related foreland basin (Rascoe and Adler, 1983; Lillie et al., 1983; Watney et al., 1991). The Arkoma basin initially experienced high rates of subsidence and subsequently was filled with siliciclastics from the Ouachitas during the Late Pennsylvanian (Watney et al., 1991). Deposition was dominated by mass flows in a slope environment (Shanmugan and Moiola, 1995). Further north, in the cratonic areas, mixed siliciclastic-carbonate platforms occurred as a series of less-subsiding depressions, namely the Anadarko, Cherokee, Sedgwick, Salina, and Forest City basins, each separated by topographic highs (Jewett, 1951; Watney et al., 1991). In particular, the marginal-marine succession analyzed in this paper accumulated in the southern part of the Forest City basin (fig. 3), which occupied part of Missouri, Nebraska, Iowa, and northeastern Kansas (Lee, 1943; Jewett, 1951).
Figure 3--Paleogeographic map of the midcontinent during the Virgilian (modified from Rascoe and Adler, 1983).
The Stull Shale Member is the uppermost unit of the Kanwaka Shale of Late Pennsylvanian (Virgilian) age, which in turn is included in the Shawnee Group (Moore, 1932, 1936, 1949) (fig. 2). In southeastern Kansas, the Stull Shale Member commonly has been confused with the Doniphan Shale Member of the Lecompton Limestone (West et al., 1989). However, detailed mapping by Maples (1991) showed that many sandy shales with orthomyalinid shell beds were part of the Stull Shale Member, because they directly underlie the fusulinid-rich Spring Branch Limestone Member of the Lecompton Limestone. The Stull Shale Member consists of trace fossil-rich sandstones overlain by fossil-poor, green-to-gray shales and sandstones, and is capped by local thin coal seams in the north (Condra and Reed, 1937) or orthmyalinid-rich and chonetid-rich shell beds in the south (Baker, 1995). The Stull Shale Member is underlain by the Clay Creek Limestone Member (Kanwaka Shale) and is overlain by the Spring Branch Limestone Member (Lecompton Limestone) (Moore, 1936, 1949).
In terms of cyclothem stratigraphy, the Stull Shale Member is an "outside shale" (nearshore/terrestrial) and is part of the Oread megacyclothem (Troell, 1969; Heckel, 1977, 1985, 1986, 1994). The well-known cyclic succession of limestones and shales cropping out in Kansas represents the deposits of an epeiric sea that covered the American midcontinent during the late Paleozoic. These deposits historically have been interpreted in terms of megacyclothems and cyclothems. A cyclothem records a transgressive-regressive event. A megacyclothem can be considered as several transgressive-regressive cycles with an overall regressive trend. According to Moore's (1936) original definition, a megacyclothem is a bundle of limestones separated by shales, each having distinctive lithologies and faunal and floral assemblages. Within this framework, limestones record open-marine stages, and shales may be either fully marine (inside shale), or restricted- to nonmarine (outside shale).
Outside shales usually have been interpreted as coastal, deltaic to nonmarine deposits, recording maximum regression (Heckel, 1985; Watney et al., 1989). Common features of outside shales are abundant plant fragments, coal seams, a sparse, low-diversity marine assemblage, thin limestones, paleosols, blocky mudstones, and channel sandstones. Although considered an outside shale, the Stull Shale Member is only partially regressive and documents a more complex history.
Outcrop Locality
This study focuses on a succession of the Stull Shale Member near the town of Waverly, Coffey County, Kansas (SW NW sec. 19, T. 19 S., R. 16 E.) (West et al., 1989). The base of the member is covered and the succession capped by the Spring Branch Limestone Member (fig. 4). At this locality, the Stull Shale Member is 5.80 m thick. Detailed facies analysis is restricted to the lower 2.75 m of the exposure, where trace fossils are preserved in tidal-flat and related facies. The upper interval of the Stull Shale Member at Waverly consists of 3.05 m of shales and orthomyalinid (bivalve) packstones and wackestones of subtidal origin, which record the onset of a transgressive event that culminated with the deposition of the overlying Spring Branch Limestone Member. Orthomyalinid bivalve shells in this unit have been extensively affected by bioerosion, displaying borings by acrothoracican barnacles, polydorid worms (ichnogenus Caulostrepsis), and ctenostomatid bryozoans (Baker, 1995). The orthomyalinid valves also provided a substrate for an encrusting biota, including algae, the fistuliporid bryozoans, Osagia, and the brachiopods Leptalosia and Derbyia (Baker, 1995). This upper part of the Stull Shale Member at Waverly has been analyzed in detail by Baker (1995) and West et al. (1996).
Figure 4--General stratigraphic column of the Stull Shale Member, Kanwaka Shale, at the Waverly trace-fossil site. See fig. 5 for legend.
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Kansas Geological Survey, Geology
Placed on web May 21, 2015; originally published 2002.
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