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Channels and Cyclothems: A Summary Model
Wanless and Weller (1932) introduced the term “cyclothem” to describe apparently repeating sequences of lithologies in coal-bearing rock sections of Pennsylvanian age (Langenheim and Nelson 1992). These authors tied such successions to sea-level fluctuations driven by the waxing and waning of polar glaciers during the Pennsylvanian, a model that has proven remarkably robust and continues in use today (e.g., de Wet et al. 1997; Heckel et al. 2007; Heckel 2008; Eros et al. 2012; Waters and Condon 2012). Challenges to the cyclothem concept reflect various attempts to outright discredit it (e.g., Wilkinson et al. 2003), to demonstrate control by local, coastal sedimentary processes (e.g., Horne et al. 1978; Ferm and Cavaroc 1979) or by structural geological movements (e.g., Ferm and Weisenfluh 1989), or to subsume it terminologically within sea-level-driven sequence stratigraphic models (e.g., Bohacs and Suter 1997). The recurrent patterns discussed here, in relation to the Galatia channel and similar features in other coals, only serve to strengthen the argument for a periodically repeating class of natural phenomena as drivers of lithological sequences in Pennsylvanian cratonic coal-bearing rock sequences. The relatively recent additions of climate (e.g., Cecil et al. 2003; Horton et al. 2007; Peyser and Poulsen 2008; Bishop et al. 2010) and of the ties between climate and sedimentation patterns (Cecil and Dulong 2003) provide a more complete framework for explaining cyclothemic patterns in space and time, particularly those permitting escape from an either–or focus on allocyclic versus autocyclic underlying controls (in the terminology of Beerbower 1964) while recognizing the role and context of each.
The modern cyclothem concept takes full account of sequence stratigraphy, including autocyclic processes, such as the formation of deltas, within a framework of sea-level change. The proximate driving force of Pennsylvanian and Early Permian sea-level change still appears to be changes in grounded ice volume, mainly in the south polar and mountainous regions of Gondwana. However, questions have been raised about the sufficiency of the volume of ice (Isbell et al. 2003; Horton and Poulsen 2009; Henry et al. 2010) and whether it was present at all (Fielding et al. 2008; Gulbranson et al. 2010) during some intervals of the Pennsylvanian (such as the Kasimovian; e.g., see Gulbranson et al. 2010), during which cyclothemic sequences nonetheless continue to be found in the equatorial regions of central and west-central Pangaea. We will take as a given that cyclothems reflect covariant changes in sea level, climate, and sediment transport volume linked to variations in ice volume (Horton et al. 2007, 2012).
There also are elements that, although varying through time, must be treated as constants because (1) they are not linked causally to the set of variables tied to cyclic global change, and (2) their effects and timing are often obscure. The most prominent of these is tectonics. It is understood that accommodation space is needed to preserve sediments as part of the geological record. However, remarkably robust correlations have proven possible between cyclothemic, glacial–interglacial cycles in the cratonic basins of the American Midcontinent (Western Interior), Illinois (Eastern Interior), and Appalachian regions (Heckel et al. 2007; Falcon-Lang et al. 2011), correlative with a nearly identical cycle sequence in the Donets Basin (Heckel et al. 2007; Heckel 2008; Eros et al. 2012; Schmitz and Davydov 2012; Davydov et al. 2013), based on marine microfossils, palynology, and radiometric dating. These correlations reveal that nearly every major cycle and many minor cycles are preserved in all these basins. This high degree of correlation leads to the conclusion that the creation of accommodation space was sufficient to be treated, on average, as a constant, even though there clearly are local effects on the thickness or areal extent of any given bed, as discussed above.
The “Typical” Cyclothem on a Cratonic Platform
A Midcontinent North American cyclothem, at its simplest, consists of a terrestrial and a marine portion. The rock units representing these markedly different settings are, in most cases, not interdigitated. The great flatness and large areal extent of the central-western equatorial Pangaean cratonic surface allowed for widespread, nearly synchronous deposition of many marine units and equally for the widespread development of soils on exposed surfaces, including Histosols (peats), under conditions of widespread, aseasonal to weakly seasonal patterns of high rainfall.
The terrestrial portion of a cyclothem consists primarily of a mineral paleosol, developed on normal marine or near-shore marine deposits, followed by patchy to widespread mineral swamp deposits, represented by organic-rich shales and terminated by a Histosol, developed in a peat-forming environment. The contacts between these units can be sharp or gradational. The basal paleosol, particularly in the later Middle Pennsylvanian and Late Pennsylvanian, and in some places, earliest Permian, shows strong evidence of a complex origin. Such paleosols are vertic and often calcic, evidencing a seasonal climate and well-drained conditions during their early formation. These initial patterns are overprint by evidence of decreasing seasonality and increasing moisture, during which the soil was gleyed to various degrees. Mineral soil formation terminates with surface flooding, locally with the development of clastic swamps and finally with the onset of peat formation (Joeckel 1995, 1999; Miller et al. 1996; Cecil et al. 2003; Hembree and Nadon 2011; Catena and Hembree 2012; Rosenau et al. 2013).
The contact between the terrestrial and marine portions of the cyclothem is marked by an often-cryptic erosional surface (a ravinement surface). Above this surface, there may be discontinuously distributed phosphate nodules (Heckel 1977), signifying a period of sediment starvation under marine water cover, or a thin, discontinuous marine bed, often just a shell hash (Bauer and DeMaris 1982), what Heckel (1986) has called the transgressive limestone. Above the ravinement surface, and transgressive limestone, if present, is generally a widespread, sheety black shale that, where it has been examined, has a terrestrial organic signature in its lower half and a marine signature in its upper half (James and Baker 1972; Banerjee et al. 2010). This black shale often contains marine invertebrates throughout. Succeeding and in sharp contact with the black shale is a marine limestone that contains a normal marine fauna. Above this limestone, variably developed is a sequence of mixed gray shales and sandstones that generally coarsen upward and rarely can contain thin, discontinuous layers or beds of organic-rich shale or coal. In most cases, these deposits appear to be of deltaic origin and strongly reflect autocyclic controls on local characteristics. The paleosol at the base of the terrestrial portion of the next sequence is developed upon these marine to brackish-water deposits. The parent material of the soil may vary considerably, reflecting the landward extent and depth of the transgression that accompanied the previous interglacial and associated sea-level highstand.
This characterization of a “cyclothem” is a generalization. Details of how this series of terrestrial–marine environments is represented in the rock record depend strongly on vagaries in a number of physical variables. These include the extent of marine transgression, the maximum water depth, the volume of sediment and location of sediment delivery points, the rate of sea-level rise and fall, the length of the exposure period of the craton during falling sea-level stages and at lowstand, the length of time and extent over which the tropical climate was suitable for peat formation during lowstand and, of course, the rate amount and areal variability in the creation of accommodation space. That regular cycles can be recognized at all, given the many variables, shows the strong influences of the most basic drivers, climate and eustasy, which overprint the entire record, allowing for the creation of accommodation space.
Cyclothem Models Do not Address Contemporaneous Channels
The traditional cyclothem model leaves out the channel deposits that form contemporaneously and continuously through the interval of paleosol development, peat formation, and early sea-level rise. Also left out, along with these channels, and of particular interest here, are the gray-shale wedges that lie immediately above, and in gradational contact with, the coal bed along the channel flanks, the splits in the coal bed that form contemporaneously with final channel filling, and the dark shales interbedded with the coal close to the channel. These deposits, which reflect the complex interaction of allocyclic (sea level and climate in particular) and autocyclic processes, are key indicators of the timing of peat or coal formation during a glacial–interglacial cycle. Such deposits appear to have existed contemporaneously with every major coal bed, to have been expressed differentially depending on the many factors described briefly above, and to have been known or understood to different degrees in each instance depending on the availability of exposures, both natural and created, by mining and road construction or in drill core.
Perhaps the most important aspect of the channel deposits is their long duration, within the framework of a glacial–interglacial cycle. As can be seen from the examples discussed above, stream downcutting began as soon as the craton was exposed during sea-level fall, in the early stages of glacial advance. As a consequence, the development of paleosols and floodplain deposits likely began under the strongly seasonal subhumid climates to, in some instances, even semi-arid climates that characterized sea-level highstands and falling stages (Cecil et al. 1985, 2003; Peyser and Poulsen 2008; Tabor and Poulsen 2008; Horton et al. 2012). The flatness of the craton in the Midcontinent through the Appalachian Basin must be kept in mind here. By the late Atokan, land-surface irregularities created by the Mississippian–Pennsylvanian sea-level drop had filled, such that the surface of the craton, deeply incised at the end of the Mississippian (Howard 1979; Kvale and Archer 2007), had become much flatter (Waters and Condon 2012). This flatness is the major factor that led to beds of huge areal extent, contemporaneous throughout their extent, most notably coals (e.g. Greb et al. 2003) but also open marine environments (Heckel 2008) and fossil soil horizons (Cecil et al. 2003). Consequently, with the shelf edge of the mid-Pangaean equatorial craton in the modern day Arkoma Basin region of Oklahoma, by later Middle Pennsylvanian (Desmoinesian/Moscovian), slopes on the surfaces leading inland from that point may have been much less than 6.3 in./mi (10 cm/km). As a consequence of this low slope, exposure of the cratonic surface during sea-level fall, or its flooding during sea-level rise, was likely rapid. Once the slow process of ice buildup began during the early phases of a glacial interval, small changes in sea level would have had large effects on the depth of water coverage on the craton and the timing and extent of surface exposure.
The channels and associated deposits discussed here are those that were initiated on the exposed cratonic surface during falling stages of sea level and that continued uninterrupted through early stages of sea-level rise. Attempts to incorporate dynamics into the cyclothem model often place channels throughout the entire sequence (e.g., Baird and Shabica 1980; Jacobson 2000), relying on a strictly autocyclic, deltaic model of deposition for an entire cyclothem under an invariant climate (e.g., Horne et al. 1978). Gray shale clastic wedges covering coal beds, and associated with channel deposits, have been explained as reflections of tectonism, again holding climate constant (e.g., Wanless 1964). There certainly are channels at many portions of any glacial–interglacial cycle, but these must not be confused as being of a single type or all reflecting deposition under a common set of circumstances.
Lowstand channels are partially contemporaneous with the paleosols that underlie coal beds, and these paleosols are indicators of the climatic conditions under which the precoal channels and floodplains formed (Feldman et al. 2005; Falcon-Lang et al. 2009). Such paleosols may have thicknesses of up to several feet (meters) and characteristics that include indicators of seasonality of moisture or rainfall (vertic features), even considerable periods when evapotranspiration exceeded rainfall (the development of soil carbonates—which, in older literature, often were referred to as “fresh-water” carbonates and envisioned to have formed in lakes, laterally equivalent to marine limestones offshore (e.g., Wanless 1964). The climate signature of these sorts of paleosols stands at odds with the coal beds that immediately supersede them stratigraphically. Formed as Histosols, the coals are indicative of perhumid to humid climates (humid during the later Middle and Late Pennsylvanian; perhumid during much of the earlier Pennsylvanian—see Cecil et al. 1985; Cecil and Dulong 2003). As the glacial maximum was approached (sea-level lowstand), the climatic seasonality under which these soils initially developed began to diminish, resulting in an overprint of a wetter climate state (Cecil et al. 2003; Hembree and Nadon 2011; Rosenau et al. 2013). This late-stage increase in rainfall and decrease in seasonality, while soils were still well drained, led to the initiation of organic buildup on the soil surface, with attendant formation of weak organic acids and the subsequent intense gleying that characterizes so many Pennsylvanian paleosols in coal-bearing sequences (Joeckel 1995, 1999; Cecil et al. 2003; Driese and Ober 2005; Hembree and Nadon 2011; Rosenau et al. 2013).
With continued high rainfall and greater uniformity of moisture distribution throughout the year, formation of swampy surface conditions ensued across much of the low-gradient, widely exposed craton, leading to the development of peat swamps. The coal beds resulting from these peat swamps were not time transgressive; they did not form as narrow belts, moving inland ahead of rising sea level, as is often portrayed (e.g., Bohacs and Suter 1998; Heckel 1995), but were time equivalent over large areas (Greb et al. 2003), indicated by such evidence as ash partings (Greb et al. 1999), widespread clastic partings that fall at consistent levels within the coal bed over its whole extent, or partings that separate benches of coal with distinct petrographic characteristics throughout their areal extents (Eble et al. 2006). As discussed above, channels already present on the landscape were considerably narrowed by the development of erosion-resistant peat deposits on their floodplains and along their banks. In addition, intense plant rooting acted to stabilize the local substrate and greatly reduce sediment input to streams throughout the drainage basins on these very flat landscapes. Channels such as the Galatia channel, the Walshville channel, the Oroville channel, and others share the common characteristic of being significantly narrower and showing no evidence of the dynamic character of their precursors on clastic floodplains. In addition, it is at this stage that the rivers appear to have carried little sediment other than very fine grained material or dissolved organic matter. Floods carrying this material into the adjacent peat swamps created the abundance of fine, dark, thin laminae of clastic material that is found in coal beds adjacent to the coal-contemporaneous channels, suggesting low-sediment to black-water river conditions.
In their final stages, the coal-contemporaneous reaches of cratonic river systems appear to have become estuarine and tidally influenced (e.g., Archer and Kvale 1993; Tessier et al. 1995) and to have widened considerably through disruption of the peat bodies along their flanks. The channels were rapidly filled with clastic material, initially clays and silts but ultimately sands, and were flanked by mudflats and a network of tidal channels. We propose that this final portion of channel life history is a reflection not only of ice melting and sea-level rise, but of the coupling of these factors to climate change (e.g., Cecil et al. 2003; Peyser and Poulsen 2008; Tabor and Poulsen 2008) and the relationship between climate and sediment transport patterns (Cecil and Dulong 2003). The filling of the main channel course with clastic material is accompanied also by the erosion of the margins of the peat body, resulting in a series of floating peat mats (Elrick et al. 2008), between which clastic material was deposited, resulting in the “splits” that line the contact between the coal body and the contemporaneous, now estuarine, and strongly tidally influenced, channel.
The Gray Shale Wedge and Its Relationship to the Channel
It is important to recognize that economically mineable, low-ash, and low- to moderate-sulfur blanket coals intrinsically have nothing to do with sea level as the driving force of their formation (Cecil et al. 1985). The peats from which these coal beds formed are a reflection of climate, specifically humid to perhumid climatic conditions (terminology from Cecil and Dulong 2003) covering large areas of the paleotropics. Such climatic conditions can occur, hypothetically, at any point in space and time. However, because of the various extrinsic drivers, most proximately related to ice volume and atmospheric CO2 (Horton et al. 2012), the peat swamps that became coals are mainly confined to the lowstand phase of the covariant, linked sea-level and climate cycles (Cecil et al. 2003), thus around the time of maximum glacial conditions.
For interpretation of the position of coal within a sea-level rise–fall cycle, the rate of sea-level rise, and the timing of sediment emplacement within the channel, the gray-shale wedge is probably the single most important feature within a cyclothemic sequence.
As discussed above, we believe the empirical evidence strongly suggests that peat accumulated under a humid to perhumid climate and that no empirical support can be adduced for rising sea level as the principal driver. In brief, if rising sea level induces peat formation, modern coastlines and coastal shelves around the world should be covered by peat swamps. These peat-accumulating environments should be advancing ahead of widespread, now flooded, trailing areas of time-transgressive blanket peat deposits, reflecting the rising sea levels of the current interglacial. Additionally, it should be considered that during the Pennsylvanian, there is no peat or coal in vast expanses of the western Pangaean tropical or equatorial regions, which many climate proxies indicate was under a strongly seasonal climate regime too dry for peat formation. Were sea level the driver, peat should be equally present throughout all coastal belts across the Pennsylvanian and Permian world, regardless of the prevailing climate. This is clearly not the case.
This model is supported further by evidence for low-sediment to black-water conditions in major peat-contemporaneous drainages. Sediment transport within channel systems is, of course, tied directly to the entry of sediment into drainage systems. As discussed by Cecil and Dulong (2003), based on empirical observations in modern tropical environments, high-rainfall, distributed throughout the year results in densities of plant cover that stabilize soils and drastically cut back on sediment movement into streams. With melting of ice, during the glacial–interglacial transition, major changes in atmospheric circulation patterns (e.g., Cecil et al. 2003; Peyser and Poulsen 2008; Horton et al. 2012) lead to increasing seasonality in the tropics (Kvale et al. 1994). This change leads directly to a reduction in vegetational cover, particularly in those areas surrounding the coastal peat-swamp ecosystems, resulting in an influx of clastics into drainages. This clastic influx is thus coincident with rising sea level and the initial conversion of river drainages into estuarine environments. Under this model, the sediment that blanketed peat-swamps adjacent to major drainages originated coincidently with rising sea level, melting ice, and climate changes from humid-perhumid to wet subhumid, and progressively to dry subhumid. Carried to the lower areas of river systems, this sediment was pushed out of the channels with flooding caused by rising base level. It was pushed outward over the swamp and moved progressively inland as the locus of such flooding moved with sea-level rise.
During this rising phase of sea level, the channel was converted to an estuary, subject to regular tidal action. This can be seen in the nature of the sediments that constitute the gray-shale wedge, particularly in the channel adjacent to the coal or peat body and in those parts of the gray-shale wedge that cover the surface of the coal bed. Such sediments bear clear evidence of rhythmicity typical of tidal deposits (Archer and Kvale 1993; Archer et al. 1995). These sediments vary in character, some evidently deposited in mud flats (Archer et al. 1994; Kvale and Mastalerz 1998) and others in tidal channels, the rate of deposition varying across the top of the peat locally. Also associated with this phase is the development of a system of tidal channels in areas immediately adjacent to the main river channel. Called “rolls” in mining parlance, these features are of limited lateral extent, have erosional bases and irregular to sinuous courses, and can contain plant fossils, often differing significantly from those buried at the top of the coal bed.
Throughout the area adjacent to the channel, abundant plant material appears to have been buried rapidly in place. The sediment surrounding these plants is the finest of the gray-shale wedge, largely claystone to fine siltstone; it is very finely laminated but preserves evidence of tidal rhythmic deposition. In addition, a transitional zone of organic shale less than 0.4 in. (1 cm) to tens of inches (centimeters) in thickness often occurs between the top of the normally bright-banded coal and the bottom of the gray-shale roof, indicating an early flooding stage that was accompanied by the onset of death of the vegetation. This transitional zone often contains abundant fallen stem material, including large lycopsid trunks that are not sediment filled, thus indicating decay in an environment where sediment input was low, even if flooding was taking place. These observations reflect the earliest phases of gray-shale wedge deposition in areas adjacent to the channel and show clearly that the gray-shale wedge was emplaced during the early stages of sea-level rise. It is at this stage that we envision the development of extensive mud flats along the margin of the channel, probably colonized by the most flood-resistant vegetation. The most dramatic evidence of the rapid early burial of vegetation is the presence of upright tree bases, nearly always of arborescent lycopsids, extending tens of inches (centimeters) to 3.3 ft or more (1 m) into the roof shales and rooted in the coal body. The gray shale onlaps the tree bases, and, in some instances, laminae in the enclosing sediments can be traced from outside a stump through the shales that filled its hollow interior. Stigmarian rooting organs of these trees, identifiable by their distinctive external patterns, have been identified as much as 3.9 in. (10 cm) deep in the coal bed, filled with roof-shale sediments, indicating that the hollows in the standing lycopsid trunks extended down into the peat and were filled as muds were deposited in and around the tree bases. In some instances, prostrate lycopsid trunks have been found still in attachment to standing tree stumps (e.g., DiMichele and DeMaris 1987). Prostrate vegetation is abundant, and stems with hollow central cavities, such as those found in calamitaleans and lycopsids, are often partially to completely filled with the roof sediment that encloses them, indicating little or no transport. Models of mud-cast logs (Gastaldo et al. 1989) demonstrate that these develop in place on the surface of the substrate. Delicate plant parts, such as the stems and foliage of tree ferns and pteridosperms, are preserved abundantly. Moreover, spatial patterns of original vegetation are preserved. All these factors point to rapid burial of the peat surface by flood-borne sediments.
Accompanying conversion of the channel to a tidal estuary, the peat body along the channel margins began to erode and be torn apart, most likely initially along the fine layers of dark clay partings in the peat in areas immediately adjacent to the channel. This probably was exacerbated by the action of daily tides against the peat margin. In addition, modern studies have shown that flooding of peats leads to an increase in methane production in the peat body, which can result in flotation of the peat. In-mine evidence of peat flotation has been found in association with channels in several different coal bodies (Elrick et al. 2008). This evidence includes the following:
- Peat or coal “flaps,” in which the detachment of peat leads to an abundance of “flaps” along the peat margin, in contact with the channel, varying in thickness from a few inches (centimeters) to 3.3 ft (1 m) or more.
- Clastic sediment deposited between these peat flaps, leading to the “splits” in the seam along the channel margin, and clastic wedges up to several feet (meters) in thickness and lateral extents of a few feet (meters) to a few hundred feet (meters) or so, measured orthogonally to the channel axis. The sediment filling these splits is identical to that constituting the roof rock that lies in gradational contact with the top of the coal bed.
- Detachment of the coal body from the seat earth. Adjacent to the channel there are areas where the coal bed is detached from the seat earth and bent and stretched or torn. The interval between the top of the seat earth and the base of the coal is filled with rock identical to the rock that fills the channel and covers the coal adjacent to the channel.
- Sharp contacts occur between peat flaps and the underlying gray shales. In areas of suspect floating peat mats, the basal contacts of the coal with the top of the underlying gray shale are sharp, entirely lacking rooting of any kind, which would be expected were these “splits” to have been splays. Splays are usually recolonized by vegetation, and with peat sitting immediately on such splits, there absolutely should have been rooting from the peat into the shale had the peat been deposited after the shale. We know of no case, among the hundreds of exposures of such splits we have observed, where any rooting occurs. Thus, the gray mudstones appear to have been deposited within the splits and not subject to recolonization by vegetation after deposition of the muds in a flood.
- Stringers of coal cross gray shales within “splits.” Dozens of examples have been observed wherein thin stringers of coal pass from the coal bench below to the coal bench above the gray shale “split” between such benches. Exhumation of these stringers shows that they are not roots but generally vitrain sheets or thicker sheets of normally bright-banded coal. This is effectively incontrovertible evidence that the mudstones of the “splits” were emplaced following some kind of disruptive ripping apart of the peat body.
- Vegetation is in the upright position upon tilted peat flaps. Upright, buried stumps of lycopsid trees have been observed above uppermost peat flaps but are tilted so that their vertical axes are orthogonal to the upper surface of the coal bed, despite its inclination. These stumps are buried in the same siltstones and claystones as the roof in areas where the coal lies horizontally and in attachment to its seat earth. They indicate that the peat was floated and distorted during sediment filling above the peat and injection into the spaces between floating flaps.
Tidal deposition continued in the gray shale wedge throughout its thickness of as much as 98.4 ft (30 m) in areas adjacent to the channel. Brackish water invertebrates appear in the higher levels, primarily inarticulate brachiopods and pelecypods of various types. At the same time, plant fossil content decreases sharply several feet (meters) above the top of the coal. The plants become increasingly scrappy and appear to be entirely allochthonous, perhaps reflecting some transport of organic matter from the shoreline, which was moving rapidly inland, flooding and drowning coastal vegetation. Evidence of normal marine salinities is very restricted in the gray shale wedges and appears to be confined to those parts that may represent the most offshore reaches, such as that represented by the so-called Essex fauna in the Francis Creek Shale, above the Colchester Coal (Johnson and Richardson 1966). Evidence of a once greater lateral extent of the gray shale wedge is represented particularly by finer grained siltstone in erosional remnants many miles (kilometers) from the main channel. The erosional remnant nature of these gray deposits is indicated by the remnants of marine shell-hash lags and the overlying and onlapping nature of the marine black shale with the erosional basal contact, as well as the still greater extent and onlap of the marine limestone above the black shale. All these combine to place the gray shale wedge and its once continuous outliers below the marine transgression that ultimately covered the peat swamp.
W. John Nelson, Scott D. Elrick, William A. DiMichele, and Philip R. Ames xxxx, Evolution of a Peat-Contemporaneous Channel: The Galatia Channel, Middle Pennsylvanian, of the Illinois Basin FINISH CITATION