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Revision as of 21:47, 11 July 2020
Observations from mines and boreholes through the Galatia channel and related features, as well as similar channels associated with older and younger coal beds, indicate that the old model of a Mississippi-style delta having natural levees and crevasse splays is due for revision. Combined with a better understanding and climate influences not available to earlier workers, these observations lead to a new model of channel development that has connotations for larger concepts of cyclic sedimentation.
Model of Channel Development
Our model is presented in 10 illustrated stages (Figures 49 to 58).
- Stage 1: Prograding deltas during early regression. To set the stage, the Houchin Creek Coal, Excello Shale, and Hanover Limestone sediments were laid down on a tectonically stable platform from lowstand (underclay and peat) through rapid marine transgression to highstand (black shale and limestone). During late highstand to early regression, clastic sedimentation resumed. A series of deltas were built into the basin, yielding upward-coarsening sediments of the Delafield Member (Figure 49). This part of the model is similar to past cyclothem models in the Illinois Basin.
- Stage 2: Valley incision and soil development during regression. Delafield sediments rapidly filled available accommodation space in the basin. With continuing marine regression, the delta platform became exposed. Rivers cut downward to base level, creating entrenched valleys. Given the extremely low gradient and the substrate of recently deposited sediments, these streams actively meandered. Outside of valleys, soil development was under way (Figure 50). Previous models (e.g., Potter 1962, 1963) placed channels in a deltaic setting and mostly did not take account of eustasy, although Hopkins (1958) interpreted channels as being cut subaerially during regression and backfilled during transgression.
- Stage 3: Aggradation and gleying during late regression. Channels developed broad meander belts. As soon as the channel bottoms were cut below sea level, suspended sediment (largely sand) began to drop out. Rivers reworked their own sandy deposits. Valleys aggraded rapidly approaching maximum lowstand, yielding an upward-fining profile (Figure 51). Approaching maximum glaciation, climate in the Illinois Basin became increasingly humid. This fostered vegetation growth, reducing soil erosion and sediment transport. Increased rainfall leached iron from soils, gleying much of the upper soil profile (Rosenau et al. 2013). Because of the lack of time and persistently wet conditions, little or no soil formation took place within the Galatia meander belt. This part of the new model is similar to previous models, although most treated paleosols in cursory fashion.
- Stage 4: Peat initiation and paludification at early lowstand. Approaching glacial maximum or lowstand, climate in the basin became humid. Constant rainfall produced lush, dense vegetation, reducing sediment flux and creating “black water” rivers of mostly plant tannin, clay, and perhaps a small bed load. Peat began to develop earliest in low-lying areas on the landscape, such as abandoned meanders. Vegetation lining the banks of the Galatia channel stabilized the banks, locking meanders into place for the duration of lowstand. Along the channel margins, fine clastics infiltrated the peat swamp, perhaps by flocculation of clay in vegetated areas along channel margins. The result was shaly coal in these channel-margin settings (Figure 52). Previous Carbondale cyclothem models did not incorporate syndepositional channels.
- Stage 5: Widespread peat accumulation at lowstand. Peat development overspread the basin except for active channels and well-drained tectonic highs (Figure 53). Naturally, the thickest peat accumulated in the lowlands flanking the Galatia channel. Periodic floods brought fine silt and clay out into the swamp, creating clastic laminae in the peat close to the channel. This process is taking place today along the Rajang River in Sarawak, Malaysia, where domed peat deposits fringe the river under a tropical, ever-wet climate (Staub and Esterle 1993). Tectonic subsidence and concurrent peat compaction created space for thick (albeit nondomed) peat deposits along the Galatia channel. Placing coal at lowstand is conceptually different from many peat or coal depositional models and is elaborated in the following section.
- Stage 6: Peat rafting and splitting during initial transgression. Glaciation abruptly ended as the polar climate warmed. During the initial stages of sea-level rise, the lower part of the Galatia channel was subjected to increasingly vigorous tidal ebb and flow (Figure 54). Buoyed by trapped air and self-generated methane, large mats of peat floated free of the substrate. At the same time, climate shifted from ever-wet to wet–dry seasonal. This led to less vegetation cover in inland source areas and a consequent increase in runoff and sediment load. Silt carried down the Galatia channel was deposited beneath floating peat layers, creating splits and major seam disruptions.
- Stage 7: Estuarine flooding during early transgression. Melting induced rapid, pulse-like transgression, drowning the Springfield peat swamp first along the Galatia channel, which became an estuary (Archer et al. 2015, 2016). The tropical climate became drier and more seasonal, which reduced the vegetation cover upstream, enhancing runoff, erosion, and fluvial sediment transport. A heavy load of gray silt (Dykersburg Shale) rapidly overwhelmed the peat, entombing tree stumps and other plant remains in place (Figure 55). Some Dykersburg sediment might have been derived from offshore and carried up the estuary by tidal currents, as is happening today in the Kampar River of Sumatra, which is located just north of the equator under perhumid conditions. The upper Kampar has a deep channel and “tea-colored” water that carries practically no bed load or suspended sediment. Cecil et al. (2003, p. 32) wrote, “The lower 85 km [52.8 mi] is exceedingly shallow and difficult to navigate even in small boats of very shallow draft” because of the sediment carried up the estuary by the tides. Placing the Dykersburg in a tidal–estuarine facies puts this relatively new concept into the larger cyclothem model and emphasizes the coastal–estuarine rather than the deltaic nature of this part of the cycle.
- Stage 8: Estuary advances inland as regression continues. Loaded with sediment, peat compacted rapidly, making space for more sediment. Basin subsidence, sea-level rise, and peat compaction worked in concert until more than 98.4 ft (30 m) of Dykersburg Shale was in place (Figure 56). The process ended when the basin area became so deep and far offshore as to be beyond the realm of coastal processes.
- Stage 9: Black shale of late transgression and highstand. The Turner Mine Shale records rapidly deepening water. The water column stratified, bottom circulation ceased, and the Illinois Basin became sediment-starved. Fine sediment that filtered down resulted in black, phosphatic shale (Figure 57). Our model is similar to previous ones, such as those of Heckel (1977, 1986, 1994).
- Stage 10: Limestone deposition at highstand to early regression. At maximum deglaciation, sea level crested and climate was seasonally dry. During highstand, bottom circulation became reestablished. Because climate was seasonally dry, evaporation was high and carbonate deposition took place as the St. David Limestone (Figure 58). Continued clastic influx during highstand led to delta progradation (Canton Shale), ending carbonate production and beginning the next cycle. This process is similar to those in previous models.
Linkage of Climate and Eustasy
We have alluded to changes in climate, together with fluctuating sea level, being instrumental in the evolution of the Galatia channel and its related features. The recurrent glacial episodes that induced sea-level changes affected climate in the tropical realm of peat production as well as in the polar and temperate realms. Here we explore how and why climate and sea-level changes were linked.
Earth’s climate belts of today are the products of unequal solar energy to the Earth’s surface. Solar radiation is most intense in the tropics, where the sun is directly overhead. Hot air rises in a belt that girdles the Earth roughly parallel to the equator. Rising air, heavily laden with moisture, cools as it rises, producing clouds and heavy rainfall. This rainy tropical belt of rising air is called the Intertropical Convergence Zone (ITCZ). In more familiar terms, this is the doldrums, that zone of light and fickle winds where sailing ships often lay becalmed for weeks at a time. The ITCZ is also a belt of low pressure that draws air from both north and south, setting up persistent winds that angle toward the equator—the trade winds. Beyond them lie the horse latitudes, which, like the doldrums, are belts of persistent high pressure, light and variable winds, and dry weather (Figures 59 and 60). Most of Earth’s largest deserts fall in the horse latitudes.
The doldrums (ITCZ), trade winds, and horse latitudes follow the sun through the seasons, migrating north during the summer[footnote 1] and south in winter. The result is that many areas of the tropics receive distinct wet and dry seasons. Wet seasons or monsoons, characterized by torrential downpours, take place when the ITCZ is overhead. Monsoons alternate with hot, dry seasons when little rain falls. Depending on local geography, areas of the tropics may receive either one or two annual monsoons, whereas other places have an ever-wet (or perhumid) climate, under which rainfall exceeds evapotranspiration every month of the year.
In the modern world, cold dense air settles over the poles and flows outward, displacing warm, moist air and generating storms in the middle latitudes. During the “ice ages,” the polar realm expanded greatly, compressing Earth’s other climate belts toward the equator. As a result, the ICTZ did not migrate as freely with the seasons as it does during the current interglacial episode. With the ITCZ nearly fixed in place during glacial episodes, much of the equatorial region experienced an ever-wet climate.
Plate-tectonic reconstructions indicate that during the Pennsylvanian, the coal-forming regions of the Illinois and Appalachian Basins, maritime Canada, Western Europe, Russia, and the Ukraine were all aligned close to the equator. Thus, a number of authors (Cecil et al. 2003; Penser and Poulsen 2009; Eros et al. 2012; Horton et al. 2012) deduced the following equation:
Maximum Glacial Ice = Low Sea Level = Ever-Wet Climate = Maximum Peat Production.
Conversely, during the interglacial episodes, sea level rose and a monsoonal regime of pronounced wet and dry seasons took hold. Less vegetation stabilized the landscape because fewer plants could tolerate the extended annual droughts. Hence, soil erosion and the sediment load in streams increased dramatically compared with the episodes of ever-wet climate, when plants carpeted the landscape:
Minimum Glacial Ice = High Sea Level = Strong Wet–Dry Seasonal Climate = Maximum Erosion and Runoff.
Peat Developed at Lowstand
Our model calls for the Springfield Coal (and, by implication, other Pennsylvanian coal seams) to be formed during eustatic lowstand under ever-wet conditions. Some previous authors, such as Flint et al. (1995), Bohacs and Suter (1997), and Heckel (2008), maintained that peat developed during transgression. According to their view, preserving a thick peat deposit requires a rising water table because otherwise peat is oxidized and lost. Flint et al. further held that most economic coal deposits developed in domed or raised mires, which exclude clastic sediment derived from nearby rivers.
However, evidence from coal-body geometry, coal petrography, geochemistry of coal and enclosing strata, and fossil-plant patterns strongly suggests that Desmoinesian coal in the Illinois Basin developed as planar, not domed, peat deposits (Cecil et al. 1985; Eble et al. 2001; Greb et al. 2002, 2003; Neuzil et al. 2005). Thus, peat accumulated at grade with the Galatia channel, with plants filtering clastics through the flanking belts now preserved as shaly coal. The Galatia was a river without banks or natural levees. Perennial flooding from the channel, coupled with an ever-wet climate, ongoing basin subsidence, and ongoing compaction of underlying sediment, maintained a consistently high water table throughout the duration of Springfield peat accumulation.
Rapid Transgression, Gradual Regression
Evidence from the Pleistocene, particularly the most recent deglaciation, indicates that melting can be surprisingly rapid, even “catastrophic.” Blanchon and Shaw (1995) inferred, based on drowned reefs in the Caribbean and Gulf of Mexico, three “catastrophic, metre-scale sea-level rise events” during the last Pleistocene deglaciation. Gregoire et al. (2012) calculated that a sea-level rise of 45.9 to 59 ft (14 to 18 m) took place within a span of about 350 years close to 14,000 years ago, and a rise of 29.5 ft (9 m) took place within 500 years about 8,200 years ago. Data from Greenland ice cores indicate two remarkably sudden Late Pleistocene warming events. One at 11,700 years B.P. lasted 60 years, and an earlier event at 14,700 years ago spanned a mere 3 years (Steffensen et al. 2008). Other evidence and examples may be found in Webster et al. (2004), Wright et al. (2009), Bird et al. (2010), and Törnqvist and Hijma (2012). In contrast, the growth of a continental ice cap requires a much longer interval of time, measured in thousands to tens of thousands of years (Muhs et al. 2011; Dutton and Lambeck 2012).
The authors cited above suggest that any triggering event that raises the sea level may set off a chain of events leading to the rapid destruction of ice caps. The trigger could be the onset of a warmer climate or the failure of ice dams that hold back large bodies of fresh water, such as glacial Lake Agassiz and Lake Missoula. The rising sea breaks up floating ice sheets and releases fleets of icebergs, which melt in warmer waters and raise the sea level further. Moreover, the loss of sea ice changes the ocean and atmospheric circulation, leading to more melting. In contrast, the growth of continental glaciers is “glacially slow” because snow and ice can accumulate only so fast. In fact, the global cooling required to bring on an “ice age” reduces the capacity of the atmosphere to hold water vapor and yield snow.
The rapid drowning and burial of the Springfield peat swamp has counterparts in other late Paleozoic deposits. Ravinement surfaces are reported from other coals, such as the Herrin Coal of Illinois (DeMaris et al. 1983), which also has a buried autochthonous flora adjacent to the Walshville paleochannel (DiMichele and DeMaris 1987). Lower Permian carbonate cycles of Kansas commonly begin with a thin transgressive lag of fish bones, ostracods, and intraclasts, implying abrupt, erosive transgression (Miller et al. 1996). The rapid marine transgression that terminated peat accumulation repeats throughout the Pennsylvanian in the Illinois Basin among coal beds that lack channels and gray shale roof. Most commonly, the coal is sharply overlain either by offshore black, phosphatic black shale or by marine limestone; at the contact, coal laminations are generally truncated, indicating some erosion of the top of the peat body. At the base of black shale, a thin pyritic shell breccia may be present (Zangerl and Richardson 1963). Where nonmarine gray shale overlies low-sulfur coal, in situ tree stumps and tidal rhythmites are commonly in evidence. We have logged examples including the Lower Block, Murphysboro, Colchester, Herrin, and Danville Coals in addition to the Springfield.
Such conditions of rapid, pulse-like sea-level rises likely also occurred during the Pennsylvanian ice age (Archer et al. 2015, 2016). Such considerations further militate against the notion of “keep up” time-transgressive peat-swamps, created by rising base level and driven across the landscape by sea-level rise (e.g. Heckel 1995). Peat formed much too slowly to keep up with the abrupt sea-level rise of a typical deglaciation.
Relationship of Effingham and Galatia Channels
The Effingham channel clearly was cut, filled, and abandoned prior to development of the Galatia channel. In fact, the Galatia channel crosses the Effingham at a right angle. Thus, the two channels represent separate cycles of sedimentation. Previously, all strata between the Houchin Creek and Springfield Coals were assumed to belong to a single cycle (Summum cyclothem), reflecting a single episode of marine transgression and regression.
In the Midcontinent Basin, the correlative interval contains two cyclothems. The minor Upper Blackjack Creek cycle (Figure 61) falls between the major Lower Fort Scott (Excello Shale) and Upper Fort Scott (Little Osage Shale, correlative with Turner Mine Shale) cycles (Heckel 1994, 2002, 2013). As Heckel (2002, p. 110) stated, “The upper part of the Blackjack Creek Limestone extends as a bed into the upper part of the Morgan School Shale in Iowa, where it contains moderately abundant conodonts [and] thus apparently represents a minor transgression.” Heckel further noted that “the Lower Fort Scott cyclothem loses both its lower and upper bounding paleosols a short distance south of the Kansas border in northern Oklahoma [see Figure 11 on p. 20 in Heckel 2013]. This supports the idea that the lower part of the Midcontinent shelf in southern Kansas and northern Oklahoma was at a lower Pennsylvanian elevation that the entire Illinois Basin” (P.H. Heckel, written communication to W.J. Nelson, June 6, 2014).
We propose the following scenario: During late highstand and early regression of the major Lower Fort Scott cycle, deltaic sediments of the Delafield Member essentially filled the Illinois Basin. As the sea level continued to fall, the Effingham channel became incised and established a meander belt. Then came the minor Upper Blackjack Creek transgression, drowning the Effingham channel and backfilling it with sediment. When sea level again declined, a new fluvial system—the Galatia channel—became established on the exposed shelf. No limestone or marine fossiliferous shale marks the Upper Blackjack Creek event in this basin because the sea-level rise was relatively brief and low in amplitude. Tidal rhythmites in the upper Effingham channel fill in the Elysium core and local, thin coal in other boreholes points to estuarine conditions, not fully marine.
As an alternate hypothesis, an autocyclic process or tectonic movement in the basin might have led to the abandonment of one channel (Effingham) and the establishment of another (Galatia) during a single eustatic cycle. Earth movements might have changed the regional gradient from southeast to southwest, inducing a change in channel orientation. This idea is not far fetched because both the Springfield and Herrin Coals thin across the La Salle Anticlinorium and other basin structures, indicating syndepositional tectonism. The channel-forming process clearly was complex and required a substantial amount of time.
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
- For simplicity, seasons are discussed in Northern Hemisphere terms.