Eastern Coals

Page 7

Eastern Coals

Zoe Gentes

Andrew Infante

Amy Lombari

Jennifer Sullivan

Dennis Titterton

16 November 2009

Abstract

Coal, formed from ancient peat deposits, requires wet, anaerobic conditions and a net accumulation of peat in order to begin forming. There are three relatively high-grade coal deposits in the Eastern United States: Kansas Coals, the Appalachian Basin, and the Illinois Basin. These coals began forming in the Carboniferous Period. The facies sequences of each of these deposits, termed cyclothems, allow interpretation of the paleoenvironments at the time of deposition. Towards the west, the pattern of peat formation is more sea level driven with more marine sediments such as limestone, whereas towards the east the main cause is thrust loading with more lacustrine and siliciclastic sediments.

Introduction

Coal is metamorphic, combustible rock that is produced from ancient peat deposits. Peat can be defined as dense, partially decayed, vascular organic matter. Peat accumulation requires an environment in which aggradation or organic matter exceeds the rate of degradation. This commonly takes place in areas containing stagnant water in which anaerobic conditions (conditions with limited oxygen) halt bacterial decay, such as are commonly found in wetlands. This organic matter must be compressed under sediment, after which time, heat, and pressure change the peat’s composition, turning it into the harder organic compound known as coal.

The coal that is found in the Eastern United States formed from peat in the Carboniferous Period (320 to 286 million years ago) (Cengage, 2003), during which there was an explosion of abundant plant growth on low, flat-lying areas with extensive river systems flowing westward, creating flood plains and deltas, to an epeiric sea. These densely vegetated flood plains were havens for peat formation. In addition to the coastal swamps and bogs created by the transgression-regression of the epeiric sea, the rivers periodically overflowed, producing lacustrine swamps, burying and compressing the peat under thick layers of sand and mud. These varying layers of sediment and coals formed specific sequences of facies called cyclothems.

With so much vegetation on the planet, the amount of atmospheric carbon dioxide was in a state of depletion, under 400 ppm (Figure 1). The only other time in Earth’s last 600 million years to see these low CO2 levels is today, the Quaternary Period. Global temperatures were also at a low point, much like they are today, and there were alternating periods of ice ages and warming. This occurs whenever a continuous continental landmass extends from one polar region to the other, while at the same time there is a large polar landmass capable of carrying heavy ice accumulations. These conditions existed 300 million years ago during the Carboniferous Period as they do today. Although different factors influenced the timing and distribution of cyclothems, it is generally believed that cycles of rising and subsiding sea levels were the primary cause. These glacio-eustatic changes appear to have been global in scope and were caused by repetitive cycles of ice expansion, then ice melting, during the Carboniferous Ice Age (Hieb, 2009).

A typical Illinois cyclothem is shown in Figure 1, and is also described by DiMichele: “The Late Desmoinesian… was a time during which classic ‘cyclothems’ developed in the midcontinent United States. In the Illinois Basin there were repeated episodes of peat formation, represented today as coals, followed by inundation and deposition of freshwater and marine elastics, and marine limestones” (DiMichele, 1994, pp. 106). These cyclothems commonly rotate between nonmarine and marine sediments that are separated by coal formations. This cycle is different for each coal deposit, but generally is driven by eustatic sea level changes or thrust loading of sediments, and sometimes by a combination of the two. These strata cycles give us an idea of what conditions were present to cause the coal formation. The different cyclothem patterns can be used to assume the paleoenvironment of each of the three relatively high-grade coal formations in the Eastern United States (Figure 2).

Kansas Coals

The westernmost coal deposits in the Eastern United States are found in Kansas. They are part of the Interior Province coal formation and result from the epeiric sea that was present in Mid-Western North America during the Carboniferous Period. This sea had transgressed and regressed many times during its existence, leaving the perfect stage for coal formation. The coal deposits correspond directly to the periods of sea transgression (Figure 3). In times of low sea level, organic material grows in abundance and creates the conditions for peat formation. The peat layer shows the beginning of the transgressing sea. Once the sea level is too high, no more organic matter can grow, and there are marine sediments deposited, forming layers like limestone. Then as the sea regresses, there are deposits of terrestrial sediment, like sandstone. The cycle begins again as the sea transgresses once more. Peat formation is not continuous, but occurs episodically with changes in sea level.

In the Pennsylvanian and Lower Permian rock succession of Kansas, there are over one hundred cyclothems (Moore, 1964, pp. 287). These marine carbonate cyclothems accumulated on a relatively stable platform affected only moderately by collision tectonics of North American margins. These cyclothems show a general sequence of non-marine sediments (sandstone, sandy shale, freshwater limestone), an underclay layer (indicative of the leaching that occurs in swamps and other wetland areas), then a layer of coal, overlaid by marine sediments (limestone and shale).

Appalachian Basin

To the east, and on the opposite end of the cyclothem spectrum, lie the Appalachian Basin coals. These are some of the most abundant coals in North America and, unlike the Kansas coals, are the result of subsidence. Tectonic instability in the area, orogeny in particular, is the main force driving this process. In Paleozoic time, the Appalachian Basin was a foreland basin bounded by the Allegheny Range to the east, the Central Pangaea Mountains to the southeast, and open westward to the sea. This resurgent basin is described as a large wetland complex drained by a fluvial system containing many channels, freshwater siliciclastic and carbonate lakes, and peat swamps on a floodplain (Garcés, 1996, pp. 305).

Sediments that were eroded and transported west of these mountains were brought into the wetland basin. Organic matter in this area was buried under these sediments and was metamorphosed when the land collapsed on itself from shifts in the water content and additional subsidence due to the added weight of transported sediments. The lacustrine deposits in the upper Middle Pennsylvanian continental cyclothems of the Appalachian Basin have been attributed to a cycle of alternating climates of wet (coal deposition) and dry (carbonate deposition) periods (Garcés, 1996, pp 305). These wet periods were periods in which flourishing plants were spreading at a rate greater than decomposition. The deposits in this area are some of the oldest and richest, and areas such as West Virginia and Pennsylvania are considered to be the coal capitols of the world. In West Virginia there are one hundred successive cyclothems, each cycle having an age of tens of thousands of years, in which there are many many layers of coal deposits.

Illinois Basin

In between the previously described basins is the Illinois Basin. This basin is also part of the Interior Basin. It is linked to the Appalachian Basin by the same orogeny processes and foreland basin development; it is also near Kansas deposits and thus affected by changes in sea level, containing some marine sediments (limestones in particular). Illinois cyclothems have been described as “an intermediate between the two end-member processes of flexural tectonics and glacial eustasy characterizing Appalachian-type cyclothems and Kansas-type cyclothems” (Klein, 1989, pp 152). Though it is too far from the epeiric sea to have coastal swamps inundated by the transgressing sea, sea level played its part through the rivers that drained through the basin and into the sea. In times of lower sea level, the river gradient steepened to accommodate the angle, which meant that the water flowed faster and less pooling occurred upstream. But in times of higher sea level, more water was present and the river became “backed up.” The river gradient became shallower and water slowed down, making pooling more likely upstream (Figure 4). These cycles repeated numerous times over a very long period of time, and “a single [Illinois] cyclothemic rock package could have formed in as little as 24,000 yr if very few gaps in sedimentation are assumed. If larger gaps existed, upper limits of 65,000-129,000 have been suggested” (DiMichele, 1996, pp.113).

Conclusion

We determined many aspects about coal deposition and paleoenvironments in the Eastern United States. We knew that the main condition for peat formation is oxygen-poor moisture in which the rate of organic accumulation exceeds the rate of degradation. In the Eastern United States, the three dominant relatively high-grade coal formations are in the Kansas, Illinois, and Appalachian basins. These coal deposits formed during the organic boom in the Carboniferous Period (286 - 360 million years ago). The predominant sequence of strata is many-layered cyclothems, caused by recurrent thrust loading and eustatic changes in sea level. These cyclothems suggest that the farther west and towards the middle of the continent the formation is, the more marine deposits it will contain and the more affected by eustatic changes the cyclothems will be; In the East, the deposits are negligibly affected by sea level and more a product of thrust loading. Following this theory, the Appalachian Basin cyclothems were based on subsidence and thrust loading patterns, the Kansas coals were based on seal level changes, and Illinois was a hybrid of the two.

References Cited

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