Coal in Pennsylvania

"the Pennsylvania coals were formed in river floodplains and coastal swamps. Wherever a swamp existed, a layer of coal-forming peat accumulated. The layer was then buried by mud and sand. This happened again and again, andas time passed, each layer of peat became coal, and the covering sediments hardened into sandstone, shale, and limestone. The final result was a sequence of rocks more than 4,000 feet thick, consisting of widespread layers of coal separated by thick intervals of other rock types. Roughly, it resembles a thick book, in which every twentieth page is made of black paper to represent the coal seams."
"All of the coal layers were originally deposited as virtually flat-lying beds. Subsequent periods of mountain building in Pennsylvania, however, have folded and broken the rocks to varying degrees. Most of the bituminouscoal fields of western and north-central Pennsylvania were only mildly affected by this mountain building. Here, the beds are only slightly folded. In the anthracite area, however, the layers have been bent into large folds and occasionally broken along great cracks or faults in the earth."

"West Virginia today is mostly an erosional plateau carved up into steep ridges and narrow valleys, but 300 million years ago, during the Carboniferous Period, it was part of a vast equatorial coastal swamp extending many hundreds of miles and barely rising above sea level. This steamy, tropical quagmire served as the nursery for Earth's first primitive forests, comprised of giant lycopods, ferns, and seed ferns.
North America was located along Earth's equator then, courtesy of the forces of continental drift. The hot and humid climate of the Middle Carboniferous Period was accompanied by an explosion of terrestrial plant life. However by the Late Carboniferous Period Earth's climate had become increasingly cooler and drier. By the beginning of the Permian Period average global temperatures declined by about 10° C.

Alternating cool and warm periods during the ensuing Carboniferous Ice Age coincided with cycles of glacier expansion and retreat. Coastlines fluctuated, caused by a combination of both local basin subsidence and worldwide sea level changes. In West Virginia a complex system of meandering river deltas supported vast coal swamps that left repeating stratigraphic levels of peat bogs that later became coal, separated by layers of fluvial rocks like sandstone and shale when the deltas were building, and marine rocks like black shales and limestones when rising seas drowned coastlands. Accumulations of several thousand feet of these sediments over millions of years produced sufficient heat and pressure to transform the soft sediments into rock and the peat layers into the 100 or so coal seams which today comprise the Great Bituminous Coalfields of the Eastern U.S. and Western Europe."

"Average global temperatures in the Early Carboniferous Period were hot- approximately 20° C (68° F). However, cooling during the Middle Carboniferous reduced average global temperatures to about 12° C (54° F). As shown on the chart below, this is comparable to the average global temperature on Earth today!
Similarly, atmospheric concentrations of carbon dioxide (CO2) in the Early Carboniferous Period were approximately 1500 ppm (parts per million), but by the Middle Carboniferous had declined to about 350 ppm -- comparable to average CO2 concentrations today!"

"Climate change during the Carboniferous Period was dominated by the great Carboniferous Ice Age. As the Earth alternately cooled then warmed, great sheets of glacial ice thousands of feet thick accumulated, then melted, then reaccumulated in synchronous cycles."


"Pangea (Greek for "all lands") is the "supercontinent" created when these two giant landmasses drifted into one another, a process that was complete by the middle of the Permian Period. ...
A broad Central Pangean mountain range formed an equatorial highland that during late Carboniferous was the locus of coal production in an equatorial rainy belt (1). This produced vast amounts of sediments which were transported to equatorial coastal regions, forming deltas which supported vast coal swamps. Throughout the Late Carboniferous (Pennsylvanian) Period, Pangea drifted northward to drier, cooler climates and by the mid-Permian North America and Northern Europe had become desert-like as continued mountain-building caused much of the interior of the vast Pangean Supercontinent to be in rain shadow.


Warm to moderate temperatures and high humidity alone do not produce all the conditions necessary for creating coal deposits. Steadily rising sea level and/or steady regional swamp subsidence are also necessary. a prerequisite to the formation of thick coal seams it is necessary that the rate of vegetable matter accumulation remain in general equilibrium with the rate of rising water levels for relatively long periods. Rise too fast, and the swamp gets drowned, rise too slowly and dead plant material is not completely submerged when it falls to the swamp floor where it will rot or oxidize rather than be preserved.


Coal seams are found in layers alternating between marine and non-marine rocks, indicating cycles of coastal transgressions and regressions played an important role in coal formation.
The Carboniferous-age rocks of the Eastern U.S. and Europe record regular cycles of advancing and retreating seas; where beds of coal, shale, limestone, and sandstones were deposited in more or less repetitive sequences. These sequences, called cyclothems, have been well-documented, particularly during the Late-Upper Carboniferous."

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"The elevated organic maturity observed in shallowly buried units from the Michigan basin implies that higher temperatures and thicker overburdens once existed in the basin. Evidence from sediment-accumulation rates, regional dips, and maturity of Pennsylvanian-age coals suggests that up to 1,000 m (3,280 ft) of sediment were removed by erosion prior to the Late Jurassic, when the basin became stable."


Subsidence in the Michigan and Illinois basins began in Late Cambrian time. The bulk of subsidence in the Michigan basin occurred from middle Ordovician through Devonian time. In Illinois rapid subsidence occurred from upper Cambrian through Ordovician time and again in Carboniferous time.
The Illinois basin continued to subside relative to its flanks during the sub-Pennsylvanian unconformity.
A second heating event occurred in the Michigan basin and the Appalachian basin in Ohio and Western Pennsylvania and Western New York. The subsidence in the Appalachian basin resembles the subsidence in Michigan in that the Cambrian and Lower Ordovicianunits are thin compared with the Middle Ordovician units. The Appalachian basin became overfilled with sediments following the Acadian orogeny and ceased to accumulate sediments after that time. Except for minor Carboniferous faulting, the Michigan basin subsided due to thermal contraction without further interruption. There is no direct igneous evidence for this heating event.
Carboniferous subsidence was related to a heating event in Southern Illinois and to the south where the basin deepened in the direction of preserved drainage patterns (Howard 1979). The Carboniferous region of subsidence is more spatially restricted than either of the two earlier events.
Glacial erosion and deposition in the Michigan basin is strongly correlated with the outcrop pattern around the basin.

Pennsylvanian rocks are preserved on the northern flank of the Illinois basin, across the Mississippi River in Iowa, and in the interior of the Michigan basin. The Pennsylvanian structural relief in these areas can be attribl:lted to dilferential subsidence following and during depOSition. The depth of burial can be constrained by the degree of organic metamorphism or rank of coals in the Pennsylvanian section.

Pennsylvanian rocks in the Michigan basin dip toward the center of the basin and extend between about 300-m elevation and sea level.

The elevation of these rocks is 200 m, similar to northern Illinois. The difference in coal rank between Moscow and northern Illinois could be attributed to either colder average climates in Moscow since Carboniferous time or a greater depth of burial and subsequent erosion in Illinois.
At end of mid-pennsylvanian, "Approximately 67% of the species in peat-forming mires, and at least half the species in clastic wetlands were eliminated by changing climatic conditions, probably protracted moisture deficits or exaggeration of seasonal dryness. "
The continued southward increase in rank was probably caused either by southwardly increasing depth of burial of the coals during coalification
Past controversy about the tectonic or eustatic origin of Pennsylvanian cyclothems is resolved by regional basin-subsidence analysis, global paleoclimate, paleogeography, and plate-tectonic evolution. Differences between marine carbonate (Kansas-type) cyclothems of the northern midcontinent and nonmarine clastic (Appalachian-type) cyclothems of the eastern United States are controlled by laterally changing flexural deformation during plate accretion into a supercontinent, coupled with superposed glacial eustasy.
Appalachian-type cyclothems accumulated in response to episodic thrust loading during plate collisions that developed a series of flexurally deformed, wide and shallow, resurgent foreland basins. ... episodic thrust loading and foreland basin subsidence of small magnitude on progressively more rigid crust.