Ph. Heckel, GLACIAL-EUSTATIC BASE-LEVEL CLIMATIC MODEL FOR LATE MIDDLE TO LATE PENNSYLVANIAN COAL-BED FORMATION IN THE APPALACHIAN BASIN, Journal of sedimentary research. Section B, Stratigraphy and global studies, 65(3), 1996, pp. 348-356
Late Middle-Late Pennsylvanian (middle Allegheny and lower Conemaugh)
marine units in the Appalachian basin represent the distal edges of gl
acial-eustatic marine incursions of the Midcontinent sea onto the detr
ital apron of the Appalachian highlands, Because these marine units ty
pically overlie coal beds, the main environment of formation of this t
ype of coal bed appears to have been that of a coastal swamp migrating
ahead of transgression, As base level rose, the water table continual
ly rose, and the swamp was further nourished and surrounding vegetatio
n kept thick enough to inhibit detrital influx by rainfall from the ex
panding nearby source of moisture, Even in parts of the Appalachian cy
clic succession with no marine units (upper Allegheny, Monongahela, Du
nkard), the coal beds represent the wettest part of the climate cycle
of Cecil (1990), sandwiched between detrital units representing season
al wet-dry climates when sediment was most readily mobilized, and anti
podal to nonmarine limestones representing the driest climates, Throug
hout this time, the Appalachian basin was in the center of the tropica
l zone of a nearly accreted megacontinent, quite distant (similar to 4
000 km) from permanent oceanic sources of moisture, Climate modeling w
ithout considering inland seas or orographic effects on winds shows th
at the tropical zone of such a large land mass would have suffered fro
m a severe deficit in rainfall and soil moisture, Although further mod
eling shows that the Appalachian orogenic highlands would have drawn i
n moisture to increase rainfall in their vicinity, they also may have
impeded the normal westward flow of moisture-laden easterly winds, The
western source for enough moisture to form coal beds was most likely
the North American Midcontinent sea, During major highstand, this sea
would have covered about 2 million km(2) and provided both high base l
evel and abundant nearby moisture for enough continual rainfall to kee
p the coastal peat swamps widespread, fresh, and relatively detritus-f
ree in the Appalachian basin, In contrast, during major lowstand it wo
uld have covered only about 200,000 km(2) with its shoreline about 150
0 km away, and it apparently provided less consistent moisture to the
Appalachian basin then, because most nonmarine limestones and paleosol
s formed there during these times contain evidence of drier climates,
Therefore, upper Allegheny, Monongahela, and Dunkard coal beds most pr
obably formed when the Midcontinent sea was approaching highstand and
stood just west of the presently preserved outcrop, Thus, they likely
represent the coastal lowland equivalent of a marine unit at highstand
farther west, This constrained time of peat formation and preservatio
n, in a variety of coastal environments prior to and during marine hig
hstand, is responsible for the discrete and widespread nature of the c
oal-bed interval in a typical depositional cycle (or cyclothem), a pat
tern that stands in contrast to the complicated relations of overlying
and underlying detrital units, which reflect the segregation of sand
and mud into a complex mosaic of local deltaic and terrestrial subenvi
ronments that developed when sea level, rainfall, and the water table
were lower.