Paleozoic Earth History - The Changing Earth: Exploring Geology and Evolution - page 546

◗ Figure 20.18 Cyclothems

Nonmarine
environment

Marine
environment

Terrestrial
sedimentation

Coal-forming
swamp

Erosion Nearshore Offshore

Disconformity
Brackish and
nonmarine shales
Marine shales
Algal limestones with
nearshore and brackish
water invertebrate fossils
Limestones with offshore
invertebrate fossils
Limestones and shale with
offshore invertebrate fossils

Progradation

Marine shales with nearshore
invertebrate fossils

Transgression

Coal
Underclay
Nonmarine shales and
sandstones
Nonmarine sandstones
Disconformity

(a) (b)

b Pennsylvania coal bed, West
Virginia.

a Columnar section of a complete cyclothem.

Marine deposition

Nonmarine
deposition

Sea level

Transgressing sea

Coal swamp

Marine sediment

Potential future coal

Nonmarine sediment
(c c Reconstruction of the evironment
of a Pennsylvania coal-forming
swamp.

d The Okefenokee Swamp, Georgia, is a modern example of a
coal-forming environment, similar to those occurring during the
Pennsylvanian Period.

convergent plate boundary), the Appalachian mobile belt
was born (Figure 20.24b). The resulting Taconic orogeny —
named after the present-day Taconic Mountains of eastern
New York, central Massachusetts, and Vermont—was the
fi rst of several orogenies to affect the Appalachian region.
The Appalachian mobile belt can be divided into
two depositional environments. The first is the extensive,
shallow-water carbonate platform (Figure 20.24a) that

formed the broad eastern continental shelf and stretched
from Newfoundland to Alabama. It formed during the
transgression of the Sauk Sea onto the craton when carbon-
ates were deposited in a vast, shallow sea. The shallow-water
depth on the platform is indicated by stromatolites, mud
cracks, and other sedimentary structures and fossils.
Carbonate deposition ceased along the east coast dur-
ing the Middle Ordovician and was replaced by deepwater