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Williston Basin
Carbonate
bottom
Land
Mountains
Evaporites
Barrier reefs
Epeiric sea
Deep ocean
hita
Mu
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Figure 20.13
Devonian Paleogeography of North America
delicate interplay between nonmarine deltaic and shallow-
marine interdeltaic and shelf environments.
For illustration, look at a typical coal-bearing cyclothem
from the Illinois Basin (
because of abrupt changes from marine to nonmarine con-
ditions or removal of some units by erosion.
Cyclothems represent transgressive and regressive
sequences with an erosional surface separating one cyclothem
from another. Thus, an idealized cyclothem passes upward
from fluvial-deltaic deposits, through coals, to detrital
shallow-water marine sediments, and fi nally to limestones
typical of an open marine environment.
Such repetitious sedimentation over a widespread area
requires an explanation. In most cases, local cyclothems of
limited extent can be explained by rapid but slight changes in
sea level in a swamp-delta complex of low relief near the sea
such as by progradation or by localized crustal movement.
Explaining widespread cyclothems is more diffi cult. The
hypothesis currently favored by most geologists is a rise and
fall of sea level related to advances and retreats of Gondwanan
◗
Figure 20.18a). Such a cyclothem
contains nonmarine units, capped by a coal unit and over-
lain by marine units. Figure 20.18c shows the depositional
environments that produced the cyclothem. The initial
units represent deltaic and fl uvial deposits. Above them is an
underclay that frequently contains root casts from the plants
and trees that comprise the overlying coal. The coal bed
results from accumulations of plant material and is overlain
by marine units of alternating limestones and shales, usually
with an abundant marine invertebrate fauna. The marine
cycle ends with an erosion surface. A new cyclothem begins
with a nonmarine deltaic sandstone. All of the beds illus-
trated in the idealized cyclothem are not always preserved
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