Geology Reference
In-Depth Information
Fig. 16.1
Location of study sites and stratigraphic units in the
Eastern Interior Basin (
EIB
) and Western Interior Basin (
WIB
) in
the central U.S. The EIB is located mostly in the state of Illinois,
but also include large parts of Indiana and western Kentucky.
The WIB contains large areas of the states of Iowa, Missouri,
Kansas, and Oklahoma. Small areas in southeastern Nebraska
and west-central Arkansas are also included in the WIB
rocks and natural outcrops are rare. Exposures are
mostly limited to quarries, open-pit and underground
coal mines, relatively rare roadcuts, and valleys of the
larger rivers. Shallow coal-exploration cores have also
provided much useful data.
The Eastern Interior Basin (EIB) is regionally
referred to as the Illinois Basin and includes parts of
the states of Illinois, Indiana, and western Kentucky as
well as small parts of southeastern Iowa (Fig.
16.1
).
The EIB began as an aulocogen in western Kentucky
during the Cambrian and evolved into a broader, intra-
cratonic basin throughout the Paleozoic (Soderberg
and Keller
1981
; Heidlauf et al.
1986
). During the
Pennsylvanian Period, there were essentially two major
depocenters, one above the old aulocogen in the Rough
Creek Graben of western Kentucky, and a second
above a semi-circular structural depression sometimes
referred to as the Fairfield Basin in southeastern Illinois
(e.g. Wanless
1975
). Pennsylvanian strata thicken
above these depocenters and thin laterally toward
the basin margins. Many of the known exposures of
rhythmites occur along the basin margins.
During Pennsylvanian deposition the EIB was
essentially a broad structural embayment that was open
to the south and connected to the Ouachita Trough on
the southern margin of the craton. Similarly, there were
times during the Pennsylvanian, when deposition was
continuous across the Midcontinent between the EIB
and WIB (Wanless and Wright
1978
; Greb et al.
2003
).
Uplift of the Pascola Arch on the southern margin of
the basin after the late Pennsylvanian resulted in a
closure of the EIB. Thus, age-equivalent rocks in the
EIB, WIB, and northeastern Arkansas are now geomor-
phically separated.
The Western Interior Coal Basin (WIB) is also
named the Forest City Basin (Fig.
16.1
). The basin
includes parts of southwestern Iowa, southeastern
Nebraska, eastern Kansas and central Missouri. The
southern extent of the Paleozoic depositional basin also
includes parts of Arkansas and Oklahoma. The WIB
has a complex Paleozoic history and initial movements
began in the Ordovician (Lee
1943
). Subsidence in
northern Kansas formed the ancestral basin, which was
subsequently bisected by uplifts along the western mar-
gin (Nemaha Anticline). During the late Mississippian
and early Pennsylvanian, widespread erosion produced
a surface with more than 70 m of relief.
Renewed uplift of the Nemaha Anticline created the
western margin and downwarping to the east created
the WIB. Pennsylvanian strata, ranging from the Atokan
to Virgilian, attain thicknesses of approximately 600 m
in the center of the basin (Anderson and Wells
1968
).
Unlike the EIB, no lower Pennsylvanian (Morrowan)
rocks have been preserved.
