Geology Reference
In-Depth Information
16.1.2 Lithostratigraphy
Kvale
1993
). In such exposures, mm- to cm-scale lamina
can be laterally traced for considerable distances before
being truncated by low-angle reactivations (Fig.
16.3
).
In the WIB, a similar stratigraphic succession can
be delineated (Fig.
16.2
). Thick sandstone units were
deposited over a regional unconformity. Locally, these
sandstone units exhibit 10-m thicknesses in surface sec-
tions and 30-m thicknesses in the subsurface. The
remaining parts of the sequence are dominated by
laterally variable gray shale that contains persistent
m-scale limestone and cm-scale coal seams. Coals are
generally thinner and limestones are better developed
than in the WIB as compared to the EIB
Throughout the study area Pennsylvanian stratigraphic
successions exhibit repetitions of lithologies. This
repetition is most notable in the marine, limestone-rich
parts of the Middle and Upper Pennsylvanian section.
In the EIB, the oscillations of nonmarine (sandstones,
nonfossiliferous shale and coals) to marine (fossili-
ferous shale and limestone) gave rise to the formerly
widespread concept of “cyclothems” (Weller
1930,
1931
; Wanless and Weller
1932
). An “ideal cyclothem”
model was developed in Illinois and was, for a brief
period, utilized as a formalized lithostratigraphic unit.
The concept was subsequently applied to the Western
Interior Basin (Moore
1935, 1964
; Moore et al.
1951
),
and attempts were also made to use cyclothems as
principal components within formal lithostratigraphic
nomenclature. Application of these lithostratigraphic
models provoked widespread debate regarding the
origin of cyclothems (e.g. Heckel
1977, 1986
). Much
of the discussion focused upon the origins of wide-
spread baselevel (eustatic) oscillations. Some workers
advocated regional basinal subsidence (e.g. Sloss
1963
) whereas other workers invoked sea level changes,
particularly those related to Gondwanan paleoglacial
cycles and the resultant glacio-eustatic variations (e.g.
Heckel
1994
).
Archer (
2008
) presented a critique of cyclothem
models noting that they oversimplify the lateral vari-
ability that is characteristic of the clastic components.
For parts of the Pennsylvanian in the EIB, the strati-
graphic section does not offer a simple fit into a
standard cyclothem model. Lower Pennsylvanian
(Morrowan) strata are dominated by thick, laterally
discontinuous and channel-filling sandstone units
rather than cyclic successions of strata (Fig.
16.2
).
These sandstone units can directly overlie the
Mississippian-Pennsylvanian regional unconformity.
Thicker sands occur where there was greater erosional
incision and relief. The lower Middle Pennsylvanian
(Atokan) section exhibits significant lateral variability
in facies and thickness. The upper Middle Pennsylvanian
(Desmoinesian) coal-bearing parts of the section
exhibits more lateral continuity and is where the con-
cept of cyclothems originated and was most applied.
Even within the Desmoinesian, the gray shale parts of
coal-bearing cycles exhibit at least some degree of
regional variability. Locally, outcrops contain thin, dis-
continuous sandstone beds and laminae (Archer and
16.1.3 Stratigraphy and Common
Lithofacies
Bundled rhythmites and tidal bundles in crossbedded
facies have been documented in several lithofacies in
the EIB and WIB (Fig.
16.2
). A variety of depositional
models (i.e., “ideal cyclothems”) have been used to
describe parts of the stratigraphic and sedimentological
successions in both basins. The marine parts are charac-
terized by laterally persistent, meter-scale limestone
units that contain a diverse suite of marine fossils, and
dark gray to black shales. Organic-rich, black-shale
beds, which are commonly only a meter or less in
thickness, are regionally widespread. Various types of
gray shale or heterolithic strata comprise the most
volumetrically dominant lithofacies relative to rhyth-
mite preservation. Marine sandstone units are rare. In
general, mostly marine facies are best developed in the
upper Middle to Upper Pennsylvanian (Desmoinesian
and younger) strata, and are poorly developed or later-
ally restricted in the lower Middle (Atokan) and Lower
Pennsylvanian (Morrowan) (Fig.
16.2
) (Greb et al.
1992
; Greb et al.
2002
).
The dominantly terrestrial parts of the Pennsylvanian
stratigraphic succession include persistent coals and
the paleosols beneath them. These tend to be laterally
restricted in the Lower Pennsylvanian in both basins,
but become widespread and can be readily correlated
directly between the interior basins in upper Middle
Pennsylvanian (Desmoinesian) and younger strata
(Wanless and Weller
1932
; Heckel
1986, 1994
).
In contrast, large-scale, trough-crossbedded, fluvial
sandstones are more locally restricted in both basins.
The thicker sandstone units (10 m+) commonly exhibit
