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lateral variability in thickness and bedding. The unique
juxtaposition along paleochannels and lateral variability
within the wedges is quite different from the typical
“cyclothemic” successions common within parts of the
Desmoinesian and younger strata. Away from the
paleochannels and gray-shale wedges, coals are over-
lain by a more common succession of limestones or
coarsening-upward, gray shale and siltstone capped by
sandstone or another coal bed. Gray shale wedges were
widely mapped in the EIB because the sulfur content
of the underlying coal seemed to bear a more-or-less
direct relationship to the relative thickness of overlying
gray-shale wedge. Lower sulfur contents were found
in coals beneath the thicker parts of the shale wedges,
and more typical higher sulfur contents were found
toward the thinning margins of the wedges (e.g. Gluskoter
and Hopkins
1970
).
Gray shale wedges in the Desmoinesian of the EIB
were formerly interpreted as (1) levee deposits of the
adjacent fluvial channels, (2) crevasse-splay deposition
into floodplain lakes, or (3) lacustrine varves (Archer
and Maples
1984
). Various types of fine-scale rhyth-
mites (Fig.
16.7
), however, are common in the shale
wedges, suggesting tidal, rather than fluvial influences.
Cyclic rhythmites in the gray shale wedges were first
noted by Kvale and Archer (
1990
, 1991). In this facies,
there is a complex continuum from thin-bedded, rippled
sandstone near the channels, to mud-draped sandstone
beds, and ultimately to mudstone-dominated, hetero-
lithic bedding, which includes flaser, wavy, and len-
ticular bedding (Reineck and Wunderlich
1968
). In
addition, mm-thick, planar sand streaks (Fig.
16.7a
) are
frequent and have been termed “pinstripe bedding”
(Kvale and Archer
1990
). A pinch-and-swell texture,
created by small-scale truncations and reactivations are
locally abundant (Fig.
16.7b
). Very similar gray-shale
dominated facies that are adjacent to paleochannels
occur in the WIB and, in may cases, directly overlie
coal seams.
As in rhythmites of the heterolithic-tidal flat facies,
there is little to no bioturbation in rhythmites of the
gray-shale wedge facies. Fine-scale depositional fabric
is commonly very well preserved (Fig.
16.7d
). In zones
where bioturbation (horizontal traces) does occur, how-
ever, the biological activity indicates a low-diversity
infauna of burrow-making organisms (Archer and
Maples
1984
). In modern settings, such organisms are
termed “opportunistic” because they commonly exhibit
a high density of one or more individual species, but
have low overall biotic diversity.
16.3
Discussion
Prior to the late 1980s, facies now interpreted as tidal
were commonly considered to have formed in nonma-
rine, fluvial-deltaic settings. Later, sedimentological
research focused on the laminae and laminae bundles
in these facies. Various types of cyclicity and related
features were used to reinterpret the depositional
setting as tidally influenced (Kvale et al.
1989
). Then,
increasing detail was focused on the types of cyclicity
that could be extracted from long, laminae-thickness
series and the remarkable apparent completeness of
tidal records preserved in some ancient tidal rhyth-
mites in the basins. In southern Illinois, a long and
continuous series of foreset thicknesses from the
Abbott Formation exhibited what appears to be one of
the most complete paleotidal records from the
Pennsylvanian (see Kvale and Archer
1991
; Archer
1996a
). Rhythmites from both basins were found to
preserve a variety of short- and longer-term tidal cycles.
Fig. 16.7
Heterolithic rhythmites from open-pit coal mines in
Indiana and Illinois. (
a
) Polished slab from Brazil Formation
(Kvale and Archer
1991
) exhibiting prominent bundling of
mostly planar to slightly rippled sandstone streaks. Note the cm-
thick dark, mud-rich zones near the
bottom
and
top
. Originally
described as neap-spring tidal cycles, the bundled could also
reflect seasonal (yearly) periodicities. (
b
) Small-scale ripples and
truncations within heterolithic rhythmites from Murphysboro
Coal in southern Illinois. Note more-organized zone at top, which
is similar to Fig.
16.7a
. (
c
) Unusual rhythmites with several
scales of periodicities. Neap-spring cycles range from about
1-cm thick at bottom to as much as 3-cm thick at top. Note the
alternation of thicker neap-spring cycles overlain by thinner
neap-spring cycles that occurs throughout the sample. This pattern
can be interpreted as related to perigean-spring tides (see Archer
1996a
). Thicker neap-spring cycles would be produced during
lunar perigee and the thinner cycles would have been deposited
during lunar apogee. Preservation of such detailed records of
paleotides is remarkable. (
d
) Polished slab of infaunally biotur-
bated heterolithic rhythmites from the EIA. Along the right side,
the only bioturbation consists of a few, tubular, 5-mm diameter
burrows. Along the
left side
, particularly in the
lower left
, exten-
sive bioturbation has selectively and nearly completely destroyed
the rhythmite fabric. Rectangle on right delineates the highest-
order cyclicity (
dark lines
), intermediate-order cyclicity (
thin,
solid lines
), and lowest-order cyclicity (
thin, dashed lines
)
