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16.2.2 Heterolithic Tidal Channel Facies
Some rhythmites in the heterolithic tidal flat facies
are predominantly silt rather than alternations of mud
and sand. Laminated siltstone facies are volumetrically
rare and are known from only a few outcrops in southern
Indiana and Kansas. Individual lamina range in thick-
ness from a few millimeters to as much as several centi-
meters. Thus these rocks contain finely interspersed
thick laminae and thin beds. Each individual lamina or
bed exhibits a relatively abrupt lower boundary and is
capped by a finer grained lamina drape. Rhythmic lami-
nation contains gradationally thickening and thinning
clay-draped laminae. In the EIB, rhythmites commonly
contain alternating thicker- and thinner laminae pairs
within laminae bundles (Fig. 16.6a, e .g. Kvale et al.
1989 ). Similar, but geologically younger facies in the
WIB do not commonly exhibit prominent laminae
pairing (Lanier et al. 1993 ).
Tidal rhythmites in the heterolithic tidal flat facies
may contain mm- and cm-scale cyclicities, which are
laterally continuous at the scale of an outcrop (Kuecher
et al. 1990 ; Kvale and Archer 1991 ). At other loca-
lities, however, erosional features and small-scale
reactivations are common in the laminated siltstones,
and rhythmites are less laterally continuous. Relatively
discontinuous rhythmites in heterolithic tidal flat facies
have been reported above coal beds (Fig. 16.6b ).
At these locations, low-angle reactivations and soft-
sediment deformation are evident. In some cases, a
single lamination has been overturned and truncated
by overlying, planar laminae (Fig. 16.6c ). These are
similar to overturned foresets that can occur in cross-
bedded sandstone. Disrupted or discontinuous rhyth-
mites in the tidal-flat facies in these situations, likely
result from syndepositional compaction of the under-
lying peat during rapid loading of tidal sediment s.
Generally there is very little internal bioturbation
(vertical burrowing) of tidal rhythmites in the hetero-
lithic tidal-flat facies. Heterolithic tidal-flat facies do
occur with well-developed bioturbation in both basins,
but in these cases (as in the modern), tidal lamination
is disrupted, so that bedding is no longer rhythmic.
Where tidal rhythmites are well-developed in modern
tidal settings, burrowing is absent or limited (Archer
2004 ). This is not to say that tidal rhythmites contain
no biogenic structures. Where clay drapes are well
developed in rhythmites, the rocks can be readily split
along this surface, and surficial (horizontal) biogenic
structures are locally common. Because of the well-
sorted, fine-grained sediment, many types of biogenic
structures (horizontal trace fossils) are unusually well
Tidal rhythmites within channel-form scours represent
fluvial channels that have been converted to tidal-
estuarine channels, tidal channels, or abandoned chan-
nels (tidal or fluvial) that filled with tidal flat facies.
Modern tidal channels may contain bedforms that
exhibit unimodal to bimodal bedding, mud-draped
foresets, or bundled foresets. Pennsylvanian hetero-
lithic tidal channel facies in the EIB and WIB, com-
monly exhibit mud-draped planar foresets. Successive
thicknesses of foresets can exhibit systematic thickening
and thinning or bundling (Fig. 16.4c ).
In western Kentucky a lower Middle Pennsylvanian
channel described by Greb and Archer ( 1995 ), exhi-
bited heterolithic laminae arranged in thickening and
thinning bundles on low-angle bedding surfaces, rather
than true foresets (Fig. 16.5 ). Thickening and thinning
laminae bundles amalgamated vertically within the
channel fill into ripple-laminated and ripple-bedded
sandstone. At first glance, stacked successions of
thickening and thinning laminae bundles are generally
similar to other documented examples of Pennsylvanian
rhythmites in which the bundling was interpreted to
represent neap-spring cycles. More detailed analyses,
however, suggested that these cm- to dm-scale bundles
could also be interpreted as annual bundles (Fig. 16.5 ).
16.2.3 Heterolithic Tidal-Flat Facies
Flat-lying heterolithic facies with tidal bedding or lam-
ination are common in the Pennsylvanian of both the
EIB and WIB. These facies are similar to the deposits
of modern tidal flats. Well-preserved successions of
tidal rhythmites are not common on all modern tidal
flats. The best-preserved rhythmites in modern tidal-
flat settings are documented in hypertidal systems
(Dalrymple and Makino 1989 ; Tessier 1993 ; Archer
2004 ). Heterolithic tidal-flat facies in the Pennsylvanian
of the EIB and WIB consist of mixed sandstone,
siltstone and shale. Extrabasinal clasts are lacking.
Sandstones within the units are commonly well sorted.
Mud-chip conglomerates can be locally evident. Within
sandstones, ripple-scale features are very common and
a great variety of ripple marks and fine-scale, exposure-
related features have been reported in both basins
(Kvale and Archer 1991 ; Lanier et al. 1993 ; Greb and
Archer 1995, 1998 ).
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