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within the cycles described in the gray-shale facies
described above, some of the rhythmites appear to
contain several “magnitudes” of periods (Fig.
16.7c
).
First of all, there appears to be a series of well-developed
neap-spring cycles. Further examination indicates that
the thickness of successive neap-spring cycles varies.
A thicker neap-spring cycle is overlain by a thinner
neap-spring cycle. The presence of these two co-
existing periods, a result of the differential alignment
of synodic and anomalistic months, represents an
apogee-perigee cycle.
Not all thickening and thinning tidal laminae bundles
represent “neap-spring” periods. Care should always
be taken to test apparent neap-spring bundling with
alternative hypotheses in mind. Figure
16.6
a illustrates
“pinstripe bedding” produced by very thin sand streaks
separated by mud drapes. As the sand streaks thin, the
drapes begin to merge and a condensed zone of amal-
gamated drapes, or “dark band” occurs. If the mud
deposition was sequentially getting thinner, a thinner
interval of amalgamated drapes would be expected. In
this case, the total thickness of the dark band appears
anomalously thick, or “over developed.” The dark bands
could represent a period significantly longer than a
week-long period of neap tides. Also, some of the
thicker sand streaks exhibit smaller-scale sub-laminae.
Thus, even the sand-rich zones show evidence that the
sand streaks might be related to multiple, rather than
single, events. If more than 7 days events are recorded
in the inferred spring- or neap-part of a tidal laminae
bundle, it might not represent a neap-spring periodicity.
It may still be tidal, but represent longer-term depo-
sitional cycles.
Figure
16.7
d exhibits a number of apparent “neap-
spring” bundles from a mine in southwestern Illinois.
There is extensive and localized bioturbation, appa-
rently by a burrowing sea anemone (the trace fossil
Conostichnus
). If an individual lamina is interpreted
as the product of daily tidal events, then the cm-scale
cycles would represent “neap-spring” cycles. It
seems unlikely that a burrowing organism, such as a
sea anemone, might have been able to tolerate such
rapid rates of deposition. In fact, most Pennsylvanian
cyclic rhythmites exhibit very little to no bioturba-
tion. An alternative interpretation, based upon bio-
turbation, would be that the apparent “neap-spring”
cycles are actually a longer-duration periodicity. The
prominent “dark band” shows some similarities with
similar features in other rhythmites (Fig.
16.7a
). This
band might suggest that the “neap-spring” cycles
within these pinstripe bedding might be yearly
cycles, related to seasonal precipitation (Kvale et al.
1994
) or some other type of regularly recurring vari-
ability. In addition, seasonal variations in fluvial
input of freshwater can dramatically affect the loca-
tion of the inland tidal limit. Thus, a particular depo-
sitional setting might oscillate between a fluvial to
an estuarine system during the course of a year, and
produce a repeated succession of tidal laminae that
perhaps mimicked neap-spring cyclicity, but actually
represented stacked spring tidal bundles that were
separated by longer hiatus.
Detailed viewing of potentially “yearly bands” in
the example indicates that some of the sand streaks
appear to contain several, very thin mud drapes. If
these streaks were the product of a semidiurnal or
diurnal tidal event, a single, simple drape would have
been preserved. Similarly, (Greb and Archer
1995,
1998
) noted these apparent “neap-spring” cycles.
Multiple drapes are difficult to explain and would
seemingly require multiple sedimentary events fol-
lowed by stillstands. The sand streaks may represent
an entire, but condensed neap-spring cycle (or longer
duration). In this case, the causality of the interior
drapes becomes easier to understand. Bundling in a
heterolithic channel facies sometimes was document-
ing longer annual or seasonal sedimentation periods.
In two Pennsylvanian examples from Kentucky,
complex internal draping within the sand lamina of
“spring-like” bundles suggested more than 7 days of
spring-tide sedimentation. Also, in the channel fill
shown in Fig.
16.5
, shaly intervals between the sandy
“spring” bundle were relatively thick or “over-developed.”
Such rhythmites can easily be misinterpreted as
neap-spring cycles if internal details of lamination
and bedding are not tested to ensure that inferred
clay-draped laminae or laminae couplets likely rep-
resent 1-day's tidal events, rather than the amalga-
mation of multiple very thin events into a single bed
or lamina.
16.3.3 Salinity of Late Paleozoic Tidal
Systems
A potential conceptual problem exists with the term
tidal when interpreting rhythmite or texturally banded
facies. To many, it suggests marine conditions or at least
