Geology Reference
In-Depth Information
Fig. 1.11
Line graph showing the changes in tidal amplitude
for the seven most dominant tidal constituents for several tidal
reference stations located along the eastern side of the Gulf of
Carpentaria (locations noted in Fig.
1.10
. Constituent data was
extracted using the Seafarer Tides software package by the
Australian National Tidal Centre, Bureau of Meteorology and
provided to Kvale (
2006
) (From Kvale (
2006
) and used by per-
mission from Marine Geology)
Formation tides were dominated by O
1
, K
1
, followed
by M
2
with very weak contributions from S
2
and N
2
.
These examples illustrate that tidal constituents
can be extracted from the rock record in well-preserved
tidal rhythmites. While it is not always possible to
draw conclusions regarding so many tidal constitu-
ents, deposits can generally be determined to be either
diurnal or semidiurnal in nature based on the absence
or occurrence of alternating thick-thin laminae. Most,
but not all, semidiurnal tidal deposits can be related
to the synodic period and the convergence of M
2
and
S
2
constituents. Exceptions of semidiurnal, tropically
driven neap-spring tides or tidal deposits, such as
Booby Island and the Abbott Sandstone, are known
and can be discerned if the tidal record is long and
clean enough. All diurnal deposits should have been
deposited in tropically driven neap-spring cycles.
Semidiurnal depositional systems that lack strong K
1
or O
1
constituents (like Effingham, England), and in
which tidal sediments were deposited only on high
intertidal zones might mimic a diurnal tidal deposit
(Archer and Johnson
1997
). In such a case, additional
outcrop work might result in the discovery of lower
intertidal or subtidal facies that would resolve the
issue.
1.4.3
Abbott Sandstone (Tradewater
Formation, Pennsylvanian, Illinois)
Figure
1.12
shows an outcrop and bundle thicknesses from
some flaggy, large-scale tidal bundles along Interstate 57
in Johnson County, Illinois (Kvale and Archer
1991
). A
histogram of bundle thicknesses indicates a strong semidi-
urnal signal throughout the record. While not as clean a
tidal record as the two previous examples, the Abbott
sandstone example appears to exhibit minimal diurnal
inequality during the neap tides. When the diurnal inequal-
ity tracks neap tides, it indicates that neap-spring cyclicity
is driven by the tropical period (e.g. Fig.
1.4
). As such, the
Abbott Sandstone tidal record resembles that of Booby
Island, Australia (Fig.
1.10
), in which M
2
, O
1
and K
1
dom-
inate the resultant tide over S
2
. There is a suggestion of a
semimonthly inequality to the Abbott sandstone record
indicating that N
2
was stronger than S
2
and sufficiently
strong to influence the tidal record.
