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that reflect intermittent migration of a sand wave
(Eriksson and Simpson
2000
). Foreset-bundle thick-
nesses, when plotted on a histogram of foreset bundle
thickness versus foreset bundle number (Fig.
15.20
),
reveal a hierarchy of diurnal, semi-monthly, and
monthly tidal periodicities. Thick-thin pairs of foreset
bundles (Fig.
15.20a
) are considered to reflect deposi-
tion from semidiurnal dominant and subordinate
flood-tidal currents, respectively. Similar thick-thin
diurnal pairs are widely developed in Holocene tidal
sediments. Cyclic variations in foreset bundle thick-
nesses record longer period changes in strength of
the dominant semidiurnal tidal currents consistent
with semi-monthly anomalistic, perigean-apogean tidal
signatures. Fast Fourier Transform analysis on the
data set reveals strong peaks at 13.11, 9.83 and 2.18
(Eriksson and Simpson
2000
). The last two peaks
are consistent with the interpretation of diurnal and
neap-spring cyclicity discussed above whereas the
13.11 peak is considered to record neap-spring cycles
in which both dominant and subordinate semi-diurnal,
subordinate-tide foreset bundles had been removed
(Fig.
15.20b
) and reveal only one well-developed peak
at 9.33 that is interpreted as a strong semi-monthly
signature. Close inspection of Fig.
15.20
b reveals that
monthly perigean-apogean cycles in the Moodies
sand wave deposit have a maximum of 20 foreset
bundles. This is a record of the minimum number of
days in the synodic month during the middle Archean
because of missing neap-tide foreset bundles espe-
cially within the apogean component of the monthly
cycle when tidal current velocities are less than during
perigee.
Fig. 15.13
Thin section photomicrograph of rhythmically bedded
siltstone/mudstone in the Coronation Formation, Government
Subgroup, Witwatersrand Supergroup.
Red arrow
indicates
correlative laminae across a break in the core
siltstone and mudstone records vertical accretion. In
both facies associations, mudstone developed during
slack water phases whereas sand and/or silt transport
took place during the ebb and flood stages. Within both
laterally and vertically accreting facies, alternating
thin-thick laminations reflect diurnal twice-daily tides.
Thinner groupings of foresets and thinner intervals
of vertically stacked sandstone/siltstone/mudstone
laminations formed during neap tides whereas thicker
groupings of foresets and laminations developed during
spring tides. Desiccated mudstone drapes on foresets
indicate that bedforms rarely were exposed during some
portion of the tidal cycle.
In the Eureka Syncline, tidal facies are represented
by mudstone-draped cross-bed foresets (Fig.
15.19
)
15.3.5 Other Precambrian Siliciclastic
Examples
The Precambrian rock record is replete with other
examples of inferred tidal facies: these are shown in
Table
15.1
in comparison to the examples discussed
above. Sedimentary structures of tidal origin are exten-
sively developed in the Big Cottonwood Formation in
Utah and include heterolithic tidal rhythmites that
record four tidally forced cycles, sigmoidal cross-bed
bundles with reactivation surfaces, tidal bedding,
current ripples with rounded crests (Chan et al.
1994
;
Sonett et al.
1996
; Ehlers and Chan
1999
). The Ortega
Quartzite and Uncompahgre Formation in New Mexico
