Geology Reference
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that provide more robust evidence for a tidal origin for
stratigraphic units ranging in age from 3.25 to 0.6 billion
years. Specifically, examples are from the Moodies
Group (~3.25 Ga) and the Witwatersrand Group
(~3.0-2.8 Ga) in South Africa; the upper Mount Guide
Group (~1.8 Ga) and the Elatina Group (0.6 Ga) in
Australia. Rhythmic bedding preserved in banded
iron formations (BIF's) of the Weeli Wolli Formation
(2.45 Ga) in the Hamersley Basin of Western Australia
is evaluated in terms of whether or not these record a
tidal signal.
et al.
1991
); and (5) ebb runoff and exposure of tidal
flats (Klein
1977
).
15.3
Facies and Facies Associations
Facies in Precambrian stratigraphic units that have
been used to infer a tidal origin include: (1) herring-
bone cross bedding and bimodal-bipolar paleocurrent
patterns that record opposing tidal currents; (2) cross-
bed foreset bundles (product of lateral accretion) and
horizontal laminae (a product of vertical accretion) that
display thick-thin pairs, reflecting diurnal inequality of
tidal current velocities, and rhythmic thickening and
thinning of laminae, reflecting fortnightly neap-spring-
neap cyclicity; (3) alternating thicker and thinner
neap-spring-neap bundles that record perigee and
apogee effects; and (4) mudstone drapes on cross-bed
and ripple-bedded foresets and between horizontally
bedded sandstone laminae that record slack-water
suspension sedimentation; (5) flaser, wavy and lenticu-
lar bedding (tidal bedding) that reflect alternating
bedload and suspension sedimentation; and (6) modi-
fied ripples, often developed and preserved because
of the development of microbial mats (microbially
induced sedimentary structures - MISS), include ladder-
backed, flat-topped, washed-out and superimposed
forms that record ebb runoff and exposure of tidal flats.
These criteria are evaluated with respect to four
Precambrian siliciclastic stratigraphic units before
discussing possible tidal signals preserved in banded
iron formations.
15.2
Tidal Processes
The gravitational attraction of the Moon (and Sun)
raises a tidal bulge in the solid Earth and in oceans.
But, because of tidal friction, there is a delay in
Earth's response causing the tidal bulge to lead the
Earth-Moon axis by a small angle (Lambeck
1980
).
The Moon exerts a torque on the tidal bulge that results
in a slowing down of Earth's rotation and an increase
in the length of the day. The torque that the Earth's
tidal bulge exerts on the Moon leads to an acceleration
of the Moon's orbital motion that causes the Moon to
retreat from Earth. The retreat curve is dependent on
the equation for the secular growth of the lunar orbit
(Lambeck
1980
). This equation requires that the
Earth-Moon distance was closer in the past than today
but less clear is the effect of a closer Earth-Moon
distance on tidal amplitudes and thus on tidal current
velocities. Based on the data presented for the five
Precambrian examples, consideration is given to
whether tidal currents were stronger than today and
whether macrotidal conditions predominated in the
Precambrian.
Interpretations of tidal processes in the rock
record are based on evidence from inferred com-
parative Holocene environments of: (1) opposing
(bimodal-bipolar) and landward-directed paleocurrents;
(2) unidirectional ebb or flood tidal currents character-
ized by semi-diurnal inequality in which successive
currents are alternately stronger and weaker (de Boer
et al.
1989
); (3) periodic acceleration and deceleration
of tidal currents in response to variations in gravita-
tional attraction of the Sun and the Moon on the Earth.
Monthly tidal signals are synodic, anomalistic or
tropical (see Kvale et al.
1999
); (4) alternating bedload
and slack-water suspension sedimentation (Dalrymple
15.3.1 Elatina Formation, Australia
The best preserved record of tidal sedimentation in the
Precambrian Era is from the Neoproterozoic (~620 Ma)
Elatina Formation in the Flinders Ranges of South
Australia (Fig.
15.1
). The Reynella Siltstone, a strati-
graphic member of the Elatina Formation near Adelaide
(Fig.
15.1
; Preiss
1987
), also contains rhythmites of
tidal origin (Williams
1989, 2000
).
The Reynella Siltstone contains graded laminae of
fine-grained sandstone and siltstone up to 2 cm thick
that commonly have a thin mudstone cap. The Elatina
rhythmites comprise graded laminae 0.2-3.0 mm thick
of very fine-grained sandstone and siltstone. Rhythmites
of the Reynella Siltstone and parts of the Elatina
