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and/or ebb tide move and deposit relatively coarse
sediment, typically sand. During high slack and low
slack, fi ne suspended sediment settles to the bottom.
The result is an alternation of sand and mud forming
what is called tidal bedding (Reineck 1967 ) . This is
one of several types of rhythmites ; sequences of sedi-
ments that are produced by cyclic conditions. Another
common type of planar rhythmites is varves, cyclic
annual sediment layers in mid-latitude lakes.
Tidal bedding units tend to be quite thin; only
millimeters to a centimeter or so in thickness (Fig. 3.16 ).
They represent individual tidal cycles and may accumu-
late in very thick successions that represent many tidal
cycles (Fig. 3.17 ). Their environment of deposition is
typically intertidal, but they may also form in very shal-
low subtidal locations. Most are heterolithic , that is,
they display more than one distinct sediment type. They
may, however, also be monolithic (Fig. 3.18 ).
The rhythmites that comprise tidal bedding may
include two to four individual layers in a semi-diurnal
setting (Archer 1998 ). The number depends on the
time-velocity asymmetry. The dominant current, ebb
or fl ood, deposits the coarser material, typically sand.
The strength of the subordinate current that determine
the presense/absence of relatively fi ne layers and the
number of elements in the individual tidal record
(Fig. 3.19 ). Such conditions can produce four lamina-
tions; two relatively thin and the other two relatively
coarse. The reduction in sedimentation by the subor-
dinate current will reduce the number from four to
three and eventually to two layers; both of the coarser
type (Fig. 3.19 ). The nature of the tidal bedding rhyth-
mites changes within the spring and neap lunar cycle
(Fig. 3.20 ). Measurement of individual layers and plot-
ting them over time shows this spring-neap cyclicity
well. Detailed analysis of these cycles permits estab-
lishing the moon-earth relationship over geologic time
(Kvale et al. 1999 ). Additionally, outcrops and preser-
vation permitting, a transition exists from a complete
spring-neap sequence of tidal bedding to one that
Fig. 3.14 Diagram showing the sequential development of
reactivation surfaces (Redrawn after Lindholm 1987 )
Fig. 3.15 Photo of a
stratigraphic sequence
showing examples of
reactivation surfaces in the
Precambrian Baraboo
Quartzite of Wisconsin, USA
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