Geology Reference
In-Depth Information
Fig. 3.13
Examples of (
a
) modern bidirectional cross-strata from Martens Plate, German Wadden Sea and (
b
) and example of
bidirectional cross-strata from 1.7 billion year old Baraboo Quartzite in Wisconsin, USA
directions (Fig.
3.13
). Depending on sediment avail-
ability and the balance between fl ood and ebb currents,
herringbone cross-strata will form. It is not common,
even in known tidal environments because herringbone
cross-strata require near equal fl ood and ebb tidal cur-
rent conditions and this condition is atypical. Generally
tidal currents display a distinct asymmetry during fl ood
and ebb conditions at a given location with the devel-
opment of mutually exclusive channels (see Chap. 2 ).
Misinterpretation of bidirectional processes can occur
when observing nested sets of trough cross-beds where
apparent herringbone may be present.
Because of time-velocity asymmetry, it is unusual
that the velocity and amount of tidal energy is the same
at any given location for fl ood and ebb currents. These
factors contribute to the other special type of bedding
features that can be attributed to tidal activity. In a tidal
cycle at a specifi c location there will be one current, it
can be either fl ood or ebb, that is dominant, and the
other is subordinate. The dominant current will move
the bedforms and produce cross-stratifi cation. The
subordinate current, moving in the opposite direction,
is not strong enough to reverse the direction of the
migrating bedforms, but it does scour the upper por-
tion of the bedforms, removing some sediment of the
upper part of the bedform and producing an erosional,
sometimes undulating surface. When the next cycle
takes place, the dominant current again forms migrat-
ing bedforms. The cycle repeats itself during each tidal
cycle. The contact that forms between successive
migrating bedforms is called a
reactivation surface
(Klein
1970
). A reactivation surface can be recognized
as one that interrupts the cross-strata that has the same
direction and inclination above and below it (Fig.
3.14
).
Reactivation surfaces are quite prominent in many
ancient stratigraphic sequences (Fig.
3.15
). They are
representative of subtidal, tidal-infl uenced environ-
ments such as channels. Although not the intent of
Klein (
1970
) the parallel or near-parallel surfaces that
separate stacked cross-sets are also produced in the
same fashion and are technically a surface that sepa-
rates two periods of active sedimentation. Excellent
reactivation surfaces can also be produced in unidirec-
tional environments such as streams (Collinson
1968
;
Nio and Yang
1991
). These occur when one megarip-
ple (dune) overtakes the other or when wave action or
water level changes lead to the erosion of the peak of
the bedform.
Horizontal Laminations
Some diagnostic sedimentary structures are based on
thin, fl at beds. Like other structures formed under tidal
infl uence, these also refl ect the fl ood, slack, ebb, slack
conditions of a tidal cycle. The currents during fl ood
