Geology Reference
In-Depth Information
gradient (and hence the energy gradient) is not per-
pendicular to the coast, but instead more or less
aligned along the main axis of the lagoon between the
mouth and the head. Other than this, the intertidal
flats display many of the features discussed in this
contribution.
crop conditions, some researchers have looked for ways
to estimate paleotidal ranges in order to solve this prob-
lem (Klein
1971
; Allen
1981
; Terwindt
1988
). In each
case, a different approach was used. Thus, Klein (
1971
)
suggested that the thickness of upward-fining sequences
approximated the paleotidal range, whereas Allen
(
1981
) used the thickness of cross-bedding sets dis-
playing mud drapes to derive at such estimates. Terwindt
(
1988
), in turn, used a complex combination of strati-
graphic criteria to reconstruct paleotidal ranges, but
submits that this was very difficult because reliable cri-
teria to identify the low-water line were lacking.
Although very persuasive, this issue has not really been
resolved to this day.
An elegant way to decide whether a tidal deposit
was formed in a back-barrier setting would be the
identification of the ancient barrier itself, or at least
remnants thereof, in the rock record. Criteria for this
have been summarized by Dickinson et al. (
1972
).
This would side-step the issue of having to estimate
paleotidal range, but would again require good expo-
sures in the field. Finally, tidal euphoria can also lead
to astonishing misinterpretations. Thus, the suppos-
edly 'transgressive-barrier and shallow shelf interpre-
tation of the lower Paleozoic Peninsula Formation,
South Africa' (Hobday and Tankard
1978
) is more
likely a large alluvial fan delta that incorporates a few
thin marine transgressions (Brian Turner, personal
communication 1993). The interpretation can be shown
to have been based on non-conclusive evidence and
that other features such as the occurrence of massive
pebble beds, ubiquitous floating pebbles, stacked lin-
guoid bars, sand-draped mud cracks, and especially
the total absence of any tidal rhythmites in any of the
excellent exposures collectively favor an alluvial ori-
gin (Flemming, unpublished).
10.6.2 Ancient Analogues
Examples of tidal deposits span the time period from
the Early Archaean to the present (Noffke et al.
2006
;
Noffke
2010
) and it would go beyond the scope of this
contribution to cite them all. This section therefore
restricts itself to a listing of important collective works
that deal with clastic tidal deposits and which also
include ancient examples. A first symposium proceed-
ings specifically dedicated to tidal research was edited
by Ginsburg (
1975
). Hobday and Errikson (
1977
) pres-
ent the results of a tidal conference with special refer-
ence to South African case studies. An exhaustive
summary of both modern and ancient examples up to
the year 1977 can be found in Klein (
1977
). Other con-
ference proceedings containing case studies from
modern and ancient tidal environments include de
Boer et al. (
1988
), Smith et al. (
1991
), Flemming and
Bartholomä (
1995
), Alexander et al. (
1998
), Park and
Davis (
2001
), and Bartholdy and Kvale (
2006
).
The cited works demonstrate that tidal-flat deposits
can generally be recognized in the rock record with
some degree of confidence. However, it is much more
difficult, and in many cases impossible, to differentiate
between micro-mesotidal back-barrier tidal flats and
macrotidal open coast counterparts. Both display
essentially the same range of surface features and
internal sedimentary structures. Being more energetic,
macrotidal flats should display a greater proportion of
larger-scale dune cross-bedding relative to smaller-
scale ripple cross-bedding on sand flats than would
be the case on mesotidal flats where dune cross-
bedding is the exception rather than the rule. Another
feature that is more prominent on macrotidal flats is
the occurrence of relatively high (>1 m) intertidal
bars entirely composed of bioclastic material (mollusc
shells) that may be found up to and even within salt
marsh deposits (e.g., Larsonneur
1994
; Schneider-
Storz et al.
2008
).
As the features addressed above are not unequivo-
cally diagnostic, in addition to requiring excellent out-
References
Alexander CR, Davis RA, Henry VJ (eds) (1998) Tidalites:
processes and products. SEPM Spec Publ 61
Allen JRL (1981) Palaeotidal speeds and ranges estimated from
cross-bedding sets with mud drapes. Nature 293:394-396
Allen PA, Homewood P (1984) Evolution and mechanics of a
Miocene tidal sandwave. Sedimentology 31:63-81
Anderson FE (1983) The northern muddy intertidal: seasonal
factors controlling erosion and deposition - a review. Can J
Fish Aquat Sci 40:143-159
Arends F (1833) Physische Geschichte der Nordsee-Küste und
deren Veränderungen durch Sturmfluthen seit der
Cymbrischen Fluth bis jetzt. Woortmann, Emden, vol 1,
