Geology Reference
In-Depth Information
Fig. 3.27
Theoretical sequence based on studies of the Wood Canyon Formation in the Precambrian of Nevada (USA) that would
equal the tidal range (From Klein
1975
)
3.4
Lack of Tidalite Production in Tidal
Environments
during fl ood and ebb conditions the possibility exists
for tidalite deposition (Fig.
3.28
). Landward of this
position the tides cause modulation of fl uvial currents
so that during ebb tide the fl uvial currents are a maxi-
mum and during fl ood tide they are a minimum. Good
examples of such estuaries are the Hudson River in
New York (USA) where tidal infl uence extends more
than 150 km up the valley but where tidalite accumula-
tion is limited to only a few kilometers. The Gironde
River in France shows some tidal infl uence all the way
to Bourdeaux, a distance of 100 km, but tidal sedimen-
tation stops only a few kilometers from its mouth
(Allen and Posamentier
1994
).
Other environments where signifi cant tidal fl ux is
present but where tidalites are absent include many
tidal fl ats. Waves in intertidal environments typically
rework sediment. In addition, burrowing organisms are
typically common in these environments. This combi-
nation of wave action and bioturbation reworks depos-
its that originally accumulated under tide-dominated
Not all tidally-infl uenced environments produce
tidalites. What is preserved in the stratigraphic record
as tidalites represents the minimum of tidal environ-
ments that existed in the geologic past. Production of
tidalites requires sediment transport. Conditions exist
whereby sediment is not moved or is actually removed
even though tidal phenomena are present. That is, too
little or too much energy may be present.
One of the most common environments where this
situation prevails is the transition from fl uvial to tidal
domination in estuaries (see Chap. 5). A wide range of
conditions may infl uence this transition including the
rate of discharge, tidal stage and lunar tidal stage.
Storm surge can also infl uence this transition. An
excellent schematic model shows the range of tide-
and fl uvial-domination (Dalrymple and Choi
2007
) .
Where the tidal currents actually reverse direction