Geology Reference
In-Depth Information
designate deposits of a tidal fl at system comprising
supratidal, intertidal, and the adjacent shallow subtidal
environments. Carbonate tidalites, formed as a result
of tidal, biogenic, chemical and diagenetic processes,
are among the most common deposits in ancient
tropical carbonate platform successions. They have
been deposited under arid or humid conditions in
carbonate platforms (Read
1985
; Pomar
2001
) associ-
ated with a variety of sedimentary basins. This chapter
fi rst summarizes the diagnostic features and charac-
terizes sedimentary facies, environments of deposition,
and stratigraphy of ancient carbonate tidalites. This
will be followed by a few illustrative examples of
ancient carbonate tidalites related to passive margins,
intracratonic, failed rift, and foreland basins.
Our understanding of the deposition and early diagen-
esis of carbonate tidal deposits grew signifi cantly during
the 1960s and 1970s as a result of comprehensive studies
of many modern shallow and marginal marine carbonate
environments (reviews in Bathurst
1975
; Tucker and
Wright
1990
; Flügel
2010
) that included studies in the
Persian Gulf (e.g. Purser
1973
), the Bahamas (e.g. Hardie
1977
), south Florida (e.g. Enos and Perkins
1977
) and
western Australia (e.g. Logan et al.
1970
) . The applica-
tion of these results and observations obtained from
modern siliciclastic tidal fl at environments (e.g. Reineck
1972
) to ancient carbonate deposits (e.g. Ginsburg
1975
;
Hardie and Shinn
1986
; Carozzi
1989
) , using Walther's
Law (Middleton
1973
) and the comparative sedimentol-
ogy approach of Ginsburg (
1974
), led to accurate inter-
pretations of facies, depositional environments and
sequences of carbonate tidal deposits in the sedimentary
record. Carbonate tidalites encompass a wide variety of
characteristic depositional and diagenetic features analo-
gous to their modern counterparts (e.g., Grotzinger
1989
;
Demicco and Hardie
1994
, Flügel
2010
) . These features
record important information on water level and tidal
range, depth and energy level, salinity, climate and sea
level history during deposition.
mally weaker ebb tidal current. A tidal cycle normally
occurs twice daily (semidiurnal), but once a day (diurnal)
or mixed diurnal and semidiurnal cycles may occur
depending on tidal regime or local conditions (Davis
1983
). The height of the water column between normal
high tide and low tide levels is known as tidal range.
During new and full moon, alignment of the moon, sun
and earth generates a greater than normal tidal range
(spring tide); conversely, during fi rst and third quarters of
a lunar cycle, a minimal tidal range (neap tide) occurs
(Davis
1983
). Based on variation of tidal range, shore-
lines are classifi ed into macrotidal (>4 m), mesotidal
(2-4 m) and microtidal (<2 m). In macrotidal areas, tide
dominates over other processes and most mesotidal and
microtidal areas are wave and storm dominated, but tide
domination may even occur in a protected microtidal
coast where wave action is limited (Dalrymple
1992
) . In
contrast to their siliciclastic counterparts, most modern
peritidal carbonate environments are microtidal (Wright
1984
; Pratt et al.
1992
) .
21.3
Depositional Environments
The daily fl uctuation of water level subdivides the tidal
system into three bathymetric belts including subtidal
(below mean low tide level), intertidal and supratidal
environments (Fig.
21.1a
) that are characterized by a
set of biological, physical and chemical processes.
The subtidal environment may extend for hundreds of
kilometers offshore depending on tectonic and geo-
graphic settings. It is the major source of sediment
for the adjacent tidal fl at setting and comprises low
energy back barrier lagoons of various salinity and
areal extent, platform margin (carbonate barriers/sand
shoals) and open marine environments (Fig.
21.1
). The
intertidal environment lies between mean low tide and
mean high tide levels (Fig.
21.1a
) and is exposed once
or twice daily during each tidal cycle. This zone in part
includes isolated ponds and meandering tidal channels
that are essentially subtidal environments occurring
within the intertidal belt (Fig.
21.2
). The supratidal
environment lies above the mean high tide level
(Fig.
21.1a
) and is fl ooded only during spring tides
(twice each month) and less frequent storm tides. It is
widespread in mainland coasts, but narrow supratidal
environment develops on channel levees and beach
ridges within the intertidal belt (Fig.
21.2b
). In an arid
climate, the supratidal environment is evaporitic and is
21.2
Tidal Processes
Tide is a periodic fl uctuation in water level in a marine
realm that is created by the gravitational pull of the moon
and sun (Davis
1983
; Dalrymple
1992
) with the moon,
being closer to earth, exerting the most gravitational force.
In a tidal cycle, rising water generates the landward fl ood
tidal current, but the fall generates the seaward and nor-
