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temperature and (5) the role of microbes in the
cementation process. These considerations and
others are beyond the scope and purpose of
this paper.
threshold, the sediment is taken out of contact
with seawater and cementation is slowed or does
not take place. When the accretion rate is retarded
or periodically stopped, cementation is enhanced.
The result of this fl uctuating process was repeat-
edly observed by the author in excavations around
the Qatar Peninsula in the mid-1960s, where
construction workers mined sand with shovels.
The mined areas revealed beachrock zones that
were suffi ciently lithifi ed as to resist removal
with picks and shovels. Such areas were generally
left and because of the removal of soft sand
around them, they could be observed in three
dimensions. The rock areas formed distinct ridges
parallel to the accreting beach.
Signifi cantly, sandy sediment in the back-beach
areas away from the active intertidal beaches was
not being cemented either by aragonite or calcite.
Lack of calcite cementation in back-beach areas is
probably due to aridity and lack of a rainfall-source
of fresh water. In contrast, back-beach areas in the
humid Bahamas are quickly lithifi ed (probably
<1000 years) by freshwater diagenesis, namely by
the precipitation of low-magnesium calcite (Halley
& Harris, 1979). Thus, varying rates of sedimenta-
tion and absence of fresh water often control for-
mation and distribution of beachrock. Numerous
examples of the overriding effect of sedimentation
rate, such as rapidly forming submarine sand spits,
are provided in Shinn (1969, 1971).
DISCUSSION
Arid versus humid climate
During the mid-1960s, the author examined many
beaches and offshore sediments in the Persian and
Arabian Gulf. The Persian Gulf climate is arid, and
salinity is elevated (Purser & Seibold, 1973). Aridity
and high salinity favour rapid calcium-carbonate
precipitation, and with little rainfall the rate of
freshwater cementation is retarded. Observations
in the Arabian Gulf were biased toward areas of
the southern windward shore (Trucial Coast),
where sedimentation and shoreline accretion are
most rapid. The author's observations in the more
humid Florida and Bahamas platform environ-
ments were generally biased toward areas of ero-
sion and locally to areas of complicated onlap
sedimentation (Strasser & Davaud, 1986).
These contrasts in sedimentation style and
diagenesis provided unique insights to the pro-
cesses of beach formation as well as to both
intertidal and submarine sediment diagenesis.
For example, cementation in the Persian Gulf is
so rapid that even sandy sediment a few centi-
metres below the intertidal surface can be rapidly
lithifi ed. The resulting rock often includes glass,
pottery, nuts and bolts, and other human art-
facts. Subtidal cementation of sandy sediment is
rapid and has been shown to transform sediment
into hard limestone in less than 30 years (Shinn,
1969). This rapidly forming submarine rock does
not contain keystone vugs. In the intertidal beach
sands that do contain keystone vugs, cementation
proceeds even more rapidly. Tidal pumping in
the intertidal beach provides a continual supply
of supersaturated seawater, and during low tide,
high evaporation and rapid degassing probably
accelerate cementation (Fig. 3).
Ginsburg (1953), Evamy (1973) and many
others have shown that the most common cement-
ing agent is fi brous aragonite. In areas of rapid
lateral beach accretion, cemented zones are
continually buried beneath sand as fast as the bio-
genic and oolitic grains are created and deposited.
In such areas, hidden from view, the continual
formation of beachrock proceeds unnoticed. If
the beach accretes rapidly beyond some unknown
Fishhook spits and 'J' shaped structures
Intertidal cementation in the Persian Gulf is
especially rapid on the protected inner margins
of curved, fi shhook-shaped accreting sand spits.
The rock forming on the sand spits is exposed to
more than a metre of tidal fl uctuation but is pro-
tected from waves that would disrupt cementation
before it could occur. The formation of new spits is
clearly documented in repeated photographs of an
area on the northeast coast of Qatar (Shinn, 1973).
Newly cemented sediment and increasingly more
cementation occur in progressively older and
older spits (Fig. 4). Microbial inducement cannot
be ruled out, but low rainfall precludes freshwa-
ter involvement in the cementation process at this
site. The large thick blocks such as those at Bimini
and elsewhere in the Caribbean were not observed.
Possibly the rapid rate of sedimentation does not
allow suffi cient subaerial exposure for thermal
fracturing and creation of thick blocks. Beachrock
that forms beneath accreting sand is not exposed
to as much sun and weathering as rock that forms
