mineralogy and habit. In sequence, beach rock cementation may commence with the precipitation of micritic
cements around in situ grains, followed by the precipitation of prismatic crystal rinds, and finally pore filling,
peloidal, spar, and infiltrating micrite cements. Changes
to the nature and characteristics of cement types can also
take place. Relic beach rock cements may show evidence
of dissolution and/or recrystallization. A beautifully illustrated and described chronosequence of carbonate fabrics
in 52 beach rock samples from the northeastern Mediterranean is presented in Desruelles et al. (2009).
Origin of cement
While earlier descriptions exist of beach rock, including
those of Darwin (Darwin, Charles (1809–1882)) and
Dana (Dana, James Dwight (1813–1895)), the first
detailed mineralogical and chemical analyses were carried
out on samples from Funafuti Atoll collected during the
Coral Boring Expeditions from 1896 to 1898. These analyses, together with thin section examination of several
“calcareous sandstones” reported by David and Sweet
(1904: 73–74), “shows the cement to be a fibrous radial
calcite” and that the beach sandstones formed through
the evaporation of calcium carbonate-charged fresh water
oozing through an island’s intertidal beach sands, where
“calcium carbonate would form and be deposited as
cement between the sandy particles.” While there have
been subsequent advocates of this fresh water lithification
process, several other mechanisms have been proposed
including: physicochemical precipitation from evaporating sea water or mixtures of meteoric water and sea water;
the escape of CO 2 or degassing from groundwaters saturated with carbonates; and precipitation directly or indirectly through biological activity, especially by microbial
cements (Khadkikar and Rajshekhar, 2003).
Detailed descriptions and illustrations of the range
beach rock cements as well as modes of beach rock formation are summarized in a series of excellent reviews over
the last several decades by Stoddart and Cann (1965) in
the 1960s, Davies and Kinsey (1973) in the 1970s, Scoffin
and Stoddart (1983) in the 1980s, Gischler and Lomando
(1997) in the 1990s, and most recently by Vousdoukas
et al. (2007). These contributions provide episodic snapshots of the status of on-going beach rock research and
its developments, especially related to the origin of beach
rock. While beach rock origin has been the main matter of
discussion over a long period of time, it is now quite clear
that there is no one unique mechanism that results in the
formation of beach rock, nor any exclusive set of environmental factors that control cementation.
Rather, beach rock formation is multigenetic, with
a number of inorganic and organic formative processes,
including direct cement precipitation from marine or
fresh or mixed marine and meteoric waters, and cementation from biological processes notably microbial (fungi
and bacteria) activity. Similarly, a range of factors appear
to control cementation, including interstitial water
temperature, pH and salinity, through-flow and tide-level
variations, the presence of calcium carbonate, organic
compounds and microbes, as well as the stability of beach
sediments, or as Gardiner (1903: 342) put it so quaintly
“where the beach is at rest so far as growth outwards is
concerned.” This list of factors is not exclusive, nor is their
relative importance known for any beach rock site.
Speed of cementation
Cementation can take place rapidly on time scales of a few
years. Examples are provided by the incorporation of
WWII relics into beach rocks on several islands across
the Pacific and on the northern Great Barrier Reef. Gardiner (Gardiner, John Stanley (1872–1946)) reports on the
removal of beach rock slabs for gravestones and building
materials on the Fijian island of Rotuma and throughout
the Maldives and their replacement by newly cemented
or case hardened bands in a few months in the same location, suggesting a sustainability in local quarrying of
beach rock. But perhaps the most famous example of
beach rock formation is that reported in 1924 by Reginald
Daly (Daly, Reginald Aldworth (1871–1957)) from the
Tortugas Marine Laboratory, where within 2 years of the
1910 storm depositing a fresh ridge of loose calcareous
sand in the vicinity of the laboratory, the deposit had been
lithified to a depth of about 0.75 m, forming a band of typical beach rock.
Age of beach rock and problems of dating
Beach rock cements are clearly younger than the grains
and clasts that make up the bulk of the rock. Moreover,
in reefal areas both the sediment and cement are composed
of calcium carbonate. Thus, dating of whole-rock samples
is problematical and will give ages based on the relative
proportions of the original skeletal components and secondary cements, both of which are likely to be quite variable. Constituent clasts may be dated to obtain a maximum
age, but to get an age closer to the time of beach rock formation, the cement must be dated. Obtaining an adequate
amount of uncontaminated cement is challenging, and
there is always the possibility of multigenerational cementation and recrystallization. However, Desruelles et al.
(2009) report on successfully extracting micrite cements
from beach rock at two sites in Turkey and obtaining
AMS dates for these cements.
Vousdoukas et al. (2007) include the estimated age of
exposed beach rocks from 20 locations around the world,
only six of which are from modern coral reef areas. They
believe that the majority of dated beach rocks are fossil
forms, 1,000–5,000 years old, but admit that the abundance of recent beach rocks is likely to be underestimated.
In spite of a large and increasing number of radiometric
dates from beach rock samples around the world, those
from modern coral reef areas have had limited success in
dating the time of formation. For example, corrected
radiocarbon dates of five whole-rock and one shell sample
BEACH ROCK
109
cements around in situ grains, followed by the precipitation of prismatic crystal rinds, and finally pore filling,
peloidal, spar, and infiltrating micrite cements. Changes
to the nature and characteristics of cement types can also
take place. Relic beach rock cements may show evidence
of dissolution and/or recrystallization. A beautifully illustrated and described chronosequence of carbonate fabrics
in 52 beach rock samples from the northeastern Mediterranean is presented in Desruelles et al. (2009).
Origin of cement
While earlier descriptions exist of beach rock, including
those of Darwin (Darwin, Charles (1809–1882)) and
Dana (Dana, James Dwight (1813–1895)), the first
detailed mineralogical and chemical analyses were carried
out on samples from Funafuti Atoll collected during the
Coral Boring Expeditions from 1896 to 1898. These analyses, together with thin section examination of several
“calcareous sandstones” reported by David and Sweet
(1904: 73–74), “shows the cement to be a fibrous radial
calcite” and that the beach sandstones formed through
the evaporation of calcium carbonate-charged fresh water
oozing through an island’s intertidal beach sands, where
“calcium carbonate would form and be deposited as
cement between the sandy particles.” While there have
been subsequent advocates of this fresh water lithification
process, several other mechanisms have been proposed
including: physicochemical precipitation from evaporating sea water or mixtures of meteoric water and sea water;
the escape of CO 2 or degassing from groundwaters saturated with carbonates; and precipitation directly or indirectly through biological activity, especially by microbial
cements (Khadkikar and Rajshekhar, 2003).
Detailed descriptions and illustrations of the range
beach rock cements as well as modes of beach rock formation are summarized in a series of excellent reviews over
the last several decades by Stoddart and Cann (1965) in
the 1960s, Davies and Kinsey (1973) in the 1970s, Scoffin
and Stoddart (1983) in the 1980s, Gischler and Lomando
(1997) in the 1990s, and most recently by Vousdoukas
et al. (2007). These contributions provide episodic snapshots of the status of on-going beach rock research and
its developments, especially related to the origin of beach
rock. While beach rock origin has been the main matter of
discussion over a long period of time, it is now quite clear
that there is no one unique mechanism that results in the
formation of beach rock, nor any exclusive set of environmental factors that control cementation.
Rather, beach rock formation is multigenetic, with
a number of inorganic and organic formative processes,
including direct cement precipitation from marine or
fresh or mixed marine and meteoric waters, and cementation from biological processes notably microbial (fungi
and bacteria) activity. Similarly, a range of factors appear
to control cementation, including interstitial water
temperature, pH and salinity, through-flow and tide-level
variations, the presence of calcium carbonate, organic
compounds and microbes, as well as the stability of beach
sediments, or as Gardiner (1903: 342) put it so quaintly
“where the beach is at rest so far as growth outwards is
concerned.” This list of factors is not exclusive, nor is their
relative importance known for any beach rock site.
Speed of cementation
Cementation can take place rapidly on time scales of a few
years. Examples are provided by the incorporation of
WWII relics into beach rocks on several islands across
the Pacific and on the northern Great Barrier Reef. Gardiner (Gardiner, John Stanley (1872–1946)) reports on the
removal of beach rock slabs for gravestones and building
materials on the Fijian island of Rotuma and throughout
the Maldives and their replacement by newly cemented
or case hardened bands in a few months in the same location, suggesting a sustainability in local quarrying of
beach rock. But perhaps the most famous example of
beach rock formation is that reported in 1924 by Reginald
Daly (Daly, Reginald Aldworth (1871–1957)) from the
Tortugas Marine Laboratory, where within 2 years of the
1910 storm depositing a fresh ridge of loose calcareous
sand in the vicinity of the laboratory, the deposit had been
lithified to a depth of about 0.75 m, forming a band of typical beach rock.
Age of beach rock and problems of dating
Beach rock cements are clearly younger than the grains
and clasts that make up the bulk of the rock. Moreover,
in reefal areas both the sediment and cement are composed
of calcium carbonate. Thus, dating of whole-rock samples
is problematical and will give ages based on the relative
proportions of the original skeletal components and secondary cements, both of which are likely to be quite variable. Constituent clasts may be dated to obtain a maximum
age, but to get an age closer to the time of beach rock formation, the cement must be dated. Obtaining an adequate
amount of uncontaminated cement is challenging, and
there is always the possibility of multigenerational cementation and recrystallization. However, Desruelles et al.
(2009) report on successfully extracting micrite cements
from beach rock at two sites in Turkey and obtaining
AMS dates for these cements.
Vousdoukas et al. (2007) include the estimated age of
exposed beach rocks from 20 locations around the world,
only six of which are from modern coral reef areas. They
believe that the majority of dated beach rocks are fossil
forms, 1,000–5,000 years old, but admit that the abundance of recent beach rocks is likely to be underestimated.
In spite of a large and increasing number of radiometric
dates from beach rock samples around the world, those
from modern coral reef areas have had limited success in
dating the time of formation. For example, corrected
radiocarbon dates of five whole-rock and one shell sample
BEACH ROCK
109