Definition
Beach rock results from lithification of unconsolidated
sediments by calcium carbonate cements in the tidal zone
of mainly tropical and subtropical beaches. Aragonite and
calcite, in a number of crystalline forms, are the primary
agents of cementation. All kinds of beach sediments can
be cemented, from fine sands to gravels of biogenic and/
or terrigenous origin. Beach rocks can also vary greatly
in texture and degree of lithification, some being quite
porous and friable, others being dense and highly indurated. Beach rock takes on the disposition of the parent
beach, including the preservation of beach slopes and stratigraphy. Outcrops usually show a number of distinct
bands that represent the bedding planes, internal laminae,
and sedimentary structures preserved during the
cementing process which can be quite rapid.
Description
Beach rocks are a common and conspicuous feature
of many island and mainland shores in reefal areas. They
vary from small discontinuous exposures of cemented
sediments to extensive outcrops tens of meters wide
and hundreds of meters long. More normally, beach
rock occurs as narrow elongate strips, 5–20 m wide and
100–200 m long, comprising an overlapping sequence
of separate bands with a definite seaward dip varying from
5 to 15
. The thickness of individual bands is usually
around 0.1–0.2 m with the total thickness of outcrops
ranging from 0.5 m to more than 2.5 m, being thickest
in areas of high tidal range. Banding is often associated
with textural bedding, while the seaward dipping layers
are characteristic of foreshore sedimentation. Different
lithofacies have been recognized both in the vertical and
within a single band.
Beach rocks generally occur in the swash zone of
a beach and frequently mobile sand or gravel obscures
the upper and/or lower portion of an outcrop. Temporary
burial and exposure of formations may result from seasonal deposition and erosion of local beach sediments, or
from alternating storm and fair weather regimes. Relic
beach rocks may be found either landward, or more commonly, seaward of the present beach.
Beach rock is not the only cemented rock on coral
coasts and reef islands. Others include conglomerates
(Conglomerates), cemented platforms (Platforms
(Cemented )), phosphate rock, Eolianite, and cay sandstone or cay rock, the last often being confused with beach
rock sensu stricto, although the distinction between beach
rock and cay rock is made quite clear by Gischler and
Lomando (1997).
Distribution
In a recent review Vousdoukas et al. (2007) note that, until
the early 1960s, the prevailing opinion had been that
beach rock formation, involving carbonate cements, was
primarily limited to tropical and subtropical coasts. However, subsequent observations have shown beach rock is
also present on temperate and higher latitude coasts with
individual occurrences reported from South Africa and
New Zealand in the southern hemisphere and Japan and
Scotland in the northern hemisphere. Nevertheless, the
great majority of beach rocks are found in low latitude
locations, although there are particularly extensive outcrops around the Mediterranean Sea. Microtidal coasts
tend to be favored, although there are many outcrops
along island shores in areas of high tidal range such as in
the central and southern Great Barrier Reef, Australia
where tidal ranges of 2.5–8 m occur. There, and in other
reef areas, beach rock is mainly associated with calcium
carbonate beaches, and on coral atolls beach rock (Beach
Rock) is generally the most common and obvious “rock”
apart from biohermal reef rock.
Grain size and composition
The grain size and composition of beach rock reflect that
of the parent beach at the time of cementation. On coral
reef coasts, beach rocks classically comprise a mix of
sand- to gravel-sized sediments made up of the skeletal
remains of calcareous organisms such as mollusks, benthonic foraminifera, coralline algae, and Halimeda, particularly in the sand to pebble size fraction. The coarsest
components are frequently whole or fragmented coral
clasts, especially of branching and foliaceous corals, and
smaller massive corals as well as reworked fragments
of reef rock and pieces of beach rock. There are also occurrences of beach rocks whose component grains are
noncarbonate; volcanoclastic grains are common on volcanic islands, and quartzose sediments on rocky continental
coasts. Many beach rocks also contain “erratic” materials,
including exotic ballast rocks and even human artifacts
and litter ranging from ancient pottery fragments to articles
of war and modern beer cans and bottle glass.
Cements
Aragonite and Calcite and especially high magnesium calcite are the predominant cementing agents of beach rocks.
Both minerals are dimorphous with the same chemical
composition (calcium carbonate) but different crystal
shape and symmetry; aragonite is orthorhombic, and calcite is trigonal. Thin section microscope and scanning
electron microscope images of beach rock cements reveal
a wide range of morphologies and fabrics. Three common
morphologies in tropical beach rocks were described by
Scoffin and Stoddart (1983) as: micritic coatings of either
aragonite or calcite on parent grains; fibrous or bladed
crusts of elongate crystals commonly as aragonite; and
classical equant crusts associated with magnesium calcite.
They also described three common cement fabrics:
isopachous fringes of uniform coatings around grains;
meniscus cements; and gravitational or pendant cements.
None of these fabrics are pore filling.
Vousdoukas et al. (2007) suggest that well-cemented
beach rocks may have undergone several diagenetic
phases, each one producing cements of a different
108
BEACH ROCK
Beach rock results from lithification of unconsolidated
sediments by calcium carbonate cements in the tidal zone
of mainly tropical and subtropical beaches. Aragonite and
calcite, in a number of crystalline forms, are the primary
agents of cementation. All kinds of beach sediments can
be cemented, from fine sands to gravels of biogenic and/
or terrigenous origin. Beach rocks can also vary greatly
in texture and degree of lithification, some being quite
porous and friable, others being dense and highly indurated. Beach rock takes on the disposition of the parent
beach, including the preservation of beach slopes and stratigraphy. Outcrops usually show a number of distinct
bands that represent the bedding planes, internal laminae,
and sedimentary structures preserved during the
cementing process which can be quite rapid.
Description
Beach rocks are a common and conspicuous feature
of many island and mainland shores in reefal areas. They
vary from small discontinuous exposures of cemented
sediments to extensive outcrops tens of meters wide
and hundreds of meters long. More normally, beach
rock occurs as narrow elongate strips, 5–20 m wide and
100–200 m long, comprising an overlapping sequence
of separate bands with a definite seaward dip varying from
5 to 15
. The thickness of individual bands is usually
around 0.1–0.2 m with the total thickness of outcrops
ranging from 0.5 m to more than 2.5 m, being thickest
in areas of high tidal range. Banding is often associated
with textural bedding, while the seaward dipping layers
are characteristic of foreshore sedimentation. Different
lithofacies have been recognized both in the vertical and
within a single band.
Beach rocks generally occur in the swash zone of
a beach and frequently mobile sand or gravel obscures
the upper and/or lower portion of an outcrop. Temporary
burial and exposure of formations may result from seasonal deposition and erosion of local beach sediments, or
from alternating storm and fair weather regimes. Relic
beach rocks may be found either landward, or more commonly, seaward of the present beach.
Beach rock is not the only cemented rock on coral
coasts and reef islands. Others include conglomerates
(Conglomerates), cemented platforms (Platforms
(Cemented )), phosphate rock, Eolianite, and cay sandstone or cay rock, the last often being confused with beach
rock sensu stricto, although the distinction between beach
rock and cay rock is made quite clear by Gischler and
Lomando (1997).
Distribution
In a recent review Vousdoukas et al. (2007) note that, until
the early 1960s, the prevailing opinion had been that
beach rock formation, involving carbonate cements, was
primarily limited to tropical and subtropical coasts. However, subsequent observations have shown beach rock is
also present on temperate and higher latitude coasts with
individual occurrences reported from South Africa and
New Zealand in the southern hemisphere and Japan and
Scotland in the northern hemisphere. Nevertheless, the
great majority of beach rocks are found in low latitude
locations, although there are particularly extensive outcrops around the Mediterranean Sea. Microtidal coasts
tend to be favored, although there are many outcrops
along island shores in areas of high tidal range such as in
the central and southern Great Barrier Reef, Australia
where tidal ranges of 2.5–8 m occur. There, and in other
reef areas, beach rock is mainly associated with calcium
carbonate beaches, and on coral atolls beach rock (Beach
Rock) is generally the most common and obvious “rock”
apart from biohermal reef rock.
Grain size and composition
The grain size and composition of beach rock reflect that
of the parent beach at the time of cementation. On coral
reef coasts, beach rocks classically comprise a mix of
sand- to gravel-sized sediments made up of the skeletal
remains of calcareous organisms such as mollusks, benthonic foraminifera, coralline algae, and Halimeda, particularly in the sand to pebble size fraction. The coarsest
components are frequently whole or fragmented coral
clasts, especially of branching and foliaceous corals, and
smaller massive corals as well as reworked fragments
of reef rock and pieces of beach rock. There are also occurrences of beach rocks whose component grains are
noncarbonate; volcanoclastic grains are common on volcanic islands, and quartzose sediments on rocky continental
coasts. Many beach rocks also contain “erratic” materials,
including exotic ballast rocks and even human artifacts
and litter ranging from ancient pottery fragments to articles
of war and modern beer cans and bottle glass.
Cements
Aragonite and Calcite and especially high magnesium calcite are the predominant cementing agents of beach rocks.
Both minerals are dimorphous with the same chemical
composition (calcium carbonate) but different crystal
shape and symmetry; aragonite is orthorhombic, and calcite is trigonal. Thin section microscope and scanning
electron microscope images of beach rock cements reveal
a wide range of morphologies and fabrics. Three common
morphologies in tropical beach rocks were described by
Scoffin and Stoddart (1983) as: micritic coatings of either
aragonite or calcite on parent grains; fibrous or bladed
crusts of elongate crystals commonly as aragonite; and
classical equant crusts associated with magnesium calcite.
They also described three common cement fabrics:
isopachous fringes of uniform coatings around grains;
meniscus cements; and gravitational or pendant cements.
None of these fabrics are pore filling.
Vousdoukas et al. (2007) suggest that well-cemented
beach rocks may have undergone several diagenetic
phases, each one producing cements of a different
108
BEACH ROCK