CARBONATE PLATFORM BEACH SEDIMENTS
The marine beach sand samples shown below well
represent a modern carbonate platform. They are all from the Bahamas
Platform, an extensive, moderately warm, shallow-water carbonate setting in the
western North Atlantic, offshore from southern Florida.
The best known Bahamian island, in a geologic sense,
is San Salvador, the first landing place of Christopher Columbus back in 1492.
The bedrock geology of San Salvador is entirely
Pleistocene and Holocene limestones. The shallow-water seas surrounding
the island have an abundance of algae, protists, invertebrates, and
vertebrates. The marine beach sediments of the island are, not
surprisingly, composed of aragonitic lime sands (CaCO3 - calcium
carbonate). The sands are derived from modern organisms' hard parts and
grains recycled from the island's bedrock & beachrock.
Marine biogenic aragonite sand from a beach at French Bay (western part of the
southern margin of San Salvador Island, eastern Bahamas). The entire
sediment sample is moderately sorted, dominated by sand plus a fair number of
granule-sized grains and a few small pebble-sized grains. The grains
range from being whitish to cream to light gray to pinkish to washed-out
reddish to reddish colored. Almost all of the grains are biogenic in
origin, but some small beach rock limestone fragments are present (= lithogenous
sediment). Most grains have irregular, glossy patches of aragonite
coatings. I haven't made a grain mount of these sediments, but
identifiable grains in the entire sample include bivalve shells, high-spired
& low-spired gastropod shells, benthic foraminifera, reddish-colored Homotrema
rubrum rotaliine foraminifera fragments, incipient oolites, echinoid
spines, scleractinian coral skeleton fragments, beach rock fragments, and
cemented grain aggregates.
Microphotograph by Sara Beth Kopczynski (Lehigh University,
Bethlehem, Pennsylvania, USA).
French Bay (looking ~E), western part of the southern margin of San Salvador
Island, eastern Bahamas.
Marine biogenic aragonite sand from just below sea level at Sandy Point (southwestern
corner of San Salvador Island, eastern Bahamas). This sediment sample is
better sorted than the first sample above. This locality is a fairly
high-energy setting, in terms of wave action, which accounts for the decent
sorting. All grains have shiny aragonite coatings, which prevents ready
identification of most grains.
Microphotograph by Sara Beth Kopczynski (Lehigh
University, Bethlehem, Pennsylvania, USA).
Sandy Point (looking ~S), southwestern corner of San Salvador Island, eastern
Bahamas.
Marine biogenic aragonite sand from Rhizo City (eastern part of southern margin of
San Salvador Island, eastern Bahamas). The entire sample consists
principally of medium to very coarse sand, granules, and small pebbles.
Some grains are lithogenous in origin (calcarenite limestone bedrock
fragments). Most of them are biogenic - scleractinian coral fragments,
mollusc shell fragments (bivalves, gastropods), echinoid spine fragments,
Homotrema rubrum foraminiferan test fragments, Archaias benthic
foraminiferan tests, and other benthic foram tests. Microphotograph by
Sara Beth Kopczynski (Lehigh University, Bethlehem, Pennsylvania, USA).
Marine biogenic aragonite sand from the beach landward from Lindsay Reef (Long Bay,
western margin of San Salvador Island, eastern Bahamas). This is a much
finer-grained beach sand than most other San Salvador beach sands. The
sample is dominated by fine sand to medium sand. There are occasional
granule-sized grains. Biogenic grains dominate - mollusc fragments,
reddish Homotrema rubrum rotaliine foraminiferan test fragments,
Archaias benthic foraminiferan tests, plus other benthic foram
tests. Many grains are not readily identifiable (without grain mount
analysis). There are some coated grain aggregates as well.
Microphotograph by Sara Beth Kopczynski (Lehigh
University, Bethlehem, Pennsylvania, USA).
Lindsay Reef Beach (looking ~NNE), southern Fernandez Bay, western
margin of San Salvador Island.
Marine biogenic aragonite sand from Sand Dollar Beach (immediately south of Rocky
Point, northwestern corner of San Salvador Island, eastern Bahamas). This
sample is dominated by coarse sand-sized, angular to subangular coated grain
aggregates. Some very coarse sand and granules are also present.
Other grain types include bivalve fragments, gastropod shells and shell
fragments, plus occasional Homotrema rubrum & Archaias &
other benthic foraminiferan tests.
Microphotograph by Sara Beth Kopczynski (Lehigh
University, Bethlehem, Pennsylvania, USA).
Sand Dollar Beach (looking ~SSW), immediately south of Rocky Point,
northwestern corner of San Salvador Island.
Marine biogenic aragonite sand from beach along Hanna Bay (northeastern San Salvador
Island, eastern Bahamas). This sand is speckled pink from the presence of
whitish aragonite sand + reddish Homotrema rubrum rotaliine
foraminiferan test fragments. The sample is dominated by angular to
subrounded, very fine to fine to medium sand. The sample is fairly rich
in benthic foraminiferan test fragments (Homotrema rubrum, Archaias,
and others). Other grain types include bivalve shell fragments, ostracod
valves, apparent bryozoan skeleton fragments, apparent siliceous sponge
spicules, and spinose/tube-shaped fragments.
Microphotograph by Sara Beth Kopczynski (Lehigh University,
Bethlehem, Pennsylvania, USA).
Hanna Bay Beach (looking ~S), northeastern San Salvador Island.
Marine biogenic aragonite sand from beach landward from Snapshot Reef (Fernandez Bay,
western margin of San Salvador Island, eastern Bahamas). The entire
sample is dominated by coarse-grained to very coarse-grained sand, plus some
medium sand and granules. Many grains are loosely-cemented to
well-cemented grain aggregates. Other readily identifiable grains include
juvenile, intact bivalve shells; mollusc shell fragments; benthic foraminiferan
tests (several species, including Archaias and Homotrema rubrum -
see both of those in the above photo); incipient, elongated oolites, linear to curvilinear
to irregularly coiled tubes, and occasional planktonic foraminiferan tests.
Microphotograph by Sara Beth Kopczynski (Lehigh
University, Bethlehem, Pennsylvania, USA).
Snapshot Reef Beach (looking ~S), Fernandez Bay, western margin of San
Salvador Island. The gently seaward-dipping limestones at this beach are Hanna
Bay Member aragonitic calcarenites (beach facies) of the upper Rice Bay
Formation (middle to upper Holocene).