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).

 


 

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