The surface bedrock geology of San Salvador consists entirely of Pleistocene and Holocene limestones.  Thick and relatively unforgiving vegetation covers most of the island’s interior (apart from inland lakes).  Because of this, the most easily-accessible rock outcrops are along the island’s shorelines.


The stratigraphic information presented here is synthesized from the Bahamian geologic literature (see list).



Stratigraphic  Succession  in the Bahamas


                                              Hanna Bay Member

Rice Bay Formation         ----------------------------

HOLOCENE (<10 k.y.)     North Point Member

                                               Cockburn Town Mbr.

Grotto Beach Formation   ----------------------------

lower Upper                          French Bay Member

PLEISTOCENE (119-131 k.y.)

Owl’s Hole Formation     Middle PLEISTOCENE

       (~215-220 k.y. & ~327-333 k.y. & ~398-410 k.y. & older)


San Salvador’s surface bedrock can be divided into two broad lithologic categories:




The limestones were deposited during sea level highstands (actually, only during the highest of the highstands).  During such highstands (for example, right now), the San Salvador carbonate platform is partly flooded by ocean water.  At such times, the “carbonate factory” is on, and abundant carbonate sediment grains are generated by shallow-water organisms living on the platform.  The abundance of carbonate sediment means there will be abundant carbonate sedimentary rock formed after burial and cementation (diagenesis).  These sea level highstands correspond with the climatically warm interglacials during the Pleistocene Ice Age.


Based on geochronologic dating on various Bahamas islands, and based on a modern understanding of the history of Pleistocene-Holocene global sea level changes, surficial limestones in the Bahamas are known to have been deposited at the following times (expressed in terms of marine isotope stages, “MIS” - these are the glacial-interglacial climatic cycles determined from δ18O analysis):


1) MIS 1 - the Holocene, <10 k.y.  This is the current sea level highstand.

2) MIS 5e - during the Sangamonian Interglacial, in the early Late Pleistocene, from 119 to 131 k.y. (sea level peaked at ~125 k.y.)

3) MIS 7 - ~215 to 220 k.y. - late Middle Pleistocene

4) MIS 9 - ~327-333 k.y. - late Middle Pleistocene

5) MIS 11 - ~398-410 k.y. - late Middle Pleistocene


Bahamian limestones deposited during MIS 1 are called the Rice Bay Formation.  Limestones deposited during MIS 5e are called the Grotto Beach Formation.  Limestones deposited during MIS 7, 9, 11, and perhaps as old as MIS 13 and 15, are called the Owl’s Hole Formation.  These stratigraphic units were first established on San Salvador Island (the type sections are there), but geologic work elsewhere has shown that the same stratigraphic succession also applies to the rest of the Bahamas.


During times of lowstands (= times of climatically cold glacial intervals of the Pleistocene Ice Age), weathering and pedogenesis results in the development of soils.  With burial and diagenesis, these soils become paleosols.  The most common paleosol type in the Bahamas is calcrete (a.k.a. caliche; a.k.a. terra rosa).  Calcrete horizons cap all Pleistocene-aged stratigraphic units in the Bahamas, except where erosion has removed them.  Calcretes separate all major stratigraphic units.  Sometimes, calcrete-looking horizons are encountered in the field that are not true paleosols.







Hanna Bay Member of the upper Rice Bay Formation at Grotto Beach.  This is the youngest bedrock unit on San Salvador Island.

These well-sorted limestones consist of sand-sized grains of aragonite (CaCO3).  On the continents, many quartz sandstones are technically called quartz arenites.  Because the sand grains making up these Bahamian rocks are calcareous (composed of calcium carbonate), the limestones are called calcarenites.  When examined microscopically, the calcareous sand grains can be seen touching each other - the rock is grain-supported.  This results in an alternative name for these Bahamian limestones - grainstones.  “Calcarenite” seems to be a more useful, more thoroughly descriptive term for these particular rocks, so I use that, versus “grainstone” (although “calcarenitic grainstone” could be used as well).  The little-used petrologic term aragonitite could also be applied to these aragonitic limestones.

Hanna Bay Member limestones are more or less planar-bedded, and gently dip toward the modern ocean.  The seaward dip, the sorting, the occurrence of more coarse-grained, finely fragmented seashell horizons, plus preserved sedimentary structures on some bedding planes such as bubble porosity and swash lines (see photo below), indicate that these Hanna Bay Member rocks represent beach deposits.  At other localities, the Hanna Bay Member includes back beach dune facies.

At the locality shown above (Grotto Beach), the lithified beach deposits are exposed two meters above current, mean sea level.  Some geologists have taken this as evidence for a mid-Holocene highstand, and that modern sea level is lower than it was during portions of the mid-Holocene.  A similar conclusion has been reached based on geologic evidence from elsewhere in the New World and the Old World.  Recent work on this very Hanna Bay Member outcrop has thrown doubt on the validity of the mid-Holocene highstand interpretation.

The aragonite sand grains in the Hanna Bay Member are principally bioclasts (worn mollusc shell fragments & coral skeleton fragments & calcareous algae fragments, etc.) and peloids (tiny, pellet-shaped masses composed of micrite/very fine-grained carbonate - some are likely microcoprolites, others are of uncertain origin).

Age: ~3200 years old, Holocene (MIS 1), although the whole-rock dates vary somewhat, depending on specific sampling site.

Locality: shoreline outcrop along the southwestern margin of Grotto Bay, southwestern San Salvador Island.



Bubble porosity and subtle swash line preserved on a calcarenite bedding plane in Hanna Bay Member limestones (upper Rice Bay Formation, Holocene, ~3.2 ky.) at Grotto Beach, southwestern San Salvador Island.



Hanna Bay Member limestones of the upper Rice Bay Formation at a Graham’s Harbour shoreline outcrop, between Singer Bar Point and the Bahamas Field Station, northern margin of San Salvador Island.  The bedding has a seaward dip (the modern ocean is to the right).  The rocks at this shoreline outcrop range from representing beach facies to back-beach dune facies (the latter is represented at this very spot).





North Point Member of the lower Rice Bay Formation at Cut Cay.  As are the Hanna Bay Member rocks shown above, the North Point Member consists of well-sorted calcarenite limestones (calcarenitic grainstones/aragonitites).

Notice that the above outcrop has horizontal bedding (at left) and low- to high-angle cross bedding (at center and right).  The grain size and sorting and cross-bedded nature of these rocks indicates they are ancient sand dune deposits (wind-blown).  Calcarenites deposited in ancient dune facies are called eolianites (“eolian” means “wind”).  The aragonite sand grains in the North Point Member are principally bioclasts and peloids.

The cross-bedding extends below modern sea level at least two meters, indicating that these sand dunes were originally deposited when sea level was lower than now.  This means that the North Point Member limestones are older than the Hanna Bay Member limestones - this is consistent with the more or less steady sea level rise throughout the Holocene.

Age: ~5300 years old, middle Holocene (MIS 1).

Locality: shoreline outcrop along the southwestern margin of Cut Cay, just north of North Point Peninsula, northeastern corner of San Salvador Island.



North Point Member of the Rice Bay Formation (middle to upper Holocene) - eolian, planar-bedded & cross-bedded aragonitic calcarenites along the western side of North Point Peninsula, northeastern corner of San Salvador Island.



North Point Member of the Rice Bay Formation (middle to upper Holocene) - eolian, aragonitic calcarenites along the eastern side of North Point Peninsula, northeastern corner of San Salvador Island.



North Point Member of the Rice Bay Formation (middle to upper Holocene) - eolian, cross-bedded, aragonitic calcarenites along the western side of North Point Peninsula, northeastern corner of San Salvador Island.  The lower portions of the crossbeds are drowned, continuing underwater for at least two meters.  They represent sand dune deposition when sea level was at least two meters below current sea level.



Eolianite/Dune facies of the COCKBURN  TOWN  MEMBER of the GROTTO BEACH  FORMATION


Cockburn Town Member (eolianite/dune facies) of the upper Grotto Beach Formation at a roadcut in southwestern San Salvador Island.

During the stillstand and regressive phases of the MIS 5e sea level highstand event, wind-blown sand dunes were present on the then-smaller land portion of ancient San Salvador.  The outcrop shown above consists of well-sorted, horizontally-bedded and variously cross-bedded aragonitic calcarenites.  These are MIS 5e sand dune deposits (eolianites).  Fossilized plant root traces are moderately common at this roadcut (but not at this very spot), which is characteristic of sand dunes deposited during the stillstand and regressive phases of Bahamian carbonate platform highstand events.

The sand grains in Cockburn Town Member eolianites are principally oolites, resulting in terrestrial oolitic limestones (oolitic calcarenites).  The oolites originally formed in the shallow water portion of the flooded carbonate platform surrounding the island.  Oolites are sand-sized, well-rounded, calcareous, concentrically-layered grains formed by rolling on the seafloor as a result of high-energy wave action in shallow water.  Fair-weather waves & storm waves deposited shallow seafloor oolites on ancient beaches, after which winds picked them up to form oolitic sand dunes further inland.

Age: ~119-122 k.y., Sangamonian, lower Upper Pleistocene (middle to late MIS 5e).

Locality: roadcut adjacent to the Watling’s Blue Hole overlook platform, southwestern San Salvador Island.






Cockburn Town Member (reef facies) of the upper Grotto Beach Formation at the type locality, Cockburn Town Fossil Reef, western margin of San Salvador Island.

The Cockburn Town Fossil Reef is one of the best-preserved and well-exposed Pleistocene fossil reefs on Earth.  It consists of non-bedded to poorly-bedded, poorly-sorted, very coarse-grained, aragonitic fossiliferous limestones (grainstones and rubblestones), representing shallow marine deposition in reef and peri-reef facies.  Cockburn Town Member reef facies rocks date to the MIS 5e sea level highstand event (~131-119 ky).

The outcrop shown above principally consists of bivalve packstone, dominated by Chione elevata (fossil cross-barred venus clam shells).




Mollusc-rich fossiliferous grainstones (above & below) from the reef facies of the Cockburn Town Member, upper Grotto Beach Formation at the Cockburn Town Fossil Reef, western margin of San Salvador Island.

Above left: Astralium phoebium fossil star snail shell.

Above center: Conus fossil cone snail shell.

Above right: Cittarium pica fossil magpie snail shell.

Below: fossil bivalves and gastropods, including Acar notabilis, Chione elevata, Ceritihium, etc.



Acropora cervicornis rubblestone (above & below) in the Cockburn Town Fossil Reef, ~125 k.y. (Sangamonian Interglacial, MIS 5e).  The thick branching structures are fossil, broken staghorn coral skeletons (Acropora cervicornis).



Porites porites - in-situ fossil clubbed finger coral colony in the Cockburn Town Fossil Reef, ~125 k.y. (Sangamonian Interglacial, MIS 5e).



Pocillopora palmata - in-situ fossil cauliflower coral colony in the Cockburn Town Fossil Reef, ~125 k.y. (Sangamonian Interglacial, MIS 5e).  This is the only coral genus that went extinct in the Caribbean at the end-Pleistocene.



Diploria strigosa - fossil brain coral in the Cockburn Town Fossil Reef, ~125 k.y. (Sangamonian Interglacial, MIS 5e).



Diploria - in-situ fossil brain coral colony in the Cockburn Town Fossil Reef.  Notice that this fossil brain coral is encrusting an irregular surface.  This surface is an unconformity and is traceable throughout the outcrop.  It represents a limestone hardground surface that formed during a short-lived, mid-5e regression at ~124 k.y., called the Devil’s Point Event.  After the event, at ~123 k.y., high sea level returned.  This coral was one of the earliest inhabitants of this locality’s shallow seafloor after the mid-5e regression.  The more deeply flooded carbonate platforms in the Bahamas, such as Mayaguana Island, were not as significantly affected by the mid-5e regression.




Devil’s Point Hardground (~123-124 k.y.) with encrusting scleractinian coral colonies (above left) and unidentified “spaghetti” encrusters (above right) and borings (below) at Cockburn Town Fossil Reef, western margin of San Salvador Island.



Cockburn Town Member of the upper Grotto Beach Formation at the Cockburn Town Fossil Reef, western margin of San Salvador Island.  The three aragonitic limestone units shown here represent a shallowing-upward (regressive) succession.

The basal unit is non-bedded, poorly-sorted, coarse-grained, shallow subtidal, fossiliferous grainstones.  The middle unit is trough cross-bedded and herringbone cross-bedded calcarenites deposited in a very shallow subtidal setting subjected to tidal pumping.  The top unit is ~planar-bedded calcarenites having parting lineation representing swash zone and supratidal beach deposits.  These three units are in conformable contact, and well display a transition from subtidal to intertidal to supratidal beach facies.





Owl’s Hole Formation at Watling’s Quarry (southwestern San Salvador Island) - this is the oldest surface-exposed stratigraphic unit on the island.  It outcrops in relatively few places (for example, Watling’s Quarry, Owl’s Hole Cave, Grotto Beach, around Stouts Lake, on the southern side of South Granny Lake, west of southern Storr’s Lake, and in the Reckley Hill Settlement Pond-Crescent Pond-Pain Pond area, and in the Dixon Hill area).  Only subaerial/terrestrial facies of the Owl’s Hole Formation are known to outcrop here.  Beach facies, shallow subtidal facies, and reef facies of the same age are presumably only now present in the subsurface.  The Owl’s Hole Formation is an upper to middle Middle Pleistocene unit.  Deposition occurred during the MIS 7, 9, 11, and perhaps 13 and 15 (see Kindler et al., 2010), highstands (= warm interglacial intervals) of the Pleistocene Ice Age.  Deposits from each highstand are generally separated by calcrete paleosols or otherwise recognizable hiatuses/truncation surfaces.  The orangish-brown areas in the upper left parts of the photo represent a weathered calcrete paleosol.

The weathered outcrop shown above is a well-sorted, aragonitic calcarenite interpreted as having formed in a dune facies (= eolianite limestones).  Sand grains in the Owl’s Hole Formation are dominated by bioclasts, as in the Rice Bay Formation, but unlike the Grotto Beach Formation (oolites are the dominant calcarenite grain type there).





The island’s stratigraphy below the Owl’s Hole Formation was revealed by a core drilled down ~168 meters (~550-feet) below the surface (for details, see Supko, 1977).  The well site was at 3 meters above sea level near Graham’s Harbour beach, between Line Hole Settlement and Singer Bar Point (northern margin of San Salvador Island).  The first 37 meters were limestones.  Below that, dolostones dominate, alternating with some mixed dolostone-limestone intervals.  Reddish-brown calcretes separate major units.  Supko (1977) infers that the lowest rocks in the core are Upper Miocene to Lower Pliocene, based on known Bahamas Platform subsidence rates.


In light of the successful island-to-island correlations of Middle Pleistocene, Upper Pleistocene, and Holocene units throughout the Bahamas (see the Bahamas geologic literature), it seems reasonable to conclude that San Salvador’s subsurface dolostones may correlate with sub-Pleistocene dolostone units exposed in the far-southeastern portions of the Bahamas Platform.


Recent field work on Mayaguana Island has resulted in the identification of Miocene, Pliocene, and Lower Pleistocene surface outcrops (see Pascal Kindler’s 2010 talk on the subject).  On Mayaguana, the worked-out stratigraphy is:

 - Rice Bay Formation (Holocene)

 - Grotto Beach Formation (Upper Pleistocene)

 - Owl’s Hole Formation (Middle Pleistocene)

 - Misery Point Formation (Lower Pleistocene)

 - Timber Bay Formation (Pliocene)

 - Little Bay Formation (Upper Miocene)

 - Mayaguana Formation (Lower Miocene)

The Timber Bay Fm. and Little Bay Fm. are completely dolomitized.  The Mayaguana Fm. is ~5% dolomitized.  The Misery Point Fm. is nondolomitized, but the original aragonite mineralogy is absent.


I wouldn’t be surprised if careful comparison of the San Salvador core (if it still exists & is accessible) with Mayaguana outcrops results in some definitive correlations (sounds like a good Ph.D. project for someone).





Paleosol - the dominant paleosol type on San Salvador Island (& other Bahamian islands) consists of hard, reddish-brown to orangish-brown colored, irregularly-sculpted crusts.  These are referred to as calcretes or caliches or terra rosas.

Calcrete paleosols cap all of the Pleistocene-aged stratigraphic units, except where removed by erosion.  The Holocene-aged units (Hanna Bay Member & North Point Member of the Rice Bay Formation) haven’t been around long enough to develop calcrete paleosols atop their outcrops.



Calcrete paleosol (orangish-brown horizon) separating eolian calcarenites of the Middle Pleistocene Owl’s Hole Formation (below) from eolian calcarenites of the Upper Pleistocene Grotto Beach Formation (above) at Watling’s Quarry, southwestern San Salvador Island.



Pisolites (above & below) - pisolites are moderately large versions of oolites - they’re >2 mm-sized, subspherical to ellipsoidal, concentrically to irregularly concentrically laminated structures, commonly composed of calcium carbonate (as these are).  They are often perceived to be biogenic in origin.  Pisolites are not uncommon below calcrete/caliche paleosol horizons.

Above: carbonate soil pisolites in basket structure underneath a calcrete paleosol capping eolian calcarenite limestones of the Grotto Beach Formation (lower Upper Pleistocene, MIS 5e, 119-131 k.y.) at Watling’s Quarry, southwestern San Salvador Island.

Below: carbonate soil pisolites exposed by erosion of a calcrete paleosol horizon.  This paleosol caps the Cockburn Town Member of the Grotto Beach Formation (lower Upper Pleistocene, MIS 5e, 119-131 k.y.) just east of “The Notch”, eastern part of southern margin of San Salvador Island.



References cited

Supko, P.R.  1977.  Subsurface dolomites, San Salvador, Bahamas.  Journal of Sedimentary Petrology 47: 1063-1077.


Kindler, P., J.E. Mylroie, H.A. Curran, J.L. Carew, D.W. Gamble, T.A. Rothfus, M. Savarese & N.E. Sealey.  2010.  Geology of Central Eleuthera, Bahamas: a Field Trip Guide.  Gerace Research Centre.  San Salvador, Bahamas.  74 pp.


Other San Salvador geology literature




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