Sea Level and the Paleoenvironmental Interpretation of the middle to late Holocene Hanna Bay Limestone, San Salvador Island, Bahamas: a High Foreshore Setting without a Eustatic Highstand

Michael Savarese (Department of Marine & Ecological Sciences, Florida Gulf Coast University, Forty Myers, Florida, USA)

15th Symposium on the Geology of the Bahamas and Other Carbonate Regions, Gerace Research Centre, San Salvador Island, Bahamas

20 June 2010

 

Looking at the middle to late Holocene Hanna Bay Limestone on San Salvador Island, Bahamas

A mid-Holocene highstand doesnít exist in Florida.  Looked at the Hanna Bay Limestone on San Salvador to check for it.

The Western Atlantic sea level curve, from 10 k.y. to now, is a relatively smooth curve, with a couple slope changes, but otherwise rising.  This info. is based on mangrove peats and Acropora coral occurrences.

Florida oyster reefs show a 3 k.y.-to-now gradual rise in sea level with no blips.

The northern Gulf of Mexico shows a few blips on the Holocene sea level curve based on info. from there.

 

A middle to late Holocene highstand has been reported globally - Gulf of Mexico, Australia, China, etc.

If this was eustatic, it suggests short period sea level fluctuations cause abrupt changes over decadal/centennial scales.  Abrupt ice sheet collapse in Antarctica or Greenland may be a factor behind these changes.  This has implications for our future and sea level rise.

 

The Hanna Bay Member consists of foreshore rocks sitting +2 m above present sea level.  Do Hanna Bay rocks really represent foreshore rocks?

 

Hanna Bay Member limestones are mostly eolian + backshore deposits + foreshore deposits.  The foreshore deposits have been questioned.  But they are there.

Hanna Bay rocks are prolific around San Salvador Island - the Hanna Bay type locality and Grotto Beach have good exposures.  Hanna Bay rocks at Grotto Beach are at +2 m above mean sea level.

 

Hanna Bay limestones have tabular, cross-laminated beds with shallow seaward dips (7į to 8į) - this is in the normal range of features for foreshore beach deposits.  Some graded bedding and some shallow angular unconformities from the erosional influence of younger beds.  Fenestral porosity is present, as are swash lines & rill lineations - both of these are hard to preserve, but they are there.  Rill lineations are returning water lineations.  This is all consistent with a foreshore interpretation.  An excellent swash line with fenestral fabric on its seaward side is preserved on a bedding plane exposure at Grotto Beach.

 

Modern beaches on the leeward coast of San Salvador have fine-grained to coarse-grained sands - lumps and bioclasts are common allochems.

Hanna Bay limestones are composed of medium- to coarse-grained sands with lumps + bioclasts as allochems.  Slightly coarser materials show up in Hanna Bay thin sections.  Otherwise itís a good correlation.

 

Ichnofauna in Hanna Bay limestones - vegemorphs.  This is contradictory information.  Skolithos and Diplocraterion are common in foreshore deposits.  Psilonichnus is a ghost crab burrow - itís a backshore trace fossil.  Vegemorphs indicate a backshore or dune setting.

 

Petrology - allochems are consistent with a wave-swept origin rather than a wind-transported origin.

Cement - freshwater phreatic + vadose cements predominate.

Exception: isopachous, acicular rim cements in intergrain pores & at basal parts of the section exposed at Grotto Beach.  Acicular rim cements are consistent with marine phreatic cement origin.

Allochems - principally bioclasts and lumps.

Intergrain cements - blocky, isopachous to meniscus (not marine phreatic in origin).

Low in the section at Grotto Beach is intergranular acicular cement (aragonite) - can get that with sea spray (also seen on the eastern side of North Point Peninsula, for example).

 

The Hanna Bay & Grotto Beach sections of Hanna Bay Limestone give different dates (whole rock radiocarbon dates).

Stratigraphically disordered dates (3 for each section) (all of these are calendar years B.P.): 

        Grotto Beach                Hanna Bay

             780 top                       4060 top

             1230                            4520

             1000 bottom               3260 bottom

This disordering is understandable given that the whole rock dates are averages of the ages of various allochems and cements.

The two sections are different enough in dates to indicate two separate highstands.

Hanna Bay limestones do represent foreshore deposits.

Cement petrography, vegemorphs, ichnofossils suggests a very short-lived sea level high - the outcrop got quickly transferred to a supratidal position.

Hanna Bay foreshore deposits are diachronous across their geographic range.

 

Sediments can get deposited in foreshore settings in short-duration, high-frequency sea level highs (high berms).

Grotto Beach has modern foreshore deposits near the Cockburn Town Fossil Reef wall - a recent high foreshore.  The same is seen at Little Exuma Island.  Do you need a sea level highstand, then?

Storm tides possibly explain this (the sediments donít indicate storm activity, though).

Perigean spring tides possibly explain this.

Another possible explanation is regional shift in oceanic circulation (e.g., Florida Current perturbations; North Atlantic Oscillations).

 

Do get local sea level highs from a combination of normal oceanographic phenomena.

A eustatic highstand is not supported based on evidence from San Salvador.

Each case of a purported mid-Holocene highstand needs to be evaluated on a case by case basis.

There are ephemeral and non-ephemeral indicators for eustatic sea level highs.

Ephemeral indicators - foreshore deposits from a perigean or spring tide.

Non-ephemeral indicators - Acropora reefs.

 

Jim Carew question: We got 2800 & 3200 yrs. B.P. dates at Grotto Beach for Hanna Bay limestones.

Mike Savarese response: It depends on where you sample the cliffs.  Foreshore deposits can be readily left behind by short-term events.

 


 

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