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 doesn’t 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 (7°
to 8°) - 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 it’s 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 - it’s 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 don’t 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.