Geology of Mayaguana Island (Bahamas) and the Timing
of Sediment Production on Carbonate Platforms
Pascal Kindler
(Section of Earth and Environmental Sciences, University of Geneva, Geneva,
Switzerland)
15th Symposium on the Geology of the Bahamas and Other
Carbonate Regions
19 June 2010
The
Bahamas Platform is subsiding 1-2 meters/million years.
The
Bahamian surface stratigraphic record is Middle Pleistocene to Holocene,
consisting of carbonate units deposited during sea level highstands.
Mayaguana
is 250 km southeast of San Salvador Island. Mayaguana is a ~flat,
medium-sized island that’s 50 km in an east-west direction and 15 km in a
north-south direction. It occupies most of its platform.
Cant
(1977) - fossil reefs were surveyed along the shoreline - they are very
extensive (much more so that the Cockburn Town Fossil Reef on San Salvador
Island).
Pierson
(1982) - did a shallow core study on Mayaguana.
Mayaguana
is sitting on the passive margin of the North American Plate, but it’s near the
Cauto-Nipe Fault & the Bahamas Escarpment (a Jurassic rift structure) &
the North American-Caribbean Plate boundary.
Mayaguana
has Rice Bay Formation rocks - Holocene oolitic & bioclastic grainstones
(beach-dune environments). There are limited exposures of Rice Bay
Formation. Ex: Flamingo Bay. The Rice Bay Formation here has
no induration and has marine phreatic rim cements.
Mayaguana
also has Grotto Beach Formation rocks (they cover almost all of the island)
with two members - a lower French Bay Member (oolitic grainstones; dunes) and
an upper Cockburn Town Member (reef + lagoon deposits). The Cockburn Town
Member is extensive on Mayaguana.
Owl’s
Hole Formation - bioclastic-peloidal grainstones in dune facies, Middle
Pleistocene in age. The Owl’s Hole Fm. is mostly at the center of the
island. Owl’s Hole Fm. rocks have phreatic caves at +18 meters (Ex:
AB Cave, Mayaguana).
So,
it has the same stratigraphy as the rest of the Bahamas islands.
There
are new units, though:
1)
Misery Point Formation - weathered bioclastic rudstones &
grainstones deposited in beach to backreef to reef to eolian settings - no
primary carbonate mineralogy anymore. Matrix is micritic. The type
locality is Misery Point Cliff on Mayaguana Island. Misery Point Cliff is
vegetated - not active. The Misery Point Fm. has three members. The
formation is capped by a paleosol & overlain by eolianites of the Owl’s
Hole Formation (= 0.7 Ma here - Middle Pleistocene). Misery Point
Formation rocks have been dated to 1.0 to 1.3 Ma (Lower Pleistocene).
2)
Timber Bay Formation - dolomite at +3 meters elevation; dolomitized
bioclastic rudstone with dissolved coral branches and sediments in-between are
preserved. This reef facies is +3 to +6 meters in terms of original
relative sea level height. 59 to 100% dolomite content. This unit
is 2.3 to 3.8 Ma (an 87Sr/86Sr date) (Pliocene).
3)
Little Bay Formation - dolomicrite exposed at +2 meters. 100%
dolomite, but originally deposited in a shoal facies. No evidence of the
precursor limestone. Dated to 5.5 to 6.5 Ma (= Messinian, Late
Miocene). The Little Bay Fm.-Timber Bay Fm. boundary is a microkarsted
contact.
4)
Mayaguana Formation - foraminiferal wackestone from lagoonal and
backreef facies, exposed at +1 meters. This is a recently designated
formation name. The Mayaguana Fm. is +10 m above relative sea level
height (from original). Maybe 5% dolomite, but otherwise not
dolomitized. Dated to 15.9 to 20.4 Ma - a biostratigraphic date from the
Burdigalian foram Miolepidocyclina (upper Lower Miocene). Dated to
18.4 to 18.7 Ma based on 87Sr/86Sr dating (Early
Miocene).
The
Owl’s Hole Formation is a big hill at the center of the island - not very
extensively exposed.
The
Miocene & Pliocene are exposed near the northwestern and northeastern
corners of the island.
Near
the northwestern corner of Mayaguana, a reentrant can be seen on a bathymetric
map. Pliocene-Miocene exposures are adjacent to this reentrant.
Northern
Hemisphere glaciation started in the Middle Pliocene.
Little
Bay has four stacked formations exposed:
-
Misery Point Fm.
-
Timber Bay Fm.
-
Little Bay Fm.
-
Mayaguana Fm.
There’s
been an average accumulation rate of 0.5 mm/1000 years. This is a low
accumulation rate for a carbonate platform - a stable platform (limited
accommodation).
The
Cockburn Town Member abuts the base of the Lower Pleistocene units at
Misery Point Cliff.
Old
units only occur along the northern shore of Mayaguana Island.
The
Pliocene-Pleistocene boundary is deeper in the southern part of the island (in
the subsurface).
Mayaguana
Bank is tilted to the south (related to tectonics & isostatic rebound after
margin collapse?).
The
Bahamas is not so tectonically quiescent, especially in the south - tilting is
seen here.
The
surface stratigraphy of the Bahamas is now known to be Lower Miocene to upper
Holocene.
On
some platforms, at some times, the carbonate factory was not on during
highstands (deeply drowned), but was on during lowstands (shallow enough
water for algae carbonate sediment production).
No
paleosols separate the Miocene-Pliocene units. This is because this
platform was never exposed. The same phenomenon is seen in the
Miocene-Pliocene of Florida.