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.