Macroporosity and Permeability Related to Callianassid
Bioturbation and Ophiomorpha Ichnofabric in Pleistocene Shallow-Marine
Carbonates: Examples from the Bahamas and South Florida
Al
Curran (Department of Geosciences, Smith College, Northampton,
Massachusetts, USA)
15th Symposium on the Geology of the Bahamas and Other
Carbonate Regions, Gerace Research Centre, San Salvador Island, Bahamas
18 June 2010
Looking
at callianassid bioturbation & Ophiomorpha ichnofabric & environments
conducive to callianassid habitation.
Ophiomorpha consists
of branching, lined burrows - the tubes are smooth on the interior and distinctly
pelletted on the outside. Callianassid shrimp
construct the lining. Ophiomorpha burrows can go quite deep.
This ichnogenus goes back to the Middle Jurassic. The burrow is the
habitation location for the shrimp - it hardly ever goes out after the larval
stage.
Ophiomorpha
architecture varies - shafts, tiered mazes, irregular boxworks. Shaft Ophiomorpha
architecture is not found in the Bahamas or in carbonates.
Callianassids
are decapod crustaceans (Thalassinidea). There are 155 living species of
Family Callianassidae. Actually, there are now >200 species.
Callianassids
are part of the marine benthos - they are mostly burrowers. They are gone
north of Myrtle Beach. There’s a high number of species in the
tropics. 95% of callianassids are intertidal to shallow marine.
Ichnofabric
- all aspects of texture & internal structure of sediments from
bioturbation.
Glypturus acanthocinus
Stimpson, 1866 is the callianassid shrimp seen on San Salvador Island.
This is the callianassid making the mounded topography in Pigeon Creek in
southeastern San Salvador Island, Bahamas - biogenic topography. Glypturus
acanthocinus is an ecosystem engineer - it lives 1 to 1.5 m below the
substrate. It’s hard to catch.
Glypturus acanthocinus
burrow architecture - a tiered spiral is shown in the literature & by resin
casts.
Callianassid
tropical habitats - backreef lagoons, adjacent to & within coral reefs,
leeward shelf of isolated platforms, platform shelves.
There’s
probably >1 species of callianassid on San Salvador Island.
Shallow
subtidal = backreef lagoonal environments & tidal channel environments.
Graham’s
Harbour - it's ~1.5 m down to bedrock at the beach; bare areas are heavily
callianassid burrowed & in grassbeds (despite what the literature says).
An
excurrent structure occurs at the tip of the mound (sand volcano).
An incurrent structure occurs at the base.
The
pockets of burrows accumulate coarser-grained sediments, but this hasn’t been
seen yet in the rock record.
Ophiomorpha walls are
quite thick - up to 1 cm thick. Can get giant callianassid Ophiomorpha
tunnels.
Ophiomorpha walls are
a mix of sediments and fecal pellets (fine-grained sediments/mud).
Some
have shafts/tunnels with blind endings (terminus structures).
Have
also seen examples from Exuma and Rum Cay and from a stromatolite-bearing tidal
channel at Lee Stocking Island (all in the Bahamas).
Some
Ophiomorpha burrows have incorporated shells.
Shafts/tunnels
are commonly 1.5 to 4 cm wide.
Favriina coprolites -
callianassid fecal pellets - have networks of holes within them - the pattern
is species-specific.
Ichnogenic
macroporosity:
1)
intraburrow porosity - tube interior
2)
interburrow porosity - between burrows (in close proximity) -
they form a framework - in such situations, the matrix is more likely to be
removed than the framework itself.
Cockburn
Town fossil reef on San Salvador Island - can see both intraburrow and
interburrow porosity. Do the porosity patterns seen here occur at depth?
Miami
Limestone & Biscayne Aquifer - the main water supply in southern Florida.
Floridan
Aquifer - a deeper aquifer than Biscayne.
Biscayne
Aquifer porosity - matrix porosity (mm scale or less) + macroporosity (caves
& Ophiomorpha burrows).
Miami
Limestone - cf. ooid shoals (intertidal/shallow subtidal of Joulter Cay -
callianassids occur there).
Miami
Zoo - the blocks that animals climb on & the facing stones there have Ophiomorpha
ichnofabric.
Miami
Limestone - air-rock - largely pore space - both intraburrow and interburrow
porosity are present from lots of Ophiomorpha.
Florida's
Alice Wainwright Park has Miami Limestone with Ophiomorpha, from Glypturus
acanthochinus callianassid shrimp - have tiered spiral burrows. There
is >1 Ophiomorpha maker species, apparently.
Examples
of ichnogenic macroporosity:
1)
Ophiomorpha macroporosity - Miami Limestone of southern Florida &
Upper Pleistocene of Bahamas.
2)
Thalassinoides
(Ex: superperm in the Ghawar Oil Field, Arabia)
3)
mangroves
