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. Thereís 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. Itís 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.
Thereís probably >1 species of callianassid on San Salvador Island.
Shallow subtidal = backreef lagoonal environments & tidal channel environments.
Grahamís 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 hasnít 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.
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)