Shallow-Marine Carbonates - Trace Fossils as Indicators of Sedimentary Environments - page 706

shallow subtidal environments is important to both sedimentologists and ichnol-
ogists because “they often occur in high densities and influence the whole sed-
imentology and geochemistry of the seabed” ( Dworschak, 2004 ). With respect
to geochemistry, some studies, mostly by marine biologists (e.g., Webb and
Eyre, 2004 ), support this statement, but data are limited. What has been clearly
documented is the many effects callianassids have on surrounding carbonate
sediments ( Tudhope and Scoffin, 1984 ), including the total transformation of
original lithofacies by burrowing activity ( Curran, 1994; Tedesco and Wanless,
1991 ; Fig. 10 ). Nonetheless, ichnologists and sedimentologists should be mind-
ful that overall knowledge of tropical callianassids remains poorly known with
respect to species identification, burrow morphology, and general ecology and
sedimentary environment effects.
There are many species of thalassinideans, and the families Callianassidae
and Upogebiidae are cosmopolitan, with decreasing species numbers toward the
poles. In a survey of extant thalassinidean diversity, Dworschak (2000)
recorded 155 species of callianassids and 139 species of upogebiids (species
numbers have increased to more than 200; Dworschak, 2010: personal commu-
nication) with subsequent new discoveries. Furthermore, most thalassinidean
species are found in shallow-marine waters, living at depths of less than
20 m, with by far the highest numbers of species found at tropical to subtropical
latitudes.
In the light of these numbers, it is no surprise that the bioturbation and
burrows of callianassids ( Ophiomorpha) dominate the ichnocoenoses of the
Skolithos Ichnofacies as it occurs in the shallow-subtidal Quaternary grain-
stones of the Bahamas, south Florida, and other similar tropical carbonate set-
tings. This is important because Ophiomorpha ichnofabrics commonly create
high levels of permeable ichnogenic macroporosity, permitting host strata to
have significant potential as carbonate aquifers and reservoirs ( Cunningham
et al., 2009, 2012 ).
The dominant ichnotaxon occurring in Late Pleistocene shallow-subtidal
grainstones of the Bahamas and south Florida is Ophiomorpha (Ichnocoenosis
1). Bahamian Ophiomorpha burrow systems are commonly robust and well pre-
served ( Fig. 10 ) and have been described in detail ( Curran, 1994, 2007; Curran
andWhite, 1991 and references therein). Ophiomorpha -bearing grainstones can
occur in association with fossil coral reefs and intervene with coral rudstone
( Curran, 1994 ). S. linearis is always present but secondary to Ophiomorpha ,
occurring in greatest densities in grainstones formed in the shallowest parts of
shallowing-upward sequences.
Ichnocoenosis 2 includes Conichnus conicus and Planolites in addition to
Ophiomorpha and Skolithos linearis . In Late Pleistocene rocks, C. conicus occurs
most commonly in grainstones characterized by tabular and trough cross-bedding
and interpreted as representing relatively rapid sediment accumulation under
shallow-subtidal, shoalingconditions influencedbynearshore and/or tidal currents
( Curran and White, 1997 ; Fig. 11 A). Planolites typically is less abundant than