Carbonate Aquifers - Trace Fossils as Indicators of Sedimentary Environments - page 844

show evidence for concentrated paleogroundwater flow through stratiform,
touching-vug, macroporous flow zones linked to secondary fabric-selective
intraburrow and interburrow macroporosity within Thalassinoides -dominated
ichnofabrics.
3.1 Biscayne Aquifer in Southeastern Florida
The Biscayne aquifer is the principal aquifer in southeastern Florida and one of
the most permeable in the world ( Parker et al., 1955 ). The main lithostrati-
graphic units of the Biscayne aquifer are the Pleistocene Fort Thompson For-
mation and Miami Limestone ( Fig. 2 ), both of which are characterized by
eogenetic karst limestone ( Cunningham et al., 2004, 2006a,b; Vacher and
Mylroie, 2002 ). These carbonate units were deposited mostly in shallow, sub-
tropical to tropical marine shelf or continental-marine transitional environ-
ments. The limestone has undergone only very shallow, near-surface burial,
and multiple widespread subaerial discontinuities associated with relative
sea-level lowstands are interspersed throughout the limestone system
( Cunningham et al., 2004, 2006a,b; Multer et al., 2002; Perkins, 1977 ); thus,
the limestone commonly has substantial primary interparticle and secondary
moldic porosity. The Biscayne aquifer is characterized as a dual-porosity pore
system (cf. Cunningham et al., 2004, 2006a,b, 2009, 2010 ), consisting of (1)
matrix porosity (mostly interparticle pores and separate vugs), which in areas
can provide much of the groundwater storage; and (2) touching-vug macro-
porosity. The touching-vugmacroporosity can create stratiform, areally extensive,
groundwater-flow pathways and less commonly observed bedding-plane and
cavernous vugs, vertical solution pipes, and solution-enlarged fractures.
The trace fossil Ophiomorpha is almost everywhere the dominant ichno-
genus in the shallow-marine, subtidal carbonate grainstone and grain-
dominated packstone of the uppermost high-frequency cycle (HFC) of the
Miami Limestone and is common in the HFCs composing the Fort Thompson
Formation, Here its presence and paleoenvironmental setting are indicators of a
Skolithos Ichnofacies ( Curran, 2007; MacEachern et al., 2007a ). These burrows
are robust, with thick walls distinctly pelleted on exterior surfaces and
smooth on the interior; their outside diameters typically are 1-3 cm or larger.
The arrangement of burrows commonly forms multigenerational, three-
dimensional, complex patterns. The burrows are filled or partly filled with
sediment, or preserved completely open. In Pleistocene carbonates of the
Bahamas and southeastern Florida, Ophiomorpha is attributed to the burrows
of callianassid shrimp by analogy with modern callianassids that dominate
deep-tier faunas of bank-top, shallow-subtidal environments throughout the
Bahamas, Florida Keys, and other similar tropical carbonate settings ( Curran,
2007; Knaust et al., 2012; Shinn, 1968 ).
Callianassids are powerful agents of bioturbation; their activities can
completely transform original depositional fabrics to distinctive ichnofabrics,