Carbonate Aquifers - Trace Fossils as Indicators of Sedimentary Environments - page 846

and their complex, pellet-lined burrows produce biologically engineered
plumbing systems in sediments at shallow depths (up to 2 m or more) beneath
the sea floor ( Curran and Martin, 2003 ). Ophiomorpha -dominated ichnofabrics
generated in tropical grainstone and grain-dominated packstone may be associ-
ated with substantial levels of touching-vug, intra- and interburrow macropo-
rosity and permeability. The burrow tubes commonly remain open or are
later washed clean or cleared of fill via fabric-selective dissolution, generating
intraburrow macroporosity. Limestone dissolution can proceed beyond the
periphery of a burrow fill and include the pellet-lined wall of the burrow, from
the inside toward the outside, so that the pelletal lining can be entirely removed
( Gameil and Sadek, 2010 ). Selective dissolution processes can also enhance the
interburrow porosity within the matrix surrounding pellet-lined burrow walls of
complex callianassid-burrow networks, resulting in large centimeter-scale inter-
burrow voids. Commonly, intra- and interburrow macroporosities develop
together, creating exceptionally permeable rock. The limestone of the Biscayne
aquifer contains prime examples of stratiform ichnogenic macroporosity and
permeability for comparison with other carbonate aquifers and reservoirs
( Cunningham and Sukop, 2011; Cunningham et al., 2009, 2010 ).

3.1.1 Ichnology in the Delineation of Cycles
The vertical arrangement of ichnofabric types, ichnofabric index measure-
ments, and assigned ichnofacies within cycles, provides criteria for use in
the delineation and correlation of HFCs within any stack of cycles comprising
the limestone of the Biscayne aquifer. In the context of ideal cycles ( Kerans and
Tinker, 1997 ), ichnofabric types and index measurements have a predictable
vertical arrangement within HFCs and HFC sets ( Cunningham et al., 2009 ).
Cunningham et al. (2009) recognized four ideal HFC types: an upward-
shallowing subtidal cycle, upward-shallowing paralic cycle, upward-shallowing
peritidal cycle, and aggradational subtidal cycle. Zones with dense concentra-
tions of Ophiomorpha (II

5) are typical in the lower to middle parts in the
upward-shallowing subtidal and paralic cycles that compose the Fort Thompson
Formation ( Figs. 2 and 3 ). The lateral continuity of the ichnofabric motifs
within and between cycles helps in correlation of individual cycles from one
borehole to another. The study area of Cunningham et al. (2004, 2006a,b)
provides an example of the use of ichnofabric types and index measurements
in the delineation and correlation of cycles. Here, a maximum Thalassinoides
ichnofabric (II

¼

5) is very widespread in wackestone and mud-dominated pack-
stone that compose an HFC assigned to marine isotope stage (MIS) 7 at the base
of the Miami Limestone ( Fig. 2 ). The occurrence of the Thalassinoides -
dominated ichnofabric contrasts with an overlying HFC comprising peloidal
grainstone and grain-dominated packstone with a maximum Ophiomorpha
ichnofabric (II

¼

¼

5). The recognition of this widespread vertical pattern in
ichnofabrics and ichnofabric index measurements increases confidence in
correlation of the two HFCs.