Carbonate Aquifers - Trace Fossils as Indicators of Sedimentary Environments - page 863

Carbonate rocks with ichnofabrics dominated by Ophiomorpha in the
Biscayne aquifer and Thalassinoides in the Edwards-Trinity aquifer system
can possess exceptionally permeable stratiform networks of cm-scale,
touching-vug ichnogenic macroporosity and permeability. The stratiform aqui-
fer zones can evolve within two different scales of shallow-marine shelf cycles
that include (1) multistoried stacks of HFCs delineated by predictable vertical
successions of lithofacies, and (2) HFSs comprised in part by thick beds that
act as aquifer zones. Burrowed zones can be laterally expansive, as in modern,
flat-topped, tropical platforms; thus, these aquifer zones can have widespread
continuity of up to the 100-km scale.
The stratiform highly permeable ichnofabric-related macroporous zones in
the Biscayne aquifer compose a Skolithos Ichnofacies and in the Edwards-
Trinity aquifer system a Thalassinoides -dominated Cruziana Ichnofacies.
Other ichnofacies recognized in the carbonate rocks of the Biscayne aquifer
are Thalassinoides -dominated Cruziana , Glossifungites , and Trypanites , but
none have been recognized as part of the Ophiomorpha -related stratiform
groundwater-flow zones. At the Real County site in central Texas, the burrows
associated with the Glossifungites Ichnofacies are filled with sediment and
would not have added any substantial input to paleogroundwater flow within
the ancestral Edwards-Trinity aquifer system.
Multiple methodologies, such as geophysical logging and tracer tests,
confirm preferential flow within many of the Ophiomorpha -dominated zones
of the Biscayne aquifer. In parts of the Biscayne aquifer, the ichnologically
associated networks of highly permeable macroporosity are related to the
fabric-selective dissolution of Ophiomorpha -dominated limestone and primary
burrow macroporosity. Five whole-core-scale-sized computer renderings of
limestone samples from the Biscayne aquifer, which are totally bioturbated
and characterized by Ophiomorpha -dominated ichnogenic macroporosity, have
extremely high intrinsic permeabilities ranging between 15 10 3 and 27 10 6

darcies.
In the Edwards-Trinity aquifer system at two study sites in central Texas,
examples of permeable Thalassinoides -dominated ichnofabrics occur within
an unsaturated part of the aquifer system. At both sites, the macroporous zones
within the Cretaceous carbonate rocks are representative of a restricted shallow-
marine, shelf-interior paleoenvironment. Lattice Boltzmann-calculated
intrinsic permeabilities for intra- and interburrow macroporosity repres-
ented in computer renderings of two whole-core-scale samples were 5.6 10 6

10 6 darcies, respectively. The Cretaceous macroporous, maximum
Thalassinoides ichnofabrics have similar intrinsic permeability values as
Pleistocene intra- and interburrow macroporosity related to Ophiomorpha -
dominated ichnofabrics in limestone of the Biscayne aquifer. These highly
permeable Thalassinoides -dominated zones could play an important role in
groundwatermovement, where present, in saturated parts of theEdwards-Trinity
aquifer system.

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