Carbonate Aquifers - Trace Fossils as Indicators of Sedimentary Environments - page 862

3.3.2 Ichnology in Defining Zones of Preferential
Groundwater Flow
The southern quarry wall shows a fabric-selective control on the development
of interburrow macroporosity and karstification. The outer surfaces of Thalas-
sinoides casts are commonly stained light brown, indicating the presence of iron
oxide ( Fig. 10 C-E). The interburrow macroporosity is in places filled or partly
filled with moderate brown-colored terra rossa clay. Lattice Boltzmann calcu-
lations of one whole-core-scale computer rendering ( Fig. 6 E and F) of an out-
crop sample (TXEA-1; Fig. 10 E) with a large volume of Thalassinoides -related
interburrow macroporosity produced an intrinsic permeability value of
1.8

10 6 darcies ( Fig. 5 and Table 1 ). This intrinsic permeability value is within
the range of Ophiomorpha -related LBM permeabilities generated for limestone
samples from the Biscayne aquifer ( Fig. 5 ) and of the same magnitude as the
intrinsic permeability calculated for the LTX-4 sample rendering ( Fig. 6 C
and D) from the Fort Terrett Formation ( Fig. 5 and Table 1 ). The extremely high
value of interburrow permeability derived from LBM, the clay plugging of
voids, and the widespread distribution of a high permeability throughout the
burrowed lithological unit 2 ( Fig. 10 ) indicate that this stratiform unit acted
to concentrate paleo-groundwater flow.

3.3.3 Influence of Ichnology on Karst
At the Travis County study site, small clay-filled paleo-caves, open caves with
speleothems, and a solution-collapse structure are associated with the burrowed
lithological unit 2 ( Fig. 10 ). The solution collapse resulted from overlying beds
falling into an area of expansive interburrow macroporosity in burrowed unit 2
( Fig. 10 A and B). The complete or partial filling of the burrow macroporosity
and paleo-caves with terra rossa clay ( Fig. 10 F) is inferred to have resulted from
transport by groundwater and deposition in the voids sometime within the
Miocene-to-Holocene time span, based on Abbott's (1975) scenario for porosity
development in the Edwards Group.

4. CONCLUSIONS
Three case studies, which include Pleistocene carbonate rocks of the Biscayne
aquifer in southern Florida and Cretaceous carbonate strata of the Edwards-
Trinity aquifer system in central Texas, demonstrate that (1) ichnology can offer
a fruitful methodology for carbonate aquifer characterization, and (2) there can
be a strong relation between ichnofabrics and groundwater flow in carbonate
aquifers. Important applications of ichnology to carbonate aquifer characteriza-
tion include its use in improving (1) the delineation of cycles, (2) the mapping of
cycles based on correlation of biogenically altered surfaces, (3) the mapping of
zones of preferential groundwater flow and paleogroundwater flow, and (4) the
understanding of the origin of karst features where related to ichnofabrics.