Carbonate Aquifers - Trace Fossils as Indicators of Sedimentary Environments - page 859

Lattice Boltzmann calculations of one whole-core-scale computer rendering of
an outcrop sample (LTX-4, Fig. 6 C and D) with a large volume of Thalassi-
noides -related intraburrow macroporosity produced an intrinsic permeability
value of 5.6 10 6 darcies, a value within a range of Ophiomorpha -related

LBM intrinsic permeabilities generated for Pleistocene limestone samples from
the Biscayne aquifer ( Fig. 5 and Table 1 ).
In the partly open or fully open burrows, the inner walls of Thalassinoides
are stained light brown ( Fig. 9 B-D). The moldic interiors of Thalassinoides
commonly are partly or completely filled ( Fig. 9 D) with a moderate brown terra
rossa clay (cf. Young, 1986 ). The clay fill within Thalassinoides tubes provides
evidence that the paleo-permeability was substantially enough within the paleo-
macropore network to allow the transport of clay carried by groundwater
through a transmissive paleo-pore network dominated by Thalassinoides .
Through time the clays settled within the network of tube-like macroporosity
to plug or partly plug it. Presumably, the clay was advected over distances of
at least outcrop scale and perhaps much more. The clay is inferred to have
entered a subsurface porosity network from an exposed paleosurface via sink-
holes or vertically oriented solution-enhanced fractures that connected the Tha-
lassinoides -related paleo-macroporosity to the paleosurface. Transport of
suspended sediment through the Fort Terrett Formation limestone is suggestive
of turbulent flow conditions ( Mahler and Lynch, 1999 ). Located about 100 m
north of the road cut shown in Figure 9 , a sinkhole in the carbonate rocks of
the Fort Terrett Formation is partly filled with terra rossa clay and may have
acted as one source of clay that fills or partly fills molds of Thalassinoides .
Alternatively, the clay was advected by groundwater flowing though a sinuous,
solution-enlarged subterranean flow path converging on or diverging from a
paleo-river valley (cf. Abbott, 1975 ).

3.2.4 Influence of Ichnology on Karst
Macroporosity associated with the Thalassinoides -dominated ichnofabric is
present generally at a cm-scale but expands to m-scale at one paleo-cave
exposed in the Highway 83 road cut. Like many Thalassinoides burrows
( Fig. 9 D), much of the paleo-cave is fully plugged with terra rossa clay. The
paleo-cave has a stratiform geometry with its ceilings, floors, and walls mainly
conforming to Thalassinoides -rich parts of HFCs B and C ( Fig. 9 A). The paleo-
cave stair-steps downward between the two HFCs, conforming to the dissolved
Thalassinoides -rich zones. Dissolution-produced dish-shaped indentations with
prominent irregular polygonal boundaries demarcate a sculptured ceiling of the
paleo-cave, and although similar in appearance to scallops, only some are asym-
metric in shape typical of scallops (cf. Palmer, 2007 ). As scallops form during
the flow of turbulent water across a bedrock surface ( Palmer, 2007 ), their minor
presence suggests cave formation when the paleo-cave was, at least at times,
filled with groundwater. This speleogenetic scenario suggests cave formation
in the phreatic groundwater zone and thus likely pre-Holocene. Speleogenesis