Carbonate Aquifers - Trace Fossils as Indicators of Sedimentary Environments - page 855

at Deering Glade is the most striking. This system makes a transverse cut
through the longitudinal barrier bar and was partly mapped by Cressler
(1993) ; the interior of one of the caves is shown in Cunningham et al. (2008 :
figs. 3B and 4). Here, the highly burrowed zone is interpreted to have been a
stabilized sand flat, later buried by southward progradation of the cross-bedded
oolitic barrier bar ( Fig. 8 ). The primary burrow porosity within the stabilized
sand-flat sediment provided a template for stratiform fabric-selective dissolu-
tion by focusing aggressive groundwater dissolution, which produced caves,
within the limits of the burrowed lithosome. The dissolution by groundwater
bypassed the cross-bedded oolitic bar because it had a relatively lower primary
porosity and permeability compared to the burrowed stabilized sand flat. The
oolite barrier-bar forms the ceilings of many of these caves.
3.2 Edwards-Trinity Aquifer System in Real County,
Central Texas
The study of outcrops in Real County focused on a freshly excavated road
cut along Highway 83 about 0.7 km north of Bee Creek (Fig. 1). Regional
observations indicate that cycle-bound Thalassinoides -dominated ichnofabrics,
likely generated by thalassinidean or thalassinidean-like crustaceans ( Goldring
et al., 2007 ), are common in parts of the Early Cretaceous succession of carbon-
ate rocks that form the Edwards Group in the eastern Edwards Plateau of central
Texas ( Rose, 1972 ). Thalassinoides -dominated ichnofabrics have a substantial
volume of associated fabric-selective macroporosity in some carbonate out-
crops atop saturated parts of the Edwards-Trinity aquifer system. Commonly,
this macroporosity is either a result of fabric-selective leaching and removal of
burrow fillings to form moldic, intraburrow macroporosity, previously referred
to as “honeycomb porosity” ( Abbott, 1975; Rose, 1972 ), or a result of dissolu-
tion of carbonate matrix surrounding burrow fills to form interburrow macro-
porosity. Honeycomb porosity is common in the Burrowed Member of the
Fort Terrett Formation in the eastern Edwards Plateau, where it occurs in the
basal parts of the Edwards-Trinity aquifer system ( Rose, 1972 ). This pore type
contributes to the Burrowed Member being the chief water-bearing zone of the
Fort Terrett Formation in parts of central Texas ( Rose, 1972 ). Barker and Ardis
(1996) suggested that the Burrowed Member of the Fort Terrett Formation may
be the most permeable part of the Edwards Group west of the intensely fractured
and highly leached Balcones fault zone. Abbott (1975) described honeycomb
porosity and permeability related to burrowing in the Kainer and Person Forma-
tions of the Edwards Group of the Edwards-Trinity aquifer system. Abbott
(1975) noted that dissolution of burrow fills in these carbonate rocks can pro-
gress well beyond the burrow walls to form caverns. Similarly, Hovorka et al.
(1998 : 30) observed solution-collapse structures in outcropping carbonate rocks
of the Edwards Group and suggested that the collapse could be related to
advanced dissolution of burrows in a subtidal stratal unit.