Carbonate Aquifers - Trace Fossils as Indicators of Sedimentary Environments - page 860

probably began with fabric-selective dissolution of the internal sediments of
Thalassinoides to produce an arrangement of molds that conforms to the strat-
iform distribution of a Thalassinoides -dominated ichnofabric confined within
the upper and lower boundaries of upward-shallowing subtidal cycles
( Fig. 9 A). As dissolution progressed beyond the internal sediment of the
burrows and into the carbonate matrix, cavern-sized voids were created. This
process of burrow-mold formation and eventual development of caverns
began during Cretaceous subaerial exposure of the Edwards Group ( Abbott,
1975 ). Abbott (1975) proposed that the most aggressive dissolution within
the ancestral Edwards aquifer occurred during or since the Miocene—an
epoch when a through-flowing dynamic groundwater system was highly
active. The filling of the macropore system at the Real County site by clay
presumably occurred within a time span bracketing the Miocene to Holocene
epochs.
3.3 Edwards-Trinity Aquifer System in Travis County,
Central Texas
Burrow-related macroporosity was investigated in and around an abandoned
quarry along Far West Boulevard in Travis County, where the dolomitic
member of the Aptian-Albian Kainer Formation ( Small et al., 1996 ) is exposed
(Fig. 1). At this locality, these carbonate rocks compose an unsaturated part of
the Edwards-Trinity aquifer system ( Barker and Ardis, 1996 ).
3.3.1 Ichnology in the Delineation of Cycles
The sequence-stratigraphic framework of Kerans (2005) was used for the Travis
County locality to infer that the lowest-order cycle shown partly in Fig. 10 is a
tens-of-meter-scale upward-shallowing HFS. Figure 10 shows two stratiform
burrowed lithological units (2-3) characterized by a Thalassinoides -dominated
ichnofabric and peloidal mudstone to wackestone depositional textures that
overlie a caprinid-rudisitid rudstone and floatstone. The very low-diversity ich-
notaxa and peloidal micrite-rich depositional textures reflect a stressed, low-
energy Thalassinoides -dominated Cruziana Ichnofacies (cf. MacEachern
et al., 2007a,b ). Above the burrowed zone, grainstone beds grade upward into
beds composed of flat-pebble conglomerate and stromatolites that cap the
upward-shallowing HFS. From bottom to top ( Fig. 10 A), the vertical lithofacies
succession (lithological units 1-6) represents the seaward progradation of (1) an
open shallow-marine, caprinid bioherm; (2) a restricted shallow-marine, shelf
interior; (3) a shallow, high-energy, coastal foreshore; and (4) mud-rich and
stromatolitic tidal to supratidal flats. The burrowed lithological units 2 and 3
( Fig. 10 A) were mapped in discontinuous outcrops over an area of about
1.6 km 2 .