Carbonate Aquifers - Trace Fossils as Indicators of Sedimentary Environments - page 858

irregular boxwork configuration of open burrows ( Fig. 9 B). Within some
cycles, Thalassinoides occurs parallel to the bedding plane of the rock, where
burrows are repeatedly branched with a slight swelling at the junctions of
the tubes and form a horizontal, regular maze ( Fig. 9 C). A high density of
Thalassinoides in the middle to lower parts of HFCs E and F ( Fig. 9 A) may
be due to a relatively concentrated abundance of deep-tiered burrow excavations
(e.g., Bromley, 1996 : fig. 4.25 ). The three-dimensional arrangement of the
Thalassinoides burrows forms an irregular boxwork of well-connected, open
tunnels and shafts with ichnofabric indices measuring as high as 5 ( Fig. 9 A).
The open, inner diameters of burrows are cylindrical in shape and most
commonly range from 0.5 to 3.5 cm in width.
3.2.2 Biogenically Altered Surfaces
Trace fossils have been shown to be useful for the genetic interpretation of dis-
continuity surfaces whether of sequence stratigraphic or autocyclic importance
( Bromley, 1975; MacEachern et al., 2007a ). A Glossifungites Ichnofacies
assemblage and discontinuity surface occur at the tops of HFCs E and F
( Fig. 9 A). During the development of the discontinuities, firm substrates existed
below the upper bounding surfaces of HFCs E and F as suggested by the pres-
ence of sharp-walled Thalassinoides , absence of borings penetrating the surface,
and by the occurrence of post-burrow excavation infillings by transgressive
sediments from superjacent cycles. Erosion of the surfaces is suggested by a
sharp facies shift across them, and their irregular and uncommon overhanging
paleotopography. Possible causative controls on the erosional surfaces include
fair-weather waves, storm waves, or tidal currents in response to a seaward shift
in paleoshoreline, followed by a period of transgression ( Gerard and Bromley,
2008 ). The association of a Glossifungites Ichnofacies assemblage with both of
the cycle-bounding surfaces could be a key to widespread correlation of HFC E
and HFC F within the Burrowed Member of the Fort Terrett Formation.
3.2.3 Ichnology in Defining Zones of Preferential
Groundwater Flow
The macroporosity associated with the interior of Thalassinoides tunnels and
shafts is likely the result of fabric-selective dissolution of an internal burrow
fill—a process proposed by Abbott (1975) . Mapping of the burrow-related
macroporosity shown in Figure 9 indicates that Thalassinoides are filled with
grainstone and grain-dominated packstone toward the southwestern limit of
the road cut outcrop as it approaches Bee Creek. From here, fabric-selective
leaching and removal of burrow fill increase toward the northeastern limit of
the road cut exposure, where most or all of the internal sediment that once filled
the interiors of Thalassinoides has been leached and removed. The grain-rich
fill in the Thalassinoides tubes has partly or entirely dissolved to produce a
tangled, serpentine arrangement of highly permeable touching-vug porosity.