Carbonate Aquifers - Trace Fossils as Indicators of Sedimentary Environments - page 849

of each cycle can provide a means for long-distance correlation of upper cycle
boundaries. Larger rhizoliths can have many shapes but are mostly circular to
irregular in cuts perpendicular to the longitudinal dimension of core samples
( Fig. 4 C) and have cylindrical to irregularly tubular shapes in length ( Fig. 4 A
and B). Longitudinally slabbed cores containing relatively large rhizoliths
commonly show downward tapering and some downward-directed bifurca-
tions ( Fig. 4 A and B). The distinctive exposure zone and accompanying
rhizoliths shown in Fig. 4 are found over a widespread area (165 km 2 ) beneath
the upper bounding surface of a stack of HFCs assigned to MIS 11 ( Figs. 2
and 4 ). These rhizoliths can penetrate downward to about 1.5 m below the
top of the uppermost HFC assigned to MIS 11. Perkins (1977) reported rhizo-
liths in some cases over 3 m in vertical length in carbonate rocks of South
Florida and suggested an association with tap-root systems. Rhizoliths occur-
ring below the MIS 11 upper surface are commonly filled with a gray to
light-brown lime mudstone that can contain the freshwater gastropod Planor-
bella ( Fig. 9 ), an occurrence that aids in the identification of the upper bounding
surface of the MIS 11 limestone.
Firmgrounds and hardgrounds uncommonly bound the tops of HFCs within
the carbonate rocks of the Biscayne aquifer. Structures that correspond to the
ichnogenus Thalassinoides have a common association with the firmgrounds
and as such are attributed to a Glossifungites Ichnofacies. The hardgrounds
have borings with morphologies ascribed to excavation by bivalves ( Gastro-
chaenolites ) or sponges ( Entobia ) or both and correspond to the Trypanites
Ichnofacies (cf. MacEachern et al., 2007a ). Correlation of some of the
firmgrounds may be over long distances (tens of kilometers). However, these
hardgrounds have only been correlated at the kilometer scale and are possibly
related to autocyclicity.
3.1.3 Ichnology in Defining Zones of Preferential
Groundwater Flow
In the study area of Cunningham et al. (2004, 2006a,b) , as shown in Fig. 1 ,
stratiform zones of concentrated groundwater flow related to inter- and intra-
burrow macroporosity are common within much of the limestone of the
Biscayne aquifer. Ophiomorpha is the dominant ichnotaxon present within
the ichnofabric of these macroporous and highly permeable zones. Cunningham
et al. (2006a,b, 2009) and Cunningham and Sukop (2011) have produced
reliable geologically based correlations of these Ophiomorpha -rich zones by
combining ichnology, lithostratigraphy, cyclostratigraphy, paleontology, and
borehole geophysics. Evidence for preferential groundwater flow within these
zones has been corroborated by multiple methods, including geophysical
borehole logs, computational methods, and hydraulic and chemical tracer tests
( Cunningham and Sukop, 2011; Cunningham et al., 2006a,b, 2009, 2010,
Renken et al., 2005, 2008; Shapiro et al., 2008 ).