Carbonate Aquifers - Trace Fossils as Indicators of Sedimentary Environments - page 848

Ichnofacies provides another means for characterization of HFCs compos-
ing the limestone of the Biscayne aquifer. In a very general sense, the dense
accumulations of Ophiomorpha in the rocks of the Miami Limestone and Fort
Thompson Formation correspond to moderate to high-energy, shallow-marine,
inner to outer shelf paleoenvironments. These attributes are characteristic of the
Skolithos Ichnofacies (cf. Curran, 2007; MacEachern et al., 2007a ). In the study
area of Cunningham et al. (2004, 2006a,b) shown in Fig. 1 , peloidal and skeletal
wackestones and packstones with a maximum Thalassinoides -dominated ichno-
fabric contribute most to the vertical thickness of the HFC assigned to MIS 7 at
the base of the Miami Limestone. These depositional textures and ichnofabrics
reflect a low- to moderate-energy, shallow-marine, inner shelf paleoenviron-
ment that is attributed to a Thalassinoides -dominated Cruziana Ichnofacies
(cf. MacEachern et al., 2007a,b ). Consistent presence in wells of this ichnofacies
within the HFC assigned to MIS 7 provides assurance in the certainty of
correlation from well to well of this HFC.
3.1.2 Biogenically Altered Surfaces
Paleosols or exposure zones commonly cap some HFCs of the Fort Thompson
Formation and Miami Limestone ( Fig. 3 ). Rhizoliths ( Klappa, 1980 ) are regu-
larly a major element of the paleosols or subaerial exposure zones ( Fig. 4 ), and
thus the common physical characteristics of these paleosols at and near the tops
FIGURE 4 Rhizoliths in a digital optical borehole-wall image and core sample of the Fort Thomp-
son Formation from test corehole G-3794 located in the Biscayne aquifer study area shown in Fig. 1 .
(A) Digital borehole image. (B) Slabbed core sample acquired from the depth interval shown by two
dashed red lines in (A). (C) Cut core taken at dashed red line in (B). Halos of relatively white-
colored, more porous, chalky-textured matrix (e.g., the area between the dashed red lines), perhaps
due to dissolution enhanced by humic acid during decomposition of roots.