Carbonate Aquifers - Trace Fossils as Indicators of Sedimentary Environments - page 852

The LBM intrinsic permeability values ( Fig. 5 ) are as much as two orders of
magnitude higher than the intrinsic permeabilities reported for rocks and uncon-
solidated deposits by Freeze and Cherry (1979 : table 2.2 ). This emphasizes the
potential for exceptionally permeable macroporosities related to Ophiomorpha -
dominated ichnofabrics in carbonate aquifers ( Fig. 6A and 6B).
In some cases, tiering has impacted the development of macroporosity and
high permeability within the carbonate rocks of the Biscayne aquifer, as in the
oolitic barrier bar within the upper part of the Miami Limestone HFC assigned
to MIS 5e ( Figs. 7 and 8 ). Tiering is the vertical partitioning of endobenthic
communities in the modern and fossil record ( Bromley, 1996 ). In parts of
the oolitic barrier bar, an upward decrease in the inner diameter of Ophiomor-
pha burrows and an upward decrease in the connectedness of Ophiomorpha -
related macroporosity have been observed in outcrops and well bores. For
the example shown in Fig. 7 , the inner diameters of Ophiomorpha burrows aver-
age about 1.5 cm near the base of the HFC assigned to MIS 5e, whereas in the
middle to upper part, the diameters average about 0.5 cm ( Fig. 7 ). It is possible
that larger and more energetic callianassids occupied the deep-tier sediment,
FIGURE 6 Cropped computer renderings of CT scans of carbonate samples with ichnogenic
macroporosity and permeability. The light-blue areas in (A), (C), and (E) represent rock matrix and
renderings used for LBM analyses (see Table 1 ). (A) Rendering of Pleistocene Miami Limestone
outcrop sample SHR-1 ( Cunningham and Sukop, 2011 ). (B) Complement of rendering in
(A). Macropores are light-blue areas. (C) Rendering of Cretaceous sample LTX-4. Outcrop shown
in Fig. 9 . (D) Complement of rendering in (C). Macropores are light-blue areas. (E) Rendering of
Cretaceous outcrop sample TXEA-1 ( Fig. 10 E). (F) Complement of rendering in (E). Macropores
are light-blue areas.