publications > paper > application of carbonate cyclostratigraphy and borehole geophysics to delineate porosity and preferential flow in the karst limestone of the Biscayne aquifer, SE Florida > pore classes
PORE CLASSESPore class I commonly includes the lower part of many of
the upward-shallowing cycles within the Fort Thompson Formation
and upper aggradational subtidal cycle of the Miami Limestone,
where the porosity and permeability are highest (Fig. 3 and Fig. 6; Table 3). Characteristic lithofacies associated with pore
class I are (1) touching-vug pelecypod rudstone and floatstone,
(2) sandy touching-vug pelecypod rudstone and floatstone, (3)
peloidal packstone and grainstone, (4) coral framestone, and (5)
laminated peloid packstone and grainstone lithofacies (Table 3).
Pore types commonly associated with specific lithofacies
include solution-enlarged fossil molds up to pebble size, irregular
vugs of uncertain origin, and molds of burrows or roots, or
irregular vugs surrounding casts of burrows or roots (Fig. 7).
Touching-vugs are the most common type of effective porosity
in this class, but conduit porosity also occurs as bedding-plane
vugs and uncommon cavernous vugs (Cunningham et al.,
2006). Atabular three-dimensional geometry regionally characterizes
the touching-vug flow zones, which are constrained
between cycle boundaries, based on porous-zone mapping in
the Lake Belt area (e.g., Cunningham et al., 2004b, 2004c,
2006). Therefore, accurate cycle correlation can produce a realistic
linkage of permeable or preferential groundwater flow zones. Groundwater flow in the touching-vug flow zones
should not be conceptually viewed as the movement of groundwater
through a system of large-scale pipes or underground
stream conduits, but more of a stratiform passage formed by
coalescence of vugs into a mostly tortuous path for the movement
of groundwater flow from vug to vug (Fig. 3). Figure 6 best exemplifies the stratiform distribution of pore class I,
notably in digital optical borehole images where the darkened
area at the base of high-frequency cycle HFC2e2 represents
touching-vug porosity (Fig. 7). Cunningham et al. (2004b,
2006) showed that pore class I has the highest porosity and permeability
of the three pore classes defined herein.
Usually assigned to pore class III are (1) mudstone and
wackestone, (2) Planorbella floatstone and rudstone, (3) peloid
wackestone and packstone, (4) conglomerate, and (5) pedogenic
limestone lithofacies (Table 3). The first two lithofacies
commonly cap upward-shallowing paralic cycles, and the third
and fourth are representative of the lower aggradational subtidal
cycle of the Miami Limestone (Fig. 3). Porosity types common
to this pore class include thin, semivertical solution pipes, and
fossil molds. The matrix porosity and permeability of these
lithofacies are low (Table 3), and the solution pipes (small-scale
conduits) and fossil molds are typically unconnected. Thus,
these lithofacies tend to retard groundwater movement and are
conceptualized as leaky, low-permeability units (Fig. 3). On a
local scale, however, pore class III can comprise bedding-plane
vugs, which may have sheet-like geometry and could represent
major conduits that are highly permeable.
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U.S. Department of the Interior, U.S. Geological Survey
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Last updated: 12 April, 2007 @ 01:29 PM(TJE)