Porosity and Permeability in Bioturbated Sediments - Trace Fossils as Indicators of Sedimentary Environments - page 812

FIGURE 2 Fluid-flow behavior in dual-permeability biogenic fabrics. (A) Slabbed core image
showing trace-fossil-associated dolomite in a limestone matrix. (B) Schematic sedimentary medium
with bioturbation shown as darker gray. (C) Within dual-permeability flow media, fluid is advected
through both the trace fossils and the matrix (dominantly by advection). The lower permeability
matrix contributes to flow only via diffusion.

fluids transported within dual-porosity media are not likely to move uni-
formly. The flow follows preferentially higher permeability beds in which
the flow paths are variably tortuous, depending on to the burrow morphology.
Notably, flow within the lower permeability rock may move 10-100 times
more slowly than in the higher permeability burrowed zones. With regard
to cryptically bioturbated intervals, where the tracemaking organism is
< 1 mm in size, reduction of heterogeneities commonly occurs and k v typi-

cally becomes equal to k h .
In general, (bioturbate) dual porosity is either created by animals moving
through the sediment, or is the result of sand-dwelling animals that selectively
ingest and rework the sediment. Therefore, dual porosity is most commonly
associated with cryptic bioturbation, the presence of Macaronichnus , and in
bioturbated sandstone that contains Thalassinoides , Ophiomorpha , Skolithos,
or Arenicolites (discussed below). Depositional settings that are associated with
biogenic dual-porosity flow media thus include foreshore, sand-dominated
intertidal flat, tidal bars, and lower shoreface deposits ( Fig. 3 ).