Alteration of Original Porosity and Permeability in Continental Deposits by Soil Biota: Concepts and Examples
Hasiotis, Stephen T.; Halfen, Alan F.; Wasserman, Hannah N.; Hirmas, Daniel; Counts, John
Alteration of original porosity and permeability of sedimentary facies deposited in terrestrial and aquatic environments can be a complex pattern to interpret for several reasons, which are unique to the continental realm. To better understand the effects of biota-sediment interactions on porosity, permeability, and hydraulic conductivity, bioturbated sedimentary fabrics must be interpreted based the integrative framework of concepts developed in ichnology, hydrogeology, sedimentology, and pedology. Animals move sediments through diffusive (local) and advective (nonlocal) excavation methods. Most animals use both methods to move sediment. In some cases different stages of an animal's life cycle use different excavation methods that result in one or both sediment distribution patterns. The depth and distribution of these bioturbation patterns is controlled by the groundwater profile. Tiering of traces is particularly prominent in terrestrial settings because the vertical distribution of soil biota is controlled by the depth and moisture content of the vadose zone (pore space dominated by air), whose traces can be assigned to one of four moisture regimes: epiterraphilic, terraphilic, hygrophilic, and hydrophilic. Aquatic environments exhibit the shallowest tiering depth, which is restricted to the hydrophilic zone. Burrowing depth in freshwater rivers and lakes is controlled mostly by bottom-water oxygen, sediment redox conditions, and the size and ability of an organism to modify its microenvironment. Plant roots also follow similar penetration patterns as do animals, with the shallowest being in aquatic settings and deepest in well-aerated vadose zones. Soil formation and, hence, paleosols, occur where sediments are subaerially exposed and contain a vadose zone. Bioturbation in a paleosol profile is an expression of the balance between deposition and pedogenesis, which can result in simple, compound, composite, or cumulative paleosols. The combination of sedimentation and pedogenesis, including groundwater-influenced bioturbation, can act to enhance or destroy paleosol horizons; these processes ultimately determine the ichnopedological fabrics (IF--trace fossils and pedogenic features) that are preserved in the stratigraphic record. The degree of interconnectedness of macrochannels and macropores produced by IF reflect the interplay between advective and diffusive pedoturbation, sedimentation rates, and groundwater hydrology.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013