Deltas - Trace Fossils as Indicators of Sedimentary Environments - page 503

mudstone matrix are often interpreted to be associated with hiatal “ Glossifun-
gites surfaces” and are characteristic of the Glossifungites Ichnofacies ( Pemberton
and Frey, 1985 ), which are commonly interpreted as markers for key
stratal surfaces of allocyclic significance ( Fig. 2 I; MacEachernet al., 2007c, 2012 ).
4.4 Oxygenation
The oxygenation of both interstitial pore waters and sea-water is relatively com-
mon first-order controls on trace-fossil distribution in subaqueous environ-
ments. Well-oxygenated sea-water may encourage intense bioturbation and
deep burrowing. Deep or shallow infaunal habitation and colonization of the
shallow, mid, and deep tiers (vertical partitioning of the substrate) can often
be used as an indicator of well-oxygenated bottom waters and pore waters in
the substrate ( Bromley, 1996 ).
4.4.1 Lower Delta Plain
Some deltaic environments may have high concentrations of sedimentary
organic matter (e.g., swamps, tidal flats, marshes, lagoons, and bay fills), which
create oxygen-poor pore-water environments. Endobenthic activity in such
organic-rich sediments is seldom limited by the absence of pore-water oxygen,
which can be introduced by bioirrigation from the overlying water column.
Bioirrigation stimulates microbial growth in the burrow wall (see Herringshaw
et al., 2010 ). It seems likely that organisms have endosymbionts that thrive in
low-oxygen sediment (e.g., some echinoids; Bromley, 1996 ) and produce
ichnofabrics with suppressed BI and diversity, independent of the bathymetry
( Ekdale and Mason, 1988 ). Tidal-flat facies containing solely Ophiomorpha
or Thalassinoides may be indicative of a stressed paleoenvironment where
pore-water oxygenation fluctuates with tidal exposure ( Fig. 2 B, C, and I;
Swinbanks and Luternauer, 1987 ).
4.4.2 Delta Front and Prodelta
Deltaic systems, being areas of dynamic coastal processes, are generally
characterized by strong bottom currents. These currents are commonly a
combination of fluvial, wave, and tidal processes, which keep mixed the water
column and bottomwater. Consequently, sea-water oxygenation is not generally
a controlling factor in the occurrence and distribution of trace fossils in deltaic
environments, especially in the delta front and prodeltaic environments.
Localized areas of bottom-water dysoxia (e.g., interdistributary areas) are
associated with a low ichnodiversity (e.g., monotaxic trace-fossil assemblages),
a low BI, and poorly developed tiering profiles. Oxygenated sea-water,