Deltas - Trace Fossils as Indicators of Sedimentary Environments - page 496

diversity of behavior and it is extremely useful to document changes throughout a
deltaic succession from, for example, prodelta to delta front. The presence or
absence of monotaxic assemblages is also highly significant. Ichnofacies com-
posed of a single ichnotaxon is usually indicative of a “stressed” paleoenviron-
ment, associated with high energy, high rates of sedimentation, and lowered
salinity (e.g., distributary channels and mouth bars at the proximal delta front).
4. CONTROLS ON DELTAIC ICHNOLOGY
The controls on benthic ecology inmarine subaqueous environments are physico-
chemical parameters including (1) hydrodynamic energy (fluvial, tidal, and
wave); (2) salinity; (3) oxygen; (4) sedimentation rate; (5) substrate consistency;
and (6) turbidity, light, and temperature. Biological parameters include the sali-
nity tolerance of the tracemaker, burrow morphology, duration of colonization
window, food supply, and trophic/feeding mode (see reviews in Gingras et al.,
2011;MacEachern et al., 2007b ; McIlroy, 2004a; Taylor et al., 2003 ). Reworking
of fluvial-derived sediments at the coastline by oceanographic processes (e.g.,
wave and tidal) controls the intensity, diversity, and distribution of trace fossils
in deltaic environments. The main paleoenvironmental controls on bioturbation
in deltaic settings are sedimentation rate, salinity, and hydrodynamic energy.
4.1 Hydrodynamic Energy and Response of Bioturbators
The distribution of trace fossils is intimately linked to hydrodynamic energy in
all depositional settings, including deltaic systems. High sedimentation rates
and erosion are commonly associated with intense hydrodynamic energy in dis-
tributary channel and delta-front facies. These facies are characterized by ver-
tical burrows conventionally interpreted as being made by suspension-feeding
organisms (e.g., Diplocraterion and Thalassinoides ; Fig. 2 G and I). Trace-fossil
diversity and intensity typically increase away from the source of intense hydro-
dynamic energy (e.g., distributary mouth and shoreface). High-energy suben-
vironments are associated with the largest grain size, so their geometry and
architecture are of particular interest in hydrocarbon reservoir characterization
( Moiola et al., 2004 ). Rapid sedimentation associated with event beds can take
place in otherwise quiescent conditions, for example, hyperpycnal-derived beds
of sand or mud in fluvial-dominated deltas ( Olariu et al., 2010 ) and storm-
induced turbidite-like sand-rich beds in the prodelta of (wave-dominated) deltas
( Pattison, 2005 ).
4.1.1 Lower Delta Plain
In the lower delta plain, distributary channels are the conduits through which
fluvial sediment is delivered to the receiving sedimentary basin. The branching
patterns and abundance of channels at the lower delta plain to proximal delta
front vary between delta types ( Olariu and Bhattacharya, 2006 ), although it