Deltas - Trace Fossils as Indicators of Sedimentary Environments - page 497

is unclear as to how these data can be used geologically, as most deltaic deposits
are cut by non-contemporaneous distributary channels during progradation
( Zaitlin et al., 1994 ). All distributary channels are, at least periodically, zones
of relatively high, albeit variable, energy. The funnel-shaped morphology of
distributary channels, and their direct connection to the ocean, means that
the currents in the channels are most likely to be tidally modulated ( Brettle
et al., 2002; Dalrymple and Choi, 2007 ). Along-strike variability in deltaic
successions can be highly significant, especially in the more proximal environ-
ments of the lower delta plain, where quiescent interdistributary areas (e.g.,
swamp, salt marshes, lakes, bays, lagoons, or tidal flats) are cut by high-energy
distributary channels, although interdistributary areas of higher energy are com-
mon in wave-dominated systems (e.g., beaches; Boyd, 2010 ).
Typical trace fossils in the high-energy environments of distributary chan-
nels and lower delta-plain deposits are Diplocraterion , Macaronichnus , Sko-
lithos , Arenicolites , and Ophiomorpha ( Fig. 2 A, C, and F). As such, the
deposits have an ichnological assemblage comparable to the Skolithos Ichnofa-
cies of Seilacher (1967) .
4.1.2 Delta Front
At the delta front, tidal and wave energy in the receiving basin significantly con-
trol the sea floor environment, as fluvial energy dissipates. Seaward of the dis-
tributary mouth bar, hydrodynamic energy dissipates rapidly. This dissipation
of hydrodynamic energy results in trace-fossil diversity and intensity Increasing
seaward.
At the distal delta front of fluvial-dominated deltas, reworking of sediment
by tides and waves is low. In wave-dominated delta fronts, physical and bio-
genic sedimentary structures have similar associations as those seen on non-
deltaic coastlines such as beaches and strandplains. In large deltas, such
coastal facies are contemporaneous with, and lateral to, deposition on the active
delta lobe ( Bhattacharya and Giosan, 2003 ).
In the moderately high-energy environments of the delta front, seaward of
the distributary mouth and above storm-wave base, distal mouth-bar deposits
are dominated by the trace fossils of sediment-feeding tracemakers such as
Teichichnus , Asterosoma , Ophiomorpha , Thalassinoides , Rosselia , and Chon-
drites (equivalent to the proximal Cruziana Ichnofacies; Fig. 2 F and I).
4.1.3 Prodelta
Hypopycnal and hyperpycnal flows from the distributary are the primary dis-
persal systems of mud and sand to the prodelta (see Fig. 3 A-C). Delivery of
mud to the distal delta can also be attributed to rapid post-event-bed deposition
in quiescent hydrodynamic conditions, or flocculation and suspended sediment
settling in quiescent conditions (e.g., slack-water tidal systems; Ichaso and Dal-
rymple, 2009; Lamb and Parsons, 2005; McIlroy et al., 2005; Sheremet et al.,