Deltas - Trace Fossils as Indicators of Sedimentary Environments - page 502

fills with Ophiomorpha ; McIlroy et al., 2005 ). Conversely, distributary channel
facies are characterized by episodic high rates of sedimentation and overall low
to moderate net accumulation, comprising a highly variable trace-fossil distri-
bution with dominantly non-bioturbated beds.
4.3.2 Delta Front and Prodelta
Sedimentation rates are typically highest in distal mouth-bar deposits and
within terminal distributary channels ( sensu Olariu and Bhattacharya, 2006 )
of the proximal delta-front facies. In areas with rapid deposition, the coloniza-
tion window is open only for short periods of time, inhibiting colonization of the
ancient sea floor and establishment of a benthic epifaunal and infaunal assem-
blage. Net sediment accumulation on the delta front is typically rapid, succes-
sions are often devoid of trace fossils or sparsely bioturbated with low-diversity
assemblages, and primary sedimentary fabrics remain intact and preserved.
Limited bioturbation may in part be a result of high sedimentation rates but also
of paleoenvironmental stress (e.g., salinity fluctuations) at the distributary
mouth. Conversely, in areas of infrequent event-bed deposition on the distal
delta front and the prodelta, BI and ichnodiversity are higher. The colonization
window in the distal prodelta is usually long enough open for biogenic rework-
ing of sediments. In the predominantly quiescent setting of the distal delta front,
bioturbation commonly disturbs and largely obscures primary sedimentary
structures, reflected in intense bioturbation. The low paleoenvironmental stress
in the distal delta front and the high organic content commonly result in diverse
ichnological assemblages of trace fossils.
4.3.3 Event-Bed Deposition and the Colonization Window
Fluvial input into the marine basins can be variable and linked to seasonal
storms. In such cases, deposition is generally episodic and event-bed deposition
is likely to dominate the delta front and prodelta settings. This inter-bedding of
fair-weather and event-bed deposits may produce “lam-scram” fabrics of alter-
nating intense to low bioturbation ( Howard, 1972 ). The thickness of event beds
is an important limiting factor on benthic ecology, as it may smother existing
infaunal communities, effectively causing defaunation if the pre-event commu-
nity is unable to escape to the new sediment/water interface ( Pollard et al., 1993;
Wheatcroft and Drake, 2003 ). If colonization from below is not possible, post-
depositional recolonization by juveniles or adult organisms is possible ( Taylor
and Goldring, 1993 ), although the new sea floor substrate may not be entirely
hospitable to deposit-feeding organisms due to a lack of deposited organic mat-
ter (see discussion in Herringshaw et al., 2010 ). Macaronichnus , Skolithos ,
Diplocraterion , Arenicolites , Rosselia , and Cylindrichnus are often found in
association with shifting substrates and event-bed deposition ( Fig. 2 G; Ekdale
and Lewis, 1991; Gingras et al., 1998 ) and have the ability to adjust to the new
sediment/water interface. Sandstone-infilled Thalassinoides burrows in a