Deltas - Trace Fossils as Indicators of Sedimentary Environments - page 501

9780444538130 ;reviewsin Buatois et al., 2005; Pemberton et al., 1992 ). In
fluvial-dominated deltas, salinity decreases markedly due to the dominance of flu-
vial outflow, resulting in freshwater, or brackish-water, lower delta plain ichnolo-
gical assemblages with low BI (e.g., BI

0-2; see Supplementary Table 1: http://

¼

booksite.elsevier.com/9780444538130 ) .
Depauperate assemblages are commonly considered to be indicative of
brackish-water settings, but an integrated approach incorporating careful sedi-
mentary facies analysis along with consideration of ichnofaunal distributions is
probably needed (cf. McIlroy, 2004a ). Persistently brackish paleoenviron-
ments, in association with deltaic deposition, are only really expected in distri-
butary channels, near-channel paleoenvironments, and potentially in restricted
lagoons ( Bhattacharya, 2010 ).

4.2.3 Prodelta
The salinity in the prodeltaic settings is predominantly marine, with episodic
brackish-water conditions during sediment-laden discharge from riverine outflow,
particularly in fluvial-dominated deltaic systems. This gives prodeltaic facies a dis-
tinctly marine ichnological assemblage with high diversity and variable BI.

4.3 Sedimentation Rate and the Colonization Window
The deceleration of fluvial currents upon meeting standing marine waters
causes rapid deposition at the basin margin to produce deltas. The fate of that
sediment is variable, from instantaneous and permanent deposition to erosion
and redistribution by waves and tides, sometimes in a multicyclic fashion.
Deltaic coastlines are all characterized by comparatively high rates of sediment
supply. The sediment source in deltas is characteristically fluvial, and rates of
shoreline progradation are largely controlled by a combination of sediment sup-
ply (usually high), accommodation space (usually low), and the relative impor-
tance of river, tide, and wave processes acting at the coastline ( Orton and
Reading, 1993; Roberts, 1997 ).
Shallow-marine successions with intense bioturbation indicate slow rates of
sedimentation relative to the rate of bioturbation. In order to assess the impor-
tance of any given highly bioturbated bed or bedset, it is important to be able to
calibrate the intensely bioturbated unit with its unbioturbated counterpart.
In deltaic successions, some facies are persistently highly bioturbated, indicat-
ing either low rates of sedimentation or optimal conditions for bioturbation, or a
combination of both. In deltaic depositional systems, the distal delta front to
prodelta and interdistributary bay fills commonly meet these criteria.

4.3.1 Lower Delta Plain
In the lower delta-plain facies, sedimentation rates are highly variable. Interdis-
tributary areas have low net accumulation rates and may have high to complete
BI, but many contain only low-diversity assemblages (e.g., abandoned channel