Deltas - Trace Fossils as Indicators of Sedimentary Environments - page 504

however, is the “norm” in all deltaic types, although pore waters in sediments
rich in organic matter may be sulfidic and hostile to endobenthos.
4.5 Substrate Consistency and Benthic Colonization Style
Substrate consistency on the sea floor in ancient deltaic setting is commonly soft
to loose, with occasional soup- and firmgrounds ( Goldring, 1995 ). The coloni-
zation of soft- and looseground substrates in the lower delta plain to prodelta
is interpreted as reflecting relatively continuous deposition or aggradation,
whereas development of a firmground can be indicative of a hiatal surface pro-
duced by erosion or non-deposition in the deltaic complex (e.g., Fig. 2 I; Glossi-
fungites surface). Firmground surfaces in the delta-front setting are commonly
colonized by Thalassinoides and may result from a hiatus or colonization of bur-
ied dewatered cohesive mudstones (e.g., concealed firmgrounds; Bromley,
1996 ). Soupgrounds are generally found in association with fluvial- and tide-
dominated deltas, especially where hyperpycnal flows cause deposition of
fluid-mud.
4.6 Turbidity, Light, and Temperature
Turbidity of the water column is a result of suspended sediment load. A high
turbidity is common in many deltaic environments, including buoyant plumes
emanating from the distributary mouth into the basin. Turbidity may reduce the
BI and diversity in deltaic environments ( MacEachern et al., 2005 ) and can also
reduce light penetration to the sediment/water interface, which may have a feed-
back into microbial food productivity for tracemaking organisms. Seasonal
temperature fluctuations of the water column triggered by flood or changes
in ocean circulation may place stress on tracemakers, reducing the BI and
ichnodiversity (e.g., fluvial-dominated and intertidal deltaic facies). Turbidity,
light, and temperature are not considered major controlling factors for the
ichnological distribution in the deltaic realm.
5. ICHNOLOGY OF DELTAIC END MEMBERS
The complex interplay between fluvial, wave, and tidal processes, and their
fluctuating environmental parameters of hydrodynamic energy and sedimenta-
tion rates are common to all deltas. Subtle variations in the relative parameters
enable characteristic facies successions and ichnological assemblages to be
defined. Salinity profiles down-dip (e.g., a salt-wedge in a tide-dominated delta
or flood discharge in a fluvial-dominated delta) are distinctive for each deltaic
type and their corresponding subenvironment (see Supplementary Table 1: http://
booksite.elsevier.com/9780444538130 and Fig. 3 ). Distributary channels are
commonly fresh to brackish and are characterized by non-bioturbated or sparsely
bioturbated facies. Delta-front facies in all three end-member deltaic types