Deltas - Trace Fossils as Indicators of Sedimentary Environments - page 491

Chapter 17
Deltas
Nicola S. Tonkin 1,2
Department of Earth Sciences, Memorial University, St. John's, Newfoundland and Labrador,
Canada
1 Corresponding author: e-mail: [e-mail]
2 Current address: Woodside Energy Ltd., Perth, Australia
1. INTRODUCTION
Subaqueous deltaic environments in marine basins are shaped by the interac-
tions of fluvial and marine hydrodynamic processes as well as the geometry
of the recipient basin. Fluvial, tidal, and wave-dominated or -influenced deltas
produce distinctive coastal morphologies, as well as having diagnostic lateral
and stratigraphic facies trends. The matrix of possible combinations of these
parameters is evident in the resultant sedimentology and ichnology. Sedimen-
tological analysis is used as the primary tool to define deltaic deposits and their
subenvironments. Ichnological characteristics may provide an excellent addi-
tional tool in recognition of deltaic subenvironments. Integration of sedimento-
logy and ichnology enables the construction of realistic paleoenvironmental
models, incorporating physico-chemical parameters such as salinity, hydrody-
namic energy, sedimentation rate, oxygenation, substrate consistency, turbidity,
light, and temperature. Such insights can be important in appreciating the facies
shifts associated with deltaic processes that are not evident from the study of
physical sedimentary structures alone. The ichnological characteristics of a del-
taic succession can be used to infer the relative importance of fluvial, tidal, or
wave processes and can aid in recognition of the key stages in the delta devel-
opment ( Galloway, 1975 ). The regressive (progradational) and transgressive
phases of the deltaic cycle are associated with changes in relative sea level
and sediment supply ( Boyd et al., 1992; Galloway, 1975; Roberts, 1997 ).
A delta is defined as the—partially subaerial—site of sediment deposition
where a river enters a standing body of water ( Galloway, 1975 ). Deltas classically
form discrete shoreline protuberances, where the alluvial system entering the sedi-
mentary basin supplies sediment more rapidly than it can be redistributed away
from the distributary mouth, by the predominant basinal processes of waves
and/or tides ( Boydet al., 1992;Galloway, 1975;OrtonandReading, 1993 ).Ancient
deltaic successions, as seen in outcrop and core, are recognized as coarsening- and