Deltaic Environments (MS:120)
Liviu Giosan
Woods Hole Oceanographic Institution, Woods Hole, MA, USA
and
Steven L. Goodbred, Jr.
Earth and Environmental Sciences, Vanderbilt University, Nashville, TN, USA
Author Contact Information:
Liviu Giosan
Woods Hole Oceanographic Institution
360 Woods Hole Rd.
Woods Hole, MA 02543 USA
tel: 508 289 2257
email: lgiosan@whoi.edu
Steven L. Goodbred Jr.
Earth & Environmental Sciences
Vanderbilt University
Nashville, Tennessee 37235-1805 USA
tel: 615-343-6424
email: steven.goodbred@vanderbilt.edu
Keywords: clinoform, continental margin, fluviodeltaic, glacioeustasy, river delta, sediment
discharge, shelf
Synopsis: Deltas are one of the most environmentally and economically important coastal
sedimentary environments, hosting significant oil, gas, and groundwater resources. Historically,
deltas have also been prized by human civilizations for their high natural and agricultural
productivity, rich biodiversity, and for the abundance of waterways that provide easy means of
transportation. Controlled in large part by sea level, climate, and tectonics, Quaternary delta
systems have experienced continuous cycles of formation, change, and destruction. Research in
recent decades has discovered new deltaic deposits from the Quaternary, which have yielded new
models for delta formation under a broad range of climatic and sea-level conditions.
Introduction
Definition
Deltas are constructional coastal landforms with both subaerial and subaqueous components that
are genetically associated with rivers discharging into a standing body of water, such as a lake,
estuary, lagoon, sea, or the open-ocean shelf (Fig. 1). A delta is usually built by a single river,
but exceptions exist (e.g., Ganges-Brahmaputra; Tigris-Euphrates). Reworking of sediments
accumulated at the river mouth by basinal processes (e.g., waves, tides, currents) should be slow
enough to allow delta building to proceed. The river is the main source for sediment delivered to
the delta, although in some wave-dominated settings (e.g., Danube, Sao Francisco, Rio Doce), a
significant portion may be transported by wave-driven currents from remote sources (Fig. 2).
Importance and significance of deltas
Deltas are amongst the most environmentally and economically important coastal sedimentary
environments. Significant oil, gas, and groundwater resources have been exploited from deltaic
formations. Historically, modern deltas have been prized by human civilizations for their high
natural and agricultural productivity, rich biodiversity, and for the abundance of waterways that
provide easy means of transportation. Deltas are ubiquitous: 21 of the world’s 25 largest rivers,
which deliver 31% of total fluvial sediment reaching the ocean, have formed well-expressed
deltas at the coast (Meade, 1996). As a result, ~ 25% of the world’s population lives within
deltaic and wetland coastal systems (Syvitski et al., 2005), some in large urban centers such as
Shanghai, Bangkok, Dhaka, Saigon, Rangoon, Lagos, Alexandria, Cairo, and New Orleans.
River deltas act as filters, repositories, and reactors for a suite of continental materials including
sediments, organic carbon, nutrients, and pollutants, significantly affecting both the regional
environment at the continent–ocean boundary as well as global biogeochemical cycles (e.g.,
McKee et al., 2004). However, deltas are fragile geomorphic features that can change
dramatically with modest modifications in their boundary conditions. Currently, the largest threat
to the world’s deltaic systems is the trapping of sediment behind dams erected on delta-building
rivers and their tributaries. Deltas also face the threat of accelerated relative sea-level rise due to
climate change and fluid extraction.
History of delta research
The term ‘delta’ was introduced by Herodotus (5th century BC), by analogy to the Greek letter ∆,
to describe the shape of the lowlands between the forking distributaries of the River Nile in
Egypt (Fig. 1g). In 1832, in his Principles of Geology, Charles Lyell first defined the modern
geological meaning of the term ‘delta’ as “an alluvial land, formed by a river at its mouth,
without reference to its precise shape.” The first comprehensive review of modern deltas was
written by Credner in 1878, where the author correctly identified many of the processes affecting
the form, structure, and evolution of deltas. Early landmark papers on deltas also include those of
Gilbert (1885), Fisk et al. (1954), and Wright and Coleman (1973), among others.
In the 1980s, research on deltas moved from the development of depositional models based on
modern highstand deltas, which are ultimately controlled by variations in sediment supply by the
delta-building rivers, to sequence stratigraphic interpretations of deltaic evolution through
multiple sea-level cycles (e.g., Van Wagoner et al., 1988). More recent research on modern
deltas has been spurred by concerns related to human influences on the larger riverine systems,
either directly through damming and water consumption, or indirectly via climate changes.
Historical stages in deltaic research can be followed in successive comprehensive volumes edited
by Morgan (1970), Broussard (1975), Coleman (1981), Oti and Postma (1995), and Giosan and
Bhattacharya (2005), as well as periodic review papers such as Bhattacharya and Walker (1992).
River Deltas
Controls and processes
The stratigraphic architecture of deltas is the end-result of complex interactions among upstream
catchment processes that regulate the location and magnitude of the fluvial sediment discharge,
and downstream basinal controls that include the shape and quantity of accommodation space for
sediment accumulation and the type and energy of coastal processes that redistribute these
sediments. Together, all of these controls determine the mode and degree of partitioning of
sediment between the delta and the wider receiving basin (e.g., Wright and Coleman, 1973).
The sediment discharge of a river is determined by (1) climate that regulates the precipitationevaporation characteristics of the watershed, and thus water flow, (2) the nature and abundance
of vegetation that may inhibit erosion, and (3) the tectonic regime and lithology that determines
elevation, slope, and degree of erodability of rocks in the watershed. Geology also controls the
water gained or lost by a river as groundwater. Accommodation space in the receiving basin is
dependent on (1) the relief characteristics of the basin, such as the extent of the shallow sector of
the basin (or shelf) and the degree of connection to the open ocean (e.g., a closed lake, indented
estuary, or a straight coast). Such basin characteristics may also change drastically as variations
in base level expose more or less of the margin. Base-level changes during the Quaternary were
predominantly driven by the growth and decay of continental ice sheets, but in the case of
enclosed basins such as the Black and Caspian Seas, major base-level changes were forced by
climatically driven fluctuations in their water budget. Additional components of relative baselevel change are caused by tectonics, compaction, and isostasy.
Sediment in suspension is discharged as a plume at the river mouth, which is the single most
important location for partitioning of sediment between the delta and the basin. Some rivers also
transport a significant fraction of coarse bedload sands to the coast, where these less mobile
sediments are preferentially stored within the delta edifice. As the river plume enters the
receiving basin, flow expansion and deceleration exerts a strong control on river-mouth
sedimentation, which is dependent on the outflow inertia, turbulent bed friction, and plume
buoyancy. The latter may be positively, neutrally, or negatively buoyant, depending on the
density contrast between the plume and basin waters. Waves and tides are also important in
controlling deposition at a river mouth, especially in high-energy coastal settings. Tidal and
wind-driven currents, as well as Coriolis rotation, may strongly influence the behavior and path
of the plume after exiting the river mouth region. The relative importance of these processes, as
well as their history (i.e., frequency of occurrence and intensity), are also important in
determining the patterns of sediment deposition across the delta and, ultimately, its stratigraphic
architecture.
Delta sediments, morphology, and morphodynamics
Deltas are complex sedimentary systems comprising a multitude of depositional environments.
At the apex of the delta plain, which is the subaerial part of a delta, the river usually bifurcates
into distributaries that, in many cases, can build their own delta lobes (Fig. 2). The number of
lobes tends to decrease in wave-dominated settings where the number of distributaries is
minimal. Lobes can rarely be distinguished in settings where tides are strong, because tidal
currents favor channel stability and suppress avulsions. The entire delta can sometimes change
location through large-scale avulsions to build separate sub-delta complexes. This is the case for
the Huanghe (Yellow) River, which completely switched its course at least twice to build deltas
in both the Bohai and South Yellow seas (Saito et al., 2001). The Mississippi River has also built
six separate sub-deltas comprising sixteen individual lobes (see review in Roberts, 1997).
Based on general morphology of the subaerial delta plain, deltas have typically been classified as
fluvial-, wave-, or tide-dominated according to the dominant processes affecting sediment
delivery, deposition, and dispersal (Fig. 3; Galloway, 1975; Orton and Reading, 1993). A typical
fluvial-dominated delta has an elongate to lobate plan-view morphology with highly crenulated
shorelines; mouth bar and channel fill sands are interspersed within fine-grained deposits of the
interdistributary region. In contrast, classic wave-dominated deltas assume a cuspate morphology
and are mainly composed of beach-ridge sands. Tide-dominated deltas, which appear mostly
irregular in plan view, are primarily composed of estuarine and tidal shoals, or islands, flanked
by mudflats. However, a continuum of settings with overlapping processes rarely makes this
classification operational at a scale larger than an individual delta lobe (Bhattacharya and
Giosan, 2003). Even at the lobe-scale, an in-depth look at wave influence on deltas shows that
morphology and facies distribution is dependent more on the influence of the wave-driven
longshore sediment transport relative to fluvial discharge, rather than on wave energy itself (Fig.
4). In multi-lobe deltas, one lobe may assume a vastly different morphology than an adjacent
lobe. Of the two youngest lobes of the Danube delta, the Chilia, which is built with ~ 60% of the
river-sediment discharge, is fluvial-dominated, whereas the St. George is clearly wavedominated (Fig. 2). Temporal variability in sediment discharge or basin energy can also result in
drastic morphological changes in a delta/lobe (Giosan et al., 2005). The Po delta lobes had been
wave-dominated before a drastic increase of sediment discharge during the Little Ice Age
(~1450-1850 AD) resulted in fluvial-dominated outbuilding phase (Correggiari et al., 2005) ).
Even without changes in total sediment discharge, the developing Chilia lobe of the Danube
delta started to show morphological modifications typical of wave-influenced settings (e.g.,
straightening of the shoreline, buildup of barrier spits and islands) due to a decrease in sediment
delivered by the river per unit shoreline (Fig. 5).
Delta morphology is also greatly controlled by geometry of the receiving basin (Wright and
Coleman, 1973). Because tides are absent in lakes, lacustrine deltas (Fig. 1a) are either fluvially
dominated, when they build in shallow, low-energy lakes, or may become wave-dominated in
larger lakes (Fig. 1b), where a longer fetch allows larger waves to form. Bayhead and lagoonal
settings are typically low-energy environments that favor fluvial and tidal influences over the
effect of waves (Fig. 1c, d, e). Morphology of deltas building along straight, open coasts is
strongly dependent on the shelf configuration (i.e., width, steepness), which changes not only
with the type of continental margin (i.e., active vs. passive) but also with sea-level changes that
regulate the vertical accommodation space and the degree of coastal indentation. For example, as
deltas prograde toward the middle and outer shelf, their morphology may be affected by
increasing wave influence associate with deeper nearshore waters. In contrast, a broad, lowgradient inner-shelf can attenuate incident waves and amplify tides, thereby favoring more
tidally influenced delta systems.
In cross section, the idealized delta morphology and stratigraphy has been described as a
prograding clinoform composed of an upward coarsening facies succession that starts with a
basal muddy prodelta, progresses into a sandy delta front and is topped by texturally variable
delta plain deposits (Fig. 6). The subaqueous portion of a delta comprises both the distal prodelta
and the steeper, prograding delta front, whereas the delta plain is predominantly subaerial. The
muddy prodelta itself often acquires a “clinoform” geometry, leading to a compound clinoform
architecture for the delta that consists of a nearshore sandy clinoform associated with the
shoreface and an offshore muddy clinoform (Fig. 7) associated with the prodelta (Kuehl et al.,
2005). Development of prodelta clinoforms is not universal and appears to be a function of
fluvial input and basin hydrodynamics (Swenson et al. 2005). In such settings, gravity-driven
flows of suspended mud commonly feed the growth of prodelta clinoforms in the presence of
episodic or periodic high-energy processes, such as spring-neap tidal cycles or during storm
events.
Morphodynamic response in deltas involves feedbacks between the evolving morphology of a
delta and its fluvial and basinal hydrodynamics. Early models identified rotation of the shoreline
as responsible for changes in longshore drift along deltaic coasts and affecting the rate of
progradation. Asymmetry of the wave climate increases the shoreline instability on the downdrift
delta wing, promoting development of spits detached from the main delta coast (Bhattacharya
and Giosan, 2003). Hydrodynamic-morphodynamic feedback loops have also been identified in
relation to bedload sedimentation at river mouths. The hydraulic groin effect of river plumes is
accompanied by the groin effect of the subaqueous delta or of the delta plain, blocking longshore
drift at the river mouth (Giosan et al., 2005). On the downdrift wing of asymmetric wavedominated deltas, periodic emergent barrier islands provide steep, continuous shorefaces for the
longshore drift, leading to rapid expansions of the subaqueous delta in the alongshore direction.
Deltas in the Quaternary
Whereas waves and tides are key controls of deltaic systems over decades to centuries, delta
evolution at longer timescales is most significantly controlled by (1) sea level, (2) climate, and
(3) tectonics. Change in these first two controls, sea level and climate, are hallmarks of the
Quaternary period and its succession of glacial and interglacial phases. As a consequence, deltaic
systems have effectively experienced continuous cycles of formation, change, and destruction
throughout the Quaternary. Because the major controls of sea level, climate, and tectonics are
interrelated, their relative impacts on deltaic systems remain unclear (Goodbred, 2003).
Nevertheless, traditional models posit that the development of major deltaic systems is restricted
to periods of sea-level highstand, with flooded continental margins and slow to moderate rates of
sea-level rise. However, in the past few decades Quaternary deltaic systems have been
discovered at a variety of shelf positions and been linked to numerous states of sea-level change.
In the past two decades, the advent of multibeam sonar imaging and improved seismic reflection
processing has allowed such deltaic shelf deposits to be discovered. A growing knowledge of
climate history has in many cases provided explanations (e.g., high sediment supply due to
increased precipitation, geomorphic landscape disequilibrium) for delta formation even under
rapid rates of sea-level rise.
Influence of climate on deltas
Deltas can largely be considered a function of (1) riverine water and sediment discharge to the
continental margin and (2) the interaction of these fluxes with coastal and marine processes. In
this way, climate plays a major role in both the catchment-based forcing of sediment production
and fluvial transport and the modulation of sea level and coastal weather conditions in the
receiving basin. As such, there is strong climatic overprinting on deltaic systems, but precise
signals are difficult to quantify because of complex feedbacks and secondary influences.
In the terrestrial realm of a fluviodeltaic dispersal system (i.e., the fluvial catchment) perhaps the
most prevalent role that climate plays lies in controlling precipitation. With regard to sediment
production, precipitation is a primary control on glacial and fluvial incision, weathering, and
slope failure. Although highly variable, sediment is readily produced across a range of climates,
from arid to humid and polar to tropical. However, one of the main factors that differs among
these settings in terms of climate is runoff (= precipitation – evapotranspiration) and thus the
ability to transport sediment to the delta. In other words, sediment can be produced in many
settings, but is most efficiently transported given sufficient water discharge, which is primarily
controlled by climate.
Other important characteristics of precipitation include its phase (liquid vs. frozen), seasonality,
and episodicity. First, the phase of precipitation controls the relative role of glacial and fluvial
erosion in sediment production, as well as the timing and nature of water discharge (i.e., the
hydrograph). Each affects sediment-load grain size and weathering characteristics and, hence, the
downstream development of delta stratigraphy. Second, the seasonality of river discharge affects
the relative interaction of fluvial and marine processes, whereby strongly seasonal systems are
subject to specific periods of river and marine-dominated conditions during the year. Similarly,
the episodic discharge typified by storm-dominated or semi-arid river systems leaves a strong
imprint on deltas, because sediments are delivered in large but infrequent pulses. Finally,
precipitation exhibits a strong secondary control on delta systems through the type and density of
catchment vegetation, which affects runoff via overland flow and evapotranspiration rates as
well as soil erosion rates.
From the marine side of a delta, climate change plays the primary role in modulating eustatic sea
level through the orbitally controlled advance and retreat of continental ice sheets. This waxing
and waning of ice sheets, a defining characteristic of the Quaternary period, has forced repeated,
generally rapid, large-magnitude cycles of sea-level change. For delta systems, being principally
coastal features, such glacioeustatic changes in sea level have been a first order control on their
development. In this way, traditional conceptual models suggest that prominent delta systems
form only under relatively stable sea-level conditions and more specifically during highstands
when fluvial sediment is likely to be trapped on the inner shelf. In contrast, during lowstands of
sea level it was considered that most fluvial sediment was discharged beyond the shelf edge,
either by off-shelf transport of gravity flows or through bypassing via canyon systems.
Intervening periods of sea-level rise were assumed to preclude the development of deltas because
of rapid coastal transgression.
Site cases from the Quaternary
Until recently, few studies have specifically investigated climate signals and impacts in delta
systems. The current knowledgebase on this subject comes from several modeling groups, some
field studies, and also by revisiting existing data with a better understanding of climate change
provided by secular records (e.g., loess deposits, speleothems, lacustrine sediments). Although
the site cases discussed below are not exhaustive, two relevant trends emerge. First, the number
of modeling studies equals or exceeds those based on field investigations, and second, the
majority of delta systems in which climate has been investigated are located in the subtropics
(the Rhine/Meuse a notable exception). Subtropical systems tend to be highly seasonal, receiving
nearly all of their precipitation during the wet summer monsoon when the intertropical
convergence zone (ITCZ) is most proximal. At orbital timescales, both the position and strength
of the ITCZ varies in response to global boundary conditions (i.e., extent of continental ice
sheets) and maximum summer insolation, respectively. Thus, such systems have exhibited
among the most prominent responses to Quaternary climate variability.
Yellow River Delta
Extensive databases from a few areas of the world have led to the discovery of significant deltaic
deposits on the middle and outer portions of the shelf (see e.g., Fig. 7). In the North Yellow Sea,
a compilation of seismic data by Liu et al. (2004) detailed a massive deltaic clinoform
comprising approximately 400 km3 of sediment from the Yellow (Huanghe) River. The
Shandong mud wedge, as the feature is known, is 20-40 m thick and situated on the mid shelf at
water depths of 30 to >50 m. Prominent within the deposit is a marine flooding surface that
indicates the delta formed in two separate stages. Dating of the two phases of transgression and
deposition suggest that these responses are associated with rapid changes in the rate of sea-level
rise during deglaciation, perhaps related to meltwater pulses. In such case, brief but rapid
transgressions were followed by progradation of the Yellow River delta, infilling
accommodation space generated by the transgression.
This early Holocene pattern of Yellow River delta formation challenges traditional models in
several ways. First, the growth occurs during a period of unstable sea level punctuated by sudden
changes, yet the location of primary deposition remains largely unchanged during this time. Liu
et al. (2004) ascribe this in large part to the stability of oceanographic circulation patterns in the
North Yellow Sea. Second, size and thickness of the delta far exceed that expected for conditions
of rapid post-glacial sea-level rise, when models might suggest formation of a thin transgressive
sand sheet. In this case, post-glacial transgression in the early Holocene coincided with
intensification of the Asian monsoon, and an apparent increase in fluvial sediment discharge.
The notion that climatically induced increases in sediment discharge may be sufficient to force
delta progradation during rapid sea level opens many new directions for research on how, where,
and when such climate-sea level-delta interactions occur.
Ganges-Brahmaputra River Delta
The Ganges and Brahmaputra rivers drain the foreslope and backslope of the Himalayan front
range, respectively. The two rivers coalesce near the Bengal margin to form the world’s largest
subaerial delta and deep-sea fan system. The rivers are also strongly driven by the summer SW
monsoon, when over 80% of water and 95% of sediments are discharged in the four to five
month wet season. Given the global significance of the Himalayan/Tibetan uplift and its
associated drainage systems, many portions of the Ganges fluvial delta system have been
independently investigated in the past 15 years, revealing important variations in glacial activity,
alluvial fan and floodplain development, delta evolution, and regional oceanography. Taken
together, the observed patterns show the immense Ganges dispersal system to respond to
millennial-scale (< 104 yr) climate change in a system-wide and largely contemporaneous
manner, and that major sedimentary signals are transferred rapidly from source to sink with little
apparent attenuation (Goodbred, 2003).
Some of the major signals of climate change recorded in the delta stratigraphy include (1) a thick
(20-30 m) coastal mangrove facies that was deposited during rapid sea-level rise in the early
Holocene and (2) a massive volume of sediment trapped during post-glacial transgression,
reflecting a sediment discharge at least 200% higher than the world’s largest modern load of 1
billion tonnes (Goodbred and Kuehl, 2000). This high discharge period corresponds to the early
Holocene hypsithermal, when peak regional insolation supported a stronger than present
monsoon and associated precipitation. In contrast, signals recorded in the delta during the Last
Glacial Maximum (18-20 ka) reflect aridification and very low river discharge. Carbonate ooids,
typical of shallow, evaporative tropical waters, formed at this time on the Bengal shelf directly
adjacent to the rivermouth. Similarly, reconstructed planktonic foraminifera assemblages from
the northern Bay of Bengal (the Ganges receiving basin) indicate peak surface-water salinities
and apparently arid conditions at the LGM. These records from the delta and margin correlate
well with geomorphic and stratigraphic results from the fluvial catchment, and together reflect
the dominance of climate change in controlling this large fluviodeltaic system during the
Quaternary (Goodbred, 2003).
Colorado, Brazos, and Trinity deltas (Gulf of Mexico)
These three rivers comprise a series of small catchments draining the semi-arid to moderately
humid hills of central Texas, USA and discharging to the northern Gulf of Mexico (Anderson
and Fillon, 2004). Presently, sediment discharge is relatively small and delta progradation is
driven by episodic floods associated with humid phases under El Niño conditions. However,
during midstands and lowstands of sea level in the Late Quaternary, these drainage basins
coalesced to form expansive fluviodeltaic sequences on the middle and outer shelf (Fig. 8). The
size of these shelf deltas and their connection to underfit streams occupying large fluvial valleys
reflect formation under higher-than-present discharge, implying significant changes in past
climate.
On the Texas shelf, the position and architecture of relict deltas also indicate their formation
during a variety of sea-level stages and transitions. Seismic-based reconstructions of the shelf
deposits reveal large, forced-regressive sequences formed during the falling stages of sea level at
Marine Isotope Stages 5 and 3 (Fig. 8; Anderson and Fillon, 2004). During these stages, the
relative highstand and midstand of sea level, respectively, placed the deltaic lobes near the
modern inner to middle shelf. Overlying these regression-phase deltas are a group of smaller
backstepping deltas formed during rapid post-glacial transgression after the Last Glacial
Maximum. Though smaller than the earlier delta complexes, formation of these systems under
rapid rates of sea-level rise was in large part a consequence of a more humid climate and
increased sediment discharge in the early Holocene. Together, delta sequences of the Texas shelf
have formed under a variety of climatic and sea-level conditions, indicating the critical role that
both of these forcings play.
Modeling Studies
Quaternary climate change has been recognized as an important control on fluviodeltaic systems.
However, it has been difficult to confidently link observed stratigraphic and sedimentological
signatures to climatic forcings, given the strong overprinting of sea level and marine processes
on delta morphology. Such limits of field investigations are reflected in the relatively limited
number of studies, many of which are discussed above. However, the exponential increase in
computing power has allowed major advances to be made in the numerical modeling of
fluviodeltaic systems. Several current models offer the opportunity to compare the consequences
of forcing of fluvial delta systems by any combination of climate, eustasy, and tectonics, thereby
helping elucidate the specific character of climate impacts on stratigraphic sequences (Overeem
et al., 2005). This is especially relevant given that climate change and glacioeustasy are phased
through most of the Quaternary (Fig. 9), making it extremely difficult to derive unique
interpretations of these forcings from field data.
One system that has been a focus of Quaternary response modeling is the Meuse river, which
drains central Europe and discharges to the southern North Sea coast. Much of the research on
this system during the past decade has had a focus on understanding the signatures of climate and
eustasy contained in the late Quaternary stratigraphy and geomorphology (e.g., Veldkamp and
Tebbens, 2001). Situated in the periglacial environments of NW Europe during the Quaternary,
climatically driven variations in the magnitude, timing, and nature of discharge appear to have
played a role in shaping the fluviodeltaic system upstream of the hinge line. Terrace
development in this river-dominated reach of the system is recognized as a primarily climatecontrolled process through both modeling and field studies. However, the downstream impacts of
such hydrological changes in the Meuse system appear to be insufficient to overcome the
dominant influence of glacioeustasy during the Quaternary. Although there are likely to be
impacts of climate in the delta, they remain indistinguishable at the resolution of current data and
modeling capabilities.
Another system, the Niger delta, has been investigated using an industry-based stratigraphic
model (van der Zwan, 2002). This investigation was aimed at testing the impact of presumed
Milankovitch-scale variations in sediment discharge over the Neogene. The orbitally linked
forcing of regional climate and sediment occurred via global climate, insolation-control of
monsoon strength, and vegetation. Results suggest that sediment supply does change due to
climatic variability at both large-scale (Ma) and Milankovitch-scale timeframes, and that there
was a modest impact on stratigraphy. However, under icehouse conditions fluctuations in
climate-forced sediment supply were greatly overshadowed by rapid glacioeustatic sea-level
change. These findings echo those of other modeling studies, in that climate is often found to be
an important, but secondary, control on fluviodeltaic sequences. Furthermore, most modeling
studies suggest that climatic signals preserved at the continental margin greatly lag the timing of
the actual forcing in the fluvial catchment (e.g., Castelltort and Van Den Driessche, 2003). This
notion conflicts with recent field evidence from several of the subtropical rivers discussed above.
Much of these discrepancies likely arise from our limited knowledge of appropriate input values
for pre-Modern climate, riverine, and coastal conditions.
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Figure Captions
Figure 1. Google Earth satellite images of deltas (http://earth.google.com/). Lacustrine deltas: (a)
Red River delta in Lake Texoma (Texas, USA; image © DigitalGlobe); (b)William River delta
in Lake Athabasca (Canada; image © DigitalGlobe and MDAEarthSat). Deltas in estuaries,
lagoons and bays: (c) bayhead delta in the Southern Bug estuary in the Black Sea (Ukraine;
image © MDAEarthSat and DigitalGlobe); (d) Colorado River delta in the Matagorda lagoon
(Texas, USA; image © DigitalGlobe); (e) Wax Lake and Atchafalaya deltas in the Atchafalaya
Bay (Louisiana, USA; image © MDAEarthSat and DigitalGlobe). The open coast, fluvialdominated (f) Balize lobe of the Mississippi delta (image © MDAEarthSat and DigitalGlobe)
and (g) the extensive, wave-dominated Nile delta in the eastern Mediterranean Sea (image ©
MDAEarthSat).
Figure 2. Morphology and depositional environments of the Danube delta in the Black Sea (after
Giosan et al., 2005): (a) channels and natural levees; (b) sandy beach ridges; (c) floodbasins,
lakes and lagoons; (d) delta lobes (the box indicates the location of Chilia III lobe described in
later in Fig. 5). The delta started as a bayhead delta (Tulcea lobe), followed by the development
of other fluvial-dominated lobes in shallow lakes and lagoons left unfilled by the Tulcea lobe
(Chilia I and II; Dunavatz). Once reaching the open coast, several wave-dominated (St. George I
and II and Sulina) or fluvial-dominated (Chilia III) lobes formed. Note that the sand comprising
the beach ridge plains built updrift of open coast lobes was delivered via the longshore drift
system.
Figure 3. Process-based ternary classification of delta morphology as a function of fluvial
discharge and wave and tidal energy (after Galloway, 1975). Satellite images of deltas are from
Google Earth (http://earth.google.com/; Balize, Ebro, Mahakam, Grijalva, and Copper images ©
MDAEarthSat and DigitalGlobe; Ganges-Brahmaputra image © MDAEarthSat)
Figure 4. Morphology and depositional environments for mud-rich wave-dominated deltas (after
Bhattacharya and Giosan, 2003) as a function of the ratio between longshore drift and fluvial
discharge (relative magnitudes indicated by black and white-filled arrows respectively). Unlike
the classical symmetric wave-dominated deltas, where sediment supplied by the feeding river is
reworked on either side by waves, asymmetric deltas develop where the longshore drift becomes
important relative to the river discharge and updrift beach ridge plains develop. The downdrift
side is a succession of elongate sandy ridges separated by mud-filled troughs. The mouth of the
river gets deflected in extreme cases of asymmetry resulting in a series of quasi-parallel sand
spits and channel fills.
Figure 5. Delta-plain evolution of the Chilia III lobe of the Danube delta (after Giosan et al.,
2005). The morphology of the lobe had been fluvial-dominated until recently, when the coast
begun to straighten and barrier spits and islands started to develop along the coast (indicated by
arrows). (Satellite image from Google Earth: http://earth.google.com/; © MDAEarthSat and
DigitalGlobe)
Figure 6. Idealized cross-section showing internal bedding geometry and facies architecture of a
prograding deltaic clinoform (after Gani and Bhattacharya, 2005).
Figure 7. Digital elevation model for the Indus shelf based on 19th century surveys showing the
compound clinform morphology for the subaqueous Indus delta. The nearshore clinoform
extends to the ~10 m isobath. Offshore clinoform lobes are indicated by arrows. Inland
geography is visualized on a satellite image from 2000 (the limits of the Indus delta and the
adjacent mud flats of the Great Rann of Kutch are indicated by the white dashed line).
Figure 8. Reconstructed Late Quaternary history for the rivers and deltas of the Colorado (CR,
CD), Brazos (BR, BD), Trinity (TR, TD), and Sabine (SR) fluviodeltaic systems on the northern
Gulf of Mexico shelf. Phases of delta progradation, incision, and backstepping are shown with
corresponding phases of sea level, and are also associated with periods of climate change and
varying sediment discharge (from Anderson and Fillon, 2004). Note larger-than-present deltaic
lobes building on the mid-shelf during MIS-3 interstade, under apparently wetter climatic
conditions. Climate and sea level are dominant controls on delta formation and have varied
somewhat in-phase during the Late Quaternary (see Fig. 9), giving rise to complex fluviodeltaic
responses.
Figure 9. SPECMAP δ18O and Milankovitch-tuned insolation records for the Late Quaternary
compiled by Imbrie, and Berger and Loutre, respectively. SPECMAP data represents a proxy for
sea-level change and insolation serves as a coarse proxy for wetness in the sub-tropical
(monsoon-influenced) northern hemisphere. Intended to show the general co-phasing of peak
insolation (generally wetter) with sea-level transgression, with the implied consequence of high
sediment discharge corresponding with rapid sea-level rise. This pattern has been observed in the
Yellow, Yangtze, and Ganges-Brahmaputra delta systems, west African rivers, and most likely in
the Texas Gulf of Mexico rivers.
b
a
c
d
e
f
g
Figure 1
marine ridges (non-Danubian sand)
marine ridges (Danubian sand)
lacustrine ridges
dune fields
a. channels and natural levees
Chilia
Ch
III
ilia
II
b. beach ridges
lia I
Chi
vatz
una
D
c. floodbasins, lakes, and lagoons
I
Sulina
St.George
Tulc
ea
ge
.G
II
r
eo
St
d. deltaic lobes
Figure 2
MAHAKAM
BALIZE-MISSISSIPPI
FLUVIAL
INPUT
RIVERDOMINATED
EBRO
WAVEDOMINATED
GRIJALVA
TIDEDOMINATED
WAVE
ENERGY
TIDAL
ENERGY
GANGES-BRAHMAPUTRA
COOPER
Figure 3
Deflected
Asymmetric
Symmetric
Lagoonal
Fluvial / Bayhead delta
Beach / Barrier
Non-deltaic
Figure 4
1830
1883
1922
Figure 5
Land
Sea
Sea Level
Clinoform
Delta plain deposits
(sand and mud)
Transgressive deposits with lag
Delta front deposits
(mostly sand)
Older deposits
Prodelta deposits
(mostly silty mud)
Shazam line
(gradational facies boundary)
Figure 6
22 oN
23 oN
24 oN
40
50
70
70 oE
100
69 oE
Indus Canyon
68 oE
10
67 oE