Boggs S. Principles of Sedimentology and Stratigraphy
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296
Chapter 9 I Marginal-Marine Environments
bar may be reworked into a ries of linear tidal ridges that replace e bar and ex
tend from within the channel mouth out onto the subaqueous delta-front platform.
The modern Ganges-Brahmaputra delta is a well-known example (Michels
et a!., 1998; Goodbred and Kuehl, 2000) of a tide-dominated delta (Fig. 9.. The
areal size of this delta is more than three times that of the Mississippi delta. It has
a mean river discharge about twice that of the Mississippi and an exceedingly high
discharge during the monsoon season when extreme ooding is common. Mean
tidal range is large, about 4 m, and tidal currents can as strong as 3.8 m/s
(Michels et a!., 1998); wave energy is relatively low. Sand transport is tense dur
ing the monsoon season, leading to deposition of sandy deposits similar to th
in braided streams. The delta is characterized by tidal-flat environments, natural
levees, and floodbasins in which fine sediment is deposited from suspension. The
strong tidal influence is manifested by a network of tidal sand bars and channels
oriented roughly parallel to the direction of tidal current flow (Fig. 9.7 A, B). Ava
riety of sediment types thus accumulate on the Ganges-Brahmaputra delta, such
as tidal-bar or tidal-ridge sands; braided, channel-fill sands; and natural levee,
tidal-flat, and oodbasin muds. The Ord Delta (Timor Sea, Australia) provides an
other example of a mode delta largely dominated by tidal processes (Fig. 9.3).
Wave-Dominated Deltas
Strong waves cause rapid diffusion and deceleration of river outflow and produce
constricted or deflected river mouths. Distributary-mouth deposits are reworked
by waves and are redistributed along the delta front by longshore currents to fo
wave-built shoreline features such as beaches, barrier bars, and spits. A smth
delta front, consisting of well-developed, coalescent beach ridges, may eventually
be
generated.
The Paraiba do Sui delta of Brazil (Fig. 9.8) displays many of the characteris
tic features of wave-dominated deltas. The Rio Paraiba do Sui coastal plain con
sists of Pleistocene and Holocene littoral marine sediments and Holocene fluvial
and lagoonal sediments (Martin et al., 1987). Ti dal range over the delta is moderate
(mesotidal); however, wave energy is extremely high. Sediment discharge from
the river is high and middle-ground bars are common at the mouth of the river.
diment is transported at high rates across the mouth, from west to east, leading
to development of amalgamated, sandy beach ridges (Bhattacharya and Giosan,
2003). Owing to
this extreme wave energy, the Paraiba do Sui delta is dominated
by high-energy environments in which sand deposition takes place. Muds accu
mulate locally in lagoons, but the interdistributary bay mud deposits characteris
tic of the Mississippi delta are absent. Paraiba do Sui delta deposits are dominated
by beach-ridge barrier sands that cover much of the delta surface adjacent to the
ocean.
Other examples of modern wave-dominated deltas include the Skeidarar
Sandur (Iceland, North Atlantic), the Punta Gorda (Belize, Gulf of Honduras), as
shown in Figure 9.3, the Tseng-wen (southwest Taiwan; Liu et a!., 2003), and the
Sao Francisco (Brazil; Dominguez, Martin, and Bittencourt, 1987; Dominguez,
1996), often cited as the classic example of a wave-dominated delta. Several addi
tional examples, from the Black Sea, Gulf of Mexico, Mediterranean Sea, South At
lantic, Bay of Bengal, and the Thyrrenian Sea, are discussed by Bhattacharya and
Giosan (2003).
Mixed-Process Deltas
The examples discussed above illustrate some differences in characteristics of
modern deltas that are shaped by processes that are predominantly fluvial, tidal,
or wave-related. Many deltas have characteristics that are transitional between
these "end-member" types. The Copper River delta in the Gulf of Alaska (Fig. 9.9)
Figure 9.7
Modern paludal and � bas1ns (<3 m)
0 Modern floodplain end de�a plain (3-7 m)
iocene uplift (5-1 0 m)
�Pleistocene uplands ( 1 20 m)
0
§Teiary uplands ( 100 m)
9.2 Deltaic Systems
297
100
The Ganges-Brahmaputra delta, a modern tide-dominated delta. A. Regional map of the
Bengal Basin showing physiography and geology of the delta and surrounding area.
[From Goodbred, S. L., and S. A. Kuehl, 2000, The significance of large sediment supply,
active tectonism, and eustasy on ma rgin sequence development: Late Quaternary stratig
raphy and evolution of the Ganges-Brahmaputra delta: Sedimentary Geology, v. 133, Fig.
2, p. 229. Reproduced by permission.] B. Space image of the delta, NASA johnson Space
Center, downloaded from the Internet 4/8/04.
298
Chapter 9 I Marginal-Marine Environments
_
�ndy ch
�
rig
Fluvial and lagoonal
""'mn'
_-.
�
Figure 9.8
The Paraiba do Sui delta, Brazil, which is characterized by sandy beach ridges oriented
roughly parallel to the shoreline. . Map of the Rio Paraiba do Sui coastal plain. [Redrawn
from Martin et al., 1987, Quaternary evolution of the central part of the Brazilian coast:
The role of telaiye sea-level variation and horeline drift, in Quaternary coastal geology of
West Africa and South America, UNESCO reports in Marine Science, No. 4 3, Fig. 16, p.
1 30. [dress: Marine Information Centre, ivision of Marine Sciences, UNESCO, Place de
Fontenoy, 75700, Paris, rance]. B. Space image of the delta and coastal plain, NASA
image downlbaded from the Internet /28/04.
provides an example of a delta that is strongly influenced by tides but also experi
ences high wave power (e.g., Galloway, 1976). Thus1 the Copper River delta
fallt
somewhere between a tide-dominated and a wave-dominated delta (see Fig. 9.3)
Tidal range on the Copper ver dela may eceed 3m, and tidal currents of up tc
2m/s have been measured. Stmng, wind-driven sweUs, ooup1ed with the west·
erly marine current, set up a net wtward longshore drift. Thus, deltaic sedi
ments are modified by both tidal and wave-related processes. Major det<
environments include (1) the subaeria• deltaic plain, consisting ot marsh
deposit!
an distributary channel fills; (2) the tidal lagoon, composed of Hdal sands
anc
mudflat deposits interlaced with a complex of tidal channel fills (Fig. 9.9); and (3:
the wind-influenced shoreface consising of marginal island, breaker bar,
ne
middle shoreface sands, lower shoreface sand and mud, and prodelta/ shelf mud
As indicated in Figure 9.3, other modem deltas influenced fairly strongly by bot�
tides and wind include the Irrawaddy (Burma), Niger (Nigeria), and
Burdekir
(Australia).
gure 9.9
9.2 Deltaic Systems
299
The Copper River delta, Gulf of Alaska. A. Physiographic map of the delta, showing tide
influenced channels and bars and wave-influenced breaker bars; based on U.S. Dept. of
Agriculture map, "The Copper River
,
" Alaska Region Rll 0-RG-1 00. B. Tidal channels and
tidal sand bars on the delta; photograph by Allan Cline.
Fan Deltas
The concept of fan deltas was intuced by Holmes (1965, p. 554). A fan delta, as
defined by Ho'lmes and mified slightly by Nemec and Stee1 �1988a), is a coastal
prism of sed•ments delivered by an alluvia1-fan..sysrem and deposited, mainly or
entirely subaqueously, t the interface between the active fan and a standing body
of water (e.g., Fig. 9.10). Fan deltas were recognized first in modern settings but
fan-deha deposits have now been rerted in many anciemt dimentary succes
sions (e.g., Nemec and Steel, 1988b; Chough and Orton, 1995).
The relationship between alluvial fans, which constitute the subaerial com
ponent cf fan deltas, and the subaqueous delta is illustrated in Figure 9.11. The al
luvial fans can include any of the fan types discussed Chapter 8 and may form
settings ranging from glacial to humid to arid. Like other deltas, the subaque
ous portion of fan deltas may be fluvial dominated, wave dominated, or tide dom
inated. diments are deposited downslope the subaqueous part of fan deltas
by prses such as slumping and debris avalanching, turbidity-current flow,
a inertia (hyperpycnal) flow that takes place particularly during flood stages;
riverrne load achieves sufficient density to overcome buoyancy and fric
onal effects at the river mouth and can transport even gravel and coarse sand
downslope (e.g., Prior and Bornhold, 1990).
Physiographic and Sediment Characteristics of Deltas
Variations in sediment input, outflow velocity, and wave and current energy cause
e depositional features of deltas to exhibit a high degree of variability from one
300
Chapter 9 I Marginal-Marine Environments
Figure 9.10
Kurobegawa fan, a large fan delta at
the mouth of the Kurobe River, Toya
ma Bay, japan Sea, off central japan.
[From Fujii, S., and N. Nasu, 1988,
Kaiteirin (submerged forest; text in
japanese), Tokyo University Press,
Color plate of Kurobegawa fan. Photo
graph courtesy of S. Fujii; reproduced
by permission of University of Tokyo
Press.]
Figure 9.1 1
Schematic diagram of a fan
delta, showing the subaerial
alluvial fan and subaqueous
delta. [After Nemec, ,
and R. ]. Steel, 1988, What
is a delta and how do we
recognize it? in Nemec, W.,
and R. ]. Steel (eds.), Fan
deltas: Sedimentology and
tectonic setting: Blackie,
Glasgow, Fig. lA, p. 6.]
delta to another. Nevertheless, all deltas can be divided into subaerial and sub
aqueous components, each ef which can be further subdivided (Fig. 9.12). The
subaerial component of deltas, called the deltaic plain, is generally larger than the
subaqueous component. It is divided to an upper delta plain, which lies largely
above high-tide level, and a lower delta plain, lying between low-tide mark and
the upper limit of tidal influence. The upper delta plat ts commonly the oldest
part of the delta and is dominated by fluvial processes. The lower delta pla is ex·
posed during low tide but is covered by water during high tide. Thus, it is sub·
jected to both fluvial and mare processes.
The upper delta plain is influenced maly by fluvial pess. Sedimen·
tion is dominated y disibutary-chael migration and associated uvial si
mentation processes such as channel and pot-bar deposition, overbank flooding,
and
crevassing to lake bass (Chapter 8). l11e principal
depositional environments
0
Fluvial gravel,
nd, mud
High tide
level
Suba
queo
us
delta
D
-
D
Bay mud, Delta front-
silt
prodelta sand,
silt, mud
8
Shelf mud,
silt
9.2 Deltaic Systems
301
Figure 9.12
Principal components of a
fluvial-dominated delta sys
tem. The physiographic sub
divisions of tide- and
wave-dominated deltas are
similar; however, sediments
on the delta front and the
lowermost deltaic plain are
reworked by tides and waves.
[Based in part on Coleman
and Prior, 1982.]
include braided channels, meandering channels, back swamps, and floodplain en
vironments such as swamps, marshes, and freshwater lakes. Therefore, upper
delta-plain sediments are predominantly fluvial sands, gravels, and muds that
may be cloly assiated with lake, swamp, and marsh deposits. The width of the
lower deltaic plain is greatest on deltas where tidal range is large. This plain in
clud the active distributary system of the delta as well as abandoned distributary
fill deposits, and it may be flanked by marginal-basin or bay-fill deposits.
Djstributary channels are numerous, but environments behveen channels make up
the largest percentage of the lower delta plain. These environments include active
ly migrang tidal channels, natural levees, interdistributary bays, bay fills
(crevasse splays), marshes, and swamps (e.g., Coleman and Prior, 1982).
The suba
q
ueous delta plain lies seaward of the lower deltaic plain below
low-tide water level. The uppermost part of the subaqueous delta, lying at water
depths down to 10m or so, is commonly called the delta front. The remaining sea
ward part of the subaqueous delta is called the prodelta, or prodelta slope. The
subaqueous delta may extend outward for distances of a fev,r kilometers to tens of
kilometers, and the prodelta may extend to water depths as much as 200-300 m.
On fluvial-dominated deltas, deposits typically consist in part of sand, and possi
bly gravel, desited near river mouths, forming distributary
-
mouth-bar deposits
such as those of the M�ssissippi delta (Fig. 9.6). On the other hand, the delta front
may be dominated by high-energy marine processes, including waves, longshore
currents, and tides. On wave- and tide-dominated deltas, sediment is reworked
and winnowed by these processes, creating well-sorted delta-front sheet sands
that are cross-bedded on a variety of scales. The finest silts and clays are trans
ported still farther seaward and settle on the prodelta on the outermost part of the
subaqueous delta. Previously deposited sediments may be reentrained, transport
, and r�deposited farther downslope on the subaqueous delta by gravity-driven
mass-movement processes such as landslides, slumps, turbidity-current flows,
and mud lows.
302 Chapter 9 I Marginal-Marine Environments
Figure 9.13
Delta cles
During the active, constructional phases of delta outbuilding, deltaic deposits pro
grade seaward, leading to generation of a coarsening-upward vertical succession
of fa cies as delta-front sands advance seaward over prodelta silts and clays. e
sedimentary succession on the progradational lobes of the fluvial-dominated Mis
sissippi ver delta provides a good example (Fig. 9.13). Active delta outbuilding
may be interrupted by major changes in the hydrologic and sediment regime re
lated to tectonism, climate variation, major diversions of rivers upstream, or
changes in sea level. Smaller-scale changes may result from processes such as
switching of delta lobes, distributaries, or tidal channels. Major changes, in parc
ular, can result in a relative rise in sea level that may halt progradational delta
growth
and bring about a transgressive phase of deposion as the shoreline ad
vances in a landward direcon. Thus, growth of deltas tends to be cyclic. During
active prograding phases, prodelta fine silts and clays are progressively overlain
by delta-front silts and sands, distributary-mouth sds, and ally marsh, fluvial,
and possibly eolian deposits as the delta builds seaward, producing a coarsening
upward regressive succession. Interruption of progradation by delta-lobe aban
donment or marine transgression brings on a destructive phase in which erosion
and redistribution of river-mou deposits predominates. Subsequent distributary
shiing or regression may bring on another phase of active progradation. Sedi
ments that make up a complete delta cycle may range in thickness from 50 to 150
m. Smaller scale cycles representing progradation of individual distributaries
range from only about 2 to 15m (Miall, 1984).
Bay and marsh deposits.
Mud, silt; bioturbated
Cvass splay deposits.
Coarsening-upward success
ions of muds to sands
lnterdisibutary bay deposits.
Mud, sand-silt stringers; shells
Overbank splay deposits.
Thin peat & soils. Thin sand,
silt, shale stringers; root traces
1 Beach & dune sands.
Mouth bar & channel deposits.
Sand, silt beds; cross-bedded
Distal bar deposits.
Sand & silt, rippled; coarsening
upward; mud laminae; shells
Prodelta deposi.
Laminated mud; silt-sand
laminae; shells
Idealized vertical succession of facies in a fluvial-dominated (Mississip
pi)
delta. Note the thickness of individual units shown in the column.
[From Coleman, J. M., 1981, Deltas: Processes of deposition and
models for exploration, 2nd ed., Fig. 4.3, p. 91, reprinted by permis
sion of IHRDC Publications, Boston.]
Slump-block deposits.
Sand & silt; flow structures
Shelf deposits.
Mud, bioturbated or laminated
Ancient Deltaic systems
9.2 Deltaic Systems
303
Ancient deltaic sediments have been reported in stratigraphic successions of most
ages, but they appear to be particularly common in rocks of Carboniferous and
Te rtiary age. Reported examples of fluvial-dominated deltaic deposits include the
Carboniferous of the British Isles (e.g., Martinsen, 1990; Pulham, 1989), the Wilcox
Group (Eocene) of Te xas (Fisher and McGowan, 1967), and the Dunvegan Forma
on (Cretaceous) of Canada (Bhattacharya and Walker, 1991). An example is pro
vided by Horne et al. (1978), who describe fluvial deltaic sediments from the
Carboniferous of Kentucky (Fig. 9.14). The deposits shown in Figure 9.14 are dis
butary-mouth-bar sandstones that grade laterally into bay-fill muds. The sand
bodies are 1.5--5 km wide and 15-25 m thick. They are widest at the base and
have gradational lower and upper contacts. Grain size increases upward in the
succession and toward the center of the bars. Fining-upward, graded beds are
common on the flanks of the bars, as are osllation- and current-rippled surfaces.
Pble-lag conglomerates are present at the bases of the channel deposits. Note
by comparison with Fig. 9.6 that these ancient river-dominated deltaic sediments
are very similar in geometry and sediment characteristics to those of the modern
Mississippi delta. Other ancient river-dominated deltas are discussed by Bhat
taarya and Walker (1991), Fisher and McGowan (1967), Galloway (1975), and
Pulham (1989).
Weise (1980) describes Upper Cretaceous sediments from the San Miguel
Formation in the subsurface of Texas that are interpreted from core and well-log
information to be wave-dominated deltaic sediments. She constructed thickness
(isopach) maps of ten sandy delta lobes having a variety of sand-body shapes
ranging from those with the characteristics of river-dominated deposits (reflecting
least influence of marine processes) to those with the characteristics of marine
wave-dominated deltas (Fig. 9.15). During periods of high sediment input and a
low rate of sea-level rise, redtribution of sediment by waves was minimal, and
lobate deltas resulted. During periods of low sediment input and high rates of sea
level rise, sediment was extensively reworked by waves to form elongate, strike
ed sandstone bodies, illustrated in Figure 9.15. Oer ancient wave-dominated
deltas are described by Bhattacharya and Giosan (2003), Elliott (1986a), Leckie and
Wal ker (1982), and Liu et a!. (2003). Howell and Flint (2003) consider the Creta
ceous strata in the Book Cliffs of Utah to be an ancient analogue to the deposits of
e Paraiba do Sui delta (Brazil), shown in Figure 9.8.
Distributary channel
lnterdistributa bay
Distributary-mouth bar sands
D
Sandstone
1�1
Bedding
Shale and siltstone
Ripples
0
�
m
Root structures
Cross-beds
�
Burrow structures
Soft-sediment
deformation structures
gure 9.14
Three-dimensional model of fluvial
dominated delta deposits from eastern
Kentucky. [After Horne, ). C., et al., De
positional models in coal exploration
and mine planning in Appalachian re
gion: Am. Assoc. Petroleum Geologists
Bull., v. 62, Fig. 6, p. 2387.]
304
apter 9 I Marginal
-
Marine Environments
Crevasse-splay
deposits
Figure 9.15
Delta plain with
marshes or ponds
/
Three-dimensional model illustrating the sand-body geometry and facies of a wave
dominated delta system in the San Miguel Formation (Cretaceous), South Texas. [After
Weise, B. R., 1980, Wave-dominated delta systems of the Upper Cretaceous San Miguel
Formation, Maverick Basin, South Texas: Bureau of Economic Geology, University of Texas
at Austin, Report of Investigations 107, Fig. 26, p. 20, reproduced by permission.]
The Eocene Misoa Formation, Maracaibo Basin, Venezuela, is a huge allu
vial-deltaic-shallow marine complex extending northeasterly about 250 km from
its source that exhibits marked characteristics of a tide-dominated delta. A small
part of this complex (64 km
2
) was reconstructed by Maguregui and Tyler (1991)
as shown in Figure 9.16. A tidal delta plain bisected by na rrow, highly sinuous
tidal channels formed low-relief highs separatg straight estuarine (river mouth)
distributary channels. The interdistributary areas have low sand content. Estuar
ine distributary systems transport and discharge sediments at the end of estuar
ies, where tidal currents redistribute these sediments to form a tidally modified
distributary-mouth-bar complex. Tidal channels within interdistributary areas
are not connected to the sediment distributary system but simply drain the tidal
plain during low-tide flow. Sands accumulated in the estuarine channels to form
estuarine distributary-channel complexes (Fig. 9.16a). Seaward, the estuarine
distributary channels diverge outward and merge into tidally modified distrib
utary-mouth-bar areas (Fig. 9.16b) in this zone of maximum tidal-current energy.
Tidal sand ridges also formed as coarsening-upward, highly bioturbated sand
bodies (Fig. 9.16c, d), which developed by lateral migration of higher energy bar
crests (sands) over lower energy muds. Note the orientation of these ridges, from
right
to left across the block, parallel
to the estuaries and perpendicular
to e
shoreline. Additional examples of ancient tide-dominated deltas are discussed by
Dalrymple et al. (2003), Mellere and Steel (1996), Roberts and Sydow (2003), and
Willis et al. (1999).
(a) Estuarine
distributary
complex
Flgure 9.16
(b) Transitional estuarine
to tidally modified
distributa mouth bar
(c) Proximal tidal
sand ridge
(d) Distal tidal
sand ridges
9.2 Deltaic Systems
305
Three-dimensional model of the delta front and lower delta plain of the tide-dominated
delta in the Misoa Formation (Eocene), Maracaibo Basin, Ve nezuela. A typical estuarine dis
tributa complex is shown in (a). A transitional lithofacies occurs farther seaward (b),
where greater influence of shallow-marine conditions and a high degree of reworking by
tidal currents modi the typical estuarine distributary-channel complex. Proximal tidal
sand-ridge facies (c) are present in areas of high sediment discharge near the ends of the
estuaries (c). Distal sand-ridge facies lie farther from the estuaries, where the sediment sup
ply is limited and tidal currents are weaker (d). Note that both the sand facies (dot pattern)
and mud facies (dark shade) are extensively bioturbated. [After Maguregui, j., and N. Tyler,
1991 , Evolution of Middle Eocene tide-dominated deltaic sandstones, Lagunillas Field,
Maracaibo Basin, western Venezuela, in Miall, A. D., and N. Ty ler (eds.), Three-dimensional
facies architecture of terrigenous clastic sediments and its implications for hydrocarbon dis
cove and recove: Concepts in sedimentology and paleontology, 3, SEPM (Society for
Sedimentary Geology), Tulsa, Okla., Fig. 16, p. 244, reproduced by permission.]
The Late Carboniferous-Permian Reinodden Formation of western Spitsber
gen provides an example of an ancient fan-delta depositional system (Kieinspehn
et al., 19). Several fan-delta successions are present in this area, one of which is
shown in Figure 9.17. The stratigraphic succession in Figure 9.17 begins with car
bonate and siliciclastic muds that were deposited in a prodelta setting. These fine
grained prodelta deposits are succeeded upward by barrier/spit and distal
mouth-bar sands, cross-bedded in part, deposited by turbidity currents and other
processes on the delta front. Sandy deposits are overlain by planar to cross-bedded
gravels that formed in fluvial channels on the fan-delta plain. A thin unit of wave
worked gravels caps the succession, representing a minor phase of marine trans
gression. The small maps in Figure 9.17 show the postulated evolution of the fan
with te as it prograded over the delta-front and prodelta environments to gen
erate the coarseng-upward succession of facies shown.