Embed Size (px)
Citation preview
1 2 34 5
Clinothem lobe growth and possible ties to downslope processes in the Gulf of PapuaThe Gulf of Papua is fed by the large-floodplain Fly River and small mountainous rivers to the north, thus creating an ideal environment where end-member cases of river systems and their deltas (e.g. the large-floodplain Brazos River and the narrow-shelved Eel River) can be stud-ied. Input from five rivers into the gulf has constructed a three-dimensional mid-shelf clino-them composed of three depositional lobes. Newly examined CHIRP (Compressed High In-tensity Radar Pulse) seismic lines from the 2004 NSF MARGINS program reveal consistent patterns of aggradation in topographic highs and progradation northeast into intervening lows. Progradation of the clinoform fills available accommodation and occurs predominantly to the northeast; however, southwest progradation is also observed. Northeast progradation of the clinoform is consistent with NCOM-predicted near-bed currents during Trade Wind conditions. The progradation of the clinoform into topographic lows has created the clinothem’s three-lobed structure. Interfingering relationships are observed at the lobe boundaries and suggest that the lobes are syndepositional. Long-lived northeast progradation and lobe inter-fingering implies that the modern-day circulation has likely been persistent through time. Ex-amining stratal relationships along the clinothem has broad implications for sequence stratig-raphy, since many studies overlook the three-dimensionality of clinoforms. Differences in along-margin geometry between the aggrading and prograding packages cause noticeable change in the across-margin geometry of clinoforms. This suggests that clinoform architecture in the Gulf of Papua cannot be interpreted simply as a rate related problem involving rates of sediment supply and relative sea level. Instead, future work interpreting clinoforms needs to account for along-margin sediment transport.
123
Scripps Institution of Oceanography, UCSD, La Jolla, CAVirginia Institute of Marine Science, College of William and Mary, Gloucester Point, VADepartment of Geosciences, Pennsylvania State University, University Park, PA Corresponding author: [email protected]*Emily A. Wei, Neal W. Driscoll, John D. Milliman, and Rudy L. Slingerland1* 1 2 3
Abstract
The Gulf of Papua Figure 1: The five river inputs into the Gulf of Papua with average water discharge.
Why is the Gulf of Papua an ideal place to study clinoforms? - rapid sediment accumulation: tropical climates --> 10 m rainfall / year - high mountains (2-4 km): steep topography induces erosion - seasonal circulation states: Monsoon season (low wave energy, NW winds, high sediment supply) Trade Winds (high wave energy, SE winds, low sediment supply spring to fall) - high energy resuspends sediment: tides (>2 m)
Introduction: What are clinoforms and why do we care? - clinoforms are building blocks of continental margins - Despite much research, processes affecting clinoform formation remain poorly understood - Stratigraphy records changes in climate
Clinoform formation and morphology are influenced by: - oceanographic conditions (waves, tides, currents) - relative sea level (eustatic sea level and tectonics) - amount of sediment delivery
Figure 3: Schematic sediment advection from the river mouth across and along clinoforms
How do you interpret clinoform geometry? - Rate-related problem: accommodation vs. sediment supply - accommodation: areas where sediment can accumulate - new accommodation is created by sea level rise or tectonic subsidence - facies change progressing from land to sea - difficult to isolate any of these variables --> why questions still exist
Sedi
men
t sup
ply
Mak
ing
spac
e
Filli
ng sp
ace
Figure 4: From Slingerland et al. (2008)
silt/clay
sand
across shelf
along shelf
gravity flows
sedim
ent plume
Sea
leve
l ris
eFa
stS
ea le
vel r
ise
slow
s do
wn
Rel
. sea
leve
l ris
ein
crea
ses
Rel
ativ
e se
a le
vel r
ise
= Su
bsid
ence
+ E
usta
tic
Low
H
igh
Sed
imen
t sup
ply
= R
el. s
ea le
vel r
ise
Sedi
men
t sup
ply
20 m
5 kmNW SE
Modern clinoform uninterpreted
27 m deep
The modern clinoform is built from five packages
Figure 2: Locations of CHIRP lines (towed and hull-mounted) and cores
Methods - High resolution Edgtech CHIRP surveys (6800 km) - Compressed High Intensity Radar Pulse - 1-6 kHz swept frequency - Hull mounted Knudsen surveys (4500 km) - 3.5 kHz
The proximal clinoform shows more complexity than the distal clinoformThough many studies have concentrated on across-margin geometry, along-shore clinoform variability is less constrained. It is important to understand along-strike sediment transport in order to unravel clinoform formation and determine how they can be used to understand sea level changes. Here we present data that shows along-strike variations from proximal to distal. Our approach to analyzing along-margin clinoform geometry is applicable to clinoforms worldwide.
seismic line 0019
Why more complexity in the proximal clinoform?Landward, sediments are exposed to increased wave energy from waves, tides, and currents - shallower profiles are subjected to large shear stresses - This can be seen in the suspended sediment concentration downslope
Walsh et al. (2004)Relict clinoform, more aggradational
incised “mesa” topography
infill from gravity flows
N
Further complexity in the lows is seen at the intersection of lobes
Blue = Most recent sediment. AggradationalOrange = Youngest clinoform. Progrades but with some aggradationRed = Older clinoform. Steeper and more progradational than orangeGreen = Transgressive fillYellow = “Mesas”: relict clinoforms that were incised by rivers and channels
Blue = Most recent sedimentPink = Infill from gravity flowsOrange = Youngest clinoformRed = Older clinoformYellow = Mesas incised by rivers
Figure 6: A Fence diagram with along-margin profiles illustrates changes in clinoform geometry from proximal to distal. The distal clinoform is characterized by the incised mesa topography, whereas sediment is thicker on the proximal lines. The profiles for the fence diagram are to the right in Figure 7.
Fly- Strickland River
6000 m /s3
Bamu2000 m /s3
Purari
2000 m /s
3
Turama
2000 m /s3
Kikori
2000 m /s
3
Central L
obe
Southe
rn Lo
be
Northern Lobe
and progrades into the lows
Along-margin advection influences across-margin clinoform architecture
Topographic controls: Sediment aggrades on the highs...
Figure 5: An uninterpreted seismic dip line to the left. In the middle are the interpreted five packages that have constructed the modern clinoform. At right, rates of relative sea level rise and sediment supply can be used to explain clinoform geometry
Orange progrades NPink fills in low
Orange progrades N into low
26 m
Orange aggrades off the mesa topography
line b0041
line d002
0
30
60
120
90
0
30
60
120
90
0 10 20D
epth
(m)
Dep
th (m
)Distance (km)
b0041: orange package progrades N
- steeper sloping orange and red - orange has more progradational component
Figure 10: The thickness and steepness of clinoforms in across-margin profile varies depending on their along-strike location. - Line b0041 is located where the orange package progrades northeast. As a result, the orange package in the dip profile is steeper and thicker - Line d002 is located on a topographic high where the orange aggrades. As a result, the orange and red packages have a gentler slope and have aggra-dational and progradational components.
Because the steepness and geometry of clinoforms changes with along-margin transport processes, down-slope clinoform geometry can be misinterpret-ed as changes in sediment supply or relative sea level. Thus, the traditional two-dimensional approach to interpreting clinoforms needs to consider along-margin sediment advection
Figure 7: The interpreted and uninterpreted profiles from the fence diagram in Figure 6 explore the along-margin clinoform from proximal to distal. The dashed boxes on the highs are shown in Figure 8a whereas the solid boxes in the lows are shown in Figure 8b. Here we can see that the along-margin profiles vary because of their depth, the amount of sediment supply, and energy from waves, tides, and currents. At right, the profiles are related to their depth on a suspended sediment concentration profile from Walsh et al. (2004)
Conclusions and Implications1. The most recent clinoform, orange, aggrades on the highs and shingles into the lows a. When it shingles into the lows, energetic nearbed currents from the SW advect sediment to the NE b. This NE advection is building the clinoform obliquely instead of directly downslope c. Topographic lows were created by the incisional mesa topography and the overlying red clinoform. Progradation of the orange clinoform causes these lows to shift obliquely to the northeast2. Complexity is observed in the lows because shingling is occuring from both the north and south; however, northeast progradation is dominant and southwest progradation is subordinate a. Shingling into the lows at the junction between two lobes causes the two lobes to interfinger. This interfingering relationship suggests that all three lobes are syndepositional3. Long-lived clinoforms shingling into the lows suggest that modern-day circulation patterns have persisted since the origin of the orange clinoform4. The traditional rate-related problem of interpreting clinoform geometry is further complicated in settings with current- controlled sediment advection. In these locations, along-margin controls on sediment dispersal need to be considered
Line h048a
Line h044a
Line g027a
a
c
topographic hightopographic high
topographic high
shingling to the Nshingling to the Sprograding orange sediment
0
60
90
3027 m
300 10 20 40 50
Orange clinoform progrades northeast
300 10 20 40 50
37.5 m0
60
90
30
Orange clinoform progrades northeast
Proximal
Distal300 10 20 40 50 60
57.8 m
0
60
90
30
300 10 20 40 50 60
Dep
th (m
)D
epth
(m)
Dep
th (m
)
Line h048a
Line h044a
Line g027a
Fig. 8a Fig. 8b
Figure 8: Comparing the relationships between topographic highs and lows and the geometry of the orange clinoform. Enlarged interpreted profiles in a) show that orange reflectors are parallel, indicating aggradation. Enlarged profiles in b) show progradation of the orange clinoform into the lows. The schematic c) describes the formation of prograding packages and interfingering. A map d) of bathymetry with NCOM-predicted near-bed currents during the Trade Wind season superposed (Slingerland et al., 2008a) shows that little to no progradation occurs on the highs but occurs in the lows. These lows serve as conduits for gravity flows (pink). A schematic e) hypothesizes how the progradation of the orange clinoform north occurs because of near-bed current-controlled transport and topographic controls.
Line b0041
Line d002
Figure 9: Though it was hypothesized that the Southern, Central, and Northern lobes were not syndepositional, seismic profiles at the junctions between lobes suggest otherwise. Profiles relating the relationship between the North and Central lobes a) show interfingering in both the proximal and distal lines. A fence diagram b) shows the relative positioning of the seismic profiles in a). Profiles of the Southern and Central Lobe relationship c) also show interfingering in the orange and red clinoforms, suggesting that the lobes were formed at the same time. The profile positions are shown in the fence diagram in d). Interfingering in the lows occurs by the same shingling processes occuring elsewhere along the clinoform. It is therefore expected to see that the lobes have interfingering relationships, were formed syndepositionally, and were formed by the same depositional processes that govern oblique progradation.
d002: orange package aggrades on high
- orange and red are less steep - orange progrades and aggrades
Dep
th (m
)
0
30
60
120
90
0
30
60
120
90Dep
th (m
)
Davies, H.L. (2012) The geology of New Guinea: the cordilleran margin of the Australian continent. Episodes 35 (1).Johnstone, E.A. (2012) Geophysical constraints on sediment dispersal systems, Unpublished thesis, University of California San Diego, La Jolla, CA.Slingerland, R., N.W. Driscoll, J.D. Milliman, S.R. Miller, and E.A. Johnstone (2008) Anatomy and growth of a Holocene clinothem in the Gulf of Papua, J. Geophys. Res. 113.Slingerland, R., R. W. Selover, A. S. Ogston, T. R. Keen, N. W. Driscoll, and J. D. Milliman (2008a), Building the Holocene clinothem in the Gulf of Papua: An ocean circulation study, J. Geophys. Res. 113.Walsh, J. P., C. A. Nittrouer, C. M. Palinkas, A. S. Ogston, R. W. Sternberg, and G. J. Brunskill (2004), Clinoform mechanics in the Gulf of Papua, New Guinea, Cont. Shelf Res., 24, 2487–2510.Wolanski, E., and D. M. Alongi (1995), A hypothesis for the formation of a mud bank in the Gulf of Papua, Geo Mar. Lett., 15(3–4), 166– 171.
Sources Cited
Acknowledgments- Research supported by National Science Foundation awards 0305699, 0305779, and 0305607 under the NSF MARGINS Initiative.- Data were collected with the R/V Melville
SummaryThis research illustrates the importance of circulation and physiography in clinoform development. Oblique sediment transport and infilling of corrugated topography gives rise to a three-dimensional clinoform where the cross-margin architecture is in large part controlled by the along-margin progradation.
0.05 m/s
Prograding to the NorthPrograding to the South
topographic hightopographic low
Near-bed currents, Trade Wind Conditions
t13s11-13: North to Central Lobe interfingering
p39s12-14: North to Central Lobe interfingering
0
30
60
120
90
0
30
60
120
90
Dep
th (m
)D
epth
(m)
3.616 km
line t21s11-13
line p39s12-14
interfingering
interfingering N
Nor
th a
nd C
entr
al L
obes
0
30
60
900
30
60
120
90
t21s1-4: South to Central Lobe downlap
interfingering
Southern lobe Central lobe
interfingering
Dep
th (m
)D
epth
(m)
0 5 10Distance (km)
0 5 10Distance (km)
N
line t21s1-4
line b0020
interfingering
2.533 km
interfingering
Sout
h an
d C
entr
al L
obes
b0020: South to Central Lobe interfingering
ab
c d
Northern lobeCentral lobe
Transect G is located at the convergence zone in the Northeast region of the Gulf of Papua
Transect G is also where suspended sedi-ment concentrations are the highest on along-margin profiles
- Aggrading orange packages are located on topographic highs and are characterized by stacking parallel reflectors - During the monsoon season, aggrading sediment is de-posited and reworked during the Trade Wind season
- Prograding orange packages are located on the flanks and are characterized by angled reflectors
NS21A-3874
Formation of aggrading and prograding packages1. Aggrading sediment is deposited on the highs during the monsoon season2. More energetic currents during the trade wind season winnow away and rework sediment on the highs3. Northeast near-bed currents advect sediment towards the lows4. Sediment progrades/shingles into the lows due to northeast advection and downslope processes5. Sediment farther north shingles into the lows due to downslope processes. The shingling of sediment from both sides of the low creates interfingering
- This map shows near-bed current predictions modeled with NCOM for Trade-wind conditions (Slingerland et al., 2008a). - Gold lines map packages that prograde to the north-east and red lines map packages that prograde to the southwest. - Most of the packages prograde to the northeast, which agrees with near-bed current predictions for the Trade wind season. - Southwestward-prograding packages exist mainly where canyon heads incise into the shelf
- Northeast progradation into the lows builds the clinoform obliquely. - This causes topographic highs and intervening lows to shift to-wards the northeast
b d
e
28 m
26 m
27 m
15 m
