Journal of the Geological Society, London, Vol. 155, 1998, pp. 217–222. Printed in Great Britain.
Rates of delta progradation during highstands: consequences for timing of deposition in deep-marine systems
PETER M. BURGESS1 & NIELS HOVIUS2 1Department of Earth Sciences, University of Liverpool, Brownlow Street, Liverpool L69 3BX, UK (e-mail: [email protected]) 2Penn State Geosciences, 442 Deike Building, University Park, Pa. 16802, USA
Abstract: Estimated times required for 24 modern river systems to form a shelf-edge delta range from 8.5 ka to 116.5 ka, depending on fluvial sediment supply, delta width, shelf volume and shelf transport rates. These values indicate that transport of sand into deep-marine systems is likely to be significant during third-order highstands of relative sea-level. Factors such as shelf transport dynamics may slow delta progradation while submarine canyons cutting the shelf may reduce the time before deep-marine deposition occurs. Interpreting ancient sand-rich deep-marine strata as lowstand deposits without sufficient palaeogeographic information may not therefore always be appropriate.
Keywords: deltas, deep-sea sedimentation, sediment transport, progradation.
A central assumption of sequence stratigraphic depositional ics, to highlight likely conditions for significant highstand models is that sedimentation increases within deep-water deep-marine deposition that may be identified in the ancient systems during relative sea-level lowstands (e.g. Posamentier & record. Vail 1988; Van Wagoner et al. 1990). For shelf-to-slope physiographies it is assumed that during periods of relative sea-level fall, sediment which would otherwise be deposited on Data the shelf is transported across the subaerially exposed shelf by Delta progradation may be considered in terms of interaction fluvial systems and deposited in a deep-marine environment between the fluvial supply rate Q and the marine transport beyond the shelf-to-slope break. Consequently increases in F rate Q . Marine transport occurs by processes such as wind- rates of deep-marine deposition, and particularly onset of T driven flows, internal waves, wave-orbital flows, tidal currents, deposition of sand-grade sediment, are thought to occur due to impinging oceanic currents and infragravity and surf-zone relative sea-level fall and thus may be correlated to type-1 processes (Nittrouer & Wright 1994). Adopting a sediment sequence boundaries. continuity approach for the shelf volume (Fig. 1), a deltaic and This assumption, although widely applied (e.g. Greenlee & marine erosion/deposition rate Q may be defined as Moore 1988; Mitchum et al. 1994; Milton & Dyce 1995), DE overlooks some fundamental observations. For example, Q =Q Q . (1) Kolla & Perlmutter (1993) demonstrate that turbidite DE F" T deposition in the Mississippi Fan continued during Holocene Delta progradation will occur when Q is positive. transgression and similar situations are described on the DE Fundamentally, when Q >0, sediment is transported along Amazon Fan (Flood et al. 1991), and the Navy Fan (Piper & T and/or across the shelf away from the delta and potentially off Normark 1983). Turbidite deposition also occurs throughout the shelf into deeper water. Although this simple statement the Holocene transgression and highstand in other Californian does not account for complications such as cross-shelf Borderland fans (Schwalbach et al. 1996) and on the Bengal variations in transport processes and grain size control on Fan (Kuehl et al. 1989; Weber et al. 1997). In these cases, local transport, it does illustrate that shelf bypass is not solely conditions such as high fluvial discharge, headward erosion of dependent on accommodation space. However, in many cases submarine canyons and/or transport of sediment into canyons deep-marine deposition will be more pronounced once a by shelf-parallel currents promote sediment transport off the shelf-edge delta has formed and fluvial sediment is transferred shelf into deep water. directly into the deep marine system via buoyant flows or Furthermore, deep-marine deposition may significantly delta-front slumping. increase after delta progradation to the shelf edge. Many Data for 24 rivers are presented in Table 1. Estimates of modern deltas are known to have prograded throughout much fluvial sediment supply, taken from Hovius (in press) were of the Holocene marine transgression and resultant highstand. This suggests that formation of shelf-edge deltas and conse- quent significant deep-marine deposition is not limited to times of relative sea-level lowstand and early transgression (Einsele 1996). This thesis is substantiated by examining modern fluvial sediment supply rates, and the physiographies, volumes and dynamics of continental shelves. We estimate shelf edge progradation times for modern delta systems and evaluate Fig. 1. Definition of a shelf system in terms of the sediment the effects of sediment supply and shelf geometry and dynam- continuity equation.
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Table 1. Fluvial supply volumes, shelf volumes and predicted progradation times
Fluvial Delta Shelf Minimum Maximum sediment system Shelf edge Shelf progradation progradation supply width width depth volume time time 3 "1 3 River QF (m ka ) (km) (km) (m) VS (m ) TMIN (ka) TMAX (ka)
Amazon* 5.75#1011 330 320 90 4.75#1012 11.6 12.7 Brahmaputra* 2.60#1011 400 210 200 8.40#1012 45.2 56.0 Mississippi 2.00#1011 250 115 130 1.87#1012 13.1 17.4 Irrawady 1.30#1011 260 185 120 2.89#1012 31.1 50.5 Indus 1.25#1011 240 115 100 1.38#1012 15.5 25.8 Orinoco* 7.50#1010 265 105 90 1.25#1012 23.4 70.1 Mackenzie* 6.25#1010 260 200 200 5.20#1012 116.5 582.4 Nile 6.25#1010 200 50 250 1.25#1012 28.0 140.0 Orange 4.55#1010 55 180 220 1.09#1012 33.5 ` Copper* 3.50#1010 25 85 200 2.13#1011 8.5 ` Danube 3.50#1010 80 112 60 2.69#1011 10.6 ` Fly 3.50#1010 70 145 65 3.30#1011 13.2 ` Krishna 3.25#1010 105 48 80 2.02#1011 8.7 ` Rhone 3.00#1010 70 48 130 2.18#1011 10.2 ` Zaire* 1.64#1010 70 70 200 4.90#1011 41.8 ` Brazos 1.55#1010 50 140 100 3.50#1011 31.6 ` Rio Grande 1.50#1010 50 80 200 4.00#1011 37.3 ` Kizil Irmak 1.15#1010 130 35 100 2.28#1011 27.7 ` Ebro 1.05#1010 35 70 125 1.53#1011 20.4 ` Volta 9.50#109 40 40 200 1.60#1011 23.6 ` Po* 9.00#109 50 240 120 7.20#1011 112.0 ` Columbia 7.50#109 60 84 165 4.16#1011 77.6 ` Colorado 6.50#109 50 105 200 5.25#1011 113.1 ` Mahakam* 6.00#109 90 40 130 2.34#1011 54.6 `
*Load unadjusted for anthropogenic influence.
derived from measured suspended load, adjusted, where may add significant volumes of coarser grained material, possible, for anthropogenic influence. Resulting sediment shortening progradation times and increasing deposition volume per kiloyear is calculated assuming an uncompacted of deep-marine sand. Overestimates may also arise from density of 2000 kg m"3. Cross-sections constructed from assuming filling of the entire rectangular shelf wedge. Deep- admiralty charts at various scales suggest shelf geometries are marine deposition will actually increase as soon as the base of approximately triangular, so shelf volume was calculated the delta front approaches the shelf edge. assuming a uniform triangular cross-section perpendicular to Predicted progradation times range from <10 ka (e.g. the shelf-to-slope break. Shelf widths were measured from Copper, Krishna) to >100 ka (e.g. Po, Mackenzie) (Table 1). river mouth to shelf-to-slope break, or if a protruding delta is Differences in predicted minimum times result from differences present, from the adjacent shoreline relatively unaffected by in sediment supply rate and/or delta and shelf geometries. deltaic progradation. The coast-parallel length component of Narrow, shallow shelves and narrow delta systems may have shelf volume was based on measured widths of delta systems. short progradation times, even for rivers with relatively small sediment loads such as the Krishna River, India.
Minimum delta progradation time
Taking QT=0, a minimum time required for deltaic progra- Complications dation to the shelf edge, filling the shelf volume over the width Modern delta systems formed during the Holocene transgres- of the delta system, can be calculated by sion. The rate of eustatic rise has been calculated at 13.6 m k "1 from 14 to 7 ka , slowing down to 2.1 m ka"1 after 7 ka (Fairbanks 1989). Thus the predicted progradation times can be assessed for accuracy by comparison with observed distances of progradation since 7 ka. Comparing the Amazon 3 where TMIN is in ka, Vs is the shelf volume in m , QF is the and Mississippi cases highlights potential complexities due to volume of fluvial sediment supplied (m3 ka"1), and C is a shelf transport dynamics. dimensionless constant to account for creation of space due The youngest lobe of the fluvial-dominated Mississippi to sediment loading and compaction. Assuming the same Delta has almost reached the shelf edge (Frazier, 1967; sediment density and implied porosity as above, C=1.4 for all Penland et al. 1988) (Fig. 2b) prograding across the systems considered. storm-dominated but low-energy epicontinental Gulf of It is important to note that this method yields only crude Mexico shelf (Johnson & Baldwin 1996; Nittrouer & Wright approximations of minimum progradation times. We have not 1994). Progradation has not been uniform across the width of included a bedload component in the sediment supply. This the delta complex however, and the next active lobe will be
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comparison of deposited marine volumes and measured fluvial input, constrained by observed flow rates, and on comparative bathymetric studies. The latter method suggests 1.6#108 m3 ka"1 for the southeast USA Atlantic shelf (Byrnes & Hiland 1995). Based on measured fluvial inputs, the Agulhas Current transports up to 5.9#1010 m3 ka"1 along the narrow eastern shelf of Southern Africa (Fleming 1981). These data, though limited in number and of uncertain accuracy, suggest that a shelf transport rate of 5#1010 m3 ka"1 probably represents a high value for modern shelves, and hence is suitable for calculating maximum delta progradation times. A maximum time for delta progradation to the shelf edge,
TMAX is given by
and calculated values for TMAX are given in Table 1. For 16 rivers this shelf transport rate is greater than the fluvial input
rate (QDE<0) and hence delta progradation is not predicted. However, actual transport rates on those specific shelves may well be lower than the value used here, allowing progradation to occur. Submarine canyons cutting into continental shelves will influence deep-marine deposition. Shelf-parallel sediment streams may be intercepted by canyons and transported off the shelf. Canyons near river mouths may capture much fluvial supply. For example, the Indus river supplies 1.25#1011 m3 ka"1 of which 2.75#1010 m3 ka"1 is funneled into the Swatch canyon and deposited on the Indus fan (Milliman et al. 1984) (Fig. 2c). A submarine canyon extends several kilometres inland up the Zaire river, so that at the coastline the river is 600 m deep (Fig. 2d). Deep-marine deposition will occur throughout relative sea-level cycles from such rivers and volumes may not increase significantly during lowstands.
Fig. 2. Delta, shelf and slope planforms, and shelf-to-slope sections from the Amazon (a), Mississippi (b), Indus (c) and Zaire (d) rivers. Sand deposition The Amazon exemplifies a system with significant shelf transport Consideration of grain size populations is also important since redistributing sediment on the shelf. The Mississippi is a fluvially the proportion of sand-grade sediment supplied to the shelf dominated system. The Indus and Zaire systems illustrate the indicates how much sand is available to be transported into importance of submarine canyons in transporting sediment off the shelf. deep-marine systems. Sequence stratigraphic models predict that deep marine sand deposition will increase during falling relative sea-level (e.g. Posamentier & Vail 1988). Table 2 shows significantly further from the shelf edge. Although more than percentages of clay, silt and sand from 16 rivers, and the half of the shelf planform is filled by Holocene delta lobes, the volume of sand supplied per kiloyear. To determine if these oldest Mississippi lobe is now submerged and isostatic loading fractions would affect significantly the depositional histories of and compaction may lead to submergence of more recent submarine fans fed from shelf-edge deltas, a sand deposition lobes. Given these observations the predicted minimum rate is calculated, assuming even deposition over the whole fan progradation time of 13.1 ka appears reasonably accurate. area, for the Amazon, Brahmaputra, Indus, Mississippi and In contrast, Amazonian sediment delivered by buoyant Nile systems (Table 2). Fan areas are taken from Reading & plume is redistributed along c. 600 km of the South American Richards (1994). For the Mississippi and Indus fans the rates Atlantic shelf by the wind driven Guiana Current, itself are very low, due to the small percentage of sand in the fluvial frequently enhanced by wave action (Johnson & Baldwin 1996) supply, but for the Amazon and Nile fans they are much (Fig. 2a). Flow velocities of 0.35–0.75 m s"1 cause sedi- higher. Rates would be still higher if deposition were confined ment transport parallel to the shelf. Transport off the shelf to individual lobes on the fans. This is particularly relevant in occurs, but northwards into the Gulf of Paria rather than the case of the Brahmaputra River, which supplies the largest eastwards to the Amazon Fan. Thus shelf transport dynamics volume of sand of all rivers discussed herein, but also has the substantially reduce shore-normal delta progradation (Burke largest fan area. To illustrate this Table 2 also shows depo- 1975; Nittrouer et al. 1986). sition rate for a restricted 100 000 km2 area, analogous in scale Consequently, constraining shelf transport rates is to a large lobe on the Pleistocene Mississippi fan (Bouma et al. important, but rates of marine sediment transport are difficult 1985. Rates for rivers delivering large sediment loads (i.e. to measure and data are sparse. Estimated rates are based on Amazon and Brahmaputra Rivers) in this case are very high,
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Table 2. Fluvial grain size fractions and predicted fan sand deposition rates
Sand Fan Whole fan Restricted supply rate surface area deposition rate depostion rate River % clay % silt %sand (m3 ka"1) (km2) (m ka"1) (m ka"1)
Brahmaputra* 5 45 45 1.17#1011 3 000 000 0.039 11.70 Amazon* 45 28 16 9.20#1010 330 000 0.278 9.20 Copper* 0 2 98 3.43#1010 3.43 Krishna 23 35 42 1.37#1010 1.37 Nile 35 45 20 1.25#1010 70 000 0.179 1.25 Orinoco* 26 67 7 5.25#109 0.53 Mississippi 50 48 2 4.00#109 300 000 0.013 0.40 Mackenzie* 45 50 5 3.13#109 0.31 Indus 20 78 2 2.50#109 1 100 000 0.002 0.25 Po* 7 70 23 2.07#109 0.21 Mahakam* 35 35 30 1.80#109 0.18 Brazos 63 27 10 1.55#109 0.16 Colorado 70 25 15 9.75#108 0.10 Rio Grande 75 21 4 6.00#108 0.06 Zaire* 36 52 2 3.28#108 0.03
*Sand supply unadjusted for anthropogenic influence.
but even some rivers with smaller loads achieve rates in excess area, but this is also dependent on climatic conditions which of 1 m ka"1. It seems reasonable to conclude from this simple may be only indirectly linked to eustatic change. This is analysis that deep-marine sand deposition via highstand shelf- illustrated by the Mississippi river where increased sediment edge deltas could be significant. supply during the Holocene transgression was due to melting ice sheets and erosion of glacial deposits (Kolla & Perlmutter 1993). Also, higher loads may be mitigated for many rivers by Significance of progradation times to third order cycles lower gradients promoting trapping of sediment in coastal Third-order relative sea-level cycles have highstands of plains. c. 1 Ma. In all modern examples discussed here, assuming low In cases where a fluvial system is capable of incising into the shelf transport rates, there would be sufficient time during continental shelf in response to a relative sea-level fall, will a third-order cycle highstand for development of shelf- the volume of sand eroded be significant relative to highstand edge deltas and consequent transport of sediment into deep sand supply? To answer this we consider the Mississippi River marine environments. If shelf transport rates were high, trans- assuming an incision distance of 350 km upstream during port off the shelf would probably occur anyway though not the last glacial lowstand occurring over an interval of necessarily oceanward of the delta. Therefore deposition in approximately 100 ka (Autin et al. 1991; Leeder & Stewart deep-marine systems should be common during third-order 1996). Assuming a relative sea-level fall of 100 m (Matthews highstands. 1990), and a linear decrease in incision amplitude to zero If higher-order relative sea-level oscillations are symmetri- at 350 km landward of the lowstand shoreline, occurring cal, their sum effect on the third-order trend will be zero. uniformly over a width of 100 km, and cutting down into a Also, if amplitudes are small relative to the third-order ampli- pure sand substrate, 1.75#1012 m3 of sand would be denuded. tudes, impact on delta progradation will be minimal. Thus Assuming a constant erosion rate this volume gives a sand higher-order cycles may interrupt delta progradation but supply rate of 1.75#1010 m3 ka"1, only four times greater should not prevent formation of highstand shelf-edge deltas. than the modern rate of sand supply from the Mississippi, or 8.75% of the total modern sediment supply. It is more realistic to assume that only a small fraction of the sediment eroded ff E ects of relative sea-level fall during incision would be sand grade or coarser, substantially Mulder & Syvitski (1996) analysed recent drainage basin reducing the estimate of sand volume. hydrology, morphometry and climate for 279 rivers to deter- These calculations suggest that incision will not always mine the likely effects of an eustatic sea-level fall on fluvial cause a significant increase of fluvial sand supply relative to sediment delivery. They found that during a lowstand only a modern highstand values. Consequently lowstand deep-marine minority of rivers would have the potential to erode across deposition may not be easily distinguished from highstand narrow shelves, supporting Schumm’s (1993) suggestion that deposition in third-order cycles. Shelf-edge delta deposition fluvial incision is not just dependent on relative sea-level fall, and consequent deep-marine deposition may be common in but also on shelf gradient relative to coastal plain gradient, and both cases. Significantly higher volumes of sand deposition potential adjustments in fluvial channel pattern. Most rivers during lowstand will occur only at points where large, merged would merge during lowstands to form mega-rivers with lower river systems debouch into deep-marine basins under favour- gradients and reduced hyperpycnal plume activity, but more able climatic conditions (Mulder & Syvitski 1996). When frequent shelf-edge deltas (relative to the Holocene) and con- relative sea-level fall is tectonically driven these preconditions sequent increased turbidity current activity. River-mouth sedi- are particularly important, since source area climate may not ment supply would increase because of increased drainage change.
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Received 29 September 1997, revised typescript accepted 28 October 1997. Scientific editing by Nick Rogers.
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