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Rates of Delta Progradation During Highstands: Consequences for Timing of Deposition in Deep-Marine Systems

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Rates of Delta Progradation During Highstands: Consequences for Timing of Deposition in Deep-Marine Systems 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. 217 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/155/2/217/4886453/gsjgs.155.2.0217.pdf by guest on 25 September 2021 218 PETER M. BURGESS & NIELS HOVIUS 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.
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