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BOULDER TRANSPORT ACROSS THE EBERSWALDE DELTA Alan D. Howard, University of Virginia Rossman Irwin, III, Smithsonian Institution Jeffrey Moore, NASA Ames.

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Presentation on theme: "BOULDER TRANSPORT ACROSS THE EBERSWALDE DELTA Alan D. Howard, University of Virginia Rossman Irwin, III, Smithsonian Institution Jeffrey Moore, NASA Ames."— Presentation transcript:

1 BOULDER TRANSPORT ACROSS THE EBERSWALDE DELTA Alan D. Howard, University of Virginia Rossman Irwin, III, Smithsonian Institution Jeffrey Moore, NASA Ames Gary Parker, University of Illinois William E. Dietrich, University of California, Berkeley

2 HiRISE Observations A recent HiRISE image (PSP_001336_1560) covers most of the Eberswalde delta at a resolution of about 0.25 m/pixel This image reveals local concentrations of boulders exceeding 1 m in size. The caption for this image suggests that the boulders “were likely too coarse to have been transported by water flowing within the channels” and suggests that the boulders are weathered fragments of lithified channel sandstones. We suggest that some of the boulder deposits are likely to be primary depositional features and that flows through the deltaic channels may have been competent to transport these boulders.

3 From Moore, Howard, Dietrich, and Schenk, 2003 Eberswalde Crater Delta

4 The volume of the delta approximately equals the volume of the incised valleys upstream

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9 Possible Origins of the Boulders Weathered fragments of indurated fine sediment Weathered fragments of indurated fine sediment Concretions Concretions Deposits from high-magnitude (catastrophic) floods Deposits from high-magnitude (catastrophic) floods Deposits from high-density flows (debris flows) Deposits from high-density flows (debris flows) The coarse end of the grain size distribution of sediment transported across the delta by normal fluvial flows The coarse end of the grain size distribution of sediment transported across the delta by normal fluvial flows

10 Some Boulders ARE Weathering Products Some boulders are clearly released from weathering of consolidated layers in the deltaic deposit Some boulders are clearly released from weathering of consolidated layers in the deltaic deposit Generally the source bed with prismatic fracturing is visible Generally the source bed with prismatic fracturing is visible Boulders are likewise prismatic Boulders are likewise prismatic Some boulders are >5 m Some boulders are >5 m Most have a reddish coloration Most have a reddish coloration

11 Boulders on Delta Surface are Different than Boulders from Weathering Boulders occur as stringers and lenses associated with sinuous channels Boulders occur as stringers and lenses associated with sinuous channels Locally boulders appear to be part of clast-supported deposit Locally boulders appear to be part of clast-supported deposit Source beds are not apparent Source beds are not apparent Boulders generally less than 2 m in size Boulders generally less than 2 m in size Boulders are light-toned Boulders are light-toned

12 Flow and Transport through Normal Fluvial Flows? Channels on the delta are m wide. Channels on the delta are m wide. Estimates from channel dimensions (width, meander wavelength) suggest discharges of m 3 /s. Estimates from channel dimensions (width, meander wavelength) suggest discharges of m 3 /s. The channel gradients are about The channel gradients are about Can such discharges carry meter-scale boulders across such a low gradient? Can such discharges carry meter-scale boulders across such a low gradient?

13 Probably! What flow conditions are necessary for boulder transport? What flow conditions are necessary for boulder transport? Flow transport competence is measured by Flow transport competence is measured by τ * = H S / (S s -1) D 50, where H is flow depth, D 50 is median bed material grain size S is gradient, S s is sediment specific gravity H is flow depth, D 50 is median bed material grain size S is gradient, S s is sediment specific gravity After Parker et al. (2007) we characterize the bankfull hydraulic geometry of streams by dimensionless depth, H *, width B *, and discharge, Q * : After Parker et al. (2007) we characterize the bankfull hydraulic geometry of streams by dimensionless depth, H *, width B *, and discharge, Q * : H * = g 1/5 H / Q, B * = g 1/5 B / Q, Q * = Q / (g 1/2 D 50 3/2 ), Q is bankfull discharge and g is gravity. Q is bankfull discharge and g is gravity.

14 Analysis We analyze flow competence in regard to two scenarios: We analyze flow competence in regard to two scenarios: A strongly bimodal size distribution in which the meter-scale boulders are being transported across a sand or fine gravel bed. Experiments suggest the coarse fraction can be mobilized at τ * = 0.01 (Wilcock and Kenworthy, 2001) A strongly bimodal size distribution in which the meter-scale boulders are being transported across a sand or fine gravel bed. Experiments suggest the coarse fraction can be mobilized at τ * = 0.01 (Wilcock and Kenworthy, 2001) A poorly-sorted gravel and sand mixture with D 90 = 1 m and D 50 = D 90 / 3.5 = 0.29 m. Experiments and field measurements suggest τ * in the range of 0.01 to 0.02 for D 90 (and τ * ≈ 0.04 for D 50 ). A poorly-sorted gravel and sand mixture with D 90 = 1 m and D 50 = D 90 / 3.5 = 0.29 m. Experiments and field measurements suggest τ * in the range of 0.01 to 0.02 for D 90 (and τ * ≈ 0.04 for D 50 ).

15 Summary of Assumptions Fully rough flow Fully rough flow Roughness height, k s = 2 D 90, that is, roughness is dominated by the boulders Roughness height, k s = 2 D 90, that is, roughness is dominated by the boulders Chezy flow resistance C z = 8.1 (H/k s ) 1/6 (Wong and Parker, 2006) Chezy flow resistance C z = 8.1 (H/k s ) 1/6 (Wong and Parker, 2006) Mean flow velocity, V = C z (g H S) 1/2 Mean flow velocity, V = C z (g H S) 1/2 Channel width, B = 50 m Channel width, B = 50 m S = S = τ * in range from 0.01 to 0.05 τ * in range from 0.01 to 0.05 S s = 2.65 S s = 2.65 Martian gravity, g = 3.8 m/s 2 Martian gravity, g = 3.8 m/s 2 D 90 = 1 m D 90 = 1 m

16 Yellow = Assumed values Green = Calculated values Reasonable results for flow depth, velocity, discharge, velocity, and width-depth ratio A low density of boulders could also contribute (e.g., sediment or tephra) τ * c90 (m) H (m) V (m/s) Q (m 3 /s) ,820 B/H H*H* B*B* Q*Q*

17 With the above assumptions and τ * c =0.01 for D 90, a reasonable value for τ * c for D 50 is predicted.

18 Also for τ * c = 0.01 for D 90, the predicted dimensionless hydraulic geometry is consistent with terrestrial values

19 The predicted width-depth ratio is also reasonable.

20 The m deep canyon upstream from the delta is a possible source of the coarse boulders.

21 The End

22 This is located on the floor of the Argyre Basin The image is about 50 km across Ridges have been attributed to being eskers, moraines, shorelines Sinuous, braided ridges Lobate debris aprons (icy!)

23 A recent HiRISE image of these ridges reveals abundant boulders up to 3 m in diameter Image width about 460 m For fluvial transport need hefty! flows, e.g., for B=200 m and τ * c = 0.01 for D 90, predict H = 8.3 m, V = 3.7 m/s, Q = 3300 m 3 /s. Alternatives?

24 Not all channels are coarse-grained. This slide shows sinuous, probably anabranching channels in the Aeolis Mensae region etched in relief by wind erosion The box shows the detailed HiRISE image on the next slide. The north-south-oriented ridges are yardangs, presumably etched into floodplain/swamp sediments

25 85 meters The sinuous channel is probably floored with partially-indurated sand or fine gravel.

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28 Arguments against Origin as Primary Concretions Concretions generally occur as irregular forms and often intergrow into irregular-shaped masses Concretions generally occur as irregular forms and often intergrow into irregular-shaped masses Concretions are often darker than their host deposit, particularly if they involve ferric minerals Concretions are often darker than their host deposit, particularly if they involve ferric minerals Goblin Valley, Utah

29 Origin from Catastrophic Flooding? Here we consider large floods, such as those with X00- year recurrence interval or Jökulhaupt-like high magnitude events Here we consider large floods, such as those with X00- year recurrence interval or Jökulhaupt-like high magnitude events Explaining the boulders from high-magnitude events faces difficulties Explaining the boulders from high-magnitude events faces difficulties Long-distance transport of coarse boulders during a single event generally requires confined flows, such as narrow bedrock valleys Long-distance transport of coarse boulders during a single event generally requires confined flows, such as narrow bedrock valleys In wide valleys or depositional plains large floods generally expose coarse sediment by scour of overbank deposits or by ramping of boulders onto the floodplain. These boulders are generally transported to that location by ~bankfull floods In wide valleys or depositional plains large floods generally expose coarse sediment by scour of overbank deposits or by ramping of boulders onto the floodplain. These boulders are generally transported to that location by ~bankfull floods In unconfined settings such as deltas overbank floods generally are not much deeper than bankfull conditions because of spreading rather than deepening of the flow In unconfined settings such as deltas overbank floods generally are not much deeper than bankfull conditions because of spreading rather than deepening of the flow

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32 More Difficulties with Catastrophic Floods Transport dominance by catastrophic floods is not compatible with observed channel morphology Transport dominance by catastrophic floods is not compatible with observed channel morphology Extremely catastrophic floods widen and straighten channels, and create diagnostic depositional and erosional forms such as longitudinal bars and flood chutes, which are not compatible with orderly development of meander bends Extremely catastrophic floods widen and straighten channels, and create diagnostic depositional and erosional forms such as longitudinal bars and flood chutes, which are not compatible with orderly development of meander bends By contrast, the boulders dominantly occur as stringers associated with sinuous channels with morphology consistent with moderate flood flows (e.g., the annual flood series). By contrast, the boulders dominantly occur as stringers associated with sinuous channels with morphology consistent with moderate flood flows (e.g., the annual flood series).

33 What about Debris Flows? Debris flows can carry large boulders because of the high flow density and high viscosity Debris flows can carry large boulders because of the high flow density and high viscosity High-mobility debris flows are similar in erosional and depositional morphology to high-magnitude floods – inconsistent with delta channel morphology High-mobility debris flows are similar in erosional and depositional morphology to high-magnitude floods – inconsistent with delta channel morphology Low-mobility debris flows terminate as lobate deposits with natural levees – again inconsistent with delta channel Low-mobility debris flows terminate as lobate deposits with natural levees – again inconsistent with delta channel Debris flow deposits are not well sorted into coarse channel and fine overbank sediments Debris flow deposits are not well sorted into coarse channel and fine overbank sediments

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36 Debris flow lobes in the Sierra Nevada Mountains


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