Continental-margin response to sea level: Theory and Experiment

Transcription

1 Continental-margin response to sea level: Theory and Experiment John B. Swenson Department of Geological Sciences and Large Lakes Observatory, University of Minnesota Duluth Chris Paola, Wonsuck Kim, Ben Sheets, Nikki Strong Saint Anthony Falls Laboratory and Dept. Geology & Geophysics University of Minnesota Lincoln Pratson Earth and Ocean Sciences, Duke University National Center for Earth-surface Dynamics

2 Topics: Relative importance of terrestrial floods and coastal storms in controlling clinoform response to sea level Controls on shoreline behavior in clinoforms: The A/S ratio revisited

3 Characteristic-event approach On GEOLOGIC TIME SCALES: Fluvial surface evolves in response to floods (e.g., Paola et al., 1992) Shallow-marine surface evolves in response to large coastal storms Floods and storms occur with known frequency and magnitude Replace stochastic forcing with simple impulse functions Storm / flood phasing unimportant Detailed knowledge of forcing unavailable in ancient systems

4 Single-equation ( integrated ) morphodynamics Objective: Express sediment flux as a function of bed elevation and its spatial derivatives Approach integrates over many characteristic events (floods and storms) and river avulsions Fluvial morphodynamics = Diffusion (e.g., Paola, 2000) q s = f(s) Shallow-marine morphodynamics = Non-linear advection / diffusion (after Coco, 1999) q s = f(h-η, S)

5 Shoreline Dynamics Shoreline position unknown a priori moving-boundary problem Collapse surf-zone geometry to a shoreface shock condition Shoreface movement driven by: Discontinuity in shore-normal flux (Swenson et al., 2000) Divergence in shore-parallel flux (Ashton et al., 2001)

6 Dimensionless numbers controlling (coupled) clinoform morphodynamics Dimensionless celerity scale: c = f ( H,D,U,I,I ) * s f, Qs Dimensionless diffusivity scale: υ ( H,D,I,I ) * = f s f, Qw H = Wave height D = Grain size U = Current I s = Storm intermittency I f = Flood intermittency Q s, Q w = Sediment & water discharge per characteristic flood Relatively small number of parameters Constrained / estimated for ancient systems Comparison between laboratory and natural scales

7 Floods vs. storms in development of highstand clinoforms: Increasing the frequency and/or magnitude of coastal storms increases sediment partitioning to the shallow-marine realm and generates rapidly expanding subaqueous deltas with broad, low-gradient topsets

8 Clinoform response to high-amplitude sea-level cycling: Flood dominated Storm dominated Increasing frequency and/or magnitude of coastal storms increases: (1) relative motion between shoreline and rollover; (2) extent of sequence boundaries; and (3) transgressive erosion

9 3D margin response to high-amplitude sea-level cycling: Flood dominated Storm dominated

10 Conclusions: Clinoform morphology is sensitive to the frequency and magnitude of floods and coastal storms Flood-dominated clinoforms: Strong sediment partitioning to the fluvial environment; Narrow subaqueous topsets; Spatially restricted sequence boundaries Storm-dominated clinoforms: Wide, low-gradient subaqueous topsets; Rapid growth of subaqueous delta; Extensive sequence boundaries / transgressive erosion First-order predictions are testable

11 Topics: Relative importance of terrestrial floods and coastal storms in controlling clinoform response to sea level Controls on shoreline behavior in clinoforms: The A/S ratio revisited (Wonsuck Kim)

12 Jervey s A/S concept in cartoon form: Shoreline trajectory controlled by A/S ratio: A = Accommodation S = Sediment supply A/S > 1: transgression A/S < 1: regression A/S = 1: vertical aggradation

13 Problems with A/S: Neither term is rigorously defined Not clear that the ratio is dimensionless depends on interpretation of terms

14 A more rigorous definition: After Swenson et al. (2000)

15 A more rigorous definition (cont.): Shoreline position ds dt s+ l mar 1 dh = q l SL sshl mar αlmar dt s Foreset slope Sediment flux at shoreline Foreset length Eustatic sea level σ dx Subsidence rate Migration rate Supply Accommodation After Swenson et al. (2000)

16 Observed shoreline (ShL) trajectory: (Jurassic Tank, 2002 sea-level run) Actual stratigraphy from scanning data s H SL Horizontal component: ds dt base level high base level low basement base level shoreline trajectory Vertical component: dh SL dt -0.8

17 Simplifying assumptions: Bypassing fluvial system: q q sshl s0 Constant subsidence rate over foreset: s+ l s mar σ dx σ ShL l mar

18 Simplified A/S relation: Use average values for all but eustatic sea level ds dt 1 dh = q l SL + σ s0 mar ShL αl dt mar Migration rate Supply Accommodation This ratio: l mar dh dt q SL s0 +σ SL is a dimensionally consistent, physically meaningful A/S ratio

19 Shoreline migration rate predicted from eustatic change only: ds/dt observed Level 0: Sea level change only The Moral of the Story: Even with fairly severe simplifying assumptions, and using mean values for all but eustatic term, the modified A- S criterion predicts shoreline behavior reasonably well Next step: Evaluate relative importance of terms we ignored or simplified ds/dt predicted