[S-CG64] [EE] Morphodynamics and Genetic Stratigraphy for Understanding Landforms and Strata

[SCG64-10] The Kinematics and Sedimentary Record of a Self-evolving Continental Slope Fed by a Prograding Shelf Delta

*Anjali M Fernandes^1,2、Kyle M. Straub² (1.University of Connecticut、2.Tulane University)

キーワード：shelf-margins, shelf-edge deltas, stacking patterns, advection length

A key issue with elucidating terrestrial environmental signals from the deep-sea stratigraphic record is the lack of quantitative theory that facilitates: 1) the separation of signals associated with local autogenics and autogenics of the upstream feeder system, and 2) the separation of autogenic signals from tectonic, climate and sea-level signals. We take the first steps towards understanding the kinematics of sedimentation and the resulting stratigraphic record associated with the coevolution of a prograding shelf delta and the down-dip continental slope fed by turbidity currents, using physical experiments. In a 26 hour experiment, we prograded a delta across a flat shelf, under conditions of constant sea-level rise to mimic pseudo subsidence. We analyze patterns of flow and sedimentation from overhead photographs and topographic scans to characterize the kinematic evolution of the slope during the delta’s approach to the shelf-edge. In this experiment, the shoreline marked the transition between a transport-limited regime and an advection-settling regime in this experiment. The continental slope experienced a gradual roughening of the surface tied to a growth of depositional topography. As the delta migrated towards the shelf-edge, the proximal slope initially exhibited the vertical growth of topography through persistent, laterally discontinuous sedimentation associated with pathways of higher flow velocity (and likely higher sediment concentrations), followed by compensational lateral stacking of lobes characterized by high sedimentation rates downstream of active delta channels. As the upstream feeder channel migrated laterally, or subaqueous flow on the slope was steered around growing topography, sediment rapidly filled in topographic lows, smoothing away topographic roughness through compensational stacking of sediment bodies. Locations farther down-dip of sediment lobes, characterized by low sedimentation rates, continued to exhibit the vertical growth of topography through persistent, laterally discontinuous sedimentation. The paths of higher velocity flow (likely carrying higher sediment concentrations) in these distal locations remained relatively unchanged for the duration of the experiment.
Our results support the hypothesis that the advection length of the modal grain-size in suspension at the transition from transport-limited to advection settling-dominated, represents the length scale that separated compensational and random or persistent stacking of deposits. Compensational stacking only occurred in zones where: 1) local flow paths were influenced by close proximity to the laterally migrating deltaic feeder channel, and/or 2) rapidly growing lobes characterized by high sedimentation rates were capable of steering flow. These sediment lobes were constructed primarily from coarse silt- the modal grain-size in suspension at the shoreline, and developed at distances bracketed by the estimated advection lengths of coarse silt. Downstream of sediment lobes, temporally persistent and laterally discontinuous sedimentation of finer sediment resulted in the continuous vertical growth of topography. In natural systems, it is likely that distal locations may never enter the “sweet spot” of high sedimentation rates associated with the characteristic advection length suspended sediment, without the influence of allogenic factors.