Experiments Evaluating the Interaction between Deformable Substrates and Prograding Clinoforms

Passive margins often contain salt and mobile shale layers that act as deformable substrates underlying coastal sediments. By understanding the interaction between deformable substrates and migrating clinoforms in simplified, experimental settings, we hope to clarify their fundamental behavior in natural deltaic settings. Substrate deformation occurs by a number of processes and can result in rugose ocean floor bathymetry which can locally trap sediment. In our experiments, we focus on how differential sediment loading on two substrates of different rheologies (Newtonian versus Bingham) affects aspects of clinoform deposition. We are particularly interested in the effects of the progradation rate and substrate rheology on the locus of clinoform deposition over time as the substrate and delta interact. Our experimental setup uses a rectangular flume that is 230 cm long, 8 cm wide and 30 cm tall. The materials we used for the mobile shale experiments involved a mixture of kaolinite and water which provide a Bingham rheology, and the sediment was a quartz sand and coal mixture. For a Newtonian rheology we used corn syrup as a deformable substrate, capturing the Newtonian rheology of subsurface salt, and the sediment consisted of a mixture of equal volumes of walnut shells and kaolinite clay. For each experiment we systematically changed sediment feed rate and substrate thickness. Deformation styles differ qualitatively between Bingham (kaolinite/mobile shale) and Newtonian (corn syrup/salt) experiments. In mobile-shale (Bingham) experiments, a bulge forms at the toe of the clinoform and the clinoform eventually overtops this bulge, creating alternating thick and thin stratigraphic accumulations down depositional dip. For the salt (Newtonian) experiments, deformation occurs behind the clinoform foreset in the form of diapirs. Preliminary results show minimal impacts on shoreline progradation rate as the delta loads mobile shale substrates. This suggests that the deformation shortens the available accommodation depth and changes in the partitioning of sediment in the topset and toeset of the clinoform. Shoreline progradation rate for the salt substrates varies with time showing a constant long term rate with periods of slowed advance. This suggests that the sediment is sinking into the substrate, creating space. The clinoform has to fill the vertical space before moving laterally rate is periodically stifled due to a major slope failure on the foreset, potentially in response to diapirism below the topset of the clinoform.