CLINOFORM DEVELOPMENT BY ADVECTION-DIFFUSION OF SUSPENDED SEDIMENT - MODELING AND COMPARISON TO NATURAL SYSTEMS

Clinoforms are the building blocks of prograding stratigraphic sequenc es. These sigmoid-shaped surfaces can be found forming today on modern deltas. Sedimentation rate profiles over the clinoform surface of the se deltas show low rates of sediment accumulation on both topset and b ottomset regions, with a maximum accumulation rate on the upper forese t region. We present a model for the formation of clinoforms that reli es on the interpretation of modem clinoform sedimentation as a result of the distribution of shear stresses at the mouth of a river. Model c linoform surfaces are generated using an equation for the conservation of suspended sediment concentration, together with a conservation of fluid equation for simple time-averaged flow velocity fields. In the m odel, suspended sediment is advected horizontally into a basin, and gr avitational settling of sediment particles is counteracted by vertical turbulent diffusion. In shallow water, shear stresses are too large t o allow deposition, and sediment bypasses the topset region. With incr easing water depth, near-bed shear stresses decrease, and sediment is allowed to deposit at the foreset region, with gradually decreasing ra tes toward deeper water. This sedimentation pattern leads to progradat ion of the clinoform surfaces through time. The clinoform surfaces pro duced by the model capture the fundamental morphological characteristi cs of natural clinoforms. These include the gradual slope rollover at the topset and bottomset, steeper foreset slopes with increased grain size, and an increase in foreset slope through time as clinoforms prog rade into deeper water. Because the parameters controlling the model c linoforms have a direct relation to physical quantities that can be me asured in natural systems, the model is an important step toward unrav eling the physical processes associated with these deposits.