Both marine and non-marine transport rates are controlled by the topographic gradient (Equation 1):
Where q is volumetric sediment flux (l2/t), h is elevation (l), x is horizontal position (l), and k is the diffusion constant (l2/t).
See Jordan and Flemings (1991) and Kaufman et al. (1991) for insight into the meaning of the diffusion constants. To generate a realistic shelf break, the `nonmarine diffusion constant' (Knonmarine) is set to a high value, while the `marine diffusion coefficient' (Kmarine) is set to a much lower value.The following figures demonstrate the effects of varying the diffusion constants. A simple rule of thumb is that to generate steeper clinoforms you should decrease the marine diffusion constant. Jordan and Flemings (1991) found reasonable values for Kmarine to be on the order of 100-1000 m2/yr, while that of the non-marine regime to be on the order of 1e4 to 1e5 m2/yr.
The library file diff1.dat is used to simulate the development of an alluvial fan. The most important controls on this model are the clastic flux and the diffusion constants (both found in the Clastics Group File).
Figure 1: Run in which marine and non-marine diffusion constants are equal. This simulates the development of an alluvial fan. Library file is diff1.dat with skip lines set at 2 and a vertical exaggeration of 200 (in
Marine deposition is simulated below with two examples. The effect of diffusion constants on the shelf topography is displayed by holding the non-marine diffusion constant at 50000 m2/yr while varying the marine diffusion constant. It is evident from Figure 2.a, that a smaller marine diffusion constant will generate much steeper shelf breaks.
Figure 2.a: Model run of marine diffusion constant = 500 m2/yr; non-marine diffusion constant = 50000 m2/yr. This simulation of a prograding delta exhibits a shelf break marked by the change in slope from near horizontal to steeply dipping. Library file is diff2.dat with vertical exaggeration set to 25.
Figure 2.b: Model run of marine diffusion constant = 5000 m2/yr; non-marine diffusion constant = 50000 m2/yr. The marine diffusion constant is higher than in figure 3.b, therefore the marine slopes are lower. Library file is diff3.dat with a vertical exaggeration of 25.
If the user wishes to change the rate of clastic flux through time, a clastic flux file can be used in place of a constant flux. If none is specified, the model uses the constant clastic sediment flux values specified by `left clastic flux' and `right clastic flux.'
Example of a clastic flux file:
Clastic flux file notes:
1) The first column is the age with 0 as the beginning of the simulation
There are two ways to calculate percent sand in the model: by linear interpolation of the diffusion values (Figure 3.a), where the highest value is set to 100% sand and the lowest to 100% shale, or as an exponential function of water depth (Figure 3.b). In order to have the model linearly interpolate values, set `cutoff for sand composition' (Compaction Group File) to a negative number. Alternatively, you may set it to a cut-off water depth value (positive number) and give the `decay constant for composition' (Compaction Group File) a value. The percent sand will then decline exponentially as a function of water depth (from 100% sand) from the cut-off water depth.
For example, if `cutoff for sand composition' is set to 10, and `decay constant for composition' is set to 0.1, then above 10 meters water depth the composition will be 100% sand and below 10 meters water depth, the composition will decline exponentially from 100%.
The `decay constant for composition' line determines the rate at which the sand composition will exponentially decline from a given cutoff depth set in `cutoff for sand composition.' Figure 3.b shows the effect of setting a cutoff depth and decay constant.
Figure 3.a: Example where sand composition is calculated as a linear interpolation between the maximum and minimum diffusion values in the model. (library file sand_linear.dat with skip lines set to 49 and composition contoured at 0.1)
Figure 3.b: Example where sand composition is defined as an exponential function of specified cut-off depth (here set to 15m). (library file sand_cutoff_15m.dat, with the same line skip and contouring as 3.a) In this example this has had the effect of distributing more sand seaward than in Figure 3.a. Dashed lines indicate increments of ten percent sand (increasing to left).
2.2 Marine Clastic Deposition
0 10.00 1e6 17.08 2e6 20.00 3e6 17.08 4e6 10.00 5e6 2.93 6e6 0.01
2) The second column is the clastic flux in L2/T
3) Fluxes between given points are linearly interpolated2.3 Composition
Last Modified: 01:47pm EST, February 22, 1996 - Steven E. Nelson
