3.09 Winnowing

Inputs: Clastic wave winnowing depths and rates

Figure 3.9.1. Winnowing of shale by waves from the left.

Figure 3.9.2 As sea level position changes so does the location of Wave Base and the zone of Winnowing.

A second set of parameters could be provided for waves from the left, when sediments were deposited from the right. At any position where winnowing is to occur, the amount of shale removed through the water column may be varied by changing the percentage winnowed with depth. This gives a means of concentrating sand at the shelf edge, provided there is enough sand entering the basin to reach the shelf edge. If there is not enough sand, a depression at that location will result. This winnowed shale is NOT accounted for and is lost from the simulation. Winnowing curves cannot be varied as a function of time, and apply for the entire simulation run.

Figure 3.9.3. Plotter for Winnowing from the left (shale deposition from the right).

Winnowing from either the left or right side of the section (or both) is input by selecting Winnowing from the SEDPAK EDIT panel. A pull down menu is displayed with two options: ...from the Left, and ...from the Right. A depth-percentage pair is entered in the data sheet or plotter, thereby enabling the amount of winnowing to be varied through the water column. Remember not to place the curves too close together so that they lie within a single column, resulting in a space-aliasing error (see Section 1.11). The user is advised to prescribe a smoothly and gradually declining winnowing curve with depth, in order to avoid production of unrealistic jagged geometries.

Figure 3.9.4. Data sheet for Winnowing from the left (shale deposition from the right).

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3.10 Depositional Parameters

Inputs: Deposition constants

Discussion: Depositional Parameters

Figure 3.10.1. Alluvial and submarine angles of the sediment surface.

When erosion is turned on, the simulation treats all clastic and carbonate sediment types equally. Any clastic sediment eroded from the sedimentary section will be added to the amount of clastic sediments being deposited in the basin for the current time step. If carbonates also are deposited, they will be eroded down to the Submarine angle of repose (s). If carbonates are deposited without clastics, erosion will not occur.

Figure 3.10.2. Dialog for Depositional Parameters.

Figure 3.10.2 shows the Depositional Parameters dialog. It is invoked by selecting Depositional Parameters from the SEDPAK EDIT menu. To avoid confusion regarding the difference between the "angles of deposition" and "angles of repose", it should be remembered that the Deep depositional angle and Shallow depositional angle are angles up to which sediment will accumulate. In contrast, the Submarine angle of repose is the angle down to which erosion will occur. Above sea level, the Alluvial angle of repose refers to both the angle up to which clastics will be deposited and, following compaction and subsidence should they lie above this angle, it is the angle down to which erosion will occur. The Alluvial angle of repose is also used for the erosion of carbonates. The Deep depositional angle and Shallow depositional angle must not exceed the Submarine angle of repose. The angles are used if a submarine angle of deposition at depth is different from the shallow angle. This is useful if turbidites are to be created. If a difference in angles is required, a value for Depth change from shallow to deep must be entered. Remember that the depositional angles apply to both sand and shale, while repose angles apply to sand, shale, and carbonates.

Erosion Enhancement refers to a constant which controls the rate of removal of both clastics and carbonates as a function of the angle of slope and the height of the sediment surface at the end of each time step, depending upon the number entered. This parameter ranges from 0.0 to 1.0 and enhances the rate at which sediment is eroded. Erosion continues to occur, even if Erosion Enhancement is set to zero. This constant is applied throughout the simulation.

Chapter 3, Section 11

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