6.15 Clastic Supply

Read Section 3.07 on Clastic Supply.

-Load Ex_Supply.db. Run the simulation.
-Click Clastic Supply and choose supply curves
-Click Options and Show Data sheet
-Change the rates for sand and shale from 0.0002 to 0.0001 using the calculator.
-Does the geometry of the simulation change?
-Increase the Depositional distance by multiplying the distances by 15 for both sand and shale.
-What is the relationship between Clastic supply and Depositional distance?

Figure 6.15.1 shows the result of changing the rate of clastic supply, whereas Figure 6.15.2 shows the result of changing the Depositional Distance.

Figure 6.15.1. Final output for Ans_Supply.db.

Figure 6.15.2. Final output for Ans_Supplydist.db.

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6.16 Depositional Distance

Read Section 3.08 on Depositional Distance, which is accessible from the SEDPAK EDIT menu.

-Load Ex_DepDist.db.
-Click Depositional distance.
The depositional distance is 50 km throughout the simulation, both for sand and shale.
-Change the depositional distance to 125 km, both for sand and shale.
-Save the file with another name.
-Compare both files.
-Now try changing the clastic supply by multiplying them by 2 using the calculator.
-Save the file.
What is the relationship between changing depositional distance and clastic supply?

Figure 6.16.1 shows the result of changing the Depositional Distance to 125 km.

Figure 6.16.1. Final output for Ans_DepDist.db.

Figure 6.16.2 shows the result of increasing the Clastic Supply rates.

Figure 6.16.2. Ans_DepDistClast.db.

Depositional Distance is closely tied to Clastic Supply (Sections 3.07 and 6.15) and often both parameters must be modified to obtain the desired sediment geometries. For example, to produce the prograding geometries, it is necessary to increase the clastic supply as a function of time, while decreasing the depositional distance. The combination of a decreasing depositional distance and increasing clastic supply rate causes the clastics to mound up until the sediment starts to bypass downslope when it exceeds the clastic depositional angles (see Section 3.10). If a ramp-like margin is desired, it can be modeled with a lower rate of sedimentation and greater depositional distance. If the margin is to mound up and prograde, then the depositional distance should be decreased, and the rate of sediment supply should be increased. In addition, the depositional distances for sand and shale can be varied as a function of time, thereby concentrating sands or shales on the shelf, at the shelf margin, or within the basin.


N. B.: It is important to remember to use a large number of columns when modeling the basin topography. If the number of columns is reduced from, for example, 200 to 10, there will be problems related to space-aliasing, which will affect the sediment geometries. There is no "critical" number of columns, but the column widths must have a greater resolution (smaller distances) than the spacing of the data values (refer to Section 8.2 for a further discussion of aliasing).

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6.17 Depositional Parameters

To understand the effect of the Depositional Parameters, first read Section 3.10.

-Load Ex_DepParm.db and study the simulation carefully.
-Open Depositional Parameters and change the Deep depositional angle and Shallow depositional angle from 0.8š to 3š.
-Save and load the file.
-Did the geometry change?

Small depositional angles prevent sediment from accumulating within the basin and cause it to bypass, even with short depositional distances. If the deep and shallow depositional angles are increased, it can be seen that with short depositional distances, a large prograding sediment wedge will develop and vice versa. Small depositional angles is shown in Figure 6.17.1 and higher angle is shown in Figure 6.17.2.

Figure 6.17.1. Final output for Ex_DepParm.db.

Figure 6.17.2. Final output for Ans_DepParm.db.

6.18 Winnowing

Read Section 3.09 on Winnowing.

Figure 6.18.1. Final output for Ex_NoWinnowRight.db.

-Open file Ex_NoWinnowRight.db.
-Watch the simulation closely, especially the distribution of sand and shale (Figure 6.18.1).
-To add the winnowing effect, click on the Constants button on the EDIT menu and turn Winnowing on.
-Click Winnowing.
-Since the sediment source is from the left, the winnowing should occur from the right. Click ...from the right.
-Open the data sheet and create a curve at locations 0 and 600.
-Type in:

for both curves on the data sheet.
-Save the file and execute the simulation. The result should look like Figure 6.18.2.
-Try to make a gradual winnowing curve by typing in:
Depth Winnowing

at both locations. The result should look like Figure 6.18.3.

Winnowing reduces the percentage of shale accumulation as a function of water depth, from sea level down to Wave Base (see Section 3.03, the Setup parameter), the depth at which waves start touching the sea floor. Wave Base is set to 100 m. Note that shale (green) and sand (yellow) are deposited on the shelf and in the basin (Figure 6.18.1).

The "jagged" sediment geometry at the shelf margin is due to the sharp change in the rate of winnowing that occurs at Wave Base, from 100% to 0% (Figure 6.18.2). In order to create smoother geometries, it is necessary for winnowing to gradually decrease down to Wave Base (Figure 6.18.3). Figure 6.18.4 has very "jagged" geometries which could be removed by gradually decreasing winnowing with depth.

Figure 6.18.2. Final output for Ans_NoWinnowRight.db.

Figure 6.18.3. Note how the amount of produces a gradual increase in sand into shallow water in Ans_NoWinnowGradual.db.

Figure 6.18.4. Position of Winnowing zone traces the position of relative sea level and so the intersection ofWave Base with the sediment surface through time for file Test_Winnow_Notchcorr.db.

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6.19 Carbonate Rates

Read Section 3.11 to become familiar with Carbonate Rates. This menu item is accessible from the SEDPAK EDIT menu. Carbonate Rates controls carbonate accumulation as a function of depth and can be varied with time.

-Load Ex_CarbRateStep.db (Figure 6.19.1).
-Open Carbonate Rates from the EDIT menu.
How thick is the carbonate from 0-5 Ma on the platform? Within the basin? From 10-5 Ma?

Figure 6.19.1. Final output for Ex_CarbRateStep.db.

Chapter 6, Section 20

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