6.20 Hardgrounds

Read Section 3.12 on Hardgrounds, which is accessible from the SEDPAK EDIT menu. This parameter provides a means to suppress carbonate accumulation during rapid sea level rises.
In this exercise the rate sea level rise is increased to produce hardgrounds.
-Load Ex_Hardground.db.
-Look closely at the simulation and the relationship between the sea level rise and the carbonate rate.
-In this file the Hardground curve is already created. Click Sea Level menu on SEDPAK EDIT menu and input the following:
Time Depth
-4.000 -135
-3.250 -60
-2.000 -60
-1.500 -20

-Save the file and run the simulation
What is the relationship between the rapid change of sea level rise and carbonate rate?

The Hardgrounds parameter slows carbonate production, creating a lag in carbonate accumulation during the rapid sea level rise. Note the variation in thickness of sediment when the variables are changed (Figure 6.20.1) and how the thickness of the carbonate layers is reduced during the rapid rise in sea level.



Figure 6.20.1. Final output for Ans_Hardground.db.


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6.21 Lagoonal Damping

Read Section 3.13 on Lagoonal Damping to become familiar with this carbonate parameter which is accessible from the SEDPAK EDIT menu.

-Open Ex_LagoonDamp.db and watch the simulation closely
-Turn on Lagoonal Damping in the Constant
-Click on Lagoonal Damping on the EDIT panel
-Open the data sheet and type the following:

Distance Damping
1 0.00
1.25 12.75
2.00 19.00
2.75 25.00
3.50 30.00
4.475 34.5
5.500 39.00
6.500 42.25
7.25 45.00
8.00 47.5
9.00 49.00
10.00 50.00


-Save the file.
-Try several other values on the data sheet


The Lagoonal Damping function enables the formation of lagoons to the leeward of buildups or reefs at the platform margin. Within the lagoon, carbonate accumulation is suppressed as a function of horizontal distance from the reef or buildup.

Note the increase in damping in the shallow water furthest from the buildups. The result is a lagoon forms between the buildups on the platform margin. Note the effects of varying the lagoonal damping across the platform (Figure 6.21.1).



Figure 6.21.1. Final output for Ans_LagoonDamp.db.

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6.22 Wave Damping

Read Section 3.14 to become familiar with Wave Damping. Wave Damping is similar to the Winnowing function associated with clastic sediments (Section 3.09). Above Wave Base, the wave damping rate is directly subtracted from the Carbonate Rate curve (Carbonate Rates, Section 3.11). Below Wave Base, carbonate production continues normally as specified by the Carbonate Rate curve. Wave Damping is also a function of the horizontal distance measured from the location at which waves touch bottom. The zone of Wave Damping changes location as relative sea level changes.

-Load Ex_WaveDampLeft.db and watch the simulation closely
-Open Wave Damping from Left
-Create a curve as follows:

Depth Rate
-50 0.01
0 0.01


-Set locations for 0 and 50 km.
-Save the file and run the program.
-When was the carbonate growth damped?
-How thick is it at each platform?


The exercise demonstrates how the carbonate at the platform margin is removed as a function of depth from sea level down to wave base. The platform and the area below Wave Base are unaffected (Figure 6.22.1).

An area below the Wave Base has to defined throughout the simulation in order to activate the Wave Damping. If the entire basin is shallower than Wave Base, the program will identify this as a Lagoon, and consequently the Wave Damping will not occur.




Figure 6.22.1. Final output for Ans_WaveDampLeft.db.

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6.23 Pelagic Deposition

Read Section 3.15 on Pelagic Deposition to become familiar with this carbonate parameter which is accessible from the SEDPAK EDIT menu.

The best way to differentiate the pelagic carbonates from benthic carbonates is to use the facies capability of SEDPAK which defines both pelagic and benthic carbonates.

-Load Ex_PelBenth.db.
-Change Display Mode to Basin-Facies.
-Select a facies definition file; choose PelBenth facies.
-When was the pelagic carbonate deposited?
-When was the benthic carbonate deposited?


The Pelagic Deposition function lays down a uniform layer of carbonate over the entire basin (i.e., it is not depth dependent) at specified time intervals. This pelagic carbonate not only supplements the carbonate accumulation specified by the Carbonate Rates curve (Section 3.11), but also deposits carbonate in areas below the zone of carbonate accumulation specified by the Carbonate Rates curve.




Figure 6.23.1. Final output for Ex_PelBenth.db.

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6.24 Carbonate Parameters

Read Section 3.16 to become familiar with the Carbonate Parameters which are accessible from the SEDPAK EDIT menu.

-Open file Ex_TalTurbDepAng.db. Run the file and watch the talus.
-Click on Carbonate Parameters.
-Change the Talus/turbidite depositional angle from 0.5š to 0.05š. Click OK.
-Save the file as TalTurbDepAng1.db.
-Load and Restart TalTurbDepAng1.db.
The final output is seen as Figure 6.24.1.
-Is there any difference between Ex_TalTurbDepAng.db and TalTurbDepAng1.db?





Figure 6.24.1. Final output for Ans_TalTurbDepAng1.db with talus/turbidite depositional angle at 0.050.



-Open file Ex_TalTurbDepAng.db. Run the file and watch the talus.
-Click on Carbonate Parameters.
-Change the Talus and turbidite penetration distance from 50 kilometers to 10 kilometers. Click OK.
-Save the file as TalTurbDepAng2.db.
The final output is seen as Figure 6.24.2.
-Is there any difference between Ex_TalTurbDepAng.db and TalTurbDepAng2.db?
From this experiments, which is more dominant, the depositional angle or the penetration distance?


Figure 6.24.2 Simulation of Ans_TalTurbDepAng2.db with talus and turbidite distance of penetration at 10 kilometers.

-Open file Ex_TalTurbDepAng.db. Run the file and watch the talus.
-Click on Carbonate Parameters.
-Change the Percent to sea from 50 to 10. Click OK.
-Save the file as TalTurbDepAng3.db.
The final output is seen as Figure 6.24.3.
What is the difference between Ex_TalTurbDepAng.db and TalTurbDepAng3.db?

 


Figure 6.24.3 Simulation of Ans_TalTurbDepAng3.db with percent of carbonate shed to the sea at 10%.


Note that the Talus and Turbidite penetration distances are affected by the Talus/Turbidite depositional angle, which determines whether or not the sediment is stable on the slope.

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6.25 Building Carbonate Geometry: Aggradation, Backstepping, and Progradation

This next exercise demonstrates how carbonate geometries are developed. The carbonate growth is primarily controlled by the depth of the platform from the sea level and the rates of accumulation with depth.

-Open Demo_CarbGeom.db and run the program.
-Until -233.7 Ma, the sea level is constantly rising. The carbonates aggrade, backstep, and catch up with the sea level (Figure 6.25.1).




Figure 6.25.1 Simulation of Ex_CarbGeom.db .




-From -233.7 to -232.9 Ma, in the deeper part of the basin the carbonate is damped as a function of the depth. The carbonate margins in each sequences step landward (Figure 6.25.2).






Figure 6.25.2 Simulation of Ex_CarbGeom.db .


-From -232.9 to -230 Ma, the relative sea level falls when the basin floor is uplifted, causing it to shallow. The carbonate begins to prograde basinward. Note that the irregular geometry overlying the backstepping margins is caused by its step-like geometry. Note also that as the sea level falls, the progradational geometries of the carbonate became steeper (Figure 6.25.3).





Figure 6.25.3 Simulation of Ex_CarbGeom.db .








Figure 6.25.3A Detail of Simulation of Ex_CarbGeom.db .



-Load Ex_CarbGeom.db from the Edit menu.
-Click on Carbonate Rates and examine the different curves on the plotter.
-How do these changes in rates produce the carbonate geometries seen in Figures 6.25.1-3

As a conclusion, the geometry of the carbonate buildups can be created by:

-Changing the sea level curve
-Changing the subsidence rates
-Changing the carbonate rates

Chapter 6, Section 26

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