3. THE SEDPAK EDIT MENU
3.04 Basin Surface
3.05 Sea Level
3.07 Clastic Supply
3.08 Depositional Distance
3.10 Depositional Parameters
3.11 Carbonate Rates
3.13 Lagoonal Damping
3.14 Wave Damping
3.15 Pelagic Deposition
3.16 Carbonate Parameters
3.18 Time Boundaries
3.19 Pseudo Wells
3.20 Out of Plane Deposition
3.21 Thermal Gradients
3.22 Surface Temperatures
3.23 Maturation Parameters
This chapter describes the initialization, in the EDIT menu, of SEDPAK variables that are used throughout the program by one or more operation modules
To run SEDPAK, type "sedpak" at the workstation prompt. The SEDPAK LAUNCH menu appears from which the SEDPAK EXEC, SEDPAK EDIT, SEDPAK FACIES, Overlay, SEDPAK Adobe, and SEDPAK Help menus can be invoked (see the discussion in Section 2.03).
The SEDPAK EDIT menu (Figure 3.0.1) is in turn invoked by pressing the SEDPAK EDIT button on the LAUNCH menu. It often takes a few moments for the EDIT menu to appear on the computer screen. Generally while waiting for the menu to appear, the EDIT button on the LAUNCH menu should not be pressed again or multiple EDIT menus will appear. Extra invocations of SEDPAK EDIT are sometimes needed, however, when editing or comparing several data files at once.
Editing a current file and creation of a new file are started in the same manner from the EDIT menu (see Section 2.06). A file must be loaded for editing by selecting Open from the File pulldown menu. Modification of a file which already exists will usually be terminated by selecting Save from the File pulldown menu. Creation of a new file is accomplished by editing a file which already exists and then saving it with a new name by use of the Save As... option from the File pulldown menu.
Each of the input parameters of the SEDPAK EDIT menu must be selected to be modified in turn. Data values are edited using plotters (Section 2.10) and data sheets (Section 2.11). Individual and groups of data values can be changed through the use of the calculator (Section 2.12).
It is important to remember that as editing is completed for a particular parameter, the OK button should be pressed, not only to close the plotter but to keep these changes. It is also advisable to select Save from the File menu in the menu bar at the top of the EDIT menu to save the changes permanently. Pressing OK will close the plotter and will only keep the changes while a particular file is open on the EDIT menu. Pressing Apply on Data Sheet will upgrade the connected plotter to display the same information graphically and pressing Apply on the plotter upgrades the plotter but does not close it. Pressing Abandon on the plotter will return the sheet to the form it had when it was opened from the EDIT menu, thus removing any changes initiated by pressing Apply. Pressing Cancel will terminate editing and close that plotter without keeping changes to the parameter. N. B. if Quit or Close is selected without saving the data, the results of the editing session are lost.
The pull-down menus on the SEDPAK EDIT menu bar are File, Option, Examine and Help. Their use is explained in Sections 2.06, 2.07, 2.08 and 2.09. When File is pulled down from the SEDPAK EDIT menu, the options are Open, Close, Save, Save As..., and Quit.
Figure 3.0.1. SEDPAK EDIT menu
Figure 3.1.1. Dialog for entering comments.
Selecting Comments from the SEDPAK EDIT menu invokes the Comments dialog (Figure 3.1.1). Comments may be used to describe a simulation run, note changed variables, and make miscellaneous notes. For instance if the cycle description portion of the calculation is used to impose a series of cycles of different frequencies it should be recorded in the Comments .
Table of Contents
This section describes the constants used in the simulation and the controls used for turning them on or off. These constants include: Carbonates, Compaction, Pelagic, Hardgrounds, Lagoonal Damping, Subsidence, Wave Damping, Winnowing, and Airy Isostasy. In addition, English or Metric units can be selected and Clastic Drection of sediment deposition can be set to Neither, Left, Rght or Both sides.
Discussion: Physical Constants and Control
Figure 3.2.1 shows the dialog for setting some of the constant values used by SEDPAK. This window is invoked by selecting Constants on the SEDPAK EDIT menu. Functions selected for Constants are shown by the buttons which appear to have been pressed in and are marked red (on).
It may be useful to turn off Compaction (button is grey) when testing the data with respect to the amount of sediment deposited and the repose angles. In all other cases, Compaction should be turned on (button is red).
The cause of a Shore Error produced during program execution can often be established by using the Constants dialog in combination with the simulation. When a Shore Error occurs, set Clastic Source to Neither and turn off Carbonates, then save the file using Save. Set the Display Controls (from the SEDPAK EXEC menu) to Plot each time step and then select Load then Restart to re-initiate the program execution. The varying position of sea level with respect to the basin surface can be viewed to determine whether sea level moves above or below this surface at the time the Shore Error occurs. By viewing the relationship of the basin to sea level, it can be determined how best to avoid a Shore Error when clastic sedimentation is turned on again.
Airy isostasy should be turned off for most simulations since the resulting geometries are unrealistic and difficult to control when it is turned on. The crust behaves rigidly when it is turned off and elastically when it is turned on. Crustal behavior is best modeled by SEDPAK with Airy Isostasy off and specifying rates of tectonic movement using the Subsidence option on the Edit menu.
Figure 3.2.1. SEDPAK Constants dialog.
Table of Contents
The set-up dialog contains:
Start time The start time for the simulation. Stop time The end time for the simulation. Duration of time steps The duration of each time step; determine the number of times steps. Number of time steps The number of time steps used by simulation; determine the duration of each time step. X-axis left The location of the left side of the simulation cross section. X-axis right The location of the right side of the simulation cross section. Width of column The width of each division of the horizontal axis; determines the number of columns. Number of columns The number of divisions on the horizontal axis; determines the width of a column. Top elevation The elevation that is used to mark the top of the simulation. Bottom elevation The elevation that is used to mark the bottom of the simulation. Sea level offset This determines the static position of the sea level curve. Positive values shift the curve upwards, negative values shift the curve downwards. Wave base The elevation at which waves touch the sea floor.
Inputs:Start Time, Stop Time and Time Steps
Figure 3.3.1. SEDPAK Setup parameters.
Discussion: Number of time steps
While experimenting with the simulation input parameters, it is sometimes useful to specify time steps of a longer duration and so ensure that the simulation runs for a smaller number of time steps (Figure 3.3.1). This is because the time taken for the program to execute is directly affected by this value. As the final parameters are converged upon for achieving the best simulation geometries, greater resolution can be attained by reducing the duration of the time steps and so increasing their number.
An important point to remember about SEDPAK is that an aliasing effect can occur in both time and/or space. Output can be adversely affected by aliasing in one of two ways: 1) the number of time steps used is too few, and 2) the number of columns defined for the width of the basin is too few (which directly affects the placement of vertices for the initial basin surface and subsidence).
The most common aliasing problem occurs when an insufficient number of time steps is used. If, for example, a section is modeled using 50 time steps, then no matter how many more sea level points are entered for the sea level curve, SEDPAK will consider only 50 sea level points for that simulation run. The result is a smoothing of the sea level history, and relatively short eustatic fluctuations may be skipped, or may be applied for longer time intervals than was intended.
Another consideration with regard to the time-aliasing problem is the times at which certain parameters are specified to start and end, i.e., when subsidence rate changes, when rates of clastic accumulation change, etc. For example, if a simulation has time steps of duration 50 ka, then SEDPAK will not be able to use two entries specified to occur at times less than 50 ka apart. Thus, if subsidence is specified to change to -0.2 m/ka at -10.500 Ma and then to -0.1 m/ka at -10.480 Ma, SEDPAK will linearly interpolate between these values and, because their times are so close together that they fall within the same time step, the subsidence rate selected is taken at intersection of the interpolated rate curve with the time boundary. Similarly, if the sedimentation rates or the sediment transport distances are at smaller time intervals than those defined by the number of time steps or column widths, then these rates may not be incorporated, or worse, could be extended over longer time intervals than intended. The best way around this time-aliasing error is to increase the number of time steps for the simulation or change the specified time for the offending parameter. It is important to remember that SEDPAK takes the input values for all parameters and performs linear interpolations between them, reading the parameter value from its intersection with time step boundaries or column boundaries.
When the Duration of Time Steps is specified on the SEDPAK Setup panel it then calculates the Number of Time Steps .
Inputs: Basin Width and Number of Columns
For example, if the left and right x-axis are assigned values of 0.0 km and 400.0 km respectively, then each column represents 4.0 km when the number of x-steps is 100.
Discussion: Number of X-Steps
While experimenting with the selection of input parameters, it may be useful to keep the number of columns used in the simulation run to the minimum needed to describe the geometries being modeled. This is because the execution time of the program is affected by this value. As the final parameter set is converged upon, greater resolution can be achieved by increasing the number of columns.
As with time, it is equally important to remember that SEDPAK produces an aliasing effect in space. This means that the output can be adversely affected if the number of columns defined for the width of the basin is too few. For example, any number of columns can be defined across a basin. The more columns used, however, the longer the program takes to run. If too few columns are used, a space aliasing problem can occur. For example, a basin that is 100 km long which is modeled using 100 columns will have a column width of 1 km. If the initial basin topography is specified by points less than 1 km apart, a space-aliasing effect will occur. SEDPAK can only handle one point per column, so if two points are specified within one column, SEDPAK will select the interpolated value at the column boundary. To get around this type of error, a greater number of columns may be specified or the distance between points may be increased.
When the Width of Columns is specified on the Setup dialog and the Number of Columns is automatically calculated and displayed.
Inputs: Top and Bottom Elevations, Sealevel Offset, and Wave Base
Discussion: Top and Bottom Elevations, Sealevel Offset, and Wave Base
These parameters are important throughout the program.
The Top Elevation and Bottom Elevation set the vertical range for the simulation output. If the Bottom Elevation is set too high, the simulation output may descend below the simulation
window. On the other hand, if the range between Top Elevation and Bottom Elevation is too large, the definition of the geometries and depositional surfaces
may be at such a fine scale that clear results cannot be seen.
Figure 3.3.2. Sea Level Offset (S0) with respect to the initial basin surface.
The Sea Level Offset is designed to allow movement of the entire sea level curve up or down with respect to the initial basin surface (Figure 3.3.3). This parameter is used to avoid a Shore Error. A Shore Error will occur at time intervals when clastics are deposited and sea level lies above the edge of the basin (no shoreline) or drops below the surface of the sediment filling the basin. Sea level is required to intersect a surface to produce a shore and there should be space available for the sediment to fill. This parameter enables the sea level curve to be dropped if it lies above the topography at any time during the simulation run. Similarly, the sea level curve can be raised so that it does not drop below the basin surface. A SEDPAK dialog will notify the user when a Shore Error occurs (Figure 3.3.3).
Figure 3.3.3. Shore Error warning dialog.
Another way to avoid a Shore Error is to reduce the clastic deposition to zero during the time interval that
the Shore Error occurs.
Wave Base is the depth at which waves touch bottom. This parameter determines the depth at which wave winnowing affects shale deposition and wave damping affects carbonate accumulation. The effects of Wave Base on winnowing by wave damping are discussed in sections 3.10 and 3.14 respectively.
Table of Contents
3.04 Basin Surface
Figure 3.4.1. Input base pairs for the initial basin surface, S0.
Figure 3.4.2. Plotter for the Basin Surface. Note initial surface.
Figure 3.4.3. Data sheet for the Basin Surface.
Inputs: Basin Surface
Discussion: Basin Surface
Selection of Basin Surface from the SEDPAK EDIT menu displays a plotter (Figure 3.4.2). Selecting Show Data Sheet from the Options pull-down or tear-off menu on the plotter invokes the data sheet (Figure 3.4.3). Note that the two columns of the data sheet are labeled "Distance" and "Depth". Several basin surfaces can be created with the Basin Surface module including the initial surface and subsequent surfaces that match unconformities (Figure 3.4.4).
The best way to begin setting up the initial basin surface for the simulation is by creating the "anchor" point(s) on the side(s) where clastics are entering the basin. If clastics were to enter the basin from both sides, then both sides would need to be anchored so they don't subside. This step ensures that there will be a shoreline throughout the simulation run. If the shoreline were not anchored, subsidence of the shelf could drop the basin surface below sea level, resulting in a Shore Error and a warning dialog to that effect (Figure 3.3.4). If clastic deposition has been turned off for one or both sides of a basin, that side or sides may subside below sea level without the Shore Error occurring (see Section 8.1).
Remember that an aliasing effect will result if distance-depth pairs either
exceed the number of columns or lie within one column (see Section 3.03). Whenever points are entered into the data sheet, select Apply so that the graph is updated to reflect the new data points. OK also must be selected from the plotter before dismissing it, in order to
keep the changes entered in the data sheet. It is also advisable to save
the file at frequent intervals by selecting Save from the File pull-down menu on the SEDPAK EDIT panel, as described in Section 2.05. If a mistake is made in entering, the data entries can be undone by pressing Abandon and the data sheet returns to its original condition without being upgraded.
If Apply has been pressed the data sheet can be dismissed with Cancel and so closed. It can then be re-opened to its original condition.
A series of new basin surfaces of different ages (Figure 3.4.4) above which all sediments will be eroded at a particular time during the simulation can be created in two ways. In the first, the Curves pull-down or tear-off menu from the plotter gives access to New Curve which can be modified to produce a new basin surface or unconformity. A curve with a different age can be created by duplicating it from one of the curves already present or by entering data for a new curve. To duplicate a curve, first select the age of the curve to be duplicated in the scroll area below the graph. Then select Copy Current Curve from the Curves pull-down menu on the plotter. The Basin surface age text area at the top of the data sheet will now read "copy of (curve age)". If the curve is unsatisfactory, this can be deleted with the Delete Current Curve function. The curve can be renamed by activating a dialog box for this purpose from the Rename Current Curve menu item. Enter the new curve age, i.e., the time at which the erosional surface is to take effect. Note that time is entered as a negative value. If a new curve is desired, first select New Curve from the Curves pull-down menu on the plotter. Enter the age for the basin surface in the Basin surface age text area at the top of the data sheet. Then enter distance-depth pairs for this new basin surface. Don't forget to click Apply on the data sheet and OK on the plotter to keep the changes. Use Save to permanently save the file.
Another method for creating unconformities uses the Surface Snapshot option activated from the File pulldown menus of the EXEC menu. This records the current basin surface of the Paused SEDPAK Simulation window. It also brings up a File dialog to which the selected surface is saved (see Section 2.06). This file can then be loaded to the Basin Surface plotter by pressing the Load button on the Basin Surface plotter. This activates the Load Basin Surface dialog box (Figure 2.6.1) and the surface file is selected and loaded to the Basin Surface plotter. This surface will contain too many points to be edited easily, so it can be simplified by selecting Simplify Current Curve from the Curves pull-down or tear off menu. The level of simplification can be set on the activated Simplify dialog box (Figure 2.10.3, see Section 2.10) and by deleting extraneous points on the Data sheet (see Section 2.11). The surface can then be edited on the Plotter and Data sheet to achieve the required unconformity surface configuration.
The erosion associated with this unconformity surface can extend below the initial basin surface and is referred to in the program as the deus ex machina function. After erosion occurs, normal deposition resumes. Should the basin margins be removed below sea level, and clastic deposition continue from that side or sides, a Shore Error will occur (see Section 8.1). Basin surface curves are identified by their age. The "oldest" curve (not necessarily the same as the simulation start time) is the initial basin surface; other curves represent erosional unconformity surfaces.
There are several important considerations regarding the initial basin surface. The section that may exist beneath this surface is not considered by the simulation. The simulation can erode below this initial basin surface (except when a younger Basin Surface cuts below it), and the surface may move up or down in response to sediment loading, erosion of sediment on its surface, faulting, regional uplift, and subsidence.
The data sources for the initial depositional surface or the basin floor may be seismic cross sections, well data, outcrops, or a combination of these sources. In all cases the problem is to extract the original topography of the initial depositional surface from a horizon that may have been deformed by tectonic movement and compaction. The best approach is to tie paleobathymetric markers for the initial surface to seismic reflectors or well tops. If paleobathymetric markers are not available, then geohistory plots derived from well data, outcrop, seismic, or a combination of these can be used to infer the topography. In many cases a seismic interpretation tied to a well can be used to derive the topography.
Figure 3.4.4. Basin Surface plotter of initial surface and subsequent unconformities.
Chapter 3, Section 5
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