Page 119 - Physical Principles of Sedimentary Basin Analysis

5.4 Erosion 101
Table 5.1. The burial history for the example shown
in Figure 5.4. The formation “X” denotes erosion.

Time Thickness
[Ma] [m] Formation Comment
−250 500 A (present-day thickness)
−225 500 B (hiatus 1)
−200 500 C (hiatus 1)
−175 500 D (hiatus 1)
−150 500 E (hiatus 1)
−125 −2000 X (hiatus 1)
−100 1000 F (present-day thickness)
−75 1000 G (hiatus 2)
−50 −1000 X (hiatus 2)
−25 1000 H (present-day thickness)
0 − −

Table 5.1 shows an example of a burial history that includes erosion. Each line in the
table represents a layer (formation) where the first column is the time when the deposi-
tion of the layer begins, the second column is the present-day thickness of the layer, and
the third column is the formation name. A deposition process lasts until the start of the
next deposition process, given by the next line in the table. The last line gives the end
of the burial history, and therefore has no thickness or formation name. Lines with neg-
ative layer thicknesses are erosion processes. There are two erosions in Table 5.1.The
first erosion process removes 2000 m, which are the four preceding layers of 500 m each.
The second one removes one layer of 1000 m. Notice that the thicknesses of the eroded
layers are not present-day thicknesses, because these thicknesses do not exist today. We
will see that these real thicknesses are just a means to assign net thicknesses to eroded
layers.
The net thicknesses for each layer need to be initialized before the simulation of the
burial history can start. A simple way to do the initialization is to assume that the present-
day porosity is 25% in all layers. The advantage of using the same initial porosity for all
layers is that the initial guess for the net layer thicknesses adds up to zero for all layers
involved in an erosion process. (Recall that the real thicknesses involved in a hiatus must
add up to zero.)
The net amount of rock in each layer is updated at the end of each forward simulation
of the burial history. A full forward simulation gives the present-day porosity (φ i ) of each
layer (i) and the net thickness for the layer becomes ζ i = (1 − φ i ) z i , where z i is
the present-day layer thickness. The layers eroded during the time span of a hiatus do not
have present-day thicknesses. The net thicknesses for these layers are not updated, and they
therefore keep their initial net thicknesses. We must therefore keep in mind that the eroded