Backstrip

Overview

The backstrip module is used to find tectonic subsidence from paleo water depths, and sediment decompaction over time.

Running backstrip

You can either run backstrip as a built-in script, specifying parameters as command-line options (...):

python -m pybacktrack.backstrip_cli ...

…or import pybacktrack into your own script, calling its functions and specifying parameters as function arguments (...):

import pybacktrack

pybacktrack.backstrip_and_write_well(...)

Note

You can run python -m pybacktrack.backstrip_cli --help to see a description of all command-line options available, or see the backstripping reference section for documentation on the function parameters.

Example

To backstrip the sunrise drill site (located on shallower continental crust), and output all available parameters (via the -d option), we can run it from the command-line as:

python -m pybacktrack.backstrip_cli \
    -w pybacktrack_examples/example_data/sunrise_lithology.txt \
    -l primary extended \
    -d age compacted_depth compacted_thickness decompacted_thickness decompacted_density decompacted_sediment_rate decompacted_depth min_tectonic_subsidence max_tectonic_subsidence average_tectonic_subsidence min_water_depth max_water_depth average_water_depth lithology \
    -slm Haq87_SealevelCurve_Longterm \
    -o sunrise_backstrip_amended.txt \
    -- \
    sunrise_backstrip_decompacted.txt

…or write some Python code to do the same thing:

import pybacktrack

# Input and output filenames.
input_well_filename = 'pybacktrack_examples/example_data/sunrise_lithology.txt'
amended_well_output_filename = 'sunrise_backstrip_amended.txt'
decompacted_output_filename = 'sunrise_backstrip_decompacted.txt'

# Read input well file, and write amended well and decompacted results to output files.
pybacktrack.backstrip_and_write_well(
    decompacted_output_filename,
    input_well_filename,
    lithology_filenames=[pybacktrack.PRIMARY_BUNDLE_LITHOLOGY_FILENAME,
                         pybacktrack.EXTENDED_BUNDLE_LITHOLOGY_FILENAME],
    sea_level_model=pybacktrack.BUNDLE_SEA_LEVEL_MODELS['Haq87_SealevelCurve_Longterm'],
    decompacted_columns=[pybacktrack.BACKSTRIP_COLUMN_AGE,
                         pybacktrack.BACKSTRIP_COLUMN_COMPACTED_DEPTH,
                         pybacktrack.BACKSTRIP_COLUMN_COMPACTED_THICKNESS,
                         pybacktrack.BACKSTRIP_COLUMN_DECOMPACTED_THICKNESS,
                         pybacktrack.BACKSTRIP_COLUMN_DECOMPACTED_DENSITY,
                         pybacktrack.BACKSTRIP_COLUMN_DECOMPACTED_SEDIMENT_RATE,
                         pybacktrack.BACKSTRIP_COLUMN_DECOMPACTED_DEPTH,
                         pybacktrack.BACKSTRIP_COLUMN_MIN_TECTONIC_SUBSIDENCE,
                         pybacktrack.BACKSTRIP_COLUMN_MAX_TECTONIC_SUBSIDENCE,
                         pybacktrack.BACKSTRIP_COLUMN_AVERAGE_TECTONIC_SUBSIDENCE,
                         pybacktrack.BACKSTRIP_COLUMN_MIN_WATER_DEPTH,
                         pybacktrack.BACKSTRIP_COLUMN_MAX_WATER_DEPTH,
                         pybacktrack.BACKSTRIP_COLUMN_AVERAGE_WATER_DEPTH,
                         pybacktrack.BACKSTRIP_COLUMN_LITHOLOGY],
    # Might be an extra stratigraphic well layer added from well bottom to ocean basement...
    ammended_well_output_filename=amended_well_output_filename)

Note

The drill site file pybacktrack_examples/example_data/sunrise_lithology.txt is part of the example data.

Backstrip output

For each stratigraphic layer in the input drill site file, backstrip can write one or more parameters to an output file.

Running the above example on the sunrise drill site:

# SiteLatitude = -9.5901
# SiteLongitude = 128.1538
# SurfaceAge = 0.0000
#
## bottom_age bottom_depth min_water_depth max_water_depth lithology
   2.000      462.000      0.000           100.000         Shale           0.20       Limestone       0.75       Dolostone       0.05      
   10.000     525.000      0.000           100.000         Shale           0.20       Limestone       0.75       Dolostone       0.05      
   24.000     822.000      0.000           100.000         Shale           0.10       Limestone       0.80       Sand            0.10      
   30.000     1062.000     0.000           100.000         Shale           0.30       Limestone       0.55       Dolostone       0.05       Sand            0.10      
   34.000     1086.000     0.000           100.000         Shale           0.20       Limestone       0.10       Sand            0.70      
   45.000     1366.000     0.000           100.000         Shale           0.10       Limestone       0.75       Dolostone       0.05       Sand            0.10      
   58.000     1442.000     0.000           100.000         Shale           0.15       Limestone       0.15       Sand            0.70      
   68.000     1494.000     50.000          200.000         Shale           0.45       Limestone       0.50       Sand            0.05      
   83.000     1521.000     20.000          200.000         Shale           0.30       Limestone       0.65       Sand            0.05      
   86.000     1545.000     20.000          200.000         Shale           0.55       Limestone       0.35       Sand            0.10      
   88.000     1582.000     20.000          200.000         Shale           0.35       Limestone       0.65      
   90.000     1620.000     20.000          200.000         Shale           0.70       Limestone       0.15       Sand            0.15      
   95.000     1890.000     20.000          200.000         Shale           0.70       Limestone       0.15       Sand            0.15      
   100.000    2036.000     20.000          200.000         Shale           0.70       Limestone       0.15       Sand            0.15      
   107.000    2062.000     20.000          200.000         Shale           0.64       Limestone       0.18       Sand            0.18      
   125.000    2066.000     0.000           100.000         Shale           0.40       Chalk           0.10       Sand            0.50      
   160.000    2068.000     0.000           100.000         Shale           0.40       Limestone       0.30       Sand            0.30      
   165.000    2130.000     0.000           100.000         Shale           0.40       Limestone       0.30       Sand            0.30      
   170.000    2176.000     0.000           100.000         Shale           0.50       Sand            0.50      
   177.000    2187.000     -10.000         25.000          Shale           0.30       Sand            0.70      
   180.000    2237.000     -10.000         25.000          Shale           0.30       Sand            0.70      
   190.000    2311.000     -10.000         20.000          Shale           0.30       Sand            0.70      

…produces an amended drill site output file, and a decompacted output file containing the decompacted output parameters like sediment thickness and tectonic subsidence.

Amended drill site output

The amended drill site output file:

# SiteLatitude = -9.5901
# SiteLongitude = 128.1538
# SurfaceAge = 0.0000
#
## bottom_age bottom_depth min_water_depth max_water_depth lithology
   2.000      462.000      0.000           100.000         Shale           0.20       Limestone       0.75       Dolostone       0.05      
   10.000     525.000      0.000           100.000         Shale           0.20       Limestone       0.75       Dolostone       0.05      
   24.000     822.000      0.000           100.000         Shale           0.10       Limestone       0.80       Sand            0.10      
   30.000     1062.000     0.000           100.000         Shale           0.30       Limestone       0.55       Dolostone       0.05       Sand            0.10      
   34.000     1086.000     0.000           100.000         Shale           0.20       Limestone       0.10       Sand            0.70      
   45.000     1366.000     0.000           100.000         Shale           0.10       Limestone       0.75       Dolostone       0.05       Sand            0.10      
   58.000     1442.000     0.000           100.000         Shale           0.15       Limestone       0.15       Sand            0.70      
   68.000     1494.000     50.000          200.000         Shale           0.45       Limestone       0.50       Sand            0.05      
   83.000     1521.000     20.000          200.000         Shale           0.30       Limestone       0.65       Sand            0.05      
   86.000     1545.000     20.000          200.000         Shale           0.55       Limestone       0.35       Sand            0.10      
   88.000     1582.000     20.000          200.000         Shale           0.35       Limestone       0.65      
   90.000     1620.000     20.000          200.000         Shale           0.70       Limestone       0.15       Sand            0.15      
   95.000     1890.000     20.000          200.000         Shale           0.70       Limestone       0.15       Sand            0.15      
   100.000    2036.000     20.000          200.000         Shale           0.70       Limestone       0.15       Sand            0.15      
   107.000    2062.000     20.000          200.000         Shale           0.64       Limestone       0.18       Sand            0.18      
   125.000    2066.000     0.000           100.000         Shale           0.40       Chalk           0.10       Sand            0.50      
   160.000    2068.000     0.000           100.000         Shale           0.40       Limestone       0.30       Sand            0.30      
   165.000    2130.000     0.000           100.000         Shale           0.40       Limestone       0.30       Sand            0.30      
   170.000    2176.000     0.000           100.000         Shale           0.50       Sand            0.50      
   177.000    2187.000     -10.000         25.000          Shale           0.30       Sand            0.70      
   180.000    2237.000     -10.000         25.000          Shale           0.30       Sand            0.70      
   190.000    2311.000     -10.000         20.000          Shale           0.30       Sand            0.70      

Note

No extra base sediment layer is added from the bottom of the drill site (2311 metres) to the total sediment thickness at the drill site (1298.15 metres), because the former (bottom of drill site) is already deeper than the latter (total sediment thickness). This happens because the default total sediment thickness grid is not as accurate near continental margins (compared to deeper ocean basins).

Decompacted output

The decompacted output file:

# SiteLatitude = -9.5901
# SiteLongitude = 128.1538
# SurfaceAge = 0.0000
#
# age       compacted_depth compacted_thickness decompacted_thickness decompacted_density decompacted_sediment_rate decompacted_depth min_tectonic_subsidence max_tectonic_subsidence average_tectonic_subsidence min_water_depth max_water_depth average_water_depth lithology
  0.000     0.000           2311.000            2311.000              2089.479            245.712                   0.000             1236.930                1336.930                1286.930                    0.000           100.000         50.000              Shale           0.20       Limestone       0.75       Dolostone       0.05       
  2.000     462.000         1849.000            1984.750              2057.304            8.922                     491.425           1038.610                1138.610                1088.610                    0.000           100.000         50.000              Shale           0.20       Limestone       0.75       Dolostone       0.05       
  10.000    525.000         1786.000            1936.640              2052.112            24.613                    562.801           956.290                 1056.290                1006.290                    0.000           100.000         50.000              Shale           0.10       Limestone       0.80       Sand            0.10       
  24.000    822.000         1489.000            1703.149              2018.885            50.826                    907.389           846.332                 946.332                 896.332                     0.000           100.000         50.000              Shale           0.30       Limestone       0.55       Dolostone       0.05       Sand            0.10       
  30.000    1062.000        1249.000            1493.707              1994.320            7.743                     1212.348          678.295                 778.295                 728.295                     0.000           100.000         50.000              Shale           0.20       Limestone       0.10       Sand            0.70       
  34.000    1086.000        1225.000            1472.172              1991.762            32.462                    1243.321          653.467                 753.467                 703.467                     0.000           100.000         50.000              Shale           0.10       Limestone       0.75       Dolostone       0.05       Sand            0.10       
  45.000    1366.000        945.000             1223.986              1937.093            7.857                     1600.399          515.129                 615.129                 565.129                     0.000           100.000         50.000              Shale           0.15       Limestone       0.15       Sand            0.70       
  58.000    1442.000        869.000             1153.349              1921.307            7.583                     1702.542          546.589                 696.589                 621.589                     50.000          200.000         125.000             Shale           0.45       Limestone       0.50       Sand            0.05       
  68.000    1494.000        817.000             1100.849              1911.363            2.517                     1778.370          443.659                 623.659                 533.659                     20.000          200.000         110.000             Shale           0.30       Limestone       0.65       Sand            0.05       
  83.000    1521.000        790.000             1074.658              1904.627            12.270                    1816.128          439.905                 619.905                 529.905                     20.000          200.000         110.000             Shale           0.55       Limestone       0.35       Sand            0.10       
  86.000    1545.000        766.000             1049.084              1900.276            26.459                    1852.938          423.355                 603.355                 513.355                     20.000          200.000         110.000             Shale           0.35       Limestone       0.65       
  88.000    1582.000        729.000             1012.260              1890.539            31.590                    1905.856          388.195                 568.195                 478.195                     20.000          200.000         110.000             Shale           0.70       Limestone       0.15       Sand            0.15       
  90.000    1620.000        691.000             968.032               1884.877            92.186                    1969.037          343.109                 523.109                 433.109                     20.000          200.000         110.000             Shale           0.70       Limestone       0.15       Sand            0.15       
  95.000    1890.000        421.000             625.158               1845.618            51.506                    2429.968          160.243                 340.243                 250.243                     20.000          200.000         110.000             Shale           0.70       Limestone       0.15       Sand            0.15       
  100.000   2036.000        275.000             412.490               1835.863            6.452                     2687.497          111.343                 291.343                 201.343                     20.000          200.000         110.000             Shale           0.64       Limestone       0.18       Sand            0.18       
  107.000   2062.000        249.000             373.048               1835.892            0.375                     2732.658          109.605                 209.605                 159.605                     0.000           100.000         50.000              Shale           0.40       Chalk           0.10       Sand            0.50       
  125.000   2066.000        245.000             367.097               1835.856            0.090                     2739.405          145.550                 245.550                 195.550                     0.000           100.000         50.000              Shale           0.40       Limestone       0.30       Sand            0.30       
  160.000   2068.000        243.000             364.308               1835.425            19.655                    2742.563          198.479                 298.479                 248.479                     0.000           100.000         50.000              Shale           0.40       Limestone       0.30       Sand            0.30       
  165.000   2130.000        181.000             276.308               1821.056            15.434                    2840.839          117.435                 217.435                 167.435                     0.000           100.000         50.000              Shale           0.50       Sand            0.50       
  170.000   2176.000        135.000             205.283               1822.654            2.458                     2918.011          116.777                 151.777                 134.277                     -10.000         25.000          7.500               Shale           0.30       Sand            0.70       
  177.000   2187.000        124.000             189.178               1820.541            26.161                    2935.218          133.251                 168.251                 150.751                     -10.000         25.000          7.500               Shale           0.30       Sand            0.70       
  180.000   2237.000        74.000              114.644               1810.669            11.696                    3013.701          74.454                  104.454                 89.454                      -10.000         20.000          5.000               Shale           0.30       Sand            0.70       
  190.000   2311.000        0.000               0.000                 0.000               0.000                     3130.665          -10.000                 20.000                  5.000                       -10.000         20.000          5.000               Shale           1.00       

The age, compacted_depth, min_water_depth, max_water_depth and lithology columns are the same as the bottom_age, bottom_depth, min_water_depth, max_water_depth and lithology columns in the input drill site (except there is also a row associated with the surface age).

The compacted_thickness column is the bottom depth of the drill site (2311 metres - noting that there is no base sediment layer in the amended drill site above) minus compacted_depth. The decompacted_thickness column is the thickness of all sediment at the associated age. In other words, at each consecutive age another stratigraphic layer is essentially removed, allowing the underlying layers to expand (due to their porosity). At present day (or the surface age) the decompacted thickness is just the compacted thickness. And note that because no extra base sediment layer was added to the bottom of the drill site (2311 metres) the thickness and density is zero there. The decompacted_density column is the average density integrated over the decompacted thickness of the drill site (each stratigraphic layer contains a mixture of water and sediment according to its porosity at the decompacted depth of the layer). The decompacted_sediment_rate column is the rate of sediment deposition in units of metres/Ma. At each time it is calculated as the fully decompacted thickness (ie, using surface porosity only) of the surface stratigraphic layer (whose deposition ends at the specified time) divided by the layer’s deposition time interval. The decompacted_depth column is similar to decompacted_sediment_rate in that the stratigraphic layers are fully decompacted (using surface porosity only) as if no portion of any layer had ever been buried. It is also similar to compacted_depth except all effects of compaction have been removed.

Finally, average_water_depth is just the average min_water_depth and max_water_depth. And min_tectonic_subsidence, max_tectonic_subsidence and average_tectonic_subsidence are obtained from min_water_depth and max_water_depth and average_water_depth by adding an isostatic correction of the decompacted sediment thickness (to obtain the deeper isostatically compensated, sediment-free water depth also known as tectonic subsidence).

Note

The output columns are specified using the -d command-line option (run python -m pybacktrack.backstrip_cli --help to see all options), or using the decompacted_columns argument of the pybacktrack.backstrip_and_write_well() function. By default, only age and decompacted_thickness are output.

Sea level variation

A model of the variation of sea level relative to present day can optionally be used when backstripping. This adjusts the isostatic correction of the decompacted sediment thickness to take into account sea-level variations.

There are two built-in sea level models bundled inside backstrip:

A sea-level model is optional. If one is not specified then sea-level variation is assumed to be zero.

Note

A built-in sea-level model can be specified using the -slm command-line option (run python -m pybacktrack.backstrip_cli --help to see all options), or using the sea_level_model argument of the pybacktrack.backstrip_and_write_well() function.

Note

It is also possible to specify your own sea-level model. This can be done by providing your own text file containing a column of ages (Ma) and a corresponding column of sea levels (m), and specifying the name of this file to the -sl command-line option or to the sea_level_model argument of the pybacktrack.backstrip_and_write_well() function.

Geohistory analysis

The Decompacting Stratigraphic Layers notebook shows how to visualize the decompaction of stratigraphic layers at a drill site.

Note

The example notebooks are installed as part of the example data which can be installed by following these instructions.

One of the examples in that notebook demonstrates decompaction of a shallow continental drill site using backstripping. The paleo water depths (blue fill) are recorded in the drill site file and the tectonic subsidence (black dashed line) is backstripped using the paleo water depths and sediment decompaction. Note that, unlike backtracking, dynamic topography does not affect tectonic subsidence (because backstripping does not have a model of tectonic subsidence). So the image below is simply plotting dynamic topography alongside backstripped tectonic subsidence.

_images/geohistory_DSDP-36-327.png