Sedimentary basins
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Accumulation of sediments requires a depression or basin to fill
Lack of a basin results in no net accumulation (Equilibrium profile and steady state).
Plate Boundaries and Basin types
Boundary type• Transform
• Divergent
• Convergent
Sedimentary Basin type• Transform Margin
• transtensional basins
• transpressional basins
• Passive Margin
• Active Margin
Passive Margin or
Rifted Margin Sedimentary Prisms
•Occur at the transition from continental to oceanic crust
•Not a plate boundary, but a crustal boundary
•Structure (geometry and components) derives from evolution of continental rift,
sedimentation and subsidence
The Passive Margin Sedimentary Prism
• Very thick sequences of alluvial, fluvial, and marginal sediments
– up to 104 m thick
– Oklahoma Andarko Basin
Convergent Margin Basins
Forearc Basin
Retroarc basin,
Trench, trench-slope basins
Accretionary wedge (prism)
Active margin sediments
Transform margin basins
Pull-apart basins (wrench basins)
Releasing bends or rhombochasms
Generally small (10’s km wide)
e.g., Los Angeles Basin, Capistrano Embayment, Late Santa Barbara
Basin analysis
• Integration of regional sedimentology and stratigraphy of related
facies environments that form a depositional system
• Requires firm understanding of forcing acting on the system
• Comprehensive data sets (seismic, field data, well logs)
• Often employs modeling to test quantitative hypotheses
Fence plots
• 3-D perspective sections plotted over a regional base map
• Integrates regional stratigraphic data
• Computer software now available to do this
Facies maps
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Isopleths are contours of surface properties of a formation
Example: % Sand content
Sediment ratio maps
Paleoenvironmental reconstruction
Isopach maps
• Subsurface formation contour maps used to plot thickness of sedimentary
units
• Provides a window onto subsurface topography
Structure
contour map
• Contours of structural data from regional outcrops or other sources
• Enables workers to “see” underlying influence of regional tectonics
Backstripping
(Geohistory Analysis)
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Remove Youngest sequence
Correct next older for compaction
Remove influence of isostacy
Iterate to generate time series
Baltimore Canyon
Trough,
NJ
Margin
Close up: Baltimore Canyon
Trough
Backstripped reconstructionBaltimore Canyon
Trough
Geohistory analysis (Backstripping)
•Quantitative stratigraphic technique to display the geologic history of a single stratigraphic section or of an
entire basin. (van Hinte, 1978)
•Construct a burial history diagram and a subsidence history diagram
•Combines stratigraphic and paleobathymetric
–To plot subsidence (burial) history of a well or measured section.
•Useful in hydrocarbon exploration
–To determine source rock maturation
–thermal history of any hydrocarbons
Geohistory (burial history) diagrams
Show interaction between:
Sediment supply
Eustatic sea level changes
Basement subsidence through time
Constructing the Burial History Diagram
•Observed and decompacted thickness of each paleo-rock column is plotted on an age/depth graph.
–These thicknesses should be plotted as depths below the seafloor.
–Depth of the seafloor at each time-interval is obtained from benthic fossil assemblages.
–Paleobathymetry is plotted relative to eustatic sealevel.
–Taken from global sea level curves (Haq et al. , 1987).
•Simply subtract the sediment load from the total subsidence at each time interval and plot the resultant line
(tectonic subsidence) on the subsidence diagram.
Determining Sediment Load
Initially, the geohistory diagram is simply a graphical
representation of the burial history.
But, the total subsidence is equal to the sum of all vertical
movement, including:
tectonic subsidence
sediment load subsidence
So, Tectonic Subsidence
= Total Subsidence - Sediment Load
Two components of subsidence
• Thermal subsidence
• Sedimentary
load subsidence
Decompaction- Running a unit's compaction history
in reverse.
Consider sedimentary rock as a mixture of sedimentary grains and interstitial pores.
Thought of as solid grains and water.
Compaction occurs by squeezing out water
The volume of sediment grains remains constant.
Porosity curves determined empirically
Decompaction begins with the oldest unit and successively adds each
younger unit to the sediment pile, thereby unburying (decompacting) the
rock column.
Sediment load is derived from unit composition and
porosity.
Model components
where:  = porosity
Zt = column thickness,
 = density
S = total decompacted thickness = (THI)
THI = decompacted thickness of interval
= [(1- )/(1- init)]*Zi
With init for :
• Sandstone = 0.5
• Shale = 0.6
• Limestone = 0.7
Density
  sed = density of sediment column (weighted average)
  w = density of pore water = 1 g/cm3
  m = density of mantle = 3.33 g/cm3
Grain densities  grains for
• SANDSTONE = 2.65 g/cm3
• LIMESTONE = 2.72 g/cm3
• SHALE = 2.72 g/cm3