Sedimentary basins

· 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

• 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)

• 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 reconstruction-Baltimore 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: f = porosity

Zt = column thickness,

r = density

S = total decompacted thickness = S(THI)

THI = decompacted thickness of interval

= [(1- f)/(1- finit)]*Zi

With finit for :

• Sandstone = 0.5
• Shale = 0.6
• Limestone = 0.7

Density

· r sed = density of sediment column (weighted average)

· r w = density of pore water = 1 g/cm3

· r m = density of mantle = 3.33 g/cm3

Grain densities r grains for

• SANDSTONE = 2.65 g/cm3

• LIMESTONE = 2.72 g/cm3

• SHALE = 2.72 g/cm3