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Basics of Basin Analysis

·         A sedimentary basin is an area in which sediments have accumulated during a particular time period at a significantly greater rate and to a significantly greater thickness than surrounding areas.

·         A low area on the Earth’s surface relative to surroundings e.g. deep ocean basin (5-10 km deep), intramontane basin (2-3 km a.s.l.)

·         Basins may be small (kms2) or large (106+ km2)

·         Basins may be simple or composite (sub-basins)

·         Basins may change in size & shape due to:

1.      erosion

2.      sedimentation

3.      tectonic activity

4.      eustatic sea-level changes

·         Basins may overlap each other in time

·         Controls on Basin Formation

1.      Accommodation Space,

a.       Space available for the accumulation of sediment

b.      T + E = S + W T=tectonic subsidence E= Eustatic sea level rise S=Rate of sedimentation W=increase in water depth

2.      Source of Sediment

a.       Topographic Controls

b.      Climate/Vegetation Controls

c.       Oceanographic Controls (Chemical/Biochemical Conditions)

·         The evolution of sedimentary basins may include:

1.      tectonic activity (initiation, termination)

2.      magmatic activity

3.      metamorphism

4.      as well as sedimentation

·         Axial elements of sedimentary basins:

1. Basin axis is the lowest point on the basement surface

2. Topographic axis is the lowest point on the depositional surface

3. Depocentre is the point of thickest sediment accumulation

·         The driving mechanisms of subsidence are ultimately related to processes within the relatively rigid, cooled thermal boundary layer of the Earth known as the lithosphere. The lithosphere is composed of a number of tectonic plates that are in relative motion with one another. The relative motion produces deformation concentrated along plate boundaries which are of three basic types:

1. Divergent boundaries form where new oceanic lithosphere is formed and plates diverge. These occurat the mid-ocean ridges.

2. Convergent boundaries form where plates converge. One plate is usually subducted beneath theother at a convergent plate boundary. Convergent boundaries may be of different types, depending on the types of lithosphere involved. This result in a wide diversity of basin types formed at convergent boundaries.

3. Transform boundaries form where plates move laterally past one another. These can be complex andare associated with a variety of basin types.

·         Many basins form at continental margins.

Using the plate tectonics paradigm, sedimentary basins have been classified principally in terms of the type of lithospheric substratum (continental, oceanic, transitional), the position with respect to a plate boundary (interplate, intraplate) and the type of plate margin (divergent, convergent, transform) closest to the basin.

·         Plate Tectonic Setting for Basin Formation

1.      Size and Shape of basin deposits, including the nature of the floor and flanks of the basin

2.      Type of Sedimentary infill

·         Rate of Subsidence/Infill

·         Depositional Systems

·         Provenance

·         Texture/Mineralogy maturity of strata

3.      Contemporaneous Structure and Syndepositional deformation

4.      Heat Flow, Subsidence History and Diagenesis

·         Interrelationship Between Tectonics - Paleoclimates - and Eustacy

1.      Anorogenic Areas------>

·         Climate and Eustacy Dominate

2.      Orogenic Areas--------->

Sedimentation responds to TectonismPlate Tectonics and Sedimentary Basin

   Types

SB = Suture Belt

RMP = Rifted margin prism

S C = Subduction complex

FTB = Fold and thrust belt

RA = Remnant arc

Wilson Cycle about opening and closing of ocean basins and creation of continental crust.

Structural Controls on Sedimentary Systems in Ba sins Forming:

Stage 1: Capacity < Sediment

Fluvial sedimentation

Stage 2: Capacity = Sediment

Fluvial lacustrine Transition

Stage 3: Capacity > Sediment

Water Volume > excess capacity

Shallow-water lacustrine sedimentation

Stage 4: Capacity >> Sediment

Water volume = excess capacity

Deep-water lacustrine sedimentation

Stage 5: Capacity > Sediment

Water volume < excess capacity

Shallow-water lacustrine sedimentation

Contributed by:

Rehan.A Farooqui

M.Sc Geology,,

University of Karachi.

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