lecture#2 fundamental geologic structure

7/29/2019 Lecture#2 Fundamental Geologic Structure

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Structural Geology

QAB1053

Lecture #2

QAB 1053 Structural Geology 1

un amen a o eo ogcStructure

Asso c. Pro f. As kur y Ab d Kadir

Geoscience & Petroleum Engineering Department

Learning Outcomes

Students should be able to:

z Know the most important concept of structuralgeology.

zDifferentiate between primary and secondary


s ruc ures.

zUnderstand more on secondary structures thatdeformed by tectonic processes.

z Appreciate the importance and usefulness ofstructural geology in petroleum geosciences.

STRUCTURAL ANALYSIS

Generally involves three successive steps:

1. Descriptive Analysis: Physical and geometricaldescription of rock structures (e.g. folds, faults,

joints, crenulations etc).


. nema c na ys s: vaua on o spacemen& change in shape, orientation and size thatrocks undergo as a result of deformation.

3. Dynamic Analysis: Reconstruct forces and

stresses which result in rock deformation andfailure.

Fundamental Structure

Three fundamental types of geologicstructures:

1. Bed contacts

2. Primar structures roduced durindeposition or emplacement of rock body.

3. Secondary (tectonic) structures produced by deformation of existing

sedimentary, igneous and metamorphicrocks.



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Bed ContactsBoundaries separating one unit rock from another,generally in two types.

1. Comformable

2. Unconformable (unconformities)

z Angular unconformity

z Nonconformity


Conformable bed contacts

zHorizontal contact between rock units with nobreak in depositional or erosional gaps.

zNo significant gaps or hiatus in geologic time


Angular unconformity

Bedding contact which discordantly cuts acrossolder strata.

zDiscordance: Strata at an angle to each other

z Contact is typically an erosional surface.


Disconformity

Erosional gap or hiatus between units withoutangular discordance, e.g. fluvial channel cuttinginto underlying sequence of horizontally beddedsequence or channel fill deposit.



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NonconformitySedimentary strata overlying igneous ormetamorphic rocks across a sharp contact, e.g.Pre-Cambrian-Paleozoic contact in Ontariorepresents an erosional hiatus of 500Ma.


Structural relations

The structural relation between bed contacts areimportant in determining;

z Sequence of events

z Relative ages of rock units


z Present of tectonic deformation/uplift

Stratigraphic Principles - Recap

These principles covered in physical geology

zPrinciple of original horizontality

zPrinciple of original lateral continuity


zPrinciple of cross-cutting

zPrinciple of inclusion

You must always remember:

Uniformitarianism the present is the keyto the past.

PRINCIPLE OF ORIGINALHORIZONTALITY

Sedimentary rocks are deposited as essentially inhorizontal layers (Steno, 1600s)

z Exception is cross-bedding (delta fore-sets, subkhaorsand dune).

z Di in sedimentar strata im lies tectonic tiltin and/orfolding of strata.



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Principle of original lateral continuity

Sedimentary strata extend as laterally continuous layers inall directions within a basin (Steno, 19600s), until:

zThey thin and pinch-out

z Grade into different type of sediment

zTerminate against the basin edges or barrier (e.g.


shoreline)

Principle of cross-cutting

Igneous intrusions and faults are youngerthan the rocks they cross-cut (J . Hutton)


Mafic dyke

sandstone

Cross-cutting Relations

Often several cross-cutting relationships are present


How many events can you identify in this outcrop?

Principle of inclusion

Inclusions within a host rock are alwaysolder than the host (J . Hutton)


Granite inclusions in basalt


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Primary Sedimentary Structures

Structures acquired or obtained during depositionof sediments or emplacement of rocks

zHorizontal bedding (stratification) is mostcommon structure in sedimentary rocks.


Laminated mudstone

Primary Sedimentary Structures


Primary Structures - FossilsFossils preserved remains of organisms, casts or moulds

z Good strain indicator

z Determine strain from change in shape of fossil

z Relative change in length of lines/angle between lines.



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Primary Igneous Structures

z Pillow lavas - record extrusion and quenching of lavafrom sea floor, fast cooling effect contact to sea water.

z Convex upper surface indicates way up.


Pillow lavas forming at MOR

Primary Igneous Structures

Flow stratification layering in volcanic rocks produced bythe emplacement of successive lava sheets/flows.Stratification of ash (tephra or pyroclastic) layers.


Lava

Ash

Importance of Primary Structures

z Paleocurrent determine paleoflow directions

zOrigin mode of deposition (slow or fast),environments (terrestrial, transition, marine)

zWay-up useful indicators of younging in


s ra grap c sequence norma or overurne

zDating allow relative ages of rocks to bedetermined based on position, cross-cuttingrelations and inclusions

z Strain indicators deformation of primarystructures allows to estimates of rock strain

Secondary Structures

zDeformation structures produced bytectonic forces and other processes incrust (intrusive), such as:

{Faults/Shear zones

{Folds

{Cleavage/foliation/lineation

zSecondary structures are of primaryinterest in structural geology



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Fractures and J oints

z Fractures (general term) surface along which rockshas broken lost cohesion.

zJ oints (specific) fractures with little or no displacementparallel to failure surface.

z Fractured system indicates brittle deformation of brittlee avour ma eras.


Fractured granite overlainby metasedimentary rocks

FaultsFracture surfaces with appreciable displacement of strataor rock units.


Shear Zones

Shear zone zone of deformed rock that is more highlystrained than surrounding rocks or host rock.

z Commonly occurs in middle or lower crust

z It can be brittle or ductile deformation


Ductile shear zone

Fault Types

Dip-slip fault slip is parallel to the fault dip direction

zNormal fault footwall block displaced up

z Reverse (thrust) fault - footwall block displaceddown



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Fault Types

Strike-slip/lateral fault slip is horizontal or parallelwith strike of fault plane.

z Right-handed or right lateral fault (dextral)

z Left-handed or left lateral fault (sinistral)


Fault TypesOblique fault combination of dip- and strike-slipmotion.

zDextral-normal

zDextral-reverse


-

z Sinistral-reverse

What is the fault type?


Specify the direction of shear



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Folds

Warping of strata produced by compressivedeformation


Fold Terminology

zHinge (Axial) plane imaginary plane bybisecting fold limbs

zHinge line trace of axial plane on fold crest

z Plunge angle of dip of hinge line


Foliation and Cleavage

z Foliation parallel alignment of planar fabric elementswithin a rock.

z Cleavage tendency of rock to break along the planarsurface.

{Type of foliation


{Resemble fractures, but are not physicaldiscontinuity.

Lineations

Subparallel to parallel alignment of elongate of linear fabricelements in a rock bodies, e.g. slickensides and grooveson the fault plane surface.



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Think & answer these questions

1. What are the differences between primaryand secondary structures?

2. How do you recognise 1o & 2o structuresin the mesostructure scale?

3. What is bed contact? Why it is veryimportant in structural geology?

4. Where you always encounter geologicalhiatus? What elements or featuressynonym to hiatus?


CONCLUSION

zThe bed contact can be conformable orunconformable.

z Stratigraphic principles and uniformitarianismare the most important rules for better


understanding of bed relationship.

z Primary structures are very important tool tointerpret historical geology.

z Secondary structures are deformed structuresby tectonic activities.

lecture#2 fundamental geologic structure

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