Lecture#2 Fundamental Geologic Structure
-
Upload
renuga-subramaniam -
Category
Documents
-
view
221 -
download
0
Transcript of Lecture#2 Fundamental Geologic Structure
-
7/29/2019 Lecture#2 Fundamental Geologic Structure
1/10
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
QAB 1053 Structural Geology 2
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).
QAB 1053 Structural Geology 3
. 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.
QAB 1053 Structural Geology 4
-
7/29/2019 Lecture#2 Fundamental Geologic Structure
2/10
Bed ContactsBoundaries separating one unit rock from another,generally in two types.
1. Comformable
2. Unconformable (unconformities)
z Angular unconformity
z Nonconformity
QAB 1053 Structural Geology 5
Conformable bed contacts
zHorizontal contact between rock units with nobreak in depositional or erosional gaps.
zNo significant gaps or hiatus in geologic time
QAB 1053 Structural Geology 6
Angular unconformity
Bedding contact which discordantly cuts acrossolder strata.
zDiscordance: Strata at an angle to each other
z Contact is typically an erosional surface.
QAB 1053 Structural Geology 7
Disconformity
Erosional gap or hiatus between units withoutangular discordance, e.g. fluvial channel cuttinginto underlying sequence of horizontally beddedsequence or channel fill deposit.
QAB 1053 Structural Geology 8
-
7/29/2019 Lecture#2 Fundamental Geologic Structure
3/10
NonconformitySedimentary strata overlying igneous ormetamorphic rocks across a sharp contact, e.g.Pre-Cambrian-Paleozoic contact in Ontariorepresents an erosional hiatus of 500Ma.
QAB 1053 Structural Geology 9
Structural relations
The structural relation between bed contacts areimportant in determining;
z Sequence of events
z Relative ages of rock units
QAB 1053 Structural Geology 10
z Present of tectonic deformation/uplift
Stratigraphic Principles - Recap
These principles covered in physical geology
zPrinciple of original horizontality
zPrinciple of original lateral continuity
QAB 1053 Structural Geology 11
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.
QAB 1053 Structural Geology 12
-
7/29/2019 Lecture#2 Fundamental Geologic Structure
4/10
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.
QAB 1053 Structural Geology 13
shoreline)
Principle of cross-cutting
Igneous intrusions and faults are youngerthan the rocks they cross-cut (J . Hutton)
QAB 1053 Structural Geology 14
Mafic dyke
sandstone
Cross-cutting Relations
Often several cross-cutting relationships are present
QAB 1053 Structural Geology 15
How many events can you identify in this outcrop?
Principle of inclusion
Inclusions within a host rock are alwaysolder than the host (J . Hutton)
QAB 1053 Structural Geology 16
Granite inclusions in basalt
-
7/29/2019 Lecture#2 Fundamental Geologic Structure
5/10
QAB 1053 Structural Geology 17
Primary Sedimentary Structures
Structures acquired or obtained during depositionof sediments or emplacement of rocks
zHorizontal bedding (stratification) is mostcommon structure in sedimentary rocks.
QAB 1053 Structural Geology 18
Laminated mudstone
Primary Sedimentary Structures
QAB 1053 Structural Geology 19
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.
QAB 1053 Structural Geology 20
-
7/29/2019 Lecture#2 Fundamental Geologic Structure
6/10
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.
QAB 1053 Structural Geology 21
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.
QAB 1053 Structural Geology 22
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
QAB 1053 Structural Geology 23
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
QAB 1053 Structural Geology 24
-
7/29/2019 Lecture#2 Fundamental Geologic Structure
7/10
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.
QAB 1053 Structural Geology 25
Fractured granite overlainby metasedimentary rocks
FaultsFracture surfaces with appreciable displacement of strataor rock units.
QAB 1053 Structural Geology 26
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
QAB 1053 Structural Geology 27
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
QAB 1053 Structural Geology 28
-
7/29/2019 Lecture#2 Fundamental Geologic Structure
8/10
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)
QAB 1053 Structural Geology 29
Fault TypesOblique fault combination of dip- and strike-slipmotion.
zDextral-normal
zDextral-reverse
QAB 1053 Structural Geology 30
-
z Sinistral-reverse
What is the fault type?
QAB 1053 Structural Geology 31
Specify the direction of shear
QAB 1053 Structural Geology 32
-
7/29/2019 Lecture#2 Fundamental Geologic Structure
9/10
Folds
Warping of strata produced by compressivedeformation
QAB 1053 Structural Geology 33
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
QAB 1053 Structural Geology 34
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
QAB 1053 Structural Geology 35
{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.
QAB 1053 Structural Geology 36
-
7/29/2019 Lecture#2 Fundamental Geologic Structure
10/10
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?
QAB 1053 Structural Geology 37
CONCLUSION
zThe bed contact can be conformable orunconformable.
z Stratigraphic principles and uniformitarianismare the most important rules for better
QAB 1053 Structural Geology 38
understanding of bed relationship.
z Primary structures are very important tool tointerpret historical geology.
z Secondary structures are deformed structuresby tectonic activities.