GY403 Structural GY403 Structural GeologyGeology

Lecture 5: Descriptive AnalysisLecture 5: Descriptive Analysis-- Field Field MappingMapping

Geological Field MappingGeological Field Mapping

Note Keeping and OrganizationNote Keeping and Organization Outcrop DescriptionsOutcrop Descriptions Components of a Geologic MapComponents of a Geologic Map Geological ContactsGeological Contacts

Primary ContactsPrimary Contacts DepositionalDepositional UnconformableUnconformable IntrusiveIntrusive

Secondary ContactsSecondary Contacts Fault ContactsFault Contacts Shear ZonesShear Zones

Primary StructuresPrimary Structures Structure ContoursStructure Contours

Field NotesField Notes

Notes must correlate Notes must correlate with map/GPS stationswith map/GPS stations

All measured structures All measured structures must be recorded for must be recorded for later stereographic later stereographic analysisanalysis

Photos must be indexed Photos must be indexed by map stationby map station

Rock/Mineral samples Rock/Mineral samples must be indexed by must be indexed by station IDstation ID

Note SketchesNote Sketches

Where needed Where needed sketches of outcrops sketches of outcrops should be included to should be included to clarify notesclarify notes

Rock Descriptions at a Map StationRock Descriptions at a Map Station

Rock Name: Igneous, Sedimentary or Metamorphic Rock Name: Igneous, Sedimentary or Metamorphic classification classification

ColorColor MineralogyMineralogy TextureTexture Primary Features (ripples, crossPrimary Features (ripples, cross--bedding, vesicles, etc.)bedding, vesicles, etc.) Gross characteristics (weathering, ridgeGross characteristics (weathering, ridge--former)former) Formation name and age (if known)Formation name and age (if known) Contacts present in outcropContacts present in outcrop

Components of a Geologic MapComponents of a Geologic Map

ExplanationExplanation Describes uses of structure symbols (bedding, lineation, etc.)Describes uses of structure symbols (bedding, lineation, etc.) Lithologic Column: chronologic order of formations (young Lithologic Column: chronologic order of formations (young

on top) and lithologic symbols (sandstone, limestone, etc.)on top) and lithologic symbols (sandstone, limestone, etc.) LineworkLinework (fault, depositional, unconformable contacts)(fault, depositional, unconformable contacts)

Title (includes geographic location)Title (includes geographic location) North Arrow (geographic and magnetic)North Arrow (geographic and magnetic) Scale (RF and graphical)Scale (RF and graphical) Border (bounding latitude and longitude of map area)Border (bounding latitude and longitude of map area) Authorship and Date of completionAuthorship and Date of completion

Geologic Map TemplateGeologic Map Template

Includes the Includes the components of a components of a complete complete geologic mapgeologic map

Mapping ContactsMapping Contacts

Conformable Conformable Depositional ContactsDepositional Contacts Bedding contacts Bedding contacts

represent changes in represent changes in depositional environmentdepositional environment

Time gap represented by Time gap represented by contact (Diastem) is contact (Diastem) is short by geological short by geological standardsstandards

UnconformitiesUnconformities

Unconformity: a depositional Unconformity: a depositional surface produced by erosion surface produced by erosion and/or periods of nonand/or periods of non--depositiondeposition

Angular UnconformityAngular Unconformity Tilted strata below unconformity Tilted strata below unconformity

surfacesurface DisconformityDisconformity

Strata is parallel to unconformity Strata is parallel to unconformity above and below unconformable above and below unconformable contactcontact

NonconformityNonconformity Rocks below unconformable Rocks below unconformable

contact can be demonstrated to contact can be demonstrated to have been uplifted and eroded have been uplifted and eroded from great depthfrom great depth

Angular Unconformity ExampleAngular Unconformity Example

A period of A period of deformation deformation preceded uplift and preceded uplift and erosionerosion

Older units below Older units below unconformity unconformity display truncation display truncation by unconformable by unconformable contactcontact

Tapeats Sandstone Fm. (C)Grand CanyonSeries Fm. (pC)

Disconformity ExampleDisconformity Example

Usually based on paleontological data since it is Usually based on paleontological data since it is difficult to recognize in the fielddifficult to recognize in the field

Redwall Ls. (M)

Muav Ls. (C)

Disconformity

Nonconformity ExampleNonconformity Example

Rocks below unconformable surface have been Rocks below unconformable surface have been deeply eroded into and brought from great deeply eroded into and brought from great depth to the surfacedepth to the surface

Town Mt. Granite (1.0 Ga)

Cap Mt. Limestone (500 Ma)

Nonconformity

Confusing Faults with Confusing Faults with UnconformitiesUnconformities

Unconformities may be confused with fault contacts Unconformities may be confused with fault contacts since both truncate contacts. Remember that a fault since both truncate contacts. Remember that a fault generally truncates strata on both sides of the contactgenerally truncates strata on both sides of the contact

Strata younger than the unconformity generally (but not Strata younger than the unconformity generally (but not alwaysalways-- see Buttress unconformity) will be parallel to see Buttress unconformity) will be parallel to the unconformitythe unconformity

An unconformable contact may display evidence of An unconformable contact may display evidence of paleopaleo--erosion (soil profiles, topographic relief, worm erosion (soil profiles, topographic relief, worm burrows, etc.) burrows, etc.)

LowLow--angle regional unconformities are impossible to angle regional unconformities are impossible to recognize at a single outcroprecognize at a single outcrop

Buttress UnconformityButtress Unconformity

Younger strata is Younger strata is actually truncated actually truncated by erosional by erosional surfacesurface-- this this scenario is scenario is generated by generated by active normal active normal fault motionfault motion

LowLow--Angle Regional UnconformityAngle Regional Unconformity

Recognition of lowRecognition of low--angle angle unconformity requires unconformity requires measured sections at measured sections at many exposuresmany exposures

Intrusive ContactsIntrusive Contacts

Produced by the intrusion of magma or some Produced by the intrusion of magma or some other liquid (hydrothermal, salt, etc.)other liquid (hydrothermal, salt, etc.)

Intrusions of magma often produce xenolithsIntrusions of magma often produce xenoliths--portions of the country rock that are stoped into portions of the country rock that are stoped into the magma and remain recognizable after the magma and remain recognizable after crystallization of the magmacrystallization of the magma

Pendants are similar to xenoliths but retain a Pendants are similar to xenoliths but retain a physical connection to the country rockphysical connection to the country rock

Intrusive Contacts: XenolithsIntrusive Contacts: Xenoliths

Xenoliths are Xenoliths are portions of the portions of the surrounding surrounding country rock that country rock that fall into the fall into the magma chamber.magma chamber.

If they do not If they do not completely melt completely melt these blocks are these blocks are preserved as preserved as xenoliths and xenoliths and they prove the they prove the intrusive nature intrusive nature of the igneous of the igneous rock.rock.

Intrusive GeometryIntrusive Geometry

Discordant: cuts across country rock bedding and/or fabric.Discordant: cuts across country rock bedding and/or fabric. Batholiths: > 100 km2 outcrop areaBatholiths: > 100 km2 outcrop area Stock: 100Stock: 100--1 km2 outcrop area1 km2 outcrop area Pluton: < 1 km2 outcrop areaPluton: < 1 km2 outcrop area Dike: tabularDike: tabular

Concordant: intrudes parallel to country rock bedding and/or Concordant: intrudes parallel to country rock bedding and/or fabricfabric Sill: tabularSill: tabular Laccolith: flat bottom, concave down upper contact, low viscositLaccolith: flat bottom, concave down upper contact, low viscosityy Lopolith: flat top, concave up bottom contact, mafic batholithLopolith: flat top, concave up bottom contact, mafic batholith

Intrusive Geometry cont.Intrusive Geometry cont.

NonNon--magmatic Intrusionsmagmatic Intrusions

Clastic dikes: intrusions Clastic dikes: intrusions of waterof water--oversaturated oversaturated clastic sediments that clastic sediments that intrude older rocks along intrude older rocks along fracturesfractures

Salt Salt DiapirsDiapirs

Anhydrite, Anhydrite, Gypsum Gypsum and Halite: and Halite: extremely extremely ductile ductile under under even even moderately moderately elevated T elevated T & P& P

Fault ContactsFault Contacts

Cataclastic Rocks: produced by the grinding action in Cataclastic Rocks: produced by the grinding action in the fault zonethe fault zone Fault Gouge: fine powderFault Gouge: fine powder Fault Breccia: angular fragments that are bound together by a Fault Breccia: angular fragments that are bound together by a

finefine--grained matrix of pulverized materialgrained matrix of pulverized material SlickensidesSlickensides MylonitesMylonites

Drag FoldingDrag Folding Ductile Shear Zones Ductile Shear Zones

Fault Breccia ExampleFault Breccia Example

Angular fragments floating in a fineAngular fragments floating in a fine--grained matrixgrained matrix

Primary Structures: may provide Primary Structures: may provide youngingyounging directiondirection

Bedding (does not provide younging direction)Bedding (does not provide younging direction) CrossCross--beddingbedding Graded bedsGraded beds Ripple marks (symmetrical = younging direction)Ripple marks (symmetrical = younging direction) Mud cracksMud cracks Sole marksSole marks Pillow structurePillow structure Vesicular lava flowsVesicular lava flows Fractional crystallizationFractional crystallization

Bedding ExampleBedding Example

Bedding represents changes in depositional Bedding represents changes in depositional environments separated by short intervals of timeenvironments separated by short intervals of time

CrossCross--beddingbedding

Truncation occurs on upper portion of bed; Truncation occurs on upper portion of bed; younging direction is on concave side of crossyounging direction is on concave side of cross--beddingbedding

Graded BedsGraded Beds

Coarse Coarse material is material is deposited at deposited at base of bed; base of bed; fine material fine material at the top of at the top of the bedthe bed

Ripple MarksRipple Marks

2 basic types:2 basic types: Asymmetric = unidirectional flowAsymmetric = unidirectional flow Symmetrical = biSymmetrical = bi--directional flowdirectional flow Symmetrical = younging directionSymmetrical = younging direction

Shallow slope faces source of flow

Mud CracksMud Cracks

Concave toward original Concave toward original to of bedto of bed

Sole MarksSole Marks

Mold made by upper unit extends into older Mold made by upper unit extends into older weathered unit, therefore, sole marks exist on weathered unit, therefore, sole marks exist on the base (lower original contact) of a bedthe base (lower original contact) of a bed

Pillow StructurePillow Structure

Pillow basalts form an original Pillow basalts form an original concave downconcave downtype structuretype structure

Vesicular Lava FlowVesicular Lava Flow

Vesicles (gas bubbles) are trapped near the top Vesicles (gas bubbles) are trapped near the top of the lava flowof the lava flow

Fractional CrystallizationFractional Crystallization

Ultramafic and mafic layers occur at the original Ultramafic and mafic layers occur at the original base of a layered intrusion, intermediate and base of a layered intrusion, intermediate and felsic units near the top.felsic units near the top.

Structural ContoursStructural Contours

Contour lines depicting the elevation of a Contour lines depicting the elevation of a contact in the in the subsurface: contact in the in the subsurface: Isolith mapIsolith map

Contour lines depicting the thickness of a unit in Contour lines depicting the thickness of a unit in the subsurface: the subsurface: Isopach mapIsopach map

Adjacent contours may be used to estimate the Adjacent contours may be used to estimate the strike & dip or bearing & plunge of planes and strike & dip or bearing & plunge of planes and lines at any point on a isolith maplines at any point on a isolith map

Estimating Strike & DipEstimating Strike & Dip

The trend of the contour line determines the strike, the The trend of the contour line determines the strike, the dip is calculated perpendicular to strike using the dip is calculated perpendicular to strike using the elevation change of adjacent contour lineselevation change of adjacent contour lines

-1200

-1300

-1400

-1500

A

B

A

B

100 ft.

80 ft.

80 ft.

N60WDip = ArcTan(100/80)=51

5143

51

45

45

Structure Contours & FaultsStructure Contours & Faults

The effects of faulting are often evident in The effects of faulting are often evident in isolith mapsisolith maps

N

-5000ft

-6000ft

-7000ft

-5000ft

-3000ft

-4000ft

-6000ft

B

A

-2000ft

0 1000 2000

Scale (feet)

Solving for the Attitude of the FaultSolving for the Attitude of the Fault

Equivalent contours are found in separate fault blocksEquivalent contours are found in separate fault blocks--the strike of the fault is the connecting contour linethe strike of the fault is the connecting contour line

N

-5000ft

-6000ft

-7000ft

-5000ft

-3000ft

-4000ft

-6000ft

B

A

-2000ft

0 1000 2000

Scale (feet)

900

CD

600

Dip of Bedding = ArcTan(1000/900)=48 SouthDip of Fault = ArcTan(1000/600) = 59 NW

N20E

Exam 1 Summary Exam 1 Summary

Be able to discuss the various types of contacts and how they maBe able to discuss the various types of contacts and how they may be recognized in the field:y be recognized in the field: DepositionalDepositional IntrusiveIntrusive UnconformableUnconformable

AngularAngular DisconformableDisconformable NonconformableNonconformable

Faulted (including fault zone rocks: gouge, breccia, mylonite)Faulted (including fault zone rocks: gouge, breccia, mylonite) Be able to sketch and define intrusive geometryBe able to sketch and define intrusive geometry Be able to discuss the required components of a geologic mapBe able to discuss the required components of a geologic map Be able to discuss and sketch the various types of primary featuBe able to discuss and sketch the various types of primary features that indicate a facing directionres that indicate a facing direction Be able to interpret structure contour mapsBe able to interpret structure contour maps Be able to discuss detailed structural analysis (Descriptive, KiBe able to discuss detailed structural analysis (Descriptive, Kinematic, Dynamic, Tectonic)nematic, Dynamic, Tectonic) Be able to discuss the components of Kinematic analysis (translaBe able to discuss the components of Kinematic analysis (translation, rotation, dilation, distortion) tion, rotation, dilation, distortion)

and give geological examples of eachand give geological examples of each Know the geologic time scaleKnow the geologic time scale