Chapter 2 Nontectonic Structures

Chapter 2Chapter 2Nontectonic Nontectonic StructuresStructures

Nontectonic StructuresNontectonic Structures

►Nontectonic structures help to Nontectonic structures help to determine the original orientation of determine the original orientation of stratastrata Primary structures such as mud cracks, Primary structures such as mud cracks,

ripples, sole marks, and vesicles help to ripples, sole marks, and vesicles help to determine original orientationdetermine original orientation

These features help us determine the These features help us determine the facing directionfacing direction of a sequence of rocks. of a sequence of rocks.

An overturned fold in An overturned fold in ScotlandScotland

Nontectonic StructuresNontectonic Structures

►Structures that formed in primary Structures that formed in primary depositional environments may mimic depositional environments may mimic tectonic structurestectonic structures Examples ductile flow in water-saturated Examples ductile flow in water-saturated

silt, glacial ice, or evaporitessilt, glacial ice, or evaporites►Primary structures are usually older Primary structures are usually older

than tectonic structures and cross-than tectonic structures and cross-cutting relationships can help cutting relationships can help differentiate between themdifferentiate between them

Tectonic vs Nontectonic Tectonic vs Nontectonic StructuresStructures

Slumping

Soft-sediment folds

Tectonic folds

Primary Sedimentary Primary Sedimentary StructuresStructures

►BeddingBedding Bedding PlanesBedding Planes Graded BedsGraded Beds Cross BeddingCross Bedding

Cross BeddingCross Bedding

Cross bedding structuresCross bedding structures

Primary Sedimentary Primary Sedimentary StructuresStructures

►MudcracksMudcracks Polygonal blocks formed by desiccation Polygonal blocks formed by desiccation

and shrinkage of saturated sedimentsand shrinkage of saturated sediments Taper down from the surface until they Taper down from the surface until they

terminateterminate The surface layer may also separate and The surface layer may also separate and

curl upwardcurl upward

Mud cracksMud cracks

Primary Sedimentary Primary Sedimentary StructuresStructures

►Ripple MarksRipple Marks CurrentCurrent (translational) ripple marks (translational) ripple marks

form under a prevailing direction of form under a prevailing direction of fluid flowfluid flow

►Asymmetrical with the steep side toward Asymmetrical with the steep side toward downstreamdownstream

►Not useful to determine facing because Not useful to determine facing because they have the same shape whether they they have the same shape whether they are upright or overturnedare upright or overturned

OscillatoryOscillatory ripple marks form with ripple marks form with back-and-forth fluid flowback-and-forth fluid flow

►Symmetrical with pointed peaks showing Symmetrical with pointed peaks showing original facing directionoriginal facing direction

Ripple MarksRipple Marks

Primary Sedimentary Primary Sedimentary StructuresStructures

►Rain ImprintsRain Imprints Can determine facing directionCan determine facing direction

►Tracks and TrailsTracks and Trails May be used to determine facing directionMay be used to determine facing direction

►Sole Marks, Scour Marks, and Flute Sole Marks, Scour Marks, and Flute CastsCasts Are later filled with sediment and can be Are later filled with sediment and can be

used to determine facing directionused to determine facing direction

Rain ImprintsRain Imprints

Sole Marks and Flute CastsSole Marks and Flute Casts

Primary Sedimentary Primary Sedimentary StructuresStructures

►Load CastsLoad Casts Form from dewatering of the underlying Form from dewatering of the underlying

sediment from the weight of the newly sediment from the weight of the newly deposited sedimentdeposited sediment

Can be used to determine the facing Can be used to determine the facing directiondirection

The broadly convex bases of the load The broadly convex bases of the load casts show the original bottom of the casts show the original bottom of the structurestructure

Load CastsLoad Casts

FossilsFossils

►FossilsFossils Provide relative agesProvide relative ages May be used to determine facing direction May be used to determine facing direction

based on fossil sequences or growing based on fossil sequences or growing conditions in life assemblagesconditions in life assemblages

May also be used as strain indicatorsMay also be used as strain indicators

Sedimentary EnvironmentsSedimentary Environments

►Sedimentary FaciesSedimentary Facies Sedimentary rock units vary laterally and Sedimentary rock units vary laterally and

vertically as paleoenvironments change.vertically as paleoenvironments change. Facies are separated by composition, Facies are separated by composition,

texture, sorting, physical and biogenic texture, sorting, physical and biogenic sedimentary structures.sedimentary structures.

Walther’s principleWalther’s principle - Only those facies that - Only those facies that once existed side-by-side can be observed once existed side-by-side can be observed vertically juxtaposed in outcrop.vertically juxtaposed in outcrop.

Sedimentary FaciesSedimentary Facies

UnconformitiesUnconformities

►Unconformity – A break in the Unconformity – A break in the sedimentary record where part of a sedimentary record where part of a stratigraphic succession is missing.stratigraphic succession is missing. Produced by erosion or nondepositionProduced by erosion or nondeposition DisconformitiesDisconformities Angular unconformitiesAngular unconformities NonconformitiesNonconformities

UnconformitiesUnconformities

Cross section through the Cross section through the Grand CanyonGrand Canyon

Primary Igneous StructuresPrimary Igneous Structures

► Igneous plutons and lava flows may Igneous plutons and lava flows may form in shapes that resemble form in shapes that resemble sedimentary features or may sedimentary features or may tectonically alter previously emplaced tectonically alter previously emplaced country rockcountry rock

Geologic Map of the Sierra NevadaGeologic Map of the Sierra Nevada

Primary Igneous StructuresPrimary Igneous Structures

►Compositional bandingCompositional banding Occurs in igneous rocks and may result Occurs in igneous rocks and may result

from crystal settling, differentiation, from crystal settling, differentiation, fractional crystallization, and multiple fractional crystallization, and multiple parallel intrusions or flows.parallel intrusions or flows.

Study of these may reveal differentiation Study of these may reveal differentiation or fractional crystallization sequencesor fractional crystallization sequences

Primary Igneous StructuresPrimary Igneous Structures

►VesiclesVesicles Cavities left by gas bubbles that form in Cavities left by gas bubbles that form in

magma due to pressure releasemagma due to pressure release Vesicles generally accumulate at the top Vesicles generally accumulate at the top

of the magma chamber providing the of the magma chamber providing the facing directionfacing direction

VesicleVesicle

Primary Igneous StructuresPrimary Igneous Structures

►Pillow StructuresPillow Structures Form from lava flows into or below waterForm from lava flows into or below water Vesicular glassy curved tops and V-Vesicular glassy curved tops and V-

shaped nonvesicular bases indicate facing shaped nonvesicular bases indicate facing directiondirection

Pillow LavasPillow Lavas

Pillow LavaPillow Lava

Primary Igneous StructuresPrimary Igneous Structures

►Contact Metamorphic ZonesContact Metamorphic Zones from from lava flowslava flows The metamorphic aureole forms beneath The metamorphic aureole forms beneath

the lava flow and can be used to the lava flow and can be used to determine facing directiondetermine facing direction

Contact MetamorphismContact Metamorphism

Gravity-Related StructuresGravity-Related Structures

►Landslide and Submarine FlowsLandslide and Submarine Flows May resemble tectonic structures because May resemble tectonic structures because

of the flow patterns of these sedimentsof the flow patterns of these sediments►OlistostromesOlistostromes are matrix supported bedded are matrix supported bedded

sedimentssediments►DiamictitesDiamictites are matrix supported sediments are matrix supported sediments

with no beddingwith no bedding►TurbiditesTurbidites are unsorted sedimentary deposits are unsorted sedimentary deposits

produced from rapid flow of sedimentproduced from rapid flow of sediment

Gravity-Related StructuresGravity-Related Structures

►Salt StructuresSalt Structures Occur in Occur in evaporiteevaporite sequences of sequences of

anhydrite, gypsum, or haliteanhydrite, gypsum, or halite These rocks flow more readily than These rocks flow more readily than

any other common rock type and can any other common rock type and can form folded structures under surface form folded structures under surface temperature and pressuretemperature and pressure

These structures that move upward These structures that move upward and gravitationally intrude overlying and gravitationally intrude overlying sediments are called sediments are called diapirsdiapirs..

Eroded salt dome in Iran and diapir Eroded salt dome in Iran and diapir evolutionevolution

Grand Saline Salt Dome in TexasGrand Saline Salt Dome in Texas

Experimentally produced Experimentally produced diapirsdiapirs

Impact StructuresImpact Structures

►Structures with circular or elliptical Structures with circular or elliptical outlines can be formed from meteorite outlines can be formed from meteorite impacts as apposed to tectonic forcesimpacts as apposed to tectonic forces Shatter cones are produced from brittle Shatter cones are produced from brittle

deformation and can be good evidence of deformation and can be good evidence of impactsimpacts

Geologic Map of Wells Creek TennesseeGeologic Map of Wells Creek Tennessee

Shatter ConeShatter Cone

Reconstruction of the impact at Reconstruction of the impact at Wells CreekWells Creek

Nontectonic StructuresNontectonic Structures

►Nontectonic structures can appear to Nontectonic structures can appear to be tectonic featuresbe tectonic features

►Nontectonic structures can also be Nontectonic structures can also be used to determine facing direction, used to determine facing direction, which can aid in the interpretation of which can aid in the interpretation of tectonic featurestectonic features