6.Sedimentary Structures

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ContentsContents

IntroductionIntroduction Unconsolidated clastic sedimentsUnconsolidated clastic sediments Sedimentary rocksSedimentary rocks DiagenesisDiagenesis Sediment transport and depositionSediment transport and deposition Sedimentary structuresSedimentary structures Facies and depositional Facies and depositional

environmentsenvironments Glacial/eolian/lacustrine Glacial/eolian/lacustrine

environmentsenvironments Fluvial/deltaic/coastal Fluvial/deltaic/coastal

environmentsenvironments Shallow/deep marine Shallow/deep marine

environmentsenvironments

Stratigraphic principlesStratigraphic principles Sequence stratigraphySequence stratigraphy Sedimentary basinsSedimentary basins Models in sedimentary Models in sedimentary

geologygeology Applied sedimentary Applied sedimentary

geologygeology ReflectionReflection

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Sedimentary structuresSedimentary structures

• Sedimentary structures occur at very different scales, from Sedimentary structures occur at very different scales, from less than a mm (thin section) to 100s–1000s of meters less than a mm (thin section) to 100s–1000s of meters (large outcrops); most attention is traditionally focused on (large outcrops); most attention is traditionally focused on the bedform-scalethe bedform-scale

• Microforms (e.g., ripples)Microforms (e.g., ripples)• Mesoforms (e.g., dunes)Mesoforms (e.g., dunes)• Macroforms (e.g., bars)Macroforms (e.g., bars)

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• LaminaeLaminae and and bedsbeds are the basic sedimentary units that are the basic sedimentary units that produce stratification; the transition between the two is produce stratification; the transition between the two is arbitrarily set at 10 mmarbitrarily set at 10 mm

• Normal gradingNormal grading is an upward decreasing grain size within is an upward decreasing grain size within a single lamina or bed (associated with a decrease in flow a single lamina or bed (associated with a decrease in flow velocity), as opposed to velocity), as opposed to reverse gradingreverse grading

• Fining-upward successionsFining-upward successions and and coarsening-upward coarsening-upward successionssuccessions are the products of vertically stacked are the products of vertically stacked individual bedsindividual beds

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Animation

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Cross stratificationCross stratification

• Cross lamination (Cross lamination (small-scale cross stratificationsmall-scale cross stratification) is ) is produced by ripplesproduced by ripples

• Cross bedding (Cross bedding (large-scale cross stratificationlarge-scale cross stratification) is ) is produced by dunesproduced by dunes

• Cross-stratified deposits can only be preserved when a Cross-stratified deposits can only be preserved when a bedform is not entirely eroded by the subsequent bedform bedform is not entirely eroded by the subsequent bedform (i.e., sediment input > sediment output)(i.e., sediment input > sediment output)

• Straight-crested bedforms lead to Straight-crested bedforms lead to planar cross planar cross stratificationstratification; sinuous or linguoid bedforms produce ; sinuous or linguoid bedforms produce trough cross stratificationtrough cross stratification

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Cross stratificationCross stratification

• The angle of climb of cross-stratified deposits increases The angle of climb of cross-stratified deposits increases with deposition rate, resulting in ‘with deposition rate, resulting in ‘climbing ripple cross climbing ripple cross laminationlamination’’

• Antidunes form cross strata that dip upstream, but these Antidunes form cross strata that dip upstream, but these are not commonly preservedare not commonly preserved

• A single unit of cross-stratified material is known as a A single unit of cross-stratified material is known as a setset; a ; a succession of sets forms a succession of sets forms a co-setco-set

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Planar stratificationPlanar stratification

• Planar lamination (or planar bedding) is formed under both Planar lamination (or planar bedding) is formed under both lower-stage and upper-stage flow conditionslower-stage and upper-stage flow conditions

• Planar stratification can easily be confused with planar Planar stratification can easily be confused with planar cross stratification, depending on the orientation of a cross stratification, depending on the orientation of a section (strike sections!)section (strike sections!)

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• Cross stratification produced by wave ripples can be Cross stratification produced by wave ripples can be distinguished from current ripples by their symmetry and distinguished from current ripples by their symmetry and by laminae dipping in two directionsby laminae dipping in two directions

• Hummocky cross stratificationHummocky cross stratification (HCS) forms during (HCS) forms during storm events with combined wave and current activity in storm events with combined wave and current activity in shallow seas (below the fair-weather wave base), and is the shallow seas (below the fair-weather wave base), and is the result of aggradation of mounds and swalesresult of aggradation of mounds and swales

• Heterolithic stratificationHeterolithic stratification is characterized by alternating is characterized by alternating sand and mud laminae or bedssand and mud laminae or beds

• Flaser bedding is dominated by sand with isolated, thin mud Flaser bedding is dominated by sand with isolated, thin mud drapesdrapes

• Lenticular bedding is mud-dominated with isolated ripplesLenticular bedding is mud-dominated with isolated ripples

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• Cross stratification produced by wave ripples can be Cross stratification produced by wave ripples can be distinguished from current ripples by their symmetry and distinguished from current ripples by their symmetry and by laminae dipping in two directionsby laminae dipping in two directions

• Hummocky cross stratificationHummocky cross stratification (HCS) forms during (HCS) forms during storm events with combined wave and current activity in storm events with combined wave and current activity in shallow seas (below the fair-weather wave base), and is the shallow seas (below the fair-weather wave base), and is the result of aggradation of mounds and swalesresult of aggradation of mounds and swales

• Heterolithic stratificationHeterolithic stratification is characterized by alternating is characterized by alternating sand and mud laminae or bedssand and mud laminae or beds

• Flaser bedding is dominated by sand with isolated, thin mud Flaser bedding is dominated by sand with isolated, thin mud drapesdrapes

• Lenticular bedding is mud-dominated with isolated ripplesLenticular bedding is mud-dominated with isolated ripples

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• Tide-influenced sedimentary structures can take different Tide-influenced sedimentary structures can take different shapes:shapes:

• Herringbone cross stratification indicates bipolar flow Herringbone cross stratification indicates bipolar flow directions, but are raredirections, but are rare

• Mud-draped cross strata are much more common, and are the Mud-draped cross strata are much more common, and are the result of alternating bedform migration during high flow result of alternating bedform migration during high flow velocities and mud deposition during high or low tide velocities and mud deposition during high or low tide (slackwater)(slackwater)

• Tidal bundles are characterized by a sand-mud couplet with Tidal bundles are characterized by a sand-mud couplet with varying thickness; varying thickness; tidal bundle sequencestidal bundle sequences consist of a series consist of a series of bundles that can be related to neap-spring cyclesof bundles that can be related to neap-spring cycles

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Gravity-flow depositsGravity-flow deposits

• Debris-flow depositsDebris-flow deposits are typically poorly sorted, matrix- are typically poorly sorted, matrix-supported sediments with random clast orientation and no supported sediments with random clast orientation and no sedimentary structures; thickness and grain size commonly sedimentary structures; thickness and grain size commonly remain unchanged in a proximal to distal directionremain unchanged in a proximal to distal direction

• TurbiditesTurbidites, the deposits formed by turbidity currents, are , the deposits formed by turbidity currents, are typically normally graded, ideally composed of five units typically normally graded, ideally composed of five units (Bouma-sequence with divisions ‘a’-‘e’), reflecting (Bouma-sequence with divisions ‘a’-‘e’), reflecting decreasing flow velocities and associated bedformsdecreasing flow velocities and associated bedforms

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Gravity-flow depositsGravity-flow deposits

• Debris-flow depositsDebris-flow deposits are typically poorly sorted, matrix- are typically poorly sorted, matrix-supported sediments with random clast orientation and no supported sediments with random clast orientation and no sedimentary structures; thickness and grain size commonly sedimentary structures; thickness and grain size commonly remain unchanged in a proximal to distal directionremain unchanged in a proximal to distal direction

• TurbiditesTurbidites, the deposits formed by turbidity currents, are , the deposits formed by turbidity currents, are typically normally graded, ideally composed of five units typically normally graded, ideally composed of five units (Bouma-sequence with divisions ‘a’-‘e’), reflecting (Bouma-sequence with divisions ‘a’-‘e’), reflecting decreasing flow velocities and associated bedformsdecreasing flow velocities and associated bedforms

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• ImbricationImbrication commonly occurs in water-lain gravels and commonly occurs in water-lain gravels and conglomerates, and is characterized by discoid (flat) clasts conglomerates, and is characterized by discoid (flat) clasts consistently dipping upstreamconsistently dipping upstream

• Sole marksSole marks are erosional sedimentary structures on a bed are erosional sedimentary structures on a bed surface that have been preserved by subsequent burialsurface that have been preserved by subsequent burial

• Scour marks (caused by erosive turbulence)Scour marks (caused by erosive turbulence)• Tool marks (caused by imprints of objects)Tool marks (caused by imprints of objects)

• Paleocurrent measurements can be based on any Paleocurrent measurements can be based on any sedimentary structure indicating a current direction (e.g., sedimentary structure indicating a current direction (e.g., cross stratification, imbrication, flute casts)cross stratification, imbrication, flute casts)

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• Trace fossilsTrace fossils (ichnofossils) are the tracks, trails or burrows (ichnofossils) are the tracks, trails or burrows left behind in sediments by organisms (e.g., feeding traces, left behind in sediments by organisms (e.g., feeding traces, locomotion traces, escape burrows)locomotion traces, escape burrows)

• Disturbance of sediments by organisms is known as Disturbance of sediments by organisms is known as bioturbationbioturbation, which can lead to the total destruction of , which can lead to the total destruction of primary sedimentary structuresprimary sedimentary structures

• Since numerous trace fossils are connected to specific Since numerous trace fossils are connected to specific depositional environments, they can be very useful in depositional environments, they can be very useful in sedimentologic interpretationssedimentologic interpretations

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• Soft-sediment deformation structures are sometimes Soft-sediment deformation structures are sometimes considered to be part of the initial diagenetic changes of a considered to be part of the initial diagenetic changes of a sediment, and include:sediment, and include:

• Slump structures (on slopes)Slump structures (on slopes)• Dewatering structures (upward escape of water, commonly Dewatering structures (upward escape of water, commonly

due to loading)due to loading)• Load structures (density contrasts between sand and Load structures (density contrasts between sand and

underlying wet mud; can in extreme cases cause mud diapirs)underlying wet mud; can in extreme cases cause mud diapirs)

6.Sedimentary Structures

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