eaes 350-61 contents introduction unconsolidated clastic sediments sedimentary rocks diagenesis...

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Contents

• Introduction• Unconsolidated clastic sediments• Sedimentary rocks• Diagenesis• Sediment transport and deposition• Sedimentary structures• Facies and depositional environments• Glacial/eolian/lacustrine

environments• Fluvial/deltaic/coastal environments• Shallow/deep marine environments

• Stratigraphic principles• Sequence stratigraphy• Sedimentary basins• Models in sedimentary

geology• Applied sedimentary geology• Reflection

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Sedimentary structures

• Sedimentary structures occur at very different scales, from less than a mm (thin section) to 100s–1000s of meters (large outcrops); most attention is traditionally focused on the bedform-scale• Microforms (e.g., ripples)• Mesoforms (e.g., dunes)• Macroforms (e.g., bars)

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Sedimentary structures

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

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

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

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Animation

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Animation

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Sedimentary structures

Cross stratification

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

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

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

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

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Sedimentary structures

Cross stratification

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

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

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

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Sedimentary structures

Planar stratification

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

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

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Sedimentary structures

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

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

• Heterolithic stratification is characterized by alternating sand and mud laminae or beds• Flaser bedding is dominated by sand with isolated, thin

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

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Sedimentary structures

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

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

• Heterolithic stratification is characterized by alternating sand and mud laminae or beds• Flaser bedding is dominated by sand with isolated, thin

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

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Sedimentary structures

• Tide-influenced sedimentary structures can take different shapes:• Herringbone cross stratification indicates bipolar flow

directions, but are rare• Mud-draped cross strata are much more common, and are

the result of alternating bedform migration during high flow velocities and mud deposition during high or low tide (slackwater)

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

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Sedimentary structures

Gravity-flow deposits

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

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

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Sedimentary structures

Gravity-flow deposits

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

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

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Turbidite

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Sedimentary structures

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

• Sole marks are erosional sedimentary structures on a bed surface that have been preserved by subsequent burial• Scour marks (caused by erosive turbulence)• Tool marks (caused by imprints of objects)

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

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Sedimentary structures

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

• Sole marks are erosional sedimentary structures on a bed surface that have been preserved by subsequent burial• Scour marks (caused by erosive turbulence)• Tool marks (caused by imprints of objects)

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

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Sedimentary structures

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

• Sole marks are erosional sedimentary structures on a bed surface that have been preserved by subsequent burial• Scour marks (caused by erosive turbulence)• Tool marks (caused by imprints of objects)

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

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Sedimentary structures

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

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

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

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Sedimentary structures

• Soft-sediment deformation structures are sometimes considered to be part of the initial diagenetic changes of a sediment, and include:• Slump structures (on slopes)• Dewatering structures (upward escape of water, commonly

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

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

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