1. 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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2. 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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5. 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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7. Animation
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8. Animation
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9. 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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19. 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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21. 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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23. 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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27. 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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31. 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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34. 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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36. 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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37. Turbidite
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39. 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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41. 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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43. 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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46. 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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49. 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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