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EaES 350-61 Contents Introduction Unconsolidated clastic sediments Sedimentary rocks Diagenesis Sediment transport and deposition Sedimentary structures.

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Presentation on theme: "EaES 350-61 Contents Introduction Unconsolidated clastic sediments Sedimentary rocks Diagenesis Sediment transport and deposition Sedimentary structures."— Presentation transcript:

1 EaES 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

2 EaES 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 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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9 EaES 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 EaES 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 EaES 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 EaES 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 EaES 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 EaES 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 EaES 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 EaES 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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39 EaES 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 EaES 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 EaES 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 EaES 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 EaES 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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