沉积构造(Sedimentary structures)-英文课件

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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
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Sedimentary structures
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Sedimentary structures
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Sedimentary structures
Gravity-flow deposits
• Debris-flow deposits are typically poorly sorted, matrixsupported 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
• 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
• 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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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
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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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
• 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
• 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
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