Docstoc

Sedimentary structures

Document Sample
Sedimentary structures Powered By Docstoc
					                                Contents

•   Introduction                           •    Stratigraphic principles
•   Unconsolidated clastic sediments       •    Sequence stratigraphy
•   Sedimentary rocks                      •    Sedimentary basins
•   Diagenesis                             •    Models in sedimentary geology
•   Sediment transport and deposition      •    Applied sedimentary geology
•   Sedimentary structures                 •    Reflection
•   Facies and depositional environments
•   Glacial/eolian/lacustrine environments
•   Fluvial/deltaic/coastal environments
•   Shallow/deep marine environments


                                   EaES 350-6                           1
                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)




                                   EaES 350-6                        2
EaES 350-6   3
EaES 350-6   4
              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




                             EaES 350-6                           5
EaES 350-6   6
Animation


            EaES 350-6   7
Animation


            EaES 350-6   8
              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
                             EaES 350-6                          9
EaES 350-6   10
EaES 350-6   11
EaES 350-6   12
EaES 350-6   13
EaES 350-6   14
EaES 350-6   15
EaES 350-6   16
EaES 350-6   17
EaES 350-6   18
               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



                              EaES 350-6                            19
EaES 350-6   20
               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!)




                              EaES 350-6                              21
EaES 350-6   22
EaES 350-6   23
EaES 350-6   24
EaES 350-6   25
               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

                              EaES 350-6                             26
EaES 350-6   27
EaES 350-6   28
EaES 350-6   29
               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

                              EaES 350-6                             30
EaES 350-6   31
EaES 350-6   32
               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




                               EaES 350-6                             33
               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



                              EaES 350-6                           34
EaES 350-6   35
               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



                              EaES 350-6                           36
EaES 350-6   37
EaES 350-6   38
                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)

                               EaES 350-6                             39
EaES 350-6   40
                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)

                               EaES 350-6                             41
EaES 350-6   42
                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)

                               EaES 350-6                             43
EaES 350-6   44
EaES 350-6   45
               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




                              EaES 350-6                            46
EaES 350-6   47
EaES 350-6   48
EaES 350-6   49
EaES 350-6   50
              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)




                             EaES 350-6                           51

				
DOCUMENT INFO