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SedPet > Sedimentary Structures > Flashcards

Sedimentary Structures Flashcards

1
Q

Tabular/lenticular seds rock layer w/ distinguishing characteristics

A

Beds

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2
Q

layers less than 1 cm are

A

laminae

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3
Q

Bed Classification by > 100 cm

A

VT

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4
Q

Bed Classification by 30-100 cm

A

T

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5
Q

Bed Classification by 10-30 cm

A

M

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6
Q

Bed Classification by 3-10 cm

A

Th

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7
Q

Bed Classification by 1-3 cm

A

VTh

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8
Q

Laminae Classification by 30-100 mm

A

VT

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9
Q

Laminae Classification by 10-30 mm

A

T

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10
Q

Laminae Classification by 3-10 mm

A

M

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11
Q

Laminae Classification by 1-3 mm

A

Th

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12
Q

Laminae Classification by<1 mm

A

Vth

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13
Q

distinct discontinuity (erosional surface) between two similar beds (composition wise)

A
  • Amalgamation surface
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14
Q

those with amalgamation surface

A
  • Amalgamated Beds
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15
Q
  • Bedding planes (BP) represent:
A

1) Plane of non-deposition, (2) abrupt change in deposition, (3) erosion surface

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16
Q

beds w/ internal layers parallel to bedding surfaces

A
  • Planar Stratified:
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17
Q

groups of similar planar beds

A
  • Bedsets
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18
Q

groups of different beds but genetically associated

A

Composite Bedsets

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19
Q

internal layers deposited at an angle to the bounding surface, sometimes referred to as set of cross-strata

A
  • Cross-stratified
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20
Q

a succession of cross-stratus

A
  • Coset
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21
Q
  • Parallel laminae common in sts, causes include:
A

o (1) beach swash n backwash,
o (2) wind transpo,
o (3) steady flow currents in upper-flow regime,
o (4), upper & lower flow regime phases during turbidity current flow and;
o (5) sheet flow

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22
Q

oscillatory equiv of plane-bed transpo in upper & lower-flow regime

A

Sheet flow:

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23
Q
  • Characterized by vertically gradual + distinct changes in grain size
A

Graded

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24
Q

Coarse to fine, bot to top

A
  • Normal Grading
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25
Q

Fine to Coarse, bot to top

A
  • Reverse/Inverse
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26
Q
  • Bouma Sequence [ideal graded bed sequence]:
A

o (A) massive + well graded,
o (B) parallel laminae,
o (C) ripple cross lamination,
o (D) parallel laminae,
o (E) stuctureless mud unit

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27
Q
  • Hsu divides sequence into two: A+B & C
A

o D rarely occurs and E maybe pelagic shale, not part of turbidte flow unit

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28
Q

repetition of graded sts/slt beds (tubidite origin) + interbeds of pelagic/hemi pelagic shale

A
  • Rythmic Bedding
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29
Q

occurs in sediment-gravity-flow deposits (debris flow) and some turbidites (residemented clg)

A
  • Inverse Gr
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30
Q
  • InvGr Causes:
A

o (1) dispersive pressures due to interparticle collisions,
o (2) kinetic sieving,
o (3) str loss of deforming clays

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31
Q
  • Beds w/o visible internal laminae
  • Common on sts
  • generated in absence of fluid-flow traction transpo
  • due to sediment grav flo or rapid material deposition from suspension
A

Massive

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32
Q

explains massive bedding of Bouma A

A
  • Rapid aggradation
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33
Q
  • internal layers/foresets dip @ an angle to the surfaces that bound the sets of cross-beds
A

Cross Bedding

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34
Q
  • called cross lamination if foresets
A

< 10 mm

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35
Q

Cross bedding are classified as either tabular, trough or festoon

A

tabular, trough or festoon

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36
Q
  • planar bounding surfaces
  • formed by migration of large 2D beforms (dunes)
  • indiv beds range from cm to m, some reaching 10 m
A
  • Tabular Cross-Bedding
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37
Q

Curved Bouding Surfaces
o originated by migration of 3d bedforms, small current ripples = small scale cross bed sets or large scale that produce large scale cross beds
o those formed large scale ranges cm to more than 4 m
o can also form by filling of scour pits n channels, point bards of meandering streams, deposition of inclined surfaces @ beaches/marine bars

A
  • Trough
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38
Q
  • inclined surfaces that separate adjacent foresets w/ similar orientations; truncates lower foreset laminae
  • formed by modification of prev. formed ripples
A

Reactiviation Surface

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39
Q

Reactivation Surface Mechanisms

A
  • erosion during decrease of water depth due to wave action/flow ‘round bedforms
  • erosion during change in flow dir (tidal reversal)
  • modification @ constant water depth and flow dir; due to erosion from rando interaction of beforms or lee of an advancing bedform eroding
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40
Q
  • general appearance of waves when viewed in outcrop sections cut normal to the wave crests
  • forms during rapid deposition during current/wave ripple migration
  • series of cross-laminae superimposes each other as ripples migrate
A

Ripple Cross

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41
Q

o Succeeding ripples travels upward
o ripple crests out of phase; seems to be climbing in a down current direction
o laminae may appear horizontal or trough shaped

A
  • Climbing-ripple lamination
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42
Q
  • Abundant sed supply + traction support
A

= ripple production and migration

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43
Q
  • ripple laminae in phase =
A

no migration

44
Q

o this type forms when traction support rate = sediment supply
o in phases lami, forms by climbing vertically

A

ripple laminae in phase

45
Q

ripple laminae in phase possible by

A
  • oscillating ripples; active but nonmigrating
  • inactive ripples passively draped by suspension settling; amplitude decreases upward
46
Q

special type of cross lamination
o Thin mud streaks in between sets of ripple laminae
o occur in ripple troughs but can be seen covering crests

A
  • Flaser
47
Q

flaser depositional conditions

A
  • traction transpo + fine sand rippling altering with quiescent mud deposition
  • such activities erode previous ripple crests, allows new ripple sands to bury + preserve with mud flasers in troughs
  • sand>mud
48
Q
  • originally called truncated wave-ripple laminae
  • undulating cross laminae sets
  • both concave up (swales) and convex up (hummocks)
  • cuts each other with curved erosional surfaces (3.16,3.17)
  • occurs in sets 15-20 cm thick
  • spacing @ centi to several meters
  • lower bounding surface is sharp; usually erosional surface w/ current-formed sole marks
  • occurs in fine sts to coards stls w/ mica and fine plant debris
  • forms in some manner under wave action
  • common in ancient sediment on shoreface and shelf
  • originates by a unidirectional + oscillatory flow related to storm activity
  • reliable indicator of shelf and shoreface envi.
A

Hummocky Cross

49
Q
  • common in modern sed envi (recent deposits)
  • form in siliciclastic + carbonate settings
  • ripples occur owing to traction transpo under unidir flow/oscillatory flow
  • common in sand size seds, also seen in finer + coarser sed
  • a bedform progression develops in granular materials undergoing traction transpo when flow conditions change from low-flow to to up-flow regime
A

Ripple Marks

50
Q

Large ripples have 2 types

A

sand waves: low, long wavelength
dunes: higher, short-wavelegnth

51
Q

ripples * @ low flow velos

A

small ripples form (0.05-0.2 m l, 0.005-0.03m h)

52
Q

ripples @ up flow velos

A

o larger ripples (0.5 m to 100m l, tens of meters m)

53
Q

ripples * @higher flow velocities

A

o dunes destroyed + eroded; phase of plane-bed sediment tranpo occurs
o may produce antidunes migrate in upcurrent dir

54
Q

o assymettrical in cross section
o gently sloping stoss
o steep lee
o called current ripples
o crests are divided into shapes:

A
  • Ripped developed by unidirectional curr flow
55
Q

ripple crests are divided into shapes

A

 straight
 sinuous
 catenary (chain like)
 linguoid (tongue like)
 lunate (crescent shape)

56
Q
  • Ripples developed by wave action
  • symmetrical in cross-sectional
  • unless unidirectional bottom current superimposed on oscillatory flow during formation
A

oscillation ripples/wave ripples

57
Q
  • Formed from regular bedding/stratification via penecontemporaneous deformation
  • penecontemporaneous deformation = deformation/alteration during or after deposition but before consolidation
  • Deformation Structures = irreg strat structure where deformation is caused by soft sediment slumping, loading, squeezing or partial liquefaction
  • Erosion structures = unconsolidated bed erosion; followed by an episode of sedimentation
A

Irregular Stratification

58
Q
  • complexly folded or intricately crumpled beds or laminations
  • confined to a single sedimentation unit
  • may appear as smol anti/synclines in cross section view (3.21)
  • Axial planes lean in paleocurrent dir
  • most common in sand - silt size siliciclastic sediment; also seen in carbonates
  • lamina traced from fold to fold; can be truncated by erosional surfaces
  • increase in complexity + amplitude from base to upper part
  • usually confined to beds < 25 cm; some reported to be several meters @ subaqueos and eolian envi
  • lateral extent variable, some reaching 750 km^2
  • common in turbidites
A

Convolute Bedding

59
Q
  • flame-shaped mud projections; extends upward from a shale unit to an overlying bed with diff composition (usually sts)
  • direction more or less upward; some overturned or bent downward
  • caused by squeezing of low-density water saturated mud upward into denser sand layer
  • orientation of overturned crests consistent with paleocurrent direction
A

Flame Structures

60
Q
  • applied to basal portion of sts beds overlying shales; broken masses of various sizes packed vertically + laterally in a mud matrix
  • resemble pillow lavas; shapes described as: pillow, hassock, kidney, ball
  • shale squeezed between pillows extend as tongues into overlying sts
A

Ball and Pillow + Pseudonodules

61
Q

sand masses in some deposits detaches from main strata, surrounded by shale, uniformed balls resembling concretions bale na isolated ang sts sa overlying bed

A
  • Pseunodules
62
Q
  • Unconsolidated sediment maty move downslope due to gravity
  • manifests itself as slumps, slides or flows
A

Synsedimentary folds, faults, and rip-up clasts

63
Q
  • Penecontemporaneous deformation of Synsedimentary folds, faults, and rip-up clasts caused by 2 types of movement
A

o décollement type of movement; called synsedimentary folds
o Products of pervasive movement; involved interior of transported mass

64
Q

cohesive clays resistant to flow
o breaks into fragments; incorporating into sand flowing as a slurry
o term commonly used for shale clasts ripped by turbidity currents
o angular to subrounded; bent or curled
o common in turbidtes

A
  • Rip-up clasts
65
Q

saucer shaped; thin, dark, subhorizontal, flat to conc up clayey laminations
o occurs in laterally extensive thick beds
o 1-50 cm with, few mm thick
o generally darker color relative to other sediments
o contain more clay, silt or organics
o common in turbidites or high concentration flow deposits

A
  • Dish structures
66
Q

associated with dishes; vertical cross cutting columns and sheets of swirled sand cutting through massive or laminated sands containing dish/convolute lamination
o Both structures considered as water escape structures
o usually result of rapid deposition
o subsequent water escape from sediment during compaction + consolidation

A
  • PIilar structures;
67
Q

Dish and Pillar * Mechanism [by Lowe & LoPiccollo]

A

o semipermeable laminations act as barriers to upward moving water carrying fine sediments, fine sediments retarded by laminations and added to then forming the laminations (happens during gradual compaction + dewatering)
o water forced to move horizontally until it fines areas of weaknesses, forceful upward escape forms pillars

68
Q

Dish and Pillar * Mechanism [by Sllen]

A

o dishes occur from a stoping process w/in a slightly cohesive water saturated bed
o shallow water filled cavities exist at lower bed parts that would eventually fail and collapse creating failure surfaces - dishes
o cavities progress upward creating a series of dishes

69
Q

sediment filled troughs
o U or V shape cross section
o cuts across beds/laminations
o filled with texturally different sediment, usually coarser than the beds they truncate
o eroded principally by currents, some may be results of sediment grav flows
o common in fluvial/tidal environments; also occurs in tubidite sediments

A

Channels

70
Q

smaller, asymmetrical, short length
o coarser or finer than substrate
o common in sandy sediment
o form owing to current scour and subsequent backfilling as current velocity decreases’
o closely spaced in a row
o prevalent in sediments of fluvial origin
o related to flute casts

A
  • Scour and fill structures
71
Q
  • laminated structures
  • composed of fine silt/clay size carbonate sediment
  • reported in siliciclastic but rare occurrences
  • some nearly flat laminations
  • most are hemispherical bodies made up of curves, crinkled or deformed laminations
A

Biogenic structures: stromatolitic bedding

72
Q

resembles stromatolites externally but lacks laminations
* lamination thickness dependent on conc. + prop. of calcium carbonate minerals fine organics, and detrital clay + silt

A
  • Thrombolite
73
Q

T or F

A

Bedding-plane markings can occur on the tops or undersides (soles) of beds.

74
Q

include positive-relief casts and various irregular markings, especially in sandstones and coarser-grained sedimentary rocks overlying shales.

A

Sole Markings

75
Q

Many sole markings are formed through a two-stage process

A

involving initial erosion of a mud substrate, creating grooves or depressions, followed by the deposition of coarser sediment that fills these features.

76
Q

Erosional markings are common on the soles of ? but can form in various environments like fluvial, tidal-flat, and shelf environments, provided the ?

A

turbidite sandstones; right conditions of erosion followed by rapid deposition are met.

77
Q

initiated by erosional events caused by objects transported by currents. These objects intermittently or continuously come into contact with the sediment at the bottom.

A

Erosional Sole Markings

78
Q

are elongate and nearly straight ridges resulting from the infilling of grooves created by objects dragged over a mud bottom while in continuous contact with the bottom.

A

Groove Casts

79
Q

Groove casts are elongated and align parallel to the ?, making them useful for determining the sense of paleoflow

A

paleocurrent direction

80
Q

These markings share an origin with groove casts but are created by tools that have intermittent contact with the sediment rather than continuous contact

A

Bounce, brush, prod, roll, and skip marks

81
Q

are positive-relief features formed when small gouge marks are filled in. They have an asymmetrical cross-section, with the deeper and broader part oriented in the downcurrent direction.

A
  • Brush and Prod Marks
82
Q

have a roughly symmetrical appearance.

A
  • Bounce Marks
83
Q

generated when a tool either bounces (saltating) or rolls across the sediment surface, leaving behind a continuous track.

A
  • Roll and Skip Marks
84
Q
  • Roll and Skip Marks
A

Current scour

85
Q

elongated welts or ridges found on sedimentary beds. They have a bulbous nose at one end that flares out toward the other end and gradually merges with the bed’s surface.

A

Flute Casts

86
Q

are rounded knobs or irregular protuberances found on the undersides (soles) of sandstone beds that are situated above shales. They have a different appearance from flute casts due to their irregular form and orientation

A

Load Casts

87
Q

Load Cast Origin

A

formed due to gravitational instability caused by denser beds overlying less dense and weaker beds.
This instability can lead to deformation of hydroplastic or uncompacted muds with excess pore pressures.
The weight of overlying sand can cause unequal sinking into the less competent mud, creating the positive-relief features seen in load casts.

88
Q

Common in modern environments and may be preserved on the top or bottom bedding surfaces of ancient sedimentary rocks, indicating subaerial exposure and desiccation.

A

o Mudcracks

89
Q

Resemble mudcracks but tend to be discontinuous and vary in shape; they are believed to be subaqueous shrinkage cracks

A

o Syneresis Cracks

90
Q

Crater-like pits with slightly raised rims, typically less than 1 cm in diameter.

A

o Raindrop and Hailstone Imprints

91
Q

Caused by bubbles breaking on the sediment surface.

A

o Bubble Imprints

92
Q

Small dendritic channels or grooves forming on beaches due to the discharge of pore waters at low tide; have low preservation potential.

A

o Rill Marks

93
Q

Thin, arcuate lines or small ridges on beaches formed by wave swash, also with low preservation potential.

A

o Swash Marks

94
Q

Forms on the bedding surfaces of parallel-laminated sandstones, consisting of subparallel ridges and grooves.

A

o Parting Lineation

95
Q

Tabular bodies of sandstone filling fractures in various types of host rocks.

A

o Sandstone Dikes

96
Q

Sand bodies injected between layers of other rock, often challenging to distinguish from normally deposited sandstone beds.

A

o Sandstone Sills

97
Q
  • Primary vs. Secondary Sedimentary Structures
A

o Primary sedimentary structures typically form at or shortly after the deposition of the host sediment.
o Secondary sedimentary structures postdate the deposition of host sediments and are often of chemical origin.

98
Q
  • Formation of Secondary Structures:
A

o Secondary structures are formed through processes such as mineral precipitation in the pores of semi-consolidated or consolidated sedimentary rock.
o Chemical replacement processes also contribute to the formation of secondary structures.

99
Q
  • Common Secondary Structure: Concretions:
A

o Concretions are common secondary structures, often composed of calcite but can include other minerals like dolomite, hematite, siderite, chert, pyrite, or gypsum

100
Q

irregularly rounded bodies with a warty or knobby surface.
o They can be composed of various minerals like chert, apatite, anhydrite, pyrite, and manganese.

A
  • Nodules
101
Q

o Sand crystals are large euhedral or subhedral crystals of calcite, barite, or gypsum filled with detrital sand inclusions.
o They form during diagenesis by growth in incompletely cemented sands.

A
  • Sand Crystals:
102
Q

o Rosettes are radially symmetric crystalline aggregates or clusters of crystals, resembling a rose shape.
o They are typically composed of minerals like barite, pyrite, or marcasite and form through cementation processes

A
  • Rosettes
103
Q

o Color banding is rhythmic layering resulting from the precipitation of iron oxide in fluid-saturated sediments.
o It forms thin, closely spaced, often curved layers with various shades of red, yellow, or brown alternating with white or cream layers

A
  • Color Banding:
104
Q

are suture-like seams in thick-bedded sedimentary rocks, resulting from irregular interlocking penetration of rock on each side.
o They are typically marked by concentrations of clay minerals, iron oxide minerals, and fine organic matter.
o Most common in limestones but also occur in sandstones, quartzites, and cherts.

A
  • Stylolites
105
Q

consists of nested sets of small concentric cones, primarily composed of calcium carbonate.
o It generally occurs in thin, persistent layers of fibrous calcite, often associated with concretions.
o Most common in shales and marly limestones and believed to form by the growth of fibrous crystals in the enclosing sediment during early diagenesis.

A
  • Cone-in-Cone Structure:

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