Sedimentary Structures Flashcards
Tabular/lenticular seds rock layer w/ distinguishing characteristics
Beds
layers less than 1 cm are
laminae
Bed Classification by > 100 cm
VT
Bed Classification by 30-100 cm
T
Bed Classification by 10-30 cm
M
Bed Classification by 3-10 cm
Th
Bed Classification by 1-3 cm
VTh
Laminae Classification by 30-100 mm
VT
Laminae Classification by 10-30 mm
T
Laminae Classification by 3-10 mm
M
Laminae Classification by 1-3 mm
Th
Laminae Classification by<1 mm
Vth
distinct discontinuity (erosional surface) between two similar beds (composition wise)
- Amalgamation surface
those with amalgamation surface
- Amalgamated Beds
- Bedding planes (BP) represent:
1) Plane of non-deposition, (2) abrupt change in deposition, (3) erosion surface
beds w/ internal layers parallel to bedding surfaces
- Planar Stratified:
groups of similar planar beds
- Bedsets
groups of different beds but genetically associated
Composite Bedsets
internal layers deposited at an angle to the bounding surface, sometimes referred to as set of cross-strata
- Cross-stratified
a succession of cross-stratus
- Coset
- Parallel laminae common in sts, causes include:
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
oscillatory equiv of plane-bed transpo in upper & lower-flow regime
Sheet flow:
- Characterized by vertically gradual + distinct changes in grain size
Graded
Coarse to fine, bot to top
- Normal Grading
Fine to Coarse, bot to top
- Reverse/Inverse
- Bouma Sequence [ideal graded bed sequence]:
o (A) massive + well graded,
o (B) parallel laminae,
o (C) ripple cross lamination,
o (D) parallel laminae,
o (E) stuctureless mud unit
- Hsu divides sequence into two: A+B & C
o D rarely occurs and E maybe pelagic shale, not part of turbidte flow unit
repetition of graded sts/slt beds (tubidite origin) + interbeds of pelagic/hemi pelagic shale
- Rythmic Bedding
occurs in sediment-gravity-flow deposits (debris flow) and some turbidites (residemented clg)
- Inverse Gr
- InvGr Causes:
o (1) dispersive pressures due to interparticle collisions,
o (2) kinetic sieving,
o (3) str loss of deforming clays
- 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
Massive
explains massive bedding of Bouma A
- Rapid aggradation
- internal layers/foresets dip @ an angle to the surfaces that bound the sets of cross-beds
Cross Bedding
- called cross lamination if foresets
< 10 mm
Cross bedding are classified as either tabular, trough or festoon
tabular, trough or festoon
- planar bounding surfaces
- formed by migration of large 2D beforms (dunes)
- indiv beds range from cm to m, some reaching 10 m
- Tabular Cross-Bedding
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
- Trough
- inclined surfaces that separate adjacent foresets w/ similar orientations; truncates lower foreset laminae
- formed by modification of prev. formed ripples
Reactiviation Surface
Reactivation Surface Mechanisms
- 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
- 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
Ripple Cross
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
- Climbing-ripple lamination
- Abundant sed supply + traction support
= ripple production and migration
- ripple laminae in phase =
no migration
o this type forms when traction support rate = sediment supply
o in phases lami, forms by climbing vertically
ripple laminae in phase
ripple laminae in phase possible by
- oscillating ripples; active but nonmigrating
- inactive ripples passively draped by suspension settling; amplitude decreases upward
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
- Flaser
flaser depositional conditions
- 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
- 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.
Hummocky Cross
- 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
Ripple Marks
Large ripples have 2 types
sand waves: low, long wavelength
dunes: higher, short-wavelegnth
ripples * @ low flow velos
small ripples form (0.05-0.2 m l, 0.005-0.03m h)
ripples @ up flow velos
o larger ripples (0.5 m to 100m l, tens of meters m)
ripples * @higher flow velocities
o dunes destroyed + eroded; phase of plane-bed sediment tranpo occurs
o may produce antidunes migrate in upcurrent dir
o assymettrical in cross section
o gently sloping stoss
o steep lee
o called current ripples
o crests are divided into shapes:
- Ripped developed by unidirectional curr flow
ripple crests are divided into shapes
straight
sinuous
catenary (chain like)
linguoid (tongue like)
lunate (crescent shape)
- Ripples developed by wave action
- symmetrical in cross-sectional
- unless unidirectional bottom current superimposed on oscillatory flow during formation
oscillation ripples/wave ripples
- 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
Irregular Stratification
- 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
Convolute Bedding
- 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
Flame Structures
- 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
Ball and Pillow + Pseudonodules
sand masses in some deposits detaches from main strata, surrounded by shale, uniformed balls resembling concretions bale na isolated ang sts sa overlying bed
- Pseunodules
- Unconsolidated sediment maty move downslope due to gravity
- manifests itself as slumps, slides or flows
Synsedimentary folds, faults, and rip-up clasts
- Penecontemporaneous deformation of Synsedimentary folds, faults, and rip-up clasts caused by 2 types of movement
o décollement type of movement; called synsedimentary folds
o Products of pervasive movement; involved interior of transported mass
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
- Rip-up clasts
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
- Dish structures
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
- PIilar structures;
Dish and Pillar * Mechanism [by Lowe & LoPiccollo]
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
Dish and Pillar * Mechanism [by Sllen]
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
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
Channels
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
- Scour and fill structures
- 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
Biogenic structures: stromatolitic bedding
resembles stromatolites externally but lacks laminations
* lamination thickness dependent on conc. + prop. of calcium carbonate minerals fine organics, and detrital clay + silt
- Thrombolite
T or F
Bedding-plane markings can occur on the tops or undersides (soles) of beds.
include positive-relief casts and various irregular markings, especially in sandstones and coarser-grained sedimentary rocks overlying shales.
Sole Markings
Many sole markings are formed through a two-stage process
involving initial erosion of a mud substrate, creating grooves or depressions, followed by the deposition of coarser sediment that fills these features.
Erosional markings are common on the soles of ? but can form in various environments like fluvial, tidal-flat, and shelf environments, provided the ?
turbidite sandstones; right conditions of erosion followed by rapid deposition are met.
initiated by erosional events caused by objects transported by currents. These objects intermittently or continuously come into contact with the sediment at the bottom.
Erosional Sole Markings
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.
Groove Casts
Groove casts are elongated and align parallel to the ?, making them useful for determining the sense of paleoflow
paleocurrent direction
These markings share an origin with groove casts but are created by tools that have intermittent contact with the sediment rather than continuous contact
Bounce, brush, prod, roll, and skip marks
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.
- Brush and Prod Marks
have a roughly symmetrical appearance.
- Bounce Marks
generated when a tool either bounces (saltating) or rolls across the sediment surface, leaving behind a continuous track.
- Roll and Skip Marks
- Roll and Skip Marks
Current scour
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.
Flute Casts
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
Load Casts
Load Cast Origin
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.
Common in modern environments and may be preserved on the top or bottom bedding surfaces of ancient sedimentary rocks, indicating subaerial exposure and desiccation.
o Mudcracks
Resemble mudcracks but tend to be discontinuous and vary in shape; they are believed to be subaqueous shrinkage cracks
o Syneresis Cracks
Crater-like pits with slightly raised rims, typically less than 1 cm in diameter.
o Raindrop and Hailstone Imprints
Caused by bubbles breaking on the sediment surface.
o Bubble Imprints
Small dendritic channels or grooves forming on beaches due to the discharge of pore waters at low tide; have low preservation potential.
o Rill Marks
Thin, arcuate lines or small ridges on beaches formed by wave swash, also with low preservation potential.
o Swash Marks
Forms on the bedding surfaces of parallel-laminated sandstones, consisting of subparallel ridges and grooves.
o Parting Lineation
Tabular bodies of sandstone filling fractures in various types of host rocks.
o Sandstone Dikes
Sand bodies injected between layers of other rock, often challenging to distinguish from normally deposited sandstone beds.
o Sandstone Sills
- Primary vs. Secondary Sedimentary Structures
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.
- Formation of Secondary Structures:
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.
- Common Secondary Structure: Concretions:
o Concretions are common secondary structures, often composed of calcite but can include other minerals like dolomite, hematite, siderite, chert, pyrite, or gypsum
irregularly rounded bodies with a warty or knobby surface.
o They can be composed of various minerals like chert, apatite, anhydrite, pyrite, and manganese.
- Nodules
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.
- Sand Crystals:
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
- Rosettes
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
- Color Banding:
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.
- Stylolites
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.
- Cone-in-Cone Structure:
