Sedimentary Structures Flashcards by Janielle Benito

Produced by erosion or scouring of muddy sediment, forming “scoop-shaped” depressions

Flute Marks / Flute Casts

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Commonly preserved as bulbous or mammilla natural casts on the bottoms of sandstone beds

Flute Marks/ Flute Casts

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They can be used to determine paleo-current directions

Flute Marks / Flute Casts

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Elongated ridges on bed undersurfaces that maybe parallel to each other or show vairation in trend formed by filling of grooves

Groove Casts

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Produced as “tools” (object such as sticks, shells, bones or pebbles) carried by current bounce skip, roll or drag along the sediment surface

Tool Marks

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Commonly preserved on lower surfaces of sandstone beds as thin ridges

Tool Marks

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Generally aligned parallel to the direction of current movement

Tool Marks

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Small-scale erosional structure less than a meter across, cutting down several centimeters and occurring on the base of or within a bed elongate in the current direction with sharp and irregular with some relief but can be smooth

Scour Marks / Scoured Surfaces

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Larger structures that are sites of sediment transport for relatively long periods of time, concave up in cross section and their fills may form elongate (shoestring) sediment bodies

Channels

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Stratification thicker than 1cm produced by changes in the pattern of sedimentation

Bedding

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Sharply defined upper and lower surfaces enclosing or bounding beds

Bedding Plane

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Indicator of the depositional conditions under which the bed formed

Bed Shape

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Stratification thinner than 1cm produced by changes in the pattern of sedimentation

Lamination

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Sets of beds in which they are genetically related to one another

Bedsets

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Refers to finely-interbedded grain sizes, such as sand and mud, and can occur at a variety of scales

Heterolithic Bedding

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Washed-out ripples produced at the boundary between lower and upper flow regime

Planar beds

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0.5 to 3.0 cm in height with wavelength of 5 to 40 cm

Ripples

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Typically found under low to moderate flow velocities in sand that is less than 0.7mm in diameter

Ripples

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Formed by the action of waves of non-cohesive sediment, typically symmetrical in shape, asymmetrical when one direction of wave motion is stronger than the other may be difficult to distinguish from straight-crested current ripples

Wave-formed ripples

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Produced by unidirectional currents, making them asymmetric with steep lee-side (downstream) and gentle stoss-side (upstream)

Current Ripples

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Over 3.0 cm in height with wavelengths of at least 40 cm

Dunes

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Typically formed under moderate to high flow velocities in relatively deep water and sand that is more than 0.2 mm in diameter

Dunes

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Dune height and spacing is related to __________

water depth

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Typically have long, straight, parallel crests with bifurcations, ripple index is high and rarely preserved

Wind Ripples

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Type of aeolian dune with a single prevailing wind direction, and limited sand supply, forming a crescent shaped dune

Barchan Dunes

Type of aeolian dune with a single prevailing wind direction, and a good supply, forming wavy- shaped dunes

Transverse Dunes

Type of aeolian dune with two prevailing winds crossing in a single direction, forming linear-shaped dunes

Seif Dunes / Linerar Dunes

Type of aeolian dune with multiple prevailing winds from multiple directions, forming star-shaped dunes

Star dunes

Internal sedimentary structure of many sand-grade, and coarser sedimentary rocks and consists of stratification at an angle to the principal bedding direction, with a height of > 6cm, and thickness of 1cm or more

Cross-bedding

Internal sedimentary structure of many sand-grade and coarser, sedimentary rocks and consists of stratification at an angle to the principal bedding direction, with a height of <6cm, and thickness of only a few mm

Cross-lamination

Cross-stratification where the inter-set boundaries are generally planar and produced by straight-crested bedforms (ripples in cross lamination and sandwaves or dunes in cross bedding)

Tabular Cross-bedding

Cross-stratification where the inter-set boundaries are scoop-shaped from curve-crested bedforms (linguoid ripples in cross-lamination and lunate and sinous dunes in cross bedding)

Trough Cross-bedding

Forms when ripples are migrating and much sediment is being deposited out of suspension, ripples will climb up the backs of those downcurrent to form climbing-ripple cross-lamination

Climbing Ripples / Ripple drift

Commonly, the laminae are not concordant with the ripple profile, and distinguished from current ripple by irregular and undulating lower set boundaries and draping foreset laminae

Wave-formed cross lamination

Cross bedding where cross-beds dips of adjacent sets oriented in opposite directions produced by reversals of currents

Herringbone Cross bedding

Cross-stratification where cross-laminated sand contains mud streak, usually the ripple troughs

Flaser bedding

Cross-stratification where mud dominates and the cross-laminated sand occurs in lenses

Lenticular bedding

Cross-stratification where thin-ripple cross-laminated sandstones alternate with mudrock

Wavy bedding

Results of storm waves and deposition in the outer shoreface or transition zone between fair weather wave-based and storm wave-base and includes hummocky cross-stratification, swaley cross-stratification and tempestites

Storm bedding

Cross-stratification characterized by gently undulating low angle (1-5 degrees) cross-lamination with the convex upward part of the hummock and concave-downward part of the swale

Hummocky Cross-stratification

Cross-stratification where hummocks are rare and the bedding mostly consists of broad concave-up laminae

Swaley Cross-stratification

Low angle planar cross-stratification formed by wave swash and backwash commonly possess primary current lineation

Beach Cross-bedding

Produced by wind action and generally forms sets which are much thicker, and the cross-beds themselves dip at higher angles (>30 degrees) and reach up to 30m high

Aeolian Cross bedding

Large-scale low angle bedding, oriented normal to medium/smaller scale cross-stratification formed through lateral accretion of point bars generally 1m or more in height and continue laterally for several meters to more than 10m

Epsilon Cross-bedding

Typical of meandering river channel sandstones but can occur in delta distributary and tidal channel deposits

Lateral Accretion

Formed when deltas build into lakes or lagoons often referred as Gilbert-type deltas, occur as wedge or fan in marginal-lacustrine or marginal-marine

Small delta cross-bedding

Large-scale that represents prograding front of the delta

Fan-delta cross-bedding

Referred to at least 50m thick commonly developed at carbonate platform margins and adjacent to reefs where the dipping beds will be made of shallow water material, reef fragments; angle of dip varies from few degrees to 30 degrees

Very large-scale cross strata and clinoforms

Low amplitude bedforms sand-grade which migrate upstream throughout deposition of the sediment on the upstream-facing slope of the bedroom

Antidune cross-bedding

Where the coarsest particle at the base give way to finer particles higher up

Normal grading

Where there are several graded units within one bed

Composite or multiple graded-bedding

Where the grain size increases upwards

Reverse or inverse grading

Have no apparent internal structure during deposition or depositional structure was destroyed by bioturbation, recrystallization, dolomitization or dewatering or formed through rapid sedimentation or "dumping"

Massive beds

Found in fine-grained rocks that form through desiccation on exposure common in marine and lacustrine shorelines and river floodplains

Shrinkage cracks/ Mudcracks

Form when strong current flows elongate and discoidal pebble can become oriented with in upstream dip

Imbrications

Incomplete polygonal patterns that form through dewatering from salinity changes or osmotic effects and occur in shallow sublittoral lacustrine deposits

Synresis cracks

May develop in carbonate sediments through early cementation and expansion of the surface crust

Polygonal cracks

Small depressions with rims, formed through the impact of rain on the soft exposed surface of fine-grained sediments

Rainspots

Cavities filled with internal sediment and sparry calcite cement

Geopetal Structure

Continuous cavities either parallel or cutting the bedding, vary considerably in size

Sheet Cracks/ Neptunian dykes

Formed by cracking of the lithified or partially lithified sediment and opening up the cavities, or through penecontemporaneous tectonic movements, early compaction and settling or slight lateral movement

Sheet Cracks/ Neptunian dykes

Formed through subaerial exposure and meteoric dissolution of a limestone surface generally under humid conditions, usually have irregular topography, with potholes or cracks

Paleokarstic surfaces

Present in limestones where there has been synsedimentary cementation so that sediment was partly or wholly lithified on the seafloor

Hardgrounds

Forms when cemented surface layer can expand and crack into polygonal pattern as a result of sedimentary cementation of carbonate sediments and can develop in shallow subtidal sediments but commonly in tidal-flat carbonates

Tepee structures

Biogenic laminated structures which have a great variety of growth forms developed through trapping and binding of carbonate particles by surficial microbial mat (algal mat composed of blue-green algae) and biochemical precipitation of carbonate

Stromatolites

A mass of sediment transported downslope where there is little internal deformation of the sediment mass, meters to kilometers in size

Slide

Used for deposits of large blocks, result of fault activity during deposition and erosion of fault scarps or collapse of carbonate platform

Megabreccia

Occurs when a sediment mass is internally deformed during downslope movement, typically shows folding, with recumbent folds, asymmetric anticlines, and synclines and thrust folds being common, found in meters to kilometers in size

Slump

Typically occurs in cross-laminated sediments, with the lamination deformed into rolls, small anticlines and sharp synclines, convolutions commonly asymmetric and overturned in paleocurrent direction

Convolute bedding

Consist of concave-up laminae (dishes), few cm across which may be separated by structureless zones (pillars)

Dish-and-pillars structures

Common on the soles of sandstone bed overlying mudrock occurring as bulbous, rounded structures, generally without any preferred orientation

Load casts

Form when mud is injected up into the sand

Flame structures

Forms as a result of loading, bed or usually of sand, can sink into underlying mud and break up into discrete masses

Ball-and-pillow structure

Local patches of cementation that form in sediments after deposition.

Nodules / Concretions

Nodules can either be (1)________ or (2)_________

1. Diagenetic nodules 2. Pedogenic nodules

Composed of fibrous crystals of calcite in fanning or conical pattern, that develops in organic-rich mudrocks during burial, and may relate to crystal growth under high pre-fluid pressure during compaction

Cone-in-cone structure

Develop within the soil of semi-arid environments where evaporation exceeds precipitation, typically found in red-bed successions in floodplain mudrocks or marine clastic sediments

Calcrete

Sutured type which are generally bedding-parallel, although they can be at high angles to the bedding too and may occur as a single sutured planes or as zones or swarms

Stylolites

Strange patterns, usually found on bedding planes resulting from precipitation of manganese and iron oxides/hydroxides and generally black that look like fossil leaves

Dendrites

Refers to the disruption of sediment by the activity of organisms and plants

Bioturbation

A type of trace fossil that is more complicated surface trails, found in symmetrical or ordered pattern, either coiled, radial, meandering, and mostly made by detritus feeders

Grazing trails

A type of trace fossil that is found to be simple to complex burrow systems but no suggestion of systematic working of sediment, and can be clay-lined or pelleted, with some made by suspension feeder

Dwelling Burrow

A type of trace fossil that is a simple to complex burrow system, often well-organized with defined branching pattern indicating systematic reworking of sediment by detritus feeders

Feeding Burrow

A type of trace fossil made by organisms in a hard substrate - cemented sediment, pebbles or fossils

Boring