Sedimentary Structures Flashcards
Produced by erosion or scouring of muddy sediment, forming “scoop-shaped” depressions
Flute Marks / Flute Casts
Commonly preserved as bulbous or mammilla natural casts on the bottoms of sandstone beds
Flute Marks/ Flute Casts
They can be used to determine paleo-current directions
Flute Marks / Flute Casts
Elongated ridges on bed undersurfaces that maybe parallel to each other or show vairation in trend formed by filling of grooves
Groove Casts
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
Commonly preserved on lower surfaces of sandstone beds as thin ridges
Tool Marks
Generally aligned parallel to the direction of current movement
Tool Marks
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
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
Stratification thicker than 1cm produced by changes in the pattern of sedimentation
Bedding
Sharply defined upper and lower surfaces enclosing or bounding beds
Bedding Plane
Indicator of the depositional conditions under which the bed formed
Bed Shape
Stratification thinner than 1cm produced by changes in the pattern of sedimentation
Lamination
Sets of beds in which they are genetically related to one another
Bedsets
Refers to finely-interbedded grain sizes, such as sand and mud, and can occur at a variety of scales
Heterolithic Bedding
Washed-out ripples produced at the boundary between lower and upper flow regime
Planar beds
0.5 to 3.0 cm in height with wavelength of 5 to 40 cm
Ripples
Typically found under low to moderate flow velocities in sand that is less than 0.7mm in diameter
Ripples
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
Produced by unidirectional currents, making them asymmetric with steep lee-side (downstream) and gentle stoss-side (upstream)
Current Ripples
Over 3.0 cm in height with wavelengths of at least 40 cm
Dunes
Typically formed under moderate to high flow velocities in relatively deep water and sand that is more than 0.2 mm in diameter
Dunes
Dune height and spacing is related to __________
water depth
Typically have long, straight, parallel crests with bifurcations, ripple index is high and rarely preserved
Wind Ripples
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)_________
- Diagenetic nodules
- 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
