Primary Sedimentary Structures

• Sedimentary Structures incorporates all kind of structures encountered within the realm of sedimentary rocks

• Among these, Deformation and Chemical structures are produced by post-depositional changes and hence termed as Secondary Structures
• Other three are included within the basket term Primary Sedimentary Structures
• By definition, primary structures includes all the structures (both inorganic as well as organic) encountered in sedimentary rocks which have formed during or just after deposition
• Primary sedimentary structures in sediment piles are of diverse origin and of different timings with respect to sedimentation
• Primary structures bear the signature of flow dynamic processes involved in sedimentation
• Syndepositional deformation structures that reflect substratum condition during deposition are also by definition primary
• Some other sedimentary structures, may be deformational, formed after deposition but prior to cementation, are called penecontemporaneous or metadepositional structures
• Together these structures complete the spectrum of primary sedimentary structures

• Primary Sedimentary Structures primarily indicate about the sediment transporting as well as depositing mechanisms and paleocurrent flow directions, with some information regarding the relative water depth and relative current velocity
• Some of them can be used to identify the tops and bottoms of beds, and thus to determine the stratigraphic tops

Genetic Classification of Primary Sedimentary Structures

Primary Physical Structures

• Usually Primary Sedimentary Structures do not have any depositional environmental bias, although very few of the structures are restricted to a particular depositional environment
• They represent a particular formative mechanism which can be shared by different depositional environments
• Primary Physical Structures includes both depositional as well as erosional structures and are classified broadly in to several descriptive categories; stratification structures and bedforms, bedding-plane markings and other structures
• Stratification structures and bedforms are genetically depositional in nature and restricted within terrigenous sediments only
• Stratification structures and bedforms are further subdivided into four descriptive categories:
• bedding and lamination
• bedforms
• cross-stratifications
• irregular stratification

Bedding and Laminations

• Beds are the basic building blocks of successions and are thus of foremost importance as primary sedimentary structure
• By definition, beds are tabular or lenticular layers of sedimentary rock that have lithologic, textural, or structural unity that clearly distinguishes them from layers above and below
• A bed is an isolated stratum produced by a single flow, bounded below and above by omission surfaces, erosional or non-depositional in nature
• Conventionally, strata thicker than 1 cm are referred to as beds
• Thickness of the bed indicates the magnitude of the event
• Depending upon the thickness of the internal layering, splitting or parting units within beds, several terms are used
• Massive - no internal structure
• Blocky
• Slabby
• Flaggy
• Laminated – fine (< 1 cm thick) internal layers or strata
• Laminae are the building blocks of bed and represent fluctuations within a single flow

• Some other informal terms are also in use
• Layers - thicker than laminae , separated by minor but distinct discontinuities in texture or composition
• Divisions - characterized by a particular association of sedimentary structures
• Bands and lenses - subdivisions of a bed based on color, composition, texture, or cementation

• Beds are produced under essentially constant physical, chemical, and biological conditions
• Beds can be produced rapidly by a single flow, which results in the formation of massive beds; sediment load is essentially bedload
• While slow deposition leads to the formation of either graded or laminated beds
• The grain size within a graded bed ranges from coarser at the bottom to finer at the top; graded beds may be used as "up indicators"
• Graded bedding implies slow deposition from a highly sediment-laden waning flow (such as turbidity current); sediment load is essentially suspension load
• Laminated beds can be of two basic kinds
• Planar laminated – internal laminae of a bed are essentially parallel to the bedding planes
• Cross-laminated – internal laminae are deposited at an angle to the bedding planes ; beds composed of cross-laminated or cross-stratified units are called cross-beds

Planar laminated

Graded Bedding

Cross-laminated

• Bounding surfaces of beds are known as bedding planes or bedding surfaces
• They represent either erosion, or non-deposition, or an abrupt change in depositional conditions
• Bedding planes can be of various types, even, wavy, or curved
• The gross geometry of a bed (tabular or lenticular) depends upon the relationship between bedding-plane surfaces, which can be either parallel or nonparallel
• Geometric forms of beds can vary enormously, some common descriptive terms include uniform-tabular, tabular-lenticular, curved tabular, wedge-shaped and irregular

• According to the Law of Horizontality beds are characterized by lateral continuity
• But, termination of beds are common enough and can be produced by one of the three reasons
• convergence and merging of upper and lower bounding surfaces (pinch-out)
• lateral gradation of the composition of a bed into another bed of different composition so that the bounding bed surfaces die out
• meeting a cross-cutting feature such as a channel, fault, or unconformity
• Pinch-outs or stratal (beds or bedsets) terminations are defined by the geometric relationship between strata and the stratigraphic surface against which they terminate
• Main types of stratal terminations are described by truncation, toplap, onlap, downlap and offlap
• Among these truncation is erosional, others are non-erosional
• These non-erosional termination surfaces are known as lapouts
• Toplap - termination of inclined strata (clinoforms) against an overlying lower angle surface
• Onlap - termination of low-angle strata against an underlying steeper stratigraphic surface
• Downlap - termination of inclined strata against an underlying lower-angle surface
• Offlap - the progressive offshore shift of the updip terminations of the sedimentary units in which each successively younger unit leaves exposed a portion of the older unit on which it lies

• Apart from the toplap, other three are grouped as baselaps, as they show relationship with the underlying strata

• Truncations , on the other hand, may lead to the formation of unconformities
• Four basic types of unconformities
• Non-conformity
• Angular unconformity
• Disconformity
• Paraconformity

• Groups of similar beds or cross-beds are called bedset (or set)
• A simple bedset consists of two or more superimposed beds characterized by similar composition, texture and internal structures
• A group of two or more sets is called cosets

• A composite bedset refers to a group of beds differing in composition, texture, and internal structures but associated genetically
• Marked discontinuities within beds are called amalgamation surfaces.

Some special kinds of beddings

Massive bedding

• Bed appears homogenous, lacking internal structure
• Can be formed primarily or by synsedimentary obliteration of internal structure
• If primary, indicates rapid deposition
• Internal lamination can be obliterated
• due to intense bioturbation
• due to liquefaction , that is, release of trapped fluid

Graded bedding

• Vertical gradations in grain size within a bed
• Basal contacts are sharp
• Normal Grading - coarse particles at the base and gradually fines towards top
• Slow but continuous deposition from a waning highly sediment-laden flow
• Sediment load is chiefly suspension load
• Larger particles settle fastest during suspension fall-out, therefore they accumulate at the bottom of the bed
• Turbidites are the common producer of such bedding
• More common variety of graded bedding
• Reverse Grading – Finer particles at the base that grades upward in to coarser particles at top
• Particles are carried as suspension load
• Concentration must be high
• Resulted through dispersive pressure and kinetic sieving
• Dispersive pressure, which acts upwards, away from the bed, is proportional to the size of the grains, pushing large grains further upward than fine grains
• Finer particles moves downward, percolating through the pore spaces of larger particles; kinetic sieving
• Common in subaerial condition, as the grain-grain collision is more fierce than subaquous conditions
• Under subaquous conditions, only restricted within rudaceous sediment-gravity flows

Normally graded sandstone

Reverse graded conglomerate

Bedforms

• Bedforms are the bedding surface features developed at the sediment-water (or air) interface due to local deposition and erosion in response to the interaction of flowing or oscillatory current of water (or air)
• A bedform is produced as soon as flow attains a force sufficient to entrain particles from the bed, and start transporting sediments through formation of a set of structures
• If buried and preserved, bedforms will form sedimentary structures
• A bedform can be made up of a single bed or succession of beds

• Conditions under which a given bedform develop depends on a combination of fluid and sediment properties:
• Nature of flow (Unidirectional or Multidirectional)
• Flow velocity (U)
• Flow depth (D)
• Water temperature (specifically fluid density and viscosity)
• Grain Size (d)
• Grain density (ρ)
• Sediment sorting and Particle shape (Considered to be of secondary importance)

• Flat and featureless
• Internally planar laminated
• Found in both silt to fine sand as well as coarse sand (d> 0.7 mm)
• Sediment is mainly transported as suspension load, rarely as bed load
• Suspension fall-out is the depositional mechanism in case of fine sandy or silty medium
• Can be observed in different environment, starting from deep marine to fluvial flood plains
• In case of bed load sediments, depositional mechanism appears to be the migration of low-relief ripples in the lower-flow regime under shallow-flow conditions, where lack of avalanche faces within the low-relief ripples prevent formation of cross-laminae
• This mechanism is more active under subaerial condition than subaqueous one
• Planar beds can also form at low velocities below the critical velocity of ripple formation
• Such plane bed only forms in coarse sand or coarser material, although rare in nature
• Can be formed by both unidirectional or oscillatory flow

Lower Flow regime Plane Beds

Planar laminae within alternate silt and fine sandstone

• Flume studies established a sequence of bedforms that can be produced under unidirectional flow depending upon velocity, grain size and depth of flow
• In sand finer than 0.7 mm the first feature to form is ripples
• Typically their spacing is 10 to 20 cm or less, and their height is less than a few centimeters
• As flow velocity increase the ripples enlarge until they form sand waves, and finally dunes, which have spacing from 0.5 to 10m or more and heights of tens of cm to a meter or more
• At higher velocities plane beds are replaced by antidunes of up to 5m spacing. Low dip angles of 10 degrees or less, eventually chutes and pool

Bedforms: via Unidirectional Flow

Ripples

• Current Ripples are the commonest bedforms encountered within the lower flow regime unidirectional flow conditions
• Formed by dominant bedload movement
• They are asymmetric with a steeper, downstream-facing lee side and a gentle upstream-facing stoss side
• Erodes on the upstream side; accretes on the downstream side
• Avalanching along the lee side takes place after sediment accumulation on crest reaches angle-of-repose
• Wavelength and height is dependant upon current velocity, grain size, water depth

• Flow detachment takes place from brink point
• Backflow operates at trough
• Ripples are smallest bedform with RI ~8 (coarse) to 20 (fine sand)
• Ripples:

angle of lee side ~ 25^o-30^o

angle of stoss side ~ 5^o-10^o

0.05 < wavelength (L) < 0.6 m 0.005 < Height (H) < 0.05 m

• Plan form: vary with flow strength and duration of flow

Types of Current Ripples

Straight crested

Sinuous crested

Linguoid

Lunate

Dunes

• Large, asymmetric bed forms
• Not “large ripples” but a dynamically different bed form
• Most common in sands coarser than 0.15 mm
• Dunes:

Dune spacing or wavelength (L) varies between 0.75 m to > 100m

Height (H) varies between 0.075 to >5 m

Ripple Index (RI) ~5 (fine sand) to 50 (gravel)

Lee slope near angle of repose (~30^o)

• Overall planform varies with flow strength

• Two-dimensional (2-D) Dune –straight/sinuous and long crested; higher ripple index; 10cm-100s m
• Three-dimensional (3-D)Dune – curved faces, crest line complex, made up of several lunate segments and comparatively shorter; scour pits in the troughs are common; lower ripple index; 10cm-10s m
• Lowest flow strength dunes have straight to sinuous crests (2-D dunes)
• Transforms gradationally into 3-D dunes with increasing flow strength
• Dune height is primarily influenced by water depth
• Ripple height, on the contrary, are influenced more by grain size

• Ripples are sometimes superimposed on dunes to form compound dunes
• Flow separation over the dune crest leads to the development of an eddy that may produce a high enough upstream velocity over the bed to produce upstream-migrating ripples (back-flow ripples).

• Dune spacing (Wavelength, L) and height (H) is dependant upon flow depth, flow velocity and grain-size of the carried sediment

• Dune spacing
• increases with depth
• increases with velocity (washing out)
• increases with increasing grain size

​

​

​

• Dune height
• increases with depth
• increases and then decreases with velocity (washing out)
• increases with decreasing grain size

• Ripples and dunes occupy separate fields in bedform stability diagram
• Sand waves appear to be the transitional phase between these two bedforms
• As flow strength increases dunes becomes longer and lower (Washed-out dunes), washing out into the next bedform, the upper flow regime planar beds

Upper Flow regime Plane Beds

• A flat bed with intense sediment transport
• Flow parallel mounds known as current lineations or parting lineations
• Characterized by flow parallel oriented ridges (with a few grain diameter height) and intervening furrows, arranged in en-echelon fashion along the length
• Parting step lineations are similar to that of parting lineation (or parting plane lineation) but the parting plane cuts across several adjacent laminations
• Sandstones preferentially break in the flow parallel direction

• Minute flow parallel eddies, moving along helically coiled path, develop on the sediment-flow interface
• Coarser grains settle between uprising eddies forming minute parallel ridges, and hence produces parting lineations
• Such ridges remain parallel to the flow direction
• Further, constituting grains acquire flow parallel preferred orientation and in turn produces parting lineations also

Antidunes

• Antidunes formed with further increase in flow strength
• Produced under rapid flow conditions (F>1)
• Antidunes – long-crested bedforms with low relief; gently sloping; undulatory; RI~7 to 100
• Shows in-phase water surface and wave propagation
• May remain stationary or move upstream by accretion
• Formed by scouring of the substrate
• Internally low angle cross beds directed upstream are discernible

• Finally, chutes & pools develop at the highest flow regime, followed by erosion

Bedform Stability Diagram

Three major controlling factors are:

• Mean velocity or stream power
• Grain size of sediment load
• Depth of the flow

Variations in field of stability of different bedforms with changing depth of flow (left) and grain-size of the sediment load (below)

• Lower plane bed only forms in coarse sand or coarser
• Ripples form only in fine sand or finer
• Dunes only forms in medium to coarse sand-size and above
• Upper plane bed and dunes stability fields become larger with increasing depth
• Antidunes require higher velocity to form with increasing depth
• F -Limits the range of velocity over which they are stable

Important points to remember

Bedforms: via Multidirectional Flow

• Multidirectional flows can be oscillatory and bidirectional
• Bidirectional flows are typically indicative of tidal influence, but the constituent bedform is asymmetric current ripples (rarely dunes) only, moving in mutually opposite direction and in course producing different kinds of cross-stratification
• Oscillatory flows typically produces wave ripples
• Wave ripples are generally symmetrical to slightly asymmetrical due to eddies

• Orbital velocity difference < 1 cm/s = symmetrical bedforms
• Orbital velocity difference > 5 cm/s = asymmetrical bedforms
• Crests are straight to sinuous, bifurcate
• Crests are sharper than troughs
• Migration less, accretion dominates

Wave ripples with sharp, straight, bifurcating crests

Wave ripples with sinuous, less-sharp bifurcating crests

Wave ripples with flat bifurcating crests

Interference & Superimposed Ripples

• Oscillatory flows are often combined with current components
• In shallower depths, wave pathway gets flattened and eventually breaks in to current after touching the sediment-water interface
• The flattened wave pathways indicate combined flow condition, that is, oscillatory motion is coupled with current component
• Breaking of wave indicate total transformation to current component near sea or lake shore

• In very shallow water, different wave sets can coexist and their interference produces a complex pattern of ripple set, known as interference ripples
• Interference ripples can be made up of both coexisting symmetrical as well as asymmetrical ripple sets
• Superimposed ripples are one kind of interference ripple where both the sets are not formed simultaneously, instead latter minor set is superimposed over the former more dominant ripple set

Interference Ripples: modern (left); ancient (right)

Superimposed Ripples:

• Minor set restricted within the troughs of major ripple set only
• Indicates occassional emergence of the crest area due to very shallow water
• Known as Ladder-back Ripples

Wave ripples vs Current ripples

Varieties of ripple indices

• First two are the most used indices and used parameters are measured from the sectional view of ripples; L = Wavelength, H = Height
• Other indices are of minor importance; used parameters are measured from the planform of the rippled surface; C_c = length of the curved crest line; d = deviation from straight line; L_max & L_min = maximum and minimum spacing between successive crest lines; Ĺ = mean spacing between linguoid ripples

Hummocks & Swales

• An undulating bedding which is thought to be formed by wave-generated oscillatory flows or combined flows (waves plus currents)
• The bedform shows dome shaped elevated areas separated by depression
• The lower bounding surface is erosional and undulatory; sediments drapes over that surface to give a undulatory dome-basin like appearance
• Domal elevated areas are known as hummocks and depressed areas in-between are swales
• Wave-current combination is most efficient in throwing sand population in suspension
• Erosion results wavy surface and vertical fall out of sand from suspension gives rise to such bedform

• Long wavelength; 1-5 m
• Low height; 10s of cm
• Directional scours may present under the lower bounding surface
• Upper surfaces are often rippled or bioturbated

• Symmetric ‘dunes’ move back and forth (rather than downstream) and produces such bedform
• Internally characterized by undulating sets of cross laminae with both concave-up (swales) and convex-up (hummocks) segments, known as Hummocky cross-strata (HCS)
• Decapping of hummocks leave only the swales

• This structure is formed by strong surges of varied direction (oscillatory flow) that are generated by relatively large storm waves
• These bedforms are common within shallow marine or lacustrine environment, between the storm water wave-base (where wave touches the sediment-water interface)

Cross-stratifications

• All stratification that is inclined from the horizontal plane due to primary processes (e.g., excluding tectonic tilting)
• Inclined strata are normally bounded between bounding surfaces which are erosional surfaces that truncate underling cross-strata, if present

• Conventionally, if the individual inclined layers are thicker than 1 cm, the cross-stratification may be referred to as cross-bedding
• Thinner inclined layering is called cross-lamination
• Cross-strata set: a single set of similar cross-strata
• Cross-strata co-set: sets of cross-strata of the same type in vertical sequence and separated by bounding surfaces

• Cross-stratification is very much common in rock record as its preservation potential is much higher than that of the bedforms themselves (because the tops of bedforms tend to be planed off by subsequent current or wind erosion)
• Cross-stratification forms primarily by migration of ripples and dunes (in water or air)
• It can also be formed by filling of scour pits and channels, by deposition on the point bars of meandering streams, and by deposition on the inclined surface of beaches and marine bars
• Cross-stratification thus can be formed under many depositional environments
• Ripple or dune migration leads to formation of dipping foreset laminae owing to avalanching or suspension settling in the zone of separation on the lee sides of these bedforms
• Horizontal bottom- and top sets, along which the inclined foreset meets, are usually produced by suspension fall-out
• If suspension load is negligible (in case of coarser than sand-sized sediments mostly), avalanching of the bedload sediment down the lee side of the ripple will produce steep and straight foreset laminae, having approximately the same angle as the angle of repose; such internal cross-strata is known as angular cross-strata
• Similar cross-strata can also form in presence of suspension load, if the height of the lee side is large compared to the total flow depth, resulting concentration of the suspension fall-out within the base of the lee slope only; as in case of large eolian dunes

• If the suspension load is considerably high, or if the height of the lee slope is small compared to flow depth, suspended sediment will pile up at the base of the lee slope rapidly enough to keep pace with growth of the avalanche deposits; causing the lower part of the foreset laminae to curve outward and approach the bottomset laminae asymptotically
• Commonest variety of cross-strata; paleotop indicator
• If the suspension load is very high, then they fall along the total foreset, over both the lee and part of crest area, hindering erosion of the crest and produce sigmoidal type of cross-strata, where both the lower as well as the higher part of the foreset laminae approach the bottom- and topset laminae asymptotically
• Common in tidally-influenced environments
• Deposition over point bars of meandering rivers typically produces large-scale sigmoidal cross-strata, known as epsilon cross-strata
• All types of cross-strata are good paleocurrent indicator

• Cross-strata are divided into two principal types on the basis of formative depositional mechanisms; Tabular cross-strata and Trough cross-strata
• Tabular cross-strata forms by migration of 2-d or straight crested ripples and dunes; avalanching is the dominant mechanism for migration; formed under lower flow regime
• Trough cross-strata, on the other hand forms by migration of 3-d ripples and dunes; formed under comparatively higher flow conditions

• Depending upon the overall geometry and the nature of the bounding surface tabular cross-strata can be of two types, planar tabular cross-strata and planar wedge-shaped cross-strata along with the distinct trough cross-strata
• Tabular cross-strata generally is characterized by broad lateral extension with respect to set thickness and essentially planar bounding surfaces; while the former variety parallel bounding surfaces, the latter shows non-parallel ones; foresets are more angular
• Trough cross-strata, on the other hand, is characterized by curved bounding surfaces having trough-shaped sets consisting of an elongate scour filled with curved foresets having asymptotic bases

• Cross-stratification reflects episodic deposition on the bedform
• Presence of heterogeneity within the sediment load made cross-stratification discernible

Formative processes for cross-stratification

• As a bed form migrates there is erosion in the trough and stoss slope and deposition on the lee slope
• To preserve internal cross-stratification requires preservation of part of the bedform

• Without net deposition (aggradation) on the bed the bedforms will migrate through each other and preserve only a thin veneer in the migrating troughs

• With net deposition the bed aggrades and the internal deposits of the bed forms are preserved
• Proportion of the preserved internal structure of a bedform depends on the stoss slope angle and the angle of climb of the bed form

• Both ripples and dunes used to show this kind of phenomenon

Ripple cross-laminations

• Ripple cross-lamination (climbing ripple cross-lamination) forms when net deposition (aggradation) takes place very rapidly during migration of current ripples
• During aggradation, ripples climb one on another in such a manner that the crests of vertically succeeding laminae are out of phase and appear to be advancing upslope
• Some ripple laminae may be in-phase (one ripple crest lies directly above the other), which indicate no migration of ripples

• Three kinds of ripple cross-laminations are observed depending upon the relationship between stoss angle and the angle of climb
• Subcritical climb : angle of climb < stoss angle
• Critical climb : angle of climb = stoss angle
• Supercritical climb : angle of climb > stoss angle

• Subcritical climb: part of lee side get preserved
• Supercritical climb: total lee side and part of stoss side gets preserved
• Critical Climb: a special case when total lee side and the crest area gets preserved
• The formation of ripple cross-lamination require an abundance of sediment, especially sediment in suspension, which quickly buries and preserves original rippled layers, along with sufficient traction current to form it
• The angle of climb increases with increasing rate of suspension fall-out
• As suspension fall-out increases erosion by traction current gets hindered and stoss sides get preserved

• Ripple laminations typically refers to cross-laminations with preserved ripple forms in sectional view

• Changes in the style of climbing ripple cross-lamination reflect changes in migration rate and/or aggradation rate over time
• Supercritical climbs can be referred as ripple laminations as ripple forms gets preserved here
• Ripple cross-laminations are common in environments characterized by rapid suspension fall-outs, such as fluvial floodplains, point bars, river deltas subject to periodic flooding and environments of turbidite sedimentation

• Flaser bedding is a type of ripple bedding in which thin streaks of mud occur between sets of cross-laminated sandy or silty sediment
• Mud concentrate mainly in the ripple troughs, but may also partly cover the crests
• Flaser bedding suggests deposition under fluctuating hydraulic conditions; periods of current activity, resulting deposition of rippled sand due to traction transport, alternate with periods of quiescence, when mud is deposited; repeated episodes of current activity impart erosion of previously deposited ripple crests, allowing new rippled sand to bury and preserve rippled beds with mud flasers in the troughs
• Lenticular bedding is formed by interbedded mud and ripple cross-laminated sand, where sand lenses are discontinuous and isolated in both vertical and horizontal directions
• Lenticular bedding indicate dominance of mud-depositing conditions interspersed by occasional traction current deposits of current or wave origin
• Wavy bedding is a typical structure, where amount of traction deposit and suspension fall-out is almost equal
• These structures indicate deposition from cyclically alternating hydraulic conditions, as common in tidally-influenced areas
• Transitions from flaser to lenticular bedding pointed towards gradual increase in suspension fall-out in expense of traction deposits

Flaser & Lenticular Beddings

Cross-stratification formed under oscillatory flows

• Internal structures attributable to oscillation are relatively less known
• To-and-fro grain movement gives rise to bimodal cross-strata because of vertical building up of bedforms
• In larger part of its operation in natural aqueous settings a current component coexists with oscillation
• Wave-current combination is most efficient in throwing sand in suspension; consequently, structures like offshoots and draping arise in wave ripples

• Erosion results wavy surface and vertical fall out of sand from suspension gives rise to wavy parallel laminations
• In case of large waves, as encountered in storms, erosion is more pronounced followed by suspension, which produce hummocky (HCS) or swaley cross-strata (SCS)
• Hummocky cross-stratification is characterized by undulating sets of cross-laminae that are both concave-up (swales) and convex-up (hummocks)
• Strong storm-wave action first erodes the seabed into low hummocks and swales that lack any significant orientation; this topography is then mantled by laminae of material swept over the hummocks and swales
• The erosion surface is, however, dome-like in geometry and thus hummocky cross-laminae dip centripetally
• Recent studies also postulated presence of minor unidirectional current component, resulting slight migration of hummocks
• Decapping of hummocks leave only the swales between the hummocks
• Between hummocky and swaley cross-stratification the latter are likely to be frequent in occurrence in the shallower domain

Some special kinds of cross-stratifications

Herringbone Cross-strata

• Herringbone cross-strata is bidirectional
• It forms by opposite migration of successive rippled bedforms one over another
• It indicates tidal influence, where both the tide and ebb current is equally strong

• Where ebb current is comparatively weak, it can not able to move ripples in opposite direction, instead induced prolonged suspension fall-out
• The result is the formation of thick mud linings, ideally in two layers over a foreset, produced by the ripple movement during tide current
• Such cross-strata also indicate tidal influence by formation of double mud-drapes

Grainflow-grainfall cross-strata

• This type of cross-strata is planar tabular in nature with alternate foresets showing contrasting character
• Foresets of one group are inversely graded and more often than not wedge downslope
• In contrast, the foresets of the other group are relatively finer grained, ungraded or normally graded and often discernibly wedging upslope
• The former group can be produced by grainflows; avalanching of material along dune slipface due to slope oversteepening (~30^o-34^o)
• The latter group is formed by grainfall; suspension fall-out of material during periods of quiescence
• Restricted within eolian environments only; indicate migration of eolian dune

Adhesion ripples & Adhesion laminae

• Adhesion ripples are not ripples in proper sense
• Bedding planes having adhesion ripples appear to be crinkled by coalescence of numerous wort-like structures together
• Wort-like feature suggests coagulation of multiple sand grains together
• Sand grains deflated by wind adhered to wet or damp substrate and produce such ripples

• Vertical aggradation of such crinkled surfaces produces adhesion laminae
• These structures are restricted within eolian environments only, where the substrate is damp

Depositional Structures in Conglomerates

• Above mentioned structures are mostly restricted within clastic arenaceous sediments
• Generally stratification is often crude and poorly developed in rudites due to the large clast size
• In case of conglomerates, on the other hand, grading, packing and orientation of the clasts gain importance
• Fabric analysis is generally popular within conglomerates
• It deals mainly with the orientation of the clasts within the conglomerates
• Clasts may be bed-parallel, imbricated or chaotic
• Bed-parallel orientation indicates deposition under high shear stress, as expected within a shooting sheet flow
• Different kinds of imbrication can develop under different conditions and can indicate the nature of flow as well as paleocurrent diretion
• Chaotic arrangement either form under free fall of clasts in a gently moving flow or by freezing of clasts within flows characterized by high matri strength

Erosional Structures

• Bedding-plane structures are mostly formed by erosion
• Erosional structures formed immediately prior to sedimentation are often preserved at top or sole of beds
• Over bedding plane such structures appear as grooves or marks or scours
• Cast of those structures, on the other hand, get preserved at the sole of the immediately overlying bed
• Both these varieties are included within the basket term sole features

• Such structures formed at top of the bed are usually low-relief
• Some other non-erosional structures, such as mud cracks or rain prints or setulfs also occurs on top of the bed
• All these structures are very useful indicators of paleotop as well as sense of paleocurrent with or without the direction

Sole marks formed by currents

Obstacle marks

Flutes

Longitudinal scours

Gutters

Sole marks formed by tools

Grooves (continuous)

Chevrons (continuous)

Prod, bounce marks (single and discontinuous)

Skip marks (repeated and discontinuous)

• Erosional structures can be grouped into two classes; current formed structures and tool formed structures
• In former case, vortices within shooting current flows erode the substrate, forming grooves or small channels or scour like structures
• In latter case, scours or depressions result from the action of objects or tools, carried by the current and intermittently or continuously making contact with the bottom

• Obstacle Marks
• Obstacle scours occur where a barrier such as a pebble or boulder has disrupted a flow
• A shadow zone of slow deposition occurs in the lee of the obstacle
• Erosion occurs on the up-current side of the obstacle
• A common example is current crescents, frequently observed in beaches
• Setulfs are one typical structure which is formed by deposition on the shadow zone of a obstacle, but without any erosion; because of its morphological resemblance to inverted flutes these features are named as setulfs; commonly observed in eolian field with slightly damped substrate

Current Crescent

Setulfs

• Flute Marks and Flute Casts
• Flute marks are special kinds of asymmetrical depression in which the steepest and deepest part of the depression is oriented upstream
• Generally developed over muddy substrate
• When filled-up, the filling forms a positive-relief structure on the sole of the following bed, with a bulbous nose oriented upstream
• Such structures are known as flute casts
• Flute casts are more common than the mark itself and occur singly or in swarms, where all of the flutes are oriented in the same general direction
• On a given sole, the flutes tend to be about the same size; however, flute casts on different beds can vary in size enormously
• The plan-view shape of flutes varies from nearly streamline, bilaterally symmetrical forms to more elongate and irregular forms, even twisted
• Common at the base of storm or other event beds and also in turbidites

• Longitudinal Scours
• Formed by helical flow eddies in fast moving shallow water
• Such longitudinal scours run parallel to the direction of current
• Parting lineations are product of such scouring
• Associated with upper flow regime plane beds
• Ridge and furrow structures also form be similar process, only the scale is larger
• Longitudinal dunes in eolian field also formed by similar process with further enlarged scale

• Gutter Marks and Gutter Casts
• Gutters are long elongated scour resembling small channels
• Concave-up base and flat topped in transverse section
• Gutter marks are rare
• Gutter cast on the sole of following bed are more common
• Different kinds of fillings of gutter can be observed

• Channel Scours
• Channel scours are similar to that of gutters, but larger in scale
• Channel scours are usually multigenerational
• Channel can expand both laterally as well as can deepens

• Sole marks formed by tools
• Tool marks can be formed by rolling, bouncing, skipping, or dragging of tools, like pieces of wood, the shells of organisms, or any similar object, over the substrate
• Generally developed over muddy substrate
• When filled-up, the filling forms a positive-relief structure on the sole of the following bed, known as casts

• Groove casts are elongate, nearly straight ridges
• Chevrons are a variety of groove casts made up of continuous V-shaped crenulations with the V pointing in a downstream direction
• Prod, bounce, skip or brush marks are all small discontinuous marks produced by tools that make intermittent contact with the bottom
• Brush and prod marks are asymmetrical in cross-section, with the deeper, broader downcurrent end
• Bounce marks are roughly symmetrical
• Roll and skip marks are repeated but discontinuous

Gutter Casts

Bounce marks

Chevron mark

Prod marks

Skip casts & Brush casts

Skip marks

Biogenic structures

• Biogenic sedimentary structures are those which are formed by living organisms, interacting with the sediment
• Organisms may be animals which walk on or burrow into the sediment, or they may be plants with roots which penetrate the sediment, or they may be bacterial colonies which trap and bind the sediment to produce layered structures
• Biogenic structures can be subdivided into two categories - trace fossils or ichnofossils and biostratifications
• Trace fossils are not true bodily preserved fossils but are simply biogenic structures that originated through the locomotion, feeding, burrowing, or resting activities of organisms
• Trace fossils are found to be distributed over arenaceous to argillaceous rocks of both carbonaceous as well as siliceous composition
• Biostratification structures are sedimentary layering produced through the activities of organisms
• Biostratification structures are only represented by stromatolites; which are mostly restricted within carbonate rocks, although rare reports of siliceous stromatolites are also available

Trace Fossils or Ichnofossils

• Trace fossils can further be subdivided into three genetic classes
• bioturbation structures (burrows, tracks, trails, root penetration structures)
• bioerosion structures (borings, scrapings, bitings)
• excrement (coprolites, such as fecal pellets or fecal castings)
• Among these, bioturbation structures are most important, along with borings as their preservation potential is higher than others
• Tracks and Trails
• Track - impression left in underlying sediment by an individual foot or podium
• Trackway - succession of tracks reflecting directed locomotion by an animal
• Trail - trace produced during directed locomotion and consisting either of a surficial groove made by an animal having part of its body in continuous contact with the substrate surface or of a continuous subsurface structure made by a mobile endobenthic organism

• Burrows and Borings
• Boring - excavation made in consolidated or otherwise firm substrates, such as rock, shell, bone, or wood
• Burrow - excavation made in loose, unconsolidated sediments
• Burrows can be highly ramified and/or interconnected, having shafts as vertical components and tunnels as horizontal components
• Some burrows used to show thickened burrow wall constructed by organisms as a structural reinforcement, known as burrow lining
• Burrow filling - sediments infilling a burrow (= burrow fill) may be either "active," if done by animals, or "passive," if done by gravity or physical processes; active fill termed "back fill” develops as the animal packed sediment behind itself during its movement through the substrate
• Such repeatative concentric laminae-like structures are known as spreite
• Bioturbate texture—gross texture or fabric imparted to sediments by extensive bioturbation; typically consists of dense, contorted, truncated, or interpenetrating burrows or other traces

Bioturbate Texture

• Root marks
• These are the traces left by the roots of plants in ancient soil zones (called paleosols)
• Rootmarks typically branch downward in a pattern resembling an up-side-down tree
• Root marks are sometimes gray or greenish, penetrating reddish-brown paleosols

Biostratification

• Biostratifications are sedimentary layering produced by the binding and trapping of sediments by organisms
• Stromatolites are only representatives of biostratification
• Stromatolites are "organo-sedimentary structures“ and formed by colonies of sediment-trapping cyanobacteria (commonly called blue-green algae)
• The laminations of stromatolites are generally less than 1 mm in thickness and are caused by concentrations of fine calcium carbonate minerals, fine organic matter, and detrital clay and silt, with rare sand-sized materials
• Common in limestone, but rarely also observed in siliciclastics
• Stromatolitic bedding ranges from nearly flat laminations to hemispherical forms
• While the former is known as microbial laminite, the latter represent stromatolite proper
• These hemispherical stromatolites can be classified into three basic types: laterally linked hemispheroids; discrete, vertically stacked hemispheroids and discrete spheroids, or spheroidal structures
• The term thrombolite represent structures that resemble stromatolites in external form and size but lack distinct laminations

Microbial Laminite

Stromatolite (sectional view)

Stromatolite (plan view)

Various forms of Stromatolites

• Stromatolites dominated the Precambrian seas
• In Phanerozoic, after the advent of metazoan grazers, they restricted themselves to certain stressed environments only
• Present day stromatolites are restricted either in stressed environments within marine realm, like hypersaline areas such as in Shark Bay, Western Australia, or in fresh water lakes

• Stromatolites represent shallow shelfal condition, within photic zone
• A new category of sedimentary structures, known as the Mat-Related Structures (MRS), have evolved since the last decade of twentieth century
• These structures are mainly produced within siliciclastic sediments, where microbial organisms impart cohesiveness into hitherto non-cohesive arenaceous sediments to produce several unique structures, unexpected to form in non-cohesive sandy substrate
• MRS includes sand cracks, sand chips, palimpsest ripple, wrinkle structures, kinneyia structures etc.

Soft-Sediment Deformation Structures

• Soft-sediment deformation (SSD) structures include structures formed by deformation during deposition (syn-sedimentary) as well as just after deposition prior to lithification (penecontemporaneous)
• Defromations producing these structures can be triggered by some external cause like seismic jerks or internal cause like overpressuring due to quick dumping
• If triggered by external cause, these structures are usually found to be restricted within a single sedimentation unit with high lateral extent and is of large-scale
• Genetic types of soft-sediment deformation structures:
• Gravitational instability
• convolute lamination
• load structures
• ball and pillow structures
• flame structures
• Downslope movement
• slides
• slumps
• growth faults

• Fluid flow
• overturned cross bedding
• Fluidisation & Liquifaction
• Sand volcanoes
• diapirs
• dish and pillar structures
• sedimentary dykes

Gravitational Instability

• Density-driven soft-sediment deformation
• Generally restricted within a particular sedimentation unit, overlying and underlying units remain undeformed

• Convolute Laminations
• Convolute lamination is a structure formed by complex folding or intricate crumpling of beds or laminations into irregular, generally small-scale anticlines and synclines
• Convolutions increase in complexity and amplitude upward from undisturbed laminae in the lower, part of the unit and may either die out in the top part of the unit or be truncated by the upper bedding surface

• Load Structures
• Due to density contrast among alternate sand and mud layers, sand layer intermittently sink into mud layer due to gravitational pull, forming load structures
• Amplitude and intensity of loading decreases gradually upward from the interface
• Quick dumping of the sand over mud also requires as it prevents the complete expulsion of water from mud layer

• Ball & Pillow Structures
• Ball and pillow structures are found in the lower part of sandstone beds, and less commonly in limestone beds, that overlie shales
• They consist of hemispherical or kidney-shaped masses of sandstone or limestone within the shale unit that show internal laminations

• Flame Structures
• Flame structures are wavy or flame-shaped tongues of mud that project upward into an overlying layer, which is commonly sandstone
• They are probably caused mainly by loading of water-saturated mud layers which are less dense than overlying sands and are consequently squeezed upward into the sand layers

Downslope Movement

• Slump Structures
• The general term slump structures has been applied to structures produced by penecontemporaneous deformation resulting from downslope movement and displacement of unconsolidated or semiconsolidated sediment, mainly under the influence of gravity
• Slump structures are either products of pervasive movement involving the interior of the transported mass, producing a chaotic mixture of different types of sediments, such as broken mud layers embedded in sandy sediment, or a decollcment type of movement in which the lateral displacement is concentrated along a sole, producing beds that are tightly folded and piled into nappe-like structures

• Large-scale Slump structures are usually related with seismic activity
• They often lead to slides

• Growth faults
• Brittle failure within soft sediment indicates cohesion to hold the grains together
• Wetted sand in aeolian dunes often fails in a brittle manner

Fluid Flow

• Overturned & Deformed Cross-beddings
• Overturning of deformation can be produced by strong shear stress exerted by the flowing fluid over semi-consolidated sediment near the surface

Fluidisation & Liquifaction

• Fluidisation and lquifaction are two important processes of soft-sediment deformation
• These two processes actually facilitated all kind of SSDs to form by reducing the cohesion among grains

• In liquifaction, increase in fluid pressure by some external shock or overpressuring, enable fluid to surround grains completely, so that grain-grain cohesion reduces and sediment starts to flow like a fluid

• In Fluidisation, fluid released by external shock or overpressuring moves upward through with great velocity through the sediment column and in consequence carried the sediment along with it
• Resulted structures include, sand volcanoes, mud diapirs, sand dykes

• Sedimentary diapiric rises are caused by fluidisation
• It can take place among sandy as well as muddy sediments
• Whenever such diapirs comes to surface through any openings mud or sand volcano result

Mud Diapir

Sand Diapir

Sand Dyke

• Dish and Pillar structures are primarily produced by liquefaction
• Dish structures are thin, dark-colored, subhorizontal, flat to concave-upward, clayey laminations that occur principally in sandstone and siltstone units
• Pillars are vertical to near-vertical, cross-cutting columns and sheets of structureless or swirled sand that cut through either massive or laminated sands
• Pillars are associated with dish structures
• These structures are commonly associated with sediment-gravity flow deposits

Miscellaneous Structures

• Some sedimentary structures are present, which does not include within the genetic classification scheme of the sedimentary structures, those are grouped under this basket term
• Some of them are bedding plane structures include desiccation cracks, rain prints, rill marks, salt pseudomorphs (generally at sole of bed)
• All these are good paleotop indicators

Desiccation cracks

Rain prints

• Both these structures indicate emergence

Rill marks

• Synaerisis cracks are another kind of cracks that form within clayey sediments by loss of pore water from clays that have flocculated rapidly
• These cracks are subaquous in nature
• Unlike desiccation cracks (continuous, polygonal in plan) they are discontinuous and can cross-cut each other; in section they are spindle-shaped whereas desiccation cracks are V-shaped
• Another important feature is rill marks, small dendritic channels or grooves that form on beaches by the discharge of pore waters at low tide, or by small streams debouching onto a sand or mud flat
• They have very low preservation potential and are seldom found in ancient sedimentary rocks
• Swash marks are very thin, arcuate lines or small ridges on a beach formed by concentrations of fine sediment and organic debris
• They are caused by wave swash and mark the farthest advance of wave uprush

Swash marks

• Salt pseudomorphs are features observed at the sole of the bed
• Crystals shaped protruberations are found
• They indicate high evaporation rate, where marine water trapped within the pore spaces crystallizes salt crystals due to evaporation of water
• During diagenesis, they got destriyed and casts of the crystal forms at the sole of bed
• Rarely they became large enough to appear at top of the beds

Primary Structures & Depositional Environments

• Although common primary structures are not environment-specific, but some rare and intrinsic structures used to show environmental bias
• Associations of common primary structures, on the other hand, indicate certain broad depositional environment
• Frequent occurrence of wave structures in a formation most certainly indicates deposition in a wave- agitated environment, viz., shallow marine or lake
• In a lake, tide, without synchronization with open ocean tide, has amplitude only up to a few centimeter, hence tidal bedforms are unexpected in lacustrine deposits.
• On the other hand, River water entering a lake is often muddy, of higher density than the lake water, producing underflows
• Such high sediment-laden underflows are often seasonal and produces graded siltsone or fine sandstone beds alternating with the normal suspension fall-out shale deposits of lake; such alternating normally graded deposits are known as verve deposits
• Beach environment within shallow marine realm can be indicated by features like current crescents, rill marks, swash marks.

• Some structures like herringbone cross-strata, sigmoids, thick-thin lamina alternations, mud drapes, double mud drapes, cyclic variation in foreset geometry indicate tidally-influenced environment
• In tidal regions the most significant features are caused by the mixing of sand- and mud-sized fractions from the asymetrical currents
• Lenticular and flaser bedding produced by interaction between wave and current actions are quite common under such circumstances
• However, the most convincing proof of tidal action is derived from demonstration of lamina-thickness variation in periodicities corresponding to those of tides

• In traction current domain it is most important to distinguish between alluvial/fluvial and eolian deposits
• As grain-flow process operate more frequently, inversely graded dune cross-strata within sandstones are restricted within eolian deposits
• Common occurrence of regular alternations between grain-flow and grain-fall strata, translatent strata , adhesion laminae or ripples are characteristics features of eolian environment
• Apart from that, aerodynamic or wind-blown ripples characteristically have very low amplitude compared to wave length; the ripple index above 30
• This is because of low impact angle involved in aeolian grain transport
• Translatent strata is produced by climbing of aerodynamic ripples, hence is characterised by very low-angle climb planes with non-discernible foresets

• Furrows, though common under glacial deposits, are equivocal as they can be generated by rock sliding during faulting or bollide impact
• Striations and facets on pebbles
• Multifaceted pebbles and striations on those facets are far more reliable indicator of glaciation than dropstones
• Dropstones may be volcanism or impact-related
• Dropstones of subaqueous environment settle vertically and deform the sediment laminae on which they settle
• Dropstones of subaerial origin however cause rupture in the underlying laminae and their curved trajectories through sediment are likely to preserved
• Varve deposites are common in case of glacial deposits
• In contrast, positive evidence for alluvial process in terms of distinctive primary structures is difficult to cite
• Identification of the deposits depend on consideration of structures along with textures, sediment body geometry, reconstruction of various alluvial architectural elements or meso-scale geomorphologic entities
• The last factor demands working out of interrelationships between various structural elements and lithologies
• Current structures in fluvial deposits yield unimodal orientation, however wide the degree of variation may be

• Finally, it can be concluded that, primary structure does help in reconstruction of paleoenvironment, but not singularly
• Primary structues indicate more about the mechanism and processes of their formation
• Paleoenvironmental reconstruction should be a holistic approach
• Not only inherent characteristics of deposits, including penecontemporaneous structures, but also their associations should be taken into consideration.