Presented by: Salman Naseer and

Ali Asghar Shahid

Bahria University Karachi

Definition: These are the structures that are formed by the sediment layers in the way they are deposited over each other.

Classified on two approaches, one is morphologic, the other genetic.

A purely morphologic classification is rather artificial and leads to grouping of unrelated structures.

Genetic classification group’s structures according to the process involved in their formation, such as, biogenetic, hydrodynamic or rheologic.

Importance of Sedimentary Structure: Study of sedimentary structures is important

because they are the most valuable features for interpreting depositional environment.

We know a lot about how most structures are formed, so finding them in the rocks can tell you a lot about the conditions of deposition.

They are much more useful than textural things like grain-size distribution and grain shape.

Primary structures

Secondary structures

Definition:

They are also known as Pre-

depositional structures.

The structures formed during deposition

without any forces applied.

Definition:

They are also known as Post-

depositional structures.

The structures formed after deposition with

some forces applied.

Physical

Chemical

Biogenic

Massive bedding: A thick bed which is

structure less.

Progradation is an example of it’s

depositional environment

Massive bedding Uses:

Shows the depositional environment

changed from a regularly repeating

depositional system to one with very little

change over time.

Graded bedding: It is bedding in which the

particles are sorted according to density,

size and shape.

Graded bedding types:

Normal Grading: If the particle size

decreases upward, the bed is said to be

normally graded.

Reversely Graded: If the particle size

increases upward, the bed is said to be

reversely graded or inversely graded.

fluvial:Diagram illustrating the formation of

a graded bed (turbidite). Slope failure

produces turbulent suspension that

moves/accelerates downslope. Once it

reaches the flat deep sea regions, it slows

down due to friction, and gradually the

sediment settles out of suspension. Larger

grain sizes settle out first, and then

successively smaller ones.

Use:indicator of upward and downward

direction in deposition

Can be useful for mapping purpose

Can also be useful for marking top and

bottom of strata.

Ripple Marks: Wavy feature formed by

wind, wave or current. Streamlines lie

parallel to a flat bed but where there is an

irregularity such as a step in the bed

caused by an accumulation of grains, the

streamlines converge and there is an

increased transport rate.

Ripple Marks types: Asymmetrical definition: These are created a one

way current( e.g. river) or the wind ( e.g. desert).This create ripple with still pointed crests and rounded troughs, it inclined in the direction of the current.

Current ripples do not form in sediment coarser than 0.6 mm in diameter (coarse sand).

Current ripples can develop in almost any environment river, delta, shoreline, offshore shelf and deep sea. • On the basis of shape, three types of current ripple are common: straight-crested, sinuous or undulatory, and linguoid ripples

Wind ripples and dunes: These are

asymmetric structures like current ripples.

Wind ripples typically have long, straight,

parallel crests with bifurcations like

waveformed ripples.

Ripple Marks types:

Symmetric definition: They are formed by

two way current, often found on beaches.

They are wave dominating. Nearly pointed

crest and rounded trough and it doesn’t

inclined in the direction of the wave. non-

cohesive sediment, medium silt to coarse

sand grades, and they are typically

symmetrical in shape.

Tidal environment and eolin environment.

Use: direction of the flow of air or water

Moreover it also tells the stregnth of wind

or water.

Cross bedding: It is formed on inclined surface during deposition by crossing a bed to another bed. The set height is generally greater than 6 cm and the individual cross-beds are many millimeters to 1 cm or more in thickness.

Most cross-stratification arises from the down current migration of ripples, dunes and sand-waves when sediment is moved up the stoss side and then avalanches down the lee-side of the structure.

Foresets- steeply dipping parts of the cross-strata, have either angular or tangential contacts with horizontal.

Bottom sets- The lower less steeply dipping parts.

Cross bedding: Common two types are,Tabular and wedge cross-bedding consists

of mostly planar cross-beds which have an angular contact with the basal surface of the set. On the bedding surface, planar cross-beds are seen as straight lines.

Trough cross-beds usually have tangential bases, and in bedding-plane view the cross-beds have a nested, curved appearance.

Use: The inclination of the cross-beds

indicates the transport direction and the

current flow (from left to right in our

diagram).The style and size of cross

bedding can be used to estimate current

velocity, and orientation of cross-beds

allows determination direction of paleoflow.

Flaser bedding: It is a bedding in which

high energy environment is involved which

forms a bedding pattern in which sand is a

dominating layer which is later infilled by a

mud layer.

These structures often are considered to

form mostly in tidally influenced

environments.

Lenticular: the bedding which is opposite to flaser bedding in which mud is the dominating layer and a thin layer of sand deposited at trough. Its bedding display is a 'lens-like' shape.

Formed during periods of slack water, mud suspended in the water is deposited on top of small formations of sand once the water's velocity has reached zero.

They are commonly found in high-energy environments such as the intertidal and supratidal zones.

Geologists use lenticular bedding to show evidence of tidal rhythm, tidal currents and tidal slack, in a particular environment.

Convolute: Convolute bedding forms when complex folding and crumpling of beds or laminations occur.

Beds of fine sand, up to a meter thick, deposited rapidly by such events as turbidity currents.

The beds have planar lower and also upper contacts, but the bed is internally folded into broad synclines and sharp to dome-shaped or even mushroom-shaped anti-clines, which usually die out upward to planarity at the upper contact.

The most common places for Convolute

bedding to materialize are in deep water

basins with turbidity currents, rivers,

deltas, and shallow-marine areas with

storm impacted conditions. This is because

these environments have high deposition

rates, which allows the sediments to pack

loosely

Sole marks : Erosional marks formed by

scouring of bed by unidirectional flows.

Forms in marine environment, This is

usually a sediment gravity flow in a

moderately deep marine environment but

strong currents in other situations can

make sole marks as well. Used for paleo

flow direction.

Sole Marks types:Flute Casts: Geometrical features produced

on a sediment bed by erosion by a strong current. Flutes are heel-shaped hollows, scoured into mud bottoms. Each hollow is generally infilled by sand, contiguous with the overlying bed.

As the current velocity declines, flute erosion ceases and the hollows are buried beneath a bed of sand.

Formed in shallow marine environments.

Uses of Flute Casts:

Indicators of turbidites, shallow marine and

non-marine environments.

Tool Marks: By mechanical disruption of

the bed by large objects carried by a

strong current. Tool marks are erosional

bottom structures.

They are irregular in shape, both in plan

and cross-section, though they are roughly

oriented parallel with the paleocurrent

fluvial deposits and marine storm deposits

They are valuable indicators of

paleocurrent direction and also give some

information about the nature of the clay

bottom and the sediment transport

mechanism operating in the flow that

deposited the overlying sand.

Groove Marks:

They are long, thin, and straight erosional

marks.

Contains mud and overlain by sand.

Few millimeters deep or wide, but they

may continue uninterrupted for a meter of

more.

They are developed in downcurrent

situation than flutes.

groove mark A linear groove, cut in a

muddy substrate by the dragging of an

object through the sediment by flowing

water. The orientation of the groove will be

parallel to the current direction.

Subsequent infilling of the groove by

sediment will result in a groove cast being

preserved on the base of the overlying

bed.

Balls and Pillows: In more extreme loading, whole masses of the overlying bed sink down into the underlying material. Masses end up with concave-up stratification that is terminated abruptly around the margins of the sunken mass. This called ball-and-pillow structure

earthquakes, erupting volcanoes, or meteoric impacts can create these formations.

Definition: These structures are formed by the chemical disruption of the sediments.

Types: Solution Structures:StylolitesMud cracksVugs

Accretionary Structures:NodulesConcretionsCrystal aggregatesVeinletsColor Banding

Stylolites: Consisting of a series of relatively small, alternating, interlocked, tooth like columns of stone; it is common in limestone, marble, and similar rock.

Insoluble minerals, such as clays, pyrite and oxides, remain within the stylolites and make them visible.

They occur most commonly in homogeneous rocks, carbonates, cherts, sandstones, but they can be found in certain igneous rocks and ice.

Their size vary from microscopic contacts between two grains (microstylolites) to large structures up to 20 m in length and up to 10 m in amplitude in ice.

Stylolites usually form parallel to bedding, because of overburden pressure.

Formed by the result of chemical solution

by groundwater circulating through semi-

consolidated or consolidated, hardened

rock.

Mud cracks: These structures form when

the clay-rich sediments found in muds dry

and shrink. As the sediment shrinks crack

begin to form in the sediment creating

polygonal patterns called mud cracks.

Mud cracks form in any environment that

allows for the wetting and subsequent

drying of sediment such as marshes,

seasonal rivers, or lake shores.

Use: They indicate that the mud

accumulated in shallow water that

periodically dried up.

Also tells us about the environment

whether it is arid or semi arid.

Vugs: They are small to medium-sized cavities inside rock that may be formed through a variety of processes.

Most commonly cracks and fissures opened by tectonic activity (folding and faulting) are partially filled by quartz, calcite, and other secondary minerals.

Vugs may also result when mineral crystals or fossils inside a rock matrix are later removed through erosion or dissolution processes, leaving behind irregular voids.

Fine crystals are often found in vugs where the open space allows the free development of external crystal form.

Nodules: In sedimentology and geology, a nodule is small, irregularly rounded knot, mass, or lump of a mineral or mineral aggregate that typically has a contrasting composition, such as a pyrite nodule in coal, a chert nodule in limestone, or a phosphorite nodule in marine shale, from the enclosing sediment or sedimentary rock.

Minerals that typically form nodules include calcite, chert, apatite (phosphorite), anhydrite, and pyrite.

Concretions: A concretion is a hard, compact mass of matter formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil. Concretions are often ovoid or spherical in shape, although irregular shapes also occur.

There is an important distinction to draw between concretions and nodules. Concretions are formed from mineral precipitation around some kind of nucleus while a nodule is a replacement body.

Color Banding: They are formed in sedimentary rocks when some variation occur in the mineral composition or due to the cementing material present in the rock.

Liesegang bands are colored bands of cement observed in sedimentary rocks that typically cut-across bedding.

These secondary sedimentary structures exhibit bands of minerals that are arranged in a regular repeating pattern.

Frequent occurrence in sedimentary rocks, rings composed of iron oxide can also occur in permeable igneous and metamorphic rocks that have been chemically weathered.

Biogenic sedimentary structures: Biogenic

structures result from bioturbation, the

post-depositional disturbance of sediments

by living organisms. This can occur by the

organisms moving across the surface of

sediment or burrowing into the first few

centimeters.

Tracks and trails: These features result

from organisms moving across the

sediment as they walk, crawl, or drag their

body parts through the sediment.

Molds: Reproduction of the inside or

outside surface of a living thing.

Cast : Duplicate of the original organism;

usually formed by replacement of inside of

living thing

Crawling traces: trails, uncomplicated pattern linear

Grazing traces: more complicated surface trails, symmetrical or ordered pattern.

Resting traces: impression of where animal rested during life (but not a fossil mold).

Dwelling structures: simple to complex burrow systems, burrows can be lined or small ball of mass.

Feeding structures: simple to complex burrow systems commonly with well organized and defined branching pattern indicating systematic reworking of sediment.

Burrow Marks:Any organism that burrows into soft

sediment can disturb the sediment and destroy many of the structures.

If burrowing is not extensive, the holes made by such organisms can later become filled with water that deposits new sediment in the holes.

Such burrow marks can be excellent top and bottom indicators.

Boring : A boring is any biogenic structure

that involves erosion of an already

consolidated substrate by an organism; the

process of forming a boring by an

organism is bioerosion.

Bioturbation: One of the agents of organic

weathering, bioturbation is the disturbance

of the soil or sediment by living things.

Bioturbation aids the penetration of air and

water and loosens sediment

Stromatolites: They are organically formed, laminated structures composed of fine silt or clay-size sediment or, more rarely, sand-size sediment. • Most ancient stromatolites occur in limestones however, have also been reported in siliciclastic sediments.

They are organosedimentary structures formed largely by the trapping and binding activities of blue-green algae

The laminated structure forms because fine sediment is trapped in the very fine filaments of algal mats.