SEDIMENTARY STRUCTURES IN SEDIMENTOLOGY/

This PPT deals with some of the important sedimentary structures encountered in nature. Sedimentary Structures are important as they help us to understand paleo environment and climate, also conditions while deposition, erosion and deformation.

1. Ritisha Singh
M.Sc II Semester

2. C O N T E N T S
1. Introduction
2. Classification
3. Inorganic Structures

i) Mechanical Structures
ii) Chemical Structures
4. Organic / Biogenic Sedimentary Structures
5. Significance In Top & Bottom Criteria
6. Conclusion
7. References

3. INTRODUCTION
1. Sedimentary rocks are derived from pre-existing rock-masses formed from the
consolidation of loose sediments or chemical precipitation from solution or an
organic matter consisting of secretions or remains of plants and animals.
2. Sediments, loosened materials that ultimately formed sedimentary rocks, are
usually deposited in layers one on top of the other. They record the conditions that
existed at the time of deposition and subsequent changes.
3. Sedimentary structure are those structure formed during sediment deposition.
4. Sedimentary structures are important attributes of sedimentary rocks. They occur
on the upper and lower surfaces of beds as well as within beds. They can be used to
deduce the processes and conditions of deposition, the directions of the currents
which deposited the sediments.

4. CLASSIFICATION
SEDIMENTARY STRUCTURES
INORGANIC STRUCTURES
ORGANIC / BIOGENIC
STRUCTURES
SECONDARY /
CHEMICAL
STRUCTURES
PRIMARY /
MECHANICAL
STRUCTURES

5. INORGANIC STRUCTURES
DEFORMED AND DISRUPTED BEDDING D

● Disrupted Bedding
● Sedimentary Sills & Dikes
BEDDING PLANE IRREGULARITIES C
● Swash Marks
● Rain Imprints
LINEAR BEDDING STRUCTURES B
● Striations
● Ripple Marks
● Spatulate Casts/ Flute Casts/ Sole Marks
A
● Laminations
● Cross Bedding
● Graded Bedding
PLANAR BEDDING STRUCTURES
1] Primary / Mechanical Structures

●Formed at the time of sedimentation.
●Mainly dependent on current velocity and rate of deposition.
Classification

6. A] Planar Bedding Structures


Arrangement of sedimentary rocks in beds or layers of varying thickness and character.
i) Laminations



●Smallest unit of beddings/ sedimentation
unit.

●Characteristics of finer grained
sediments, notably shales and siltstones.

●0.05 to 1.00 millimeter thick.

●They may be continuous & distinct or
discontinuous & obscure.

7. Descriptive terms for lamina continuity, shape, and geometry

These terms are useful for all levels of stratification (e.g., laminae, laminasets, beds, bedsets).
Image Source :
https://www.researchgate.net/publication/276394310_Captu
ring_Key_Attributes_of_Fine-Grained_Sedimentary_Rocks_In_
Outcrops_Cores_and_Thin_Sections_Nomenclature_and_Des
cription_Guidelines

8. ii) Bedding
Thicker than 1 cm and consists of beds.
a)Cross Beddings




●Useful character in ascertain current
directions.

●Consists of inclined dipping bedding,
bounded by sub horizontal surfaces.

●Each of these units is termed a set. Vertically
contiguous sets are termed co-sets.

●Inclined bedding is referred to as a foreset.
Foreset may grade down with decreasing dip
angle into a bottom set or toe set.
Fig. Trough And Planar Cross Beddings
Image Source: Sedimentary Structures
http://epgp.inflibnet.ac.in/epgpdata/uploads/epgp_cont
ent/S000010ES/P001694/M020139/ET/149450443204
7.N012.ES-251Sedimentarystructures.pdf

9. TYPES OF CROSS BEDDINGS
Commonly found in Arenaceous Sediments Oolitic limestone Some
type of Clastic limestone
IImage Source : Chapter 4, Sedimentary Structures,
https://uomosul.edu.iq/public/files/datafolder_2888/_20
191023_080450_330.pdf

10. b) Graded Beddings




●Sedimentation units, marked by gradation in
grain size, from coarse to fine, upward from the
base to top of the unit.

● Graded bedding is produced as sediment settles
out of suspension, normally during the waning
phase of a turbidity flow.

● The lower part of a graded bed is normally
massive, the upper part may exhibit the Bouma
sequence of sedimentary structures.

●A characteristic sequence of sedimentary
structures occurring in sedimentary rocks
deposited in areas of deep water sedimentation
by turbidity currents
Image Source : https://pin.it/2qsnFAr

11. Graded Bedding

12. B] Linear Bedding Structures


Linear arrangement and patterns of sediments.
i) Striations



● A groove, created by a geological
process, on the surface of a rock or a
mineral.

● In structural geology, striations are
linear furrows, or linear marks,
generated from fault movement. The
striations’ direction reveal the movement
direction in the fault plane.

●Glacial striations, can occur in areas
subjected to glaciation

13. Glacial striations cut and abraded into the surface of a limestone pavement, Hawkes
Bay, Newfoundland island. Photo by Andy Fyon, June 16, 2011.
Image Source : https://www.ontariobeneathourfeet.com/glacial-striation

14. Striated, glossy surface of the Corona Heights Fault (USA) (courtesy of J. E. Samuelson).
Image Source :
https://www.researchgate.net/figure/Principal-slip-zones-PSZs-in-natural-faults-a-Sketch-of-a-normal-fault-highlighting_fig2_333433701

15. ii) Ripple Marks
●When a current flowing over bed of sans reaches a certain velocity, the sand particles begin
to move, and a rippling appears on the surface of sand.
●A product of both air and water action.

16. ●Symmetrical ripples, also called oscillation or
vortex ripples, are commonly produced in
shallow water by the orbital motion of waves.
●Asymmetric ripples, show a clearly
differentiated low angle stoss side and
steep-angle lee side. Internally they are
cross-laminated, with the cross-lamina
concordant with the lee face. Produced by
unidirectional traction currents as,eg., in a river
channel.

17. iii) Flute or Spatulate Casts
●Flute casts are scoop-shaped structures on the soles (undersides) of beds
●Features represent sediments that filled depressions on the immediately subjacent bedding
plane.
● 1-5 cm wide and 5-20 cm long, and typically occur as groups
●Formed by localized scouring action of a current moving over an unconsolidated mud bottom
●The rounded part of the flute Points to the upcurrent end whereas the flared end points to the
downcurrent (excellent paleocurrent indicators).





Image Source :
https://prezi.com/yamerjyilwga/flute-casts/
Image Source :
https://burmesegeologist.wordpress.com/2009/12/18
/weekly-geology-photo-02/

18. Flute Casts
Image Source :
https://www.researchgate.net/figure/Flute-casts-common-sole-marks-often-used-as-evidence-for-ero
sion-by-turbidity-currents_fig5_348187577

19. C] Bedding Plane Irregularities


Irregular structures formed during sedimentation.
i) Swash Marks



●Swash is the motion of water up a beach,
after the breaking of a wave.

●Swash mark is a miniature arcuate ridge
1-2 millimeters high, representing the
upper limit of swash after each wave
break.

●With a falling tide, numerous swash
traces leave an intersecting arcuate
pattern.


Swash marks - Whiritoa Beach (Nueva Zelanda
Image Source : https://www.flickr.com/photos/banco_imagenes_geologicas/8557283887

20. ii) Rain Imprints
●Small rounded or ovate (due to windblown rain) depressions range from 2 to 10 mm in diameter,
with raised rims.
● They typically occur as closely spaced group.
●Formed by impact of rain drops on soft mud and mark the top of sediments.
●Good indicator of subaerial exposures especially in arid environments.

21. D] Deformed And Disrupted Bedding


Bedding may be disrupted or deformed prior to the consolidation of materials; the
deformation occurs prior to the deposition of the next overlying bed.
i) Mud or Desiccation Cracks




●Downward-tapering cracks in mud, which
are infilled by sand and displaying polygonal
pattern in plain view.
●They form subaerially when mud exposed to
air is dewatered, shrinks and leaves a crack.
●They are commonly preserved on the tops of
bedding surfaces as positive-relief fillings of
the original cracks.
●They occur in both siliciclastic and carbonate
mud.

22. Image Source: https://uomosul.edu.iq/public/files/datafolder_2888/_20191023_080450_330.pdf

23. ii) Sedimentary Dike
●A dike is a body of rock, either sedimentary or igneous, that cuts vertically across the layers of
its surroundings. Always younger than the body of rock that they have intruded into.
●Often referred to as clastic or sandstone dikes, sedimentary dikes occur whenever sediment and
minerals build up and lithify in a rock fracture.
●Clastic dikes can form in several ways: Through fracturing and liquefaction associated with
earthquakes, the passive deposition of sediment into pre-existing fissures, the injection of
sediment into a not-yet-cemented, overlying material.

24. Image Source: https://www.thoughtco.com/what-are-dikes-and-how-do-they-form-3893130

25. INORGANIC STRUCTURES
COMPOSITE STRUCTURES C
● Cone-in-cone
ACCRETIONARY STRUCTURES B
● Nodules
A
● StylolitesSOLUTION STRUCTURES
2] Secondary/ Chemical Structures

●The structures formed by chemical processes such as
oxidation, reduction, precipitation and evaporation,etc.
Classification

26. A] Solution Structures



Stylolites



●Suture like seams of clay or other
insoluble material that commonly occur in
limestones owing to pressure solution.

●They are generally marked by
concentrations of insoluble constituents
such as clay minerals, iron oxide minerals,
and fine organic matter.

27. B] Accretionary Structures



Nodules



● Spherical or irregular rounded bodies having a
warty or knobby surface.
● They generally have no internal structure except
the preserved remnants of original bedding or
fossils.
●The nodules appear to form by partially or
completely replacing minerals of the host rock
rather than by simple precipitation of mineral
into available pore space.
●Common minerals that make up nodules include
chert (e.g. chert nodules in limestones), apatite
(e.g. phosphorite nodules), anhydrite, pyrite, and
manganese

28. C] Composite Structures



Cone-In-Cone



● Cone-shaped forms consisting of
concentric cones nested inside each
other, composed, in most examples,
of carbonate minerals with thin
layers of clay between cones.

●They generally occur in, persistent
layer of fibrous calcite and are most
common in shales and marly
limestones.
Image Source : https://uomosul.edu.iq/public/files/datafolder_2888/_20191023_080450_330.pdf

29. ORGANIC/ BIOGENIC STRUCTURES
These are formed as a result of direct or indirect effect of organic activity.
i) Stromatolites
● Organically formed, laminated structures composed of fine silt or clay-size sediment or,
more rarely, sand-size sediment.
●Formed largely by the trapping and binding activities of blue-green algae (cyanobacteria).
●Most ancient stromatolites occur in limestones
●They occur mainly in the shallow subtidal, intertidal, and supratidal environments.

30. ii) Trace Fossils
●These are records of life and events that took place in-situ during or soon after the deposition of
sediments.
●They include footprints,crawling marks,tracks,trails, burrows and coprolites..





FOOTPRINTS AND BURROWS

31. COPROLITES
TRACKS

32. SIGNIFICANCE IN TOP & BOTTOM CRITERIA
In any sedimentary sequence, the rock beds exist in order, from the bottom to the top
of the column. The bottom most bed is naturally the oldest one while the top most 's the
youngest. But where the sedimentary columns have undergone very severe
diastrophism, it is difficult to determine the order of superposition. The following
criteria may be used for determining the order of superposition in the areas, which
have been tectonically deformed.

33. i) Ripple Marks






ii) Current Beddings
DETERMINATION OF
THE ORDER
OF SUPERPOSITION
IN THE FIELD

34. iii) Graded Bedding





iv) Mud Cracks
DETERMINATION OF
THE ORDER
OF SUPERPOSITION
IN THE FIELD

35. v) Local Unconformities





vi) Fossils
DETERMINATION OF
THE ORDER
OF SUPERPOSITION
IN THE FIELD

36. CONCLUSION
●The formation of sedimentary structures involves the cooperative operations of
processes like weathering, erosion, transportation and deposition of sediments as
well as solution and precipitation and also the accumulation of organic remains.

● The sedimentary structures are large scale features and the study of them help us
to understand the environment of deposition, ancient life activities, stratigraphic
order of beds, palaeo-current flow direction, etc

37. REFERENCES
●Sedimentary Rocks by F.J. Pettijohn
●A Textbook of Geology by G.B. Mahapatra
●Sedimentary Structures Notes by eGyaankosh
●Sedimentary Structures Notes by ePathshala
●Sedimentary Structures https://uomosul.edu.iq/public/files/datafolder_2888/_20191023_080450_330.pdf
●Research Gate
●DBpedia
●Springer