STRUCTURES IN SEDIMENTARY ROCKS

1. STRUCTURES IN SEDIMENTARY ROCKS Definition: Features within sedimentary rocks produced during or just after sediment deposition are called sedimentary structures.  • Structures in sedimentary rocks can be divided into: Primary structures (formed during deposition), Secondary structures (formed after deposition) and Biogenic structures.

2. Significance Sedimentary structures provide clues about the environment of deposition or can serve to tell which side originally faced up where tectonics have tilted or overturned sedimentary layers. OR • The primary structures show (a) palaeo-current condition, (b) rate of supply of sediments, (c) mode of transportation, (d) environment of deposition and (e) top and bottom of beds etc.

3. I. Primary structures Major Primary Sedimentary Structures includes: • Bedding, • Ripple marks and dunes • Cross-bedding, • Graded bedding, • Mud Cracks/Dessication Cracks, • Fossils and • Rain Drop Imprints

4. II. Secondary structures • Secondary sedimentary structures are those which formed sometime after deposition and are the indicators of circumstances after deposition. • These secondary sedimentary structures are largely of chemical origin, formed by precipitation of mineral substances in the pores of semi-consolidated or consolidated sedimentary rock or by chemical replacement processes.

5. These include: • Concretions • Solution structures • Oolitic Structures • Pisolitic Structures • Geode

6. III. Biogenic structures • Biogenic sedimentary structures are biologically produced structures that include: • tracks, • trails, • burrows, • borings, • fecal pellets and • other traces made by organisms. These structures are the examples of trace fossils (also called ichnofossils).

7. I. Primary structures • 1: Bedding or Stratification • Series of horizontal layers within an outcrop of rock are called bedding. • Bedding (or Stratification) is defined as the deposition of sediments or fragments in the series of beds that differ from each other by mineralogy, rock type, color or other characteristics.

9. The bed thickness may vary from mm to several meters and the thinner ones are referred to as laminae. Fine laminae are characteristic of suspension deposits. • Each bed represents a period of deposition such that the upper bedding plane represents the cessation of that period of deposition or perhaps a change in the type of sediment being deposited i.e. a line of parting which is the bedding plane usually forms during the time gap or composition change

10. Cross-bedding Cross-bedding or cross stratification is an arrangement of beds or laminations in which one set of layers or beds is inclined relative to the others.

11. Cross-beds usually curve at the bottom edge, becoming tangent to the lower bed surface. • The upper edge of individual inclined cross-beds is usually at a steep angle to the overlying bedding plane. • Hence, cross-beds may be used as indicators of ancient current directions.

12. These structures results in shallow-water and wind formed deposits when the stream suffer repeated changes in the velocity and direction of flow of stream or wind carrying sediments.

13. Cross-lamination • Cross laminations commonly present in granular sedimentary rocks that consist of tabular, irregularly lenticular or wedge-shaped beds lying essentially parallel to the general stratification and • which themselves show a pronounced laminated structure in which the laminae are steeply inclined to the general bedding.

14. In cross- lamination, the height of the individual set is less than 6 cm and the thickness of the cross-laminae is only a few millimeters, and cross-bedding, where the set height is generally greater than 6 cm and the individual cross-beds are many millimeters to1 cm or more in thickness. • Cross-lamination forms when deposition takes place very rapidly during migration of current or wave ripples.

15. A series of cross-laminae are produced by superimposing migrating ripples (Fig.).

16. Graded bedding Graded bedding • In graded bending structures, beds with particle of finer size occurred mostly at top and the beds with particles of coarser size occurred at base.

17. When sediments settle down in the basin or lake, heavier particles settled down first then the lighter ones according to their specific gravity. • Thus a sequence formed is called Graded bedding. • These show progressive change in grain size from bottom to top of a bed (usually fining-upward).

18. Ripple Marks and Dunes • Ripple marks are the small ridges formed on top of beds by moving wind or water. • There are two types: asymmetric wave ripples and symmetric current ripples. • Environments where the current is in one direction, such as rivers, produce asymmetric ripples.

19. The longer flank of such ripples is oriented opposite to the direction of the current. Wave ripples occur in environments where currents occur in all directions

21. Dunes develop with increasing flow strength beyond the upper limit of ripples. • They are similar in form to ripples (i.e., asymmetric bedwaves) but are larger than ripples with lengths ranging from greater than 0.75 m to in excess of 100 m and heights ranging from greater than 0.075 m to >5 m.

22. Mud Cracks/Dessication Cracks • These are the polygonal cracks formed in drying mud. These form in sediments that are sometimes exposed to air (subaerial exposure such as dry lake beds and tidal flats).

23. Rain Imprints • Rain drop impressions are a geological feature characterized by small hole like pits with slightly raised edges that are the result of the impact of rain on soft sediment surfaces.

24. II. Secondary structures • Secondary sedimentary structures are those which formed sometime after deposition and are the indicators of circumstances after deposition. • These secondary sedimentary structures are largely of chemical origin, formed by precipitation of mineral substances in the pores of semi-consolidated or consolidated sedimentary rock or by chemical replacement. processes.

25. These include: 1: Concretions: (a) Oolitic Structures (b) Pisolitic Structures (c) Geode 2: Solution structures

26. Concretions • Most concretions are composed of calcite, but concretions composed of dolomite, hematite, siderite, chert, pyrite, and gypsum are also known. • In carbonate based rocks such as limestone or chalk, chert or flint concretions are common, while terrestrial sandstones can have iron concretions.

27. Shapes of these masses range from spherical to disc shaped, cone-shaped, and pipe-shaped and spherical to elliptical bodies, usually of diverse chemical nature than the rocks in which they occur. • They form by precipitation of mineral matter around some kind of nucleus, such as a shell fragment, inside burrows or around plant roots, gradually building up a globular mass

29. Oolitic Structures Oolites are small (0.1-1mm), concentrically layered, spherical grains composed of primary carbonate materials or replacement phases.

30. Pisolitic Structures • Individual size of a concretion is like that of a pea nut. Lime stone and Bauxite shows both structure. Fig. Pisolitic structure in Bauxite

31. Geode • It is a structure formed by the depositional activity of ground water. It is a cavity in rocks, lined with well developed crystals of quartz projecting towards the center.

32. Solution structures • Solution structures are irregular openings commonly in calcareous rocks and are produced due to ground water action e.g. Vugs and stylolite. • Vugh is a cavity in a rock usually with lining of crystalline minerals. The term used especially of cavities in mineral veins (Fig).

33. Stylolites is diagenetic structure and is common in carbonate rocks and is found in many limestones, dolomites and bedded siderites. A stylolite is a surface marked by interlocking or mutual interpenetration of two sides (Fig).

34. The tooth like projections of one side fit into the sockets of like dimension on the other. • These are irregular planes where material was dissolved into the pore fluids in the rock. • This can result in the precipitation of a certain chemical species producing colouring and staining of the rock, or the formation of concretions.

36. Structures Generated by Deformation • After deposition, physical processes can deform the sediment, producing another class of secondary structures. • e.g; • when the pore fluid pressure in a sand layer surpasses a critical point, the sand can break through overlying clay layers and flow through, forming discordant bodies of sedimentary rock called sedimentary dykes.

37. The same process can form mud volcanoes on the surface where they broke through upper layers. • The mud produced by mud volcanoes is most typically formed as hot water, which has been heated deep below the earth's surface, begins to mix and blend with subterranean mineral deposits, thus creating the mud slurry exudate. • This material is then forced upwards through a geological fault or fissure due to local subterranean pressure imbalances. About 86% of the gas released from these structures is methane, with much less carbon dioxide and nitrogen emitted.

38. Density contrasts can also cause small-scale faulting, even while sedimentation progresses. • Such faulting can also occur when large masses of non-lithified sediment are deposited on a slope, such as at the front side of a delta or the continental slope. • Instabilities in such sediments can result in the deposited material to slump, producing fissures and folding. The resulting structures in the rock are synsedimentary folds and faults, which can be difficult to distinguish from folds and faults formed by tectonic forces acting on lithified rocks.

39. III. Biogenic structures • Biogenic sedimentary structures are biologically produced structures that include tracks, trails, burrows, borings, fecal pellets and other traces made by organisms. • These structures are the examples of trace fossils (also called ichnofossils).

40. The burrowing, boring, feeding and locomotion activities of organisms can produce a variety of trails, depressions, and open burrows and borings in mud or semi-consolidated sediment bottoms. • Filling of these depressions and burrows with sediment of a different type or with different packing creates structures that may be either positive-relief features, such as trails on the base of overlying beds, or features that show up as burrow or bore fillings on the tops of the underlying mud bed.

41. These include: • Burrows and Borings • Tracks and Trails • Stromatolites

42. Burrows • Burrows are excavations made by animals into soft sediment. • -used by organisms for dwellings, or may be produced as a subterranean organism moves through the soil or sediment in search of food. Burrows are commonly filled in by sediment of a different color or texture than the surrounding sediment. • Burrow fillings may become cemented and hard

44. Borings • Borings are holes made by animals into hard material, such as wood, shells, rock, or hard sediment. • Borings are usually circular in cross-section. • Most trace fossils are burrows of molluscs or arthropods. • This burrowing is called bioturbation. • can be a valuable indicator of the biological and ecological environment that existed after the sediment was deposited.

45. Tracks • Tracks and trailsare markings indicative of the some animal over soft sediments. • Tracks or footprints are impressions on the surface of a bed of sediment produced by the feet of animals. • Examples include dinosaur footprints or bird tracks. • In some cases, tracks are found as sole marks on the bottoms of beds, where sediment has infilled the tracks, and preserved them as casts.

47. Trails • Trails are the winding passages through which long bodied and short bodied animals moved. • In other words, trails are groove-like impressions on the surface of a bed of sediment produced by an organism which crawls or drags part of its body. • Trails may be straight or curved

49. Stromatolites

50. They develop through the trapping and binding of carbonate particles by a surficial microbial mat (‘algal mat’) composed mainly of cyanobacteria (formerly called blue-green algae) and other microbes, and the biochemical precipitation of carbonate. • Stromatolites are finely laminated algal accumulations (>10cm in diameter) that result when algae grow upwards trapping carbonate mud into thin layers.