GEOLOGY. Subject : GEOLOGY (For under graduate student.) Paper No. : Paper 02 Introduction to Geology 02

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GEOLOGY Subject : GEOLOGY (For under graduate student.) Paper No. : Paper 02 Introduction to Geology 02 Topic No. & Title : 56 Structure of Sedimentary Rocks Frequently Asked Questions FAQ s Que 01.

1 GEOLOGY Subject : GEOLOGY (For under graduate student.) Paper No. : Paper 02 Introduction to Geology 02 Topic No. & Title : 56 Structure of Sedimentary Rocks Frequently Asked Questions FAQ s Que 01. What are sediments? Ans. Sediment is a solid material that has settled down from the state of suspension in a liquid. Any of the layered rocks resulting from the consolidation of sediment is called sedimentary rock. Sediments are transported by wind, glaciers and also by water in four different ways sliding, rolling, saltation and suspension. Que 02. Explain sediments structure. Ans. Sedimentary Structure signifies some large scale features developed in the rock masses during their formation. The structures of sediments are best studied in outcrops rather than in hand specimens or thin sections. The small scale geometrical aspects of the constituent grains are commonly distinguished as Texture. Sedimentary structures are formed during or after deposition and also during diagnosis. Structures in sedimentary rocks are visible because of local variation in texture and composition. Que 03. Explain about bedding Ans. It is expressed by rock unit of generally tabular or lenticular forms that have same litho logy and structure and are thus set off from other strata with which they are interleaved. The most nearly universal primary structure of sedimentary rocks in bedding of stratification. 1

2 Scale of thickness (in cm) Stratum > 1 Thickly laminated Thinly laminated < 0.2 Very thinly 1 3 bedded Thinly bedded 3-10 Medium bedded Very thickly bedded Beds differ from the layers above and below by virtue of some discontinuity in rock types, internal structure or texture. The lower boundary of the bed is called sole. Different parts of beds separated by minor but distinct discontinuities in texture or composition are called layers. Que 04. Give an account on classification of sedimentary structures. Ans. Inorganic Structures 2 Mechanical Chemical Bedding Geometry Solution structures Lamination Stylolites Wavy bedding Vugs, oolicasts Bedding internal Accretionary structures structures Cross bedding Nodules Ripple bedding Concretions Graded bedding Growth bedding Bedding plane markings Composite structures (on sole) Scour marks Geodes Tool marks Septaria Cone-in-cone Bedding plane markings (on surface) Pits and prints Mud cracks Deformed bedding Condute bedding Load cast

3 3 Ball and pillow structures Organic Structures Biogenic Structures Petrification Stromatolites Trail fossils Que 05. What are the types of Sedimentary Structures? Ans. Sedimentary structures can be classified into two types: 1. Morphologic Classification base on the form or geometry and place of occurrence of sedimentary structures 2. Genetic Classification based on the process involved in the formation of sedimentary structures such as biogenic, hydrodynamic etc. Since, in classification, it is more meaningful to use a system involving both genetic and morphologic grouping, sedimentary structures are classified as mechanical, chemical and biogenic structures. Que 06. Give a note on Sedimentary Structures Based On Bedding Geometry. Ans. Types of Sedimentary Structures Based on bedding geometry: 1. Lamination: It may be formed due to transitory phases or minor fluctuations in velocity or the depositing current. It is characterised by fine grained sediments, notably silt stone and shale. Beds less than 1 cm are called as laminae. 2. Wave bedding: It is characterised by inter-bedded rippled sands and mud layers, completely out of phase with each other. Que 07. Give a note on Sedimentary Structures Based On Bedding Internal Structures Ans. Types of Sedimentary Structures based on Bedding Internal Structure: 1. Cross bedding: It is an internal feature and is a product of migration of a mega ripple or sand wave. Cross bedding means bedding with a high initial dip. There are two types of cross bedding: a) Tabular cross bedding b) Trough cross bedding

4 2. Ripple Bedding: It is applied to micro cross lamination produced by ripple migration. They are undulations of the sediment surface produced as wind or water moves across sand. Ripple bedding may be: a) Asymmetrical ripples ( Current ripple marks) b) Symmetrical ripples ( Wave ripple marks) 3. Graded bedding: Graded beds are sedimentation units marked by a gradation in grain size, from coarse to fine, upward from base to the top of the unit. Graded beds are deposited from wavy current and many range in thickness from a cm or less to 1 or more meter. Graded materials may be silt, sand or in rare cases, gravels. Usually, most graded beds are sandstone and range from a few cm to m in thickness. 4. Growth Bedding: This term is applied for the stratification produced by insitu activity of organisms or by chemical precipitation on surfaces of accumulation Que 08. Give a note on Sedimentary Structures Based On Bedding Plane Markings. Ans. Types of Sedimentary Structures based on Bedding Plane Markings: When beds separate readily along bedding planes, the surfaces produced commonly display various markings and structures. Markings found on the lower side of the beds are known as sole marks. 1. Scour marks: They are mainly due to current action. They are produced by cutting or scouring action of a water current flowing over the bottom. 2. Tool marks: It differs from scour marks as being produced by the object carried with the bed rather than the flow itself. The tools may be wood fragments, shells, pebbles etc. Que 09. Give a note on Sedimentary Structures Based On Bedding Plane Markings (Surface Surface). Ans. Types of Sedimentary Structures based on Bedding Plane Markings (on surface): 1. Pits and Prints: rain, hail and spray impressions are small circular to ellipsoidal pits formed in wet mud by the agents. Those marks indicate environment or on-shore environment of depression. 4

5 2. Mud Cracks: It is an irregular fracture in a crudely polygonal pattern formed by the shrink age of clay, silt or mud in the course of drying under the influence of atmospheric surface condition. Que 10. Give a note on Sedimentary Structures Based Deformed Bedding Structures. Ans. Types of Sedimentary Structures based on Deformed Bedding Structures: 1. Como lute bedding: A single thin sandstone or siltstone layer, siliceous or calcareous silt get folded and the internal laminations may get contorted. Deformation appears as highly contorted folded and disrupted layers. 2. Load Cast: These are structures produced by sinking of heavier into lighter sediments. They are somewhat irregular bulbous or mammillary features on the base of a sandstone bed that over sites shale. 3. Ball and Pillow Structures: Some Sandstones like some subaqueous lava flows, display a pillow structure. The sand appears as numerous closely packed ball or pillow like forms and termed as Pseudo nodules. They are not a primary depositional feature but are instead a product of deformational feature but are instead a product of deformation which took place before the deposition of the overlying strata. Que 11. Explain about Secondary Sedimentary Structures. Ans. Secondary Sedimentary Structures Chemical Origin: 1. Stylolites: - A stylolitic seam is a surface marked by interlocking or mutual interpretations of two sides. The tooth like projections of one side fit into socket like dimension on the other. In cross section, the Stylolitic surface resembles a suture. They are very abundant in carbonate rocks and may also occur in some sandstone and quartzite. They even occur in gypsum and probably in anhydrite and salt. 2. Vugs: - Vugs are the pores unrelated to primary fabric which is the result of solution activity in limestone. The actual pore may have had a complex history. A skeletal element, for example, may be removed by solution, leaving the mold, which may be enlarged by further solution and converted to an irregular rug. 3. Oolicasts: - These structures are displayed by a rock having oolitic texture. Oolites are small spherical, accretionary bodies generally ranging from 0.25 mm to 2mm in diameter. 5

6 If the diameter is greater than 2mm, the texture is called pisolites. Oolites appear to grow outward from a centre, which is formed by the solution activity in carbonate rocks. Partial to complete solution of the ooid leaves a void that may later be filled by inward growth of crystalline matter such as silica in calcareous ooids. This is called as oolicasts Que 12. Explain about Accretionary structures Ans. Accretionary Structures Accretion, as defined by Tod is a term used based on growth manner. Accretion grows from the centre outward in a regular manner. Accretion grows from the centre outward in a regular manner. 1. Nodules: local chemical precipitation and reaction create structures commonly referred to as nodules. They occur in host sediments of virtually any composition. A nodule is generally spherical or irregular rounded in shape. These are typically solid replacement bodies of chert or iron oxides formed during diagenesis of sedimentary rock. 2. Concretions: It consists of round or irregular masses of more resistant rock formed as a result of precipitating around a core material usually of fossil or grain of different composition. Que 13. Explain about Composite Structures. Ans. Composite Structures 1. Geodes: Geodes are essentially rock cavities with internal crystal formation or concentric banding. The exterior of the most common geode is generally limestone or related carbonate rock while the interior contains quartz crystals and chalcedony deposits. Geodes are characteristic of certain limestone beds but are rarely found in shale s. The formation of a geode is first the origin of cavity and filling of it by diagenesis like re-crystallization in the central area, silicification of the exterior, followed by the central portion and subsequent precipitation of crystalline filling. 2. Septaria: Septaria are large ( cm) distinctly oblate nodules characterised by series of radiating cracks that widen towards the centre and die out near the margin that is crossed by a series of cracks concentric with the margin. 3. Cone in cone structure: This structure is typical to certain black shale and is characterized by an abundance of right circular cone with cone axis perpendicular to the bedding. 6

7 Que 14. Explain about Biogenic Structures. Ans. Biogenic Structures These structures are formed as a result of direct or indirect effect of organic activity. 1. Petrifaction: It is the replacement of inorganic matter by organic matter. It is a cell to cell replacement where the original structure of the organic matter is preserved. Example: Petrified wood, petrified shells 2. Straomatolites: It is a laminated structure composed of particulate sand, silt and clay sized sediments which have been formed by the trapping and binding of detrital sedimentary particles by an algal mat. 3. Trace fossils: These are the records of life and events that took place in-situ during or soon after the deposition of sediments. They include foot prints, crawling marks, tracks, trails and burrows. Que 15. Give an account on applications of sedimentary structures Ans. Applications Of Sedimeary Structures 1. At the time of sedimentation. 2. Biological structures are useful in places to determine stratigraphic order in a vertical overturned sequence. 3. Trace fossils can be mapped and used to define major facies belts. 4. Ripple bedding is useful in determining stratigraphic order, in giving evidence of the direction of current flow and in indicating flow conditions. 5. They reflect upon the nature of the environment during sedimentation. 7

ESC102. Sedimentary Rocks. Our keys to the past. Monday, February 11, 13

ESC102. Sedimentary Rocks. Our keys to the past. Monday, February 11, 13 ESC102 Sedimentary Rocks Our keys to the past Sedimentary Rocks Sedimentary rocks are rocks that form through the accumulation of sediment and the process of lithification. Lithification occurs after deposition