Top and bottom criteria

STRATIGRAPHY- TOP AND BOTTOM CRITERIA

1. TOP AND BOTTOM CRITERIA Guided By: Dr. Poonam Bhatnagar Submitted By: Amit kr. Mishra M.Sc. Geology GOVT. HOLKAR SCIENCE COLLEGE, INDORE (M.P.) 1

2. CONTENT: Introduction Bed Bedding plane Lamination Law of super Position Factors to recognise top & bottom criteria Unconformity Fossils Ripple marks Cross Bedding Graded Bedding Pillow structure Sun cracks & Rain prints Drag fold Position of cleavage in folded rocks. Reference 2

3. INTRODUCTION: In 1948 top and bottom criteria given by Robert Shark. When the sedimentary rocks are deposited then the older bed deposited in lower portion while the younger bed deposited in upper portion and with the help of these phenomenon age and order of super position. 3

4. Due to the tectonic movement order of beds will change sometime these beds become inclined, vertical and overturned and in this stage determination of top and bottom criteria, igneous rocks and metamorphic rock also used in determination of top and bottom criteria. 4

5. BEDS:- A layer of rock separated from the layer above and below by bedding plane. Beds vary in thickness from 1cm to many tens of meter. Deposition of sediment into beds or layers is called as stratification. Different beds are distinguished from each other by variations in grain size or texture , difference in mineral composition , variation in thickness. 5

6. BEDDING PLANE: It defines the top and bottom of a bed. Represents a change in the nature of sedimentation. A change in the rate or type of sedimentation. A pause where no sediment is deposited. A period of erosion where some sediment is removed. 6

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8. LAW OF SUPERPOSITION: First proposed by Nicolaus Steno in 17 th century. If one bed of sediment lies on the top of another ,then the one above must be the younger. This assumed the beds have not been overturned due to earth movement. 8

9. FACTORS TO RECOGNISE TOP AND BOTTOM CRITERIA: Some imp factor to recognise top & bottom criteria Position of cleavage in folded rocks Unconformity Ripple Marks Rain prints Pillow structure Graded bedding Cross bedding Drag fold Fossils 9

10. UNCONFORMITY: An  unconformity  is a buried erosional  or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. Clear angular unconformities provide unequivocal evidence of the relative age of two rock sequences. 10

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13. FOSSIL: A fossil is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in amber, hair, petrified wood, oil, coal, and DNA remnants. 13

14. RIPPLE MARKS: Ripple marks are the wavy undulations seen on the surface of bedding plane. They are produced by the action of waves and currents in shallow water. This structure may also be formed on the surface of deposits formed by wind. Ripple marks are of 2 types: asymmetrical marks symmetrical mark 14

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16. CROSS BEDDING: Also known as current bedding and false bedding. large scale it is termed dune bedding, small scale it is termed cross lamination. In each case the sediments is being moved and accumulated at an angle to the principle bedding direction. Current bedding is commonly found in shallow water and wind formed deposits This structure indicates rapid changes in velocity and direction of flow of stream or wind carrying sediments. 16

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19. GRADED BEDDING: Each bed shows a gradation in grain size. Coarse grain are situated at bottom and fine grains at above. The graded bedding results from rapid sedimentation in water The bottom of a graded bed generally lies on shale and may consist of a coarse grain It than shows an upward transition towards finer material , at tops it commonly ends in silt and clay particles. 19

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22. PILLOW STRUCTURE: Pillows are flat where as top side of pillows are convex. Pillow Structures  are subaqueous lava flow. Shape of pillow is ellipsoidal. These pillows are in few feet to some long inches. 22

23. MUD CRACKS AND RAIN PRINT: Formed when sediments is exposed to the atmosphere . Often found in the fine grained sedimentary rocks. The sediments drives out and shrinks as water is evaporated from it . Contraction centers develop and a polygonal pattern of cracks develop. Rain print  are a geological feature characterized by small crater-like pits with slightly raised edges that are the result of the impact of raindrop impacts on soft sediment surfaces 23

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26. DRAG FOLD: A minor  fold , usually one of a series, formed in an incompetent bed lying between more competent beds produced by movement of the competent beds in opposite directions relative to one another. Drag folds  may also develop beneath a thrust sheet. With the help of drag fold we can identified the synclinal axis younger beds are found. 26

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28. POSITION OF CLEAVAGE IN FOLDED ROCKS: When the cleavage is perpendicular to the bedding and that place where cleavages are tilted and in that place fold limbs are formed, then we can identified the top and bottom criteria. 28

29. REFERENCE: BILLINGS (2018), STRUCTURAL GEOLOGY, PEARSON, 81-93. https://www.fossilera.com/fossils Images: google photos 29

30. Thank You. 30