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
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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.
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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.
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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.
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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.
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8. LAW OF SUPERPOSITION:
• First proposed by Nicolaus Steno in 17th
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.
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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
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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.
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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.
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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.
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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.
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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.
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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
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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.
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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.
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29. REFERENCE:
• BILLINGS (2018), STRUCTURAL GEOLOGY,
PEARSON, 81-93.
• https://www.fossilera.com/fossils
• Images: google photos
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30. Thank You.
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