Chapter Six
Sediments & Sedimentary Rocks
Sediment
• Sediment - loose, solid particles originating from:
– Weathering and erosion of pre-existing rocks
– Chemical precipitation from solution, including
secretion by organisms in water
• Classified by particle size
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Boulder - >256 mm
Cobble - 64 to 256 mm
Pebble - 2 to 64 mm
Sand - 1/16 to 2 mm
Silt - 1/256 to 1/16 mm
Clay - <1/256 mm
SEDIMENTATION AND SEDIMENTARY ROCKS
I. INTRODUCTION
IMPORTANCE OF SEDIMENTARY ROCKS:
i) 75% OF ROCKS EXPOSED AT THE EARTH’S SURFACE
ii) SOURCE OF FOSSIL FUELS, IRON AND ALUMINUM
ORES, ALUMINUM ORES, AND GROUNDWATER
iii) RECORD OF EARTH’S HISTORY
TYPES OF SEDIMENT
II ORIGIN OF SEDIMENTARY ROCKS:
A. INTRODUCTION
1. REVIEW OF WEATHERING PROCESSES:
2. CLASSIFICATION OF SEDIMENTS
a) DETRITAL SEDIMENT: PREEXISTING FRAGMENTS OF PREEXISTING
IGNEOUS, SEDIMENTARY AND METAMORPHIC ROCKS
b) CHEMICAL SEDIMENT: PRECIPITATED FROM WATER OR EXTRACTED
BY ORGANISMS AND DEPOSITED LATER
B. SEDIMENT TRANSPORT AND TEXTURE
1. INTRODUCTION
i) TRANSPORTATION AND DEPOSITION OF CHEMICAL
SEDIMENTS
ii) TRANSPORTATION AND DEPOSITION OF DETRITAL
SEDIMENTS
2. GRAIN SIZE – CONTROLLED BY:
a) NATURE OF PARENT ROCKS
b) NATURE AND ENERGY LEVEL OF TRANSPORT MEDIUM
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1) SORTING
WELL-SORTED: WIND MOST SELECTIVE
POORLY-SORTED: GLACIERS AND FLOODING
RIVERS SORT
POORLY
2) CURRENT VELOCITY
Grain size
• 3) GRAIN SHAPE - DEPENDS ON:
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a) TRANSPORT MEDIA: SWIFTLY FLOWING RIVERS
BOUNCE PEBBLES
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AND SAND GRAINS AROUND VIGOROUSLY
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b) DISTANCE FROM PARENT ROCK (AT THE BASE IF A
GLACIER MAY
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BE GROUND TO A ROCKY POWDER)
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c)MINERAL HARDNESS:
Sediment in a Stream
C. SEDIMENTARY STRUCTURES:
1. BEDDING (STRATIFICATION): ARRANGMENT OF SEDIMENT
PARTICLES INTO DISTINCT LAYERS
a) CHANGES IN SEDIMENT:
b) CHANGES IN TRANSPORT ENERGY
2. GRADED BEDDINGS: SEDIMENT LAYER (FORMED BY A SINGLE
DEPOSITONAL EVENT) IN WHICH PARTICLE SIZE VARIES
GRADUALLY WITH THE COARSEST PARTICLES ON THE BOTTOM
Development of a bedding plane
Develop. Bedding plane-contd.
Develop. Bedding plane contd.
Graded bedding of sediment
Development of cross-bedding
Cementation & Recrystallization
• 4. CEMENTATION: PRECIPITATION OF DISSOLVED IONS IN THE PORE
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SPACE
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a) CALCIUM CARBONATE
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b) SILICA
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c) IRON COMPOUNDS
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• 5.CLASTIC TEXTURE: FORMED BY COMPACTION AND CEMENTATION
OF
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• SEDIMENT PARTICLES HAS CLASTIC TEXTURE
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• 6.RECRYSTALLIZATION: RECRYSTALLIZATION OF CERTAIN UNSTABLE
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MINERALS INTO NEW, MORE STABLE MINERALS
III. CLASSICATION OF SEDIMENTARY ROCKS:
DETRITAL SEDIMENTARY ROCKS:
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MUDSTONES
SANDSTONES
CONGLOMERATES
BRECCIAS
A. DETRITAL SEDIMENTARY ROCKS:
1. INTRODUCTION: CLASSIFICATION BASED ON
PARTICLE SIZE
a) ALL DETRITAL ROCKS ARE CLASTIC
b) SAND AND SILT PREDOMINANTLY QUARTZ
c) FINER-SIZED PARTICLES OF CLAY MINERALS
2. MUDSTONES
a) MORE THAN HALF OF ALL SEDIMENTARY ROCKS
b) CONTAIN THE SMALLEST PARTICLES (0.004 mm IN DIAMETER)
c) ENVIRONMENTS OF DEPOSITION: LAKES, LAGOONS, DEEP OCEAN
BASINS, RIVER FLOODPLAINS
d) COLOR VARIETY OF SHALE REPRESENTS MINERAL COMPOSITION
e) PRACTICAL USES OF SHALE: BRICKS, CERAMICS, CEMENT, AND OIL
SHALE
3.SANDSTONES:
a) 25% OF ALL SEDIMENTARY ROCKS
b) SANDSTONE PARTICLES (1/16-2 MM IN DIAMETER)
c) PRACTICAL USES OF SANDSTONES: BUILDINGS AND RESERVOIR
FOR FOSSIL FUELS AND GROUNDWATER
4.CONGLOMERATES AND BRECCIAS
a) GRAINS LARGER THAN 2 MM
b) CONGLOMERATES HAVE ROUNDED GRAINS
c) BRECCIAS HAVE ANGULAR GRAINS
B. CHEMICAL SEDIMENTARY ROCKS
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INORGANIC CHEMICAL SEDIMENTARY ROCKS
a) LIMESTONE (INORGANIC)
i) FORMATION
ii) OOLITIC LIMESTONE
iii) TUFA
iv) TRAVERTINE
Cross-bedding & mudcracks
3. CROSS-BEDDING: SEDIMENTARY LAYERS DEPOSITED AT AN
ANGLE TO THE UNDERLYING SET OF BEDS
4. SURFACE SEDIMENTARY FEATURES
a) RIPPLE MARKS: SMALL SURFACE RIDGES PRODUCED WHEN
WATER OR WIND FLOWS OVER SEDIMENT AFTER IT IS
DEPOSITED
b) MUDCRACKS: OCCUR ON THE TOP OF A SEDIMENT LAYER
WHEN MUDDY SEDIMENT DRIES AND CONTRACTS
Origin of mud cracks
Asymmetric and symmetric ripples
D. LITHIFICATION: TURNING SEDIMENT INTO
SEDIMENTARY ROCK
• 1. DEFINITION OF DIAGENESIS: CHANGES IN THE
SEDIMENT DUE TO INCRESED HEAT, PRESSURE, AND
CIRCULATING GROUNDWATER
• 2. DEFINITION OF LITHIFICATION: END RESULT OF
DIAGENESIS
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• 3. COMPACTION: DIAGENETIC PROCESS BY WHICH
THE WEIGHT OF OVERLYING MATERIALS REDUCES
THE VOLUME OF SEDIMENTARY BODY
Lithification of sediment
Initial deposits of flat/tabular clay
Formation of ooliths
Locations of subsurface evaporite
Formation of coal from swamp deposits
Formation of coal –contd.
Common geological environment
Geology at a glance
Marine sedimentary environment
Sedimentary facies formation
Sedi. Facies formation – contd.
Landword Migration
Chapter Summary
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% of sedimentary rocks in outer 10 miles
Basic classification of sedimentary rocks
Most sedi. Rocks – Chemical or detrital
Definition of sedimentary rocks
Sorting by wind, glaciers (well sorted vs poor sorting)
Degree of sedimentary particle rounding
Bedding plane, sedimentary structure, occurrence
Diagenesis, Lithification, Cementation
Composition of detrital sedimentary rocks
Environment – deposition of shale
Breccia and Conglomerate
Oolitic Limestone
Definition of Sedimentary Facies – Characteristics that
distinguish one from another