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PETROLOGY OF SEDIMENTARY ROCKS
Dr. Sugeng S Surjono
Lab. Sedimentografi
Jurusan Teknik Geologi, Fakultas Teknik
Universitas Gadjah Mada
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INTRODUCTION
Three Rock Types:
Igneous, Sedimentary, Metamorphic rocks
Sedimentary rocks : rocks formed at the surface of the earth under lowtemperature and low-pressure, result from the accumulation and solidification
of sediments, material transported in water, air or ice (Raymond, 1995).
Origin of sedimentary rocks:
- Formation of source rocks/sediment source :
intrusion, metamorphism, volcanism, tectonic uplift
- Weathering :
physical and chemical breakdown of source rocks
- Erosion and Transportation
agent of transportation : water, wind, ice
- Deposition
material is deposited within depositional basins
- Diagenesis
sediment is covered by successive layer of younger sediment; increased
temperature and pressure leading to consolidation and lithification of the
sediment into sedimentary rocks
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INTRODUCTION
Sedimentary rocks are characterized by :
-Presence of layers
-Presence of transported grains
-Sedimentary structures
-Fossils
Types of Sedimentary Rocks: (Tucker, 1991)
Siliciclastic (fragmental) : - Conglomerates & breccias
- Sandstones
- Mudrocks
Biogenic, biochemical and organic :
- limestones & dolomites
- cherts
- phosphates
- coal
- oil shale
Chemical
: - evaporites
- ironstones
Volcaniclastic
: (e.g.) ignimbrites, tuffs, hyaloclastites
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Sedimentary rocks
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Clastic (siliciclastic) rocks (80-85% of the stratigraphic
record)
Carbonate sediments and rocks (10-15% of the
stratigraphic record)
Volcaniclastic sediments and rocks
Others (< 5% of the stratigraphic record) :
-
Organic (carbonaceous) sediments and rocks
Evaporites
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SILICICLASTIC SEDIMENTARY ROCKS
Clast (from the Greek klastos, meaning ‘broken’) is the technical term for broken
fragment within sedimentary rocks. It is also called as terrigenous grains
Because most terrigenous grains are composed in part of silica, they are
often referred to as siliciclastic grains.
Siliciclastic sedimentary rocks are composed by clasts that originated from
transportation and deposition of pre-existing rocks within depositional environments.
Mechanism involved in the transportation include the wind, glaciers, river currents,
waves, tidal currents, debris flow and turbidity currents (Tucker, 1991).
Two important features of siliciclastic sediments related to depositional processes
and diagenesis are sedimentary textures and structures.
DESCRIPTION OF SEDIMENTARY ROCKS (Prothero & Schwab, 2005):
-Color
-Sedimentary textures
-Sedimentary structures
-Composition
-Fossil contents
-Geometry of sedimentary rocks
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Basic components of
siliciclastic sedimentary rock are :
-clasts or fragments
-matrix
-cements
Color
-Color usually reflects some aspect of the rock’s composition (bulk color can
reflect the color of major mineralogical components)
-Color of rock controlled by color of clast, matrix and cement
-Color is not treated as an independent property, however, but as an aspect
of sedimentary rock composition
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Sedimentary textures
Textures refers to the size, morphology, and arrangement
(fabric) of siliciclastic grains that make up a sedimentary
rock.
Grain size
-Grain or siliciclastic particles range in size from clay to
boulder
-The grade scale most widely used by sedimentologist is
the Udden-Wenthworth scale
-The Udden-Wentworth grain-size scale is based on factors
of two:  = -log2 d ; where d is grain size in mm
- It extends from <1/256 mm (0.0039) to >256 mm and is
divided into four major size categories (clay, silt, sand, and
gravel) that can be further subdivided
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Mud
Udden-Wenthworth
grain-size scale for
sediments and the
equivalent phi scale
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Grain size parameters : mean, sorting, skewness, kurtosis
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Grain-size (particle-size, granulometric) analysis
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The old-fashioned way: direct measurement (gravel) and sieve/pipette
analysis (sand and mud)
The modern technology: laser particle sizing (sand and mud)
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Grain size parameters
Graphical method
Graphic Mean
Standard deviation
Skewness
Kurtosis
(Mz) =
(σ1) =
16   50   84
3
 84  16
4
 95   5
+
(SK1) =  84  16  2 50
2( 84  16)
(KG) =
6,6
+
 95   5  2 50
2( 95   5)
 95   5
2,44( 75   25)
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Grain size parameters
Moment measures (mathematic method)
•
First moment: mean (cf. median, mode)
• Premier measure of the grain size
xø=
•
 fm
N
Second moment: variance (cf. standard deviation)
• Measure of the degree of sorting
σø =
 f (m  X )
( = standard deviation)
2
100
• Third moment : Skewness
- Measure of the symmetry of the grain-size distribution
Skø =
 f (m  X )
3
100 3
• Fourth moment : Kurtosis
- Measure of the sharpness or peakedness of a grain-size
frequency curve
f (m  X ) 4

Kø =
100 4
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SK1 class :
+1,0 - +0,3
+0,3 - +0,1
+0,1 - -0,1
-0,1 - -0,3
-0,3 - -1,0
very fine-skewed
fine-skewed
near-symmetrical
coarse-skewed
very coarse-skewed
KG class:
<0,67
very platykutic
0,67 – 0,90 platykurtic
0,90 – 1,11 mesokurtic
1,11 – 1,50 leptokurtic
1,50 – 3,00 very leptokurtic
>3,00
extremely leptokurtic
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Grain Morphology
Three aspects of grain morphology are the shape, sphericity and
roundness.
• The shape or form of grain is measured by various ratios of the long,
intermediate and short axes.
• Sphericity is a measure of how closely the grain shape approaches
that of a sphere.
• Roundness is concerned with the curvature of the corners of a grain
and six classes from very angular to well rounded.
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Grain shape classification
Roundness and Sphericity
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Grain Fabric
Fabric for grain in sedimentary rock refers to their orientation and
packing and to the nature of contacts between them.
• Grain Packing is a function of
the size and shape of grains
and postdepositional physical
and chemical processes that
bring about compaction of
sediment.
• Grain orientation is mainly a
function of the physical
processes and condition
operating at the time of
deposition
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Sedimentary structures
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Sedimentary structures occur at very different scales, from less
than a mm (thin section) to 100s–1000s of meters (large
outcrops); most attention is traditionally focused on the
bedform-scale : Microforms (e.g., ripples) ;Mesoforms (e.g.,
dunes); Macroforms (e.g., bars)
The majority of structures form by physical processes, before,
during and after sedimentation. Other result from organic and
chemical processes
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Laminae and beds are the
basic sedimentary units that
produce stratification; the
transition between the two
is arbitrarily set at 10 mm
Normal grading is an
upward decreasing grain
size within a single lamina
or bed (associated with a
decrease in flow velocity),
as opposed to reverse
grading
Fining-upward
successions and
coarsening-upward
successions are the
products of vertically
stacked individual beds
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Cross stratification
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Cross lamination (small-scale cross stratification) is
produced by ripples
Cross bedding (large-scale cross stratification) is produced
by dunes
Cross-stratified deposits can only be preserved when a bedform
is not entirely eroded by the subsequent bedform (i.e., sediment
input > sediment output)
Straight-crested bedforms lead to planar cross stratification;
sinuous or linguoid bedforms produce trough cross
stratification
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Low angle planar cross-bedding, Kali Ngalang-Gunung Kidul, YK
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Cross stratification
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The angle of climb of cross-stratified deposits increases with
deposition rate, resulting in ‘climbing ripple cross lamination’
Antidunes form cross strata that dip upstream, but these are not
commonly preserved
A single unit of cross-stratified material is known as a set; a
succession of sets forms a co-set
Planar stratification
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Planar lamination (or planar bedding) is formed under both lowerstage and upper-stage flow conditions
Planar stratification can easily be confused with planar cross
stratification, depending on the orientation of a section (strike
sections!)
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Cross stratification produced by wave ripples can be
distinguished from current ripples by their symmetry and by
laminae dipping in two directions
Hummocky cross stratification (HCS) forms during storm
events with combined wave and current activity in shallow seas
(below the fair-weather wave base), and is the result of
aggradation of mounds and swales
Heterolithic stratification is characterized by alternating
sand and mud laminae or beds
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Flaser bedding is dominated by sand with isolated, thin mud drapes
Lenticular bedding is mud-dominated with isolated ripples
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Gravity-flow deposits
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Debris-flow deposits are
typically poorly sorted, matrixsupported sediments with
random clast orientation and
no sedimentary structures;
thickness and grain size
commonly remain unchanged
in a proximal to distal
direction
Turbidites, the deposits
formed by turbidity currents,
are typically normally graded,
ideally composed of five units
(Bouma-sequence with
divisions ‘a’-‘e’), reflecting
decreasing flow velocities and
associated bedforms
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Imbrication commonly occurs in water-lain gravels and
conglomerates, and is characterized by discoid (flat) clasts consistently
dipping upstream
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Sole marks are erosional sedimentary structures on a bed surface
that have been preserved by subsequent burial
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Scour marks (caused by erosive turbulence)
Tool marks (caused by imprints of objects)
Paleocurrent measurements can be based on any sedimentary
structure indicating a current direction (e.g., cross stratification,
imbrication, flute casts)
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Trace fossils (ichnofossils) are the tracks, trails or burrows left
behind in sediments by organisms (e.g., feeding traces, locomotion
traces, escape burrows)
Disturbance of sediments by organisms is known as bioturbation,
which can lead to the total destruction of primary sedimentary
structures
Since numerous trace fossils are connected to specific depositional
environments, they can be very useful in sedimentologic
interpretations
Soft-sediment deformation structures are sometimes considered to be
part of the initial diagenetic changes of a sediment, and include:
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Slump structures (on slopes)
Dewatering structures (upward escape of water, commonly due to loading)
Load structures (density contrasts between sand and underlying wet mud;
can in extreme cases cause mud diapirs)
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Clastic (siliciclastic) rocks
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Sandstones (20-25% of the stratigraphic record) can be subdivided
according to the Pettijohn classification, based on texture and
composition (relative proportions of quartz, feldspar, and lithic
fragments)
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Quartz arenite: quartz-dominated
Arkosic arenite: feldspar-dominated
Lithic arenite: dominance of lithic fragments
Wacke: significantly matrix-supported (>15% mud)
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Quartz wacke
Greywacke (feldspathic or lithic wacke)
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Classification of Sandstone (Pettijohn, 1975)
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Clastic (siliciclastic) rocks
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Mudstones (60% of the stratigraphic record) are also known
as mudrocks or shales and commonly exhibit a distinct fissility
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Claystone
Siltstone
Conglomerates are consolidated gravels; breccias are
conglomerates with dominantly angular clasts
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Clast-supported conglomerates
Matrix-supported conglomerates
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Halang Formation, Panujah, Slawi, Central Jawa
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Alternating of siltstone and claystone, Sambipitu Formation-Wonogiri
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Black shale (upper part) overlying above thickly bedded fine sandstone
Sambipitu Formation-Wonogiri
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Normal grading sandstone, from coarse sst to fine sst, ambipitu Formation-Wonogiri
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Conglomerate in Kali Ngalang, Gunung Kidul. Nglanggran Formation
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Breccia, in Kali Ngalang, Gunung Kidul. Nglanggran Formation
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Siliciclastic rocks classification (Pettijohn, 1975)
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CARBONATE SEDIMENT AND ROCKS
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Principal minerals: calcite, aragonite (unstable), and dolomite
(diagenetic)
Principal rocks: limestone (>50% CaCO3) and dolomite (dolostone)
(CaMg(CO3)2)
Formation of carbonate sediments and rocks occurs by means of two
main processes:
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Biomineralization of CaCO3 by organisms
Direct chemical precipitation

Ca2  2HCO3  CaCO3  H2CO3
Biogenic carbonate formation occurs by a wide range of organisms (e.g.,
molluscs, corals, forams, algae, bacteria, and many others)
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Most organisms initially form unconsolidated carbonate sediments
Coral reefs and microbial mats (e.g., stromatolites) are examples of more
solid carbonate structures
Chemical precipitation produces non-skeletal carbonate grains of
various sizes (e.g., ooids, pisoids, micrite)
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CARBONATE SEDIMENT AND ROCKS
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Biogenic carbonate formation occurs by a wide range of organisms
(e.g., molluscs, corals, forams, algae, bacteria, and many others)
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Most organisms initially form unconsolidated carbonate sediments
Coral reefs and microbial mats (e.g., stromatolites) are examples of
more solid carbonate structures
Chemical precipitation produces non-skeletal carbonate grains of
various sizes (e.g., ooids, pisoids, micrite)
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After Scholle, 2003
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CARBONATE SEDIMENT AND ROCKS

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Carbonate sand usually consists either of
(fragmented) skeletal remains or non-skeletal
grains
Carbonate mud (micrite) is commonly the product
either of chemical precipitation or algal/bacterial
activity
Dunham classification of carbonate rocks:
 Texturally-based subdivision (cf. clastics):
mudstone, wackestone, packstone, grainstone,
rudstone
 Organically bound framework during formation:
boundstone
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Modified of Dunham Classification
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ORGANIC (CARBONACEOUS) SEDIMENTS AND ROCKS
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Peat and organic-rich clastic sediments form in relatively anaerobic
(reducing) environments (e.g., mires, lakes, oceans)
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Minerotrophic peat: mostly nutrient-rich, groundwater-fed mires (e.g.,
floodplains, delta plains, coastal plains)
Ombrotrophic peat: mostly nutrient-poor, rainwater-fed mires (e.g.,
relatively high, flat terrains)
Gyttja: organic-rich lake sediment
Sapropel: organic-rich marine sediment
Coal consists primarily of solid organic matter; the remainder is
known as ‘ash’
Carbonaceous shales have a lower proportion of solid organic
matter
Oil shales (may be formed in anaerobic lake and marine
environments) contain organic matter that can be driven off as
liquid or gas by heating
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Thickly bedded coal, in between shale – siltstone (Bontang, East Kalimantan)
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EVAPORITES
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Dissolved salts precipitate out of sea water due to concentration
(brine formation) during evaporation (1 km of sea water --> 12 m of
evaporites)
Evaporites commonly lithify into consolidated rocks upon formation
Least soluble compounds precipitate first:
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CaCO3 (calcium carbonate)
CaSO4 (calcium sulphate: gypsum or anhydrite)
NaCl (halite: rock salt)
Other, less stable (highly soluble) chlorides
Dissolved salts precipitate out of sea water due to concentration
(brine formation) during evaporation (1 km of sea water --> 12
m of evaporites)
Evaporites commonly lithify into consolidated rocks upon
formation
Least soluble compounds precipitate first:
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CaCO3 (calcium carbonate)
CaSO4 (calcium sulphate: gypsum or anhydrite)
NaCl (halite: rock salt)
Other, less stable (highly soluble) chlorides
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VOLCANICLASTIC SEDIMENTS AND ROCKS
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Lava (cooled magma flows) produces volcaniclastic sediment upon
weathering
Pyroclastic material or tephra (ejected particulate material) can be
subdivided into different compositional categories:
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Mineral grains
Lithic fragments
Vitric material (volcanic glass or pumice)
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Pillow lava, Watuadeg-Bantul, DIY
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Kekar tiang, lava. Kali Grindulu, Pacitan
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Tephra of Merapi pyroclastic flow mechanism, Kaliadem, Sleman
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