PETROLOGY OF SEDIMENTARY ROCKSSugeng S SuryonoLab. SedimentografiJurusan Teknik Geologi, Fakultas TeknikUniversitas Gadjah Mada

INTRODUCTIONThree Rock Types: Igneous, Sedimentary, Metamorphic rocksOrigin of sedimentary rocks:- Formation of source rocks : 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 rocksSedimentary rocks : rocks formed at the surface of the earth under low-temperature and low-pressure, result from the accumulation and solidification of sediments, material transported in water, air or ice (Raymond, 1995).

INTRODUCTIONTypes of Sedimentary Rocks: (Tucker, 1991)Siliciclastic (fragmental) : - Conglomerates & breccias - Sandstones - Mudrocks Biogenic, biochemical and organic : - limestones & dolomites - cherts - phosphates - coal - oil shaleChemical : - evaporites - ironstonesVolcaniclastic : (e.g.) ignimbrites, tuffs, hyaloclastitesSedimentary rocks are characterized by :Presence of layersPresence of transported grainsSedimentary structuresFossils

Sedimentary rocksClastic (siliciclastic) rocks (80-85% of the stratigraphic record)Carbonate sediments and rocks (10-15% of the stratigraphic record) Volcaniclastic sediments and rocksOthers (< 5% of the stratigraphic record) :Organic (carbonaceous) sediments and rocksEvaporites

SILICICLASTIC SEDIMENTARY ROCKSClast (from the Greek klastos, meaning broken) is the technical term for broken fragment within sedimentary rocks. It is also called as terrigenous 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).DESCRIPTION OF SEDIMENTARY ROCKS (Prothero & Schwab, 2005):ColorSedimentary texturesSedimentary structuresComposition Fossil contentsGeometry of sedimentary rocksTwo important features of siliciclastic sediments related to depositional processes and diagenesis are sedimentary textures and structures.Because most terrigenous grains are composed in part of silica, they are often referred to as siliciclastic grains.

ColorColor usually reflects some aspect of the rocks composition (bulk color can reflect the color of major mineralogical components)Color of rock controlled by color of clast, matrix and cementColor is not treated as an independent property, however, but as an aspect of sedimentary rock composition

Basic components of siliciclastic sedimentary rock are :-clast or fragment-matrix-cements

Sedimentary texturesTextures 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 256 mm and is divided into four major size categories (clay, silt, sand, and gravel) that can be further subdivided

Udden-Wenthworth grain-size scale for sediments and the equivalent phi scaleMud

Grain-size (particle-size, granulometric) analysisThe old-fashioned way: direct measurement (gravel) and sieve/pipette analysis (sand and mud) The modern technology: laser particle sizing (sand and mud)Grain size parameters : mean, sorting, skewness, kurtosis

Moment measures (mathematic method)First moment: mean (cf. median, mode)Premier measure of the grain sizex = Grain size parametersSk = Second moment: variance (cf. standard deviation)Measure of the degree of sorting = ( = standard deviation) Third moment : Skewness - Measure of the symmetry of the grain-size distribution Fourth moment : Kurtosis - Measure of the sharpness or peakedness of a grain-size frequency curveK =

Graphical method

Graphic Mean

Standard deviation

Skewness

Kurtosis Grain size parameters(1) = + + (SK1) = (Mz) = (KG) =

SK1 classification :+1,0 - +0,3 very fine-skewed+0,3 - +0,1 fine-skewed+0,1 - -0,1 near-symmetrical-0,1 - -0,3 coarse-skewed-0,3 - -1,0 very coarse-skewed KG classification:3,00 extremely leptokurtic

Grain MorphologyThree 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.

Grain shape classificationRoundness and Sphericity classification

Grain FabricFabric 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

Sedimentary structures occur at very different scales, from less than a mm (thin section) to 100s1000s 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)Sedimentary structuresThe majority of structures form by physical processes, before, during and after sedimentation. Other result from organic and chemical processesAlthough there is no generally accepted classification of sedimentary structures, the four main groups are (a) erosional, (b) depositional, (c) post depositional and (d) biogenic structures (Tucker, 1991)

Laminae and beds are the basic sedimentary units that produce stratification; the transition between the two is arbitrarily set at 10 mmNormal grading is an upward decreasing grain size within a single lamina or bed (associated with a decrease in flow velocity), as opposed to reverse gradingFining-upward successions and coarsening-upward successions are the products of vertically stacked individual beds

Cross stratification

Cross lamination (small-scale cross stratification) is produced by ripplesCross bedding (large-scale cross stratification) is produced by dunesCross-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

Cross stratification

Cross stratification

The angle of climb of cross-stratified deposits increases with deposition rate, resulting in climbing ripple cross laminationAntidunes 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-setPlanar stratification

Planar lamination (or planar bedding) is formed under both lower-stage and upper-stage flow conditionsPlanar stratification can easily be confused with planar cross stratification, depending on the orientation of a section (strike sections!)

Cross stratification produced by wave ripples can be distinguished from current ripples by their symmetry and by laminae dipping in two directionsHummocky 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 bedsFlaser bedding is dominated by sand with isolated, thin mud drapesLenticular bedding is mud-dominated with isolated ripples

Gravity-flow deposits

Debris-flow deposits are typically poorly sorted, matrix-supported sediments with random clast orientation and no sedimentary structures; thickness and grain size commonly remain unchanged in a proximal to distal directionTurbidites, 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

Imbrication commonly occurs in water-lain gravels and conglomerates, and is characterized by discoid (flat) clasts consistently dipping upstream

Sole marks are erosional sedimentary structures on a bed surface that have been preserved by subsequent burialScour 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)

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 structuresSince numerous trace fossils are connected to specific depositional environments, they can be very useful in sedimentologic interpretationsSoft-sediment deformation structures are sometimes considered to be part of the initial diagenetic changes of a sediment, and include: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)

Clastic (siliciclastic) rocks

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)Quartz arenite: quartz-dominatedArkosic arenite: feldspar-dominatedLithic arenite: dominance of lithic fragmentsWacke: significantly matrix-supported (>15% mud)Quartz wackeGreywacke (feldspathic or lithic wacke)

Classification of Sandstone (Pettijohn, 1975)

Clastic (siliciclastic) rocks

Mudstones (60% of the stratigraphic record) are also known as mudrocks or shales and commonly exhibit a distinct fissilityClaystoneSiltstone

Conglomerates are consolidated gravels; breccias are conglomerates with dominantly angular clastsClast-supported conglomeratesMatrix-supported conglomerates

Conglomerate

Siliciclastic rocks classification (Pettijohn, 1975)

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:Biomineralization of CaCO3 by organismsDirect chemical precipitation CARBONATE SEDIMENT AND ROCKSBiogenic carbonate formation occurs by a wide range of organisms (e.g., molluscs, corals, forams, algae, bacteria, and many others)Most organisms initially form unconsolidated carbonate sedimentsCoral reefs and microbial mats (e.g., stromatolites) are examples of more solid carbonate structuresChemical precipitation produces non-skeletal carbonate grains of various sizes (e.g., ooids, pisoids, micrite)

Biogenic carbonate formation occurs by a wide range of organisms (e.g., molluscs, corals, forams, algae, bacteria, and many others)Most organisms initially form unconsolidated carbonate sedimentsCoral reefs and microbial mats (e.g., stromatolites) are examples of more solid carbonate structuresChemical precipitation produces non-skeletal carbonate grains of various sizes (e.g., ooids, pisoids, micrite)CARBONATE SEDIMENT AND ROCKS

Carbonate sand usually consists either of (fragmented) skeletal remains or non-skeletal grainsCarbonate 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, rudstoneOrganically bound framework during formation: boundstoneCARBONATE SEDIMENT AND ROCKS

ORGANIC (CARBONACEOUS) SEDIMENTS AND ROCKS

Peat and organic-rich clastic sediments form in relatively anaerobic (reducing) environments (e.g., mires, lakes, oceans)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 sedimentSapropel: organic-rich marine sedimentCoal consists primarily of solid organic matter; the remainder is known as ashCarbonaceous shales have a lower proportion of solid organic matterOil shales (may be formed in anaerobic lake and marine environments) contain organic matter that can be driven off as liquid or gas by heating

EVAPORITES

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 formationLeast soluble compounds precipitate first:CaCO3 (calcium carbonate)CaSO4 (calcium sulphate: gypsum or anhydrite)NaCl (halite: rock salt)Other, less stable (highly soluble) chloridesDissolved 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 formationLeast soluble compounds precipitate first:CaCO3 (calcium carbonate)CaSO4 (calcium sulphate: gypsum or anhydrite)NaCl (halite: rock salt)Other, less stable (highly soluble) chlorides

VOLCANICLASTIC SEDIMENTS AND ROCKS

Lava (cooled magma flows) produces volcaniclastic sediment upon weatheringPyroclastic material or tephra (ejected particulate material) can be subdivided into different compositional categories:Mineral grainsLithic fragmentsVitric material (volcanic glass or pumice)