Sedimentary rocks and the origin of sedimentary strata

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1 Sedimentary Rocks and the Origin of Sedimentary Strata Basins to Bedding Sedimentary Rocks • Sedimentary rocks are those rocks which form at or near the earth's surface primarily through: • Deposition of weathered silicate material by water, wind, or ice (detrital, clastic, terrigenous) • Direct inorganic chemical precipitation from water • Precipitation by organic processes Sedimentary Rocks • Three end-member types: • T=Terrigenous • Residual and secondary weathering products (siliciclastic) • Allogenic (extra-basinal) origin • A= Allochemical • Chemical or biochemical particles, shell fragments • Authigenic (form within basin) but locally reworked • O= Orthochemical • Primary chemical precipitation from dissolved ions • Authigenic (form within basin of deposition), no reworking IO= Impure orthochemical IA= Impure allochemical Sedimentary Rocks • T: Terrigenous • Most mudrocks, sandstones, and conglomerates • 65% to 75% of sedimentary strata • IA: Impure Allochemical • Very fossiliferous shale, sandy fossiliferous or oolitic limestones • 10-15% of sedimentary strata • IO: Impure Orthochemical • Clay-rich microcrystalline limestones • 2-5% of sedimentary strata • A: Allochemical rocks • Fossiliferous, oolitic, pellet, or intraclastic limestone or dolomite • 10-15% of sedimentary strata • O: Orthochemical Rocks • Microcrystalline limestone, chert, anhydrite, crystalline dolomite • 2-8% of sedimentary strata Sedimentary Rocks: Terrigenous • Terrigenous (clastic, detrital) sediments and rocks • Also called siliciclastic since most particles are silicate mineral grains • Grains created by weathering • Transported by surface processes • Water, wind, ice • Deposited as horizontal, stratified layers in sedimentary basins • Buried and lithified by • Compaction • Cementation Sedimentary Rocks: Allochemical • Allochemical (mainly carbonate) sediments and rocks • Dominantly biologic origin (shells or bones) • Carbonate systems develop where siliciclastic sourcelands are low and/or very distant • The water is shallow marine • Climates are tropical to subtropical Sedimentary Rocks: Orthochemical • Orthochemical (chemical precipitate) sediments and rocks • Dominated by limestones and dolostones of precipitate origin • Also includes evaporites, chert, and iron formations • Precipitate from marine or nonmarine waters due to chemical changes Sedimentary Depositional Environments • In geology depositional environments are defined by processes and products • Physical processes determine: • Grain size, sorting, rounding • Bedding style (including sedimentary structures) and geometry • Biological processes determine: • Fossil content • Biological disruption of original stratification • Chemical processes determine: • Types of minerals formed at the site of deposition and during burial • Study of modern depositional environments used to infer how ancient rocks formed (“present is key to past”) Sedimentary Depositional Environments: Main Types • Continental (above sea level) • Fluvial (stream); stream channel and floodplain • Glacial; direct deposits and outwash • Lacustrine (lake) • Transitional (Continental and Marine) • Delta • Estuary and lagoon • Beach • Marine (below sea level) • Shallow sea (shelf) and reefs • Submarine canyons (submarine “deltas”) • Pelagic environments; abyssal plains 10 Sedimentary Basins • Sedimentary rocks form in basins • Areas of the earth’s surface subject to long term (millions to tens of millions of years) subsidence resulting in space to accommodate sediment (not subject to erosion) 27 Carbonates • Make up 10-15% of sedimentary rocks • Excellent indicators of depositional environments; integral to study of past environments and earth history • Important reservoirs for oil and gas • Carbonates (>50% primary carbonate minerals) ▫ Limestone (CaCO3)  Chemical  biochemical ▫ Dolomite (CaMg(CO3)2)  Chemical 28 Carbonate Sediment: Origin • Most primary carbonate sediments form as biogenic particles in shallow marine environments (secreted as shells of invertebrates and algae) • Warm water (tropical; 30oN to 30oS latitude) • Shallow shelf; within the photic zone (mostly 0.004 mm) Forms by precipitation (often as cement) or recrystallization Allochems • • • Lime mud ( environment) 36 Carbonate Rock Classification • Based on depositional texture (mainly proportion of allochems) • Two main classification schemes: • Folk • % and type of allochem • Micrite vs sparite matrix • Dunham • Abundance of allochems (ratio grains:mud) • Original components bound together • Both overlook some types of carbonates 37 Carbonate Rock Classification: Dunham • Dunham Classification • Texture and allochem type incorporated into classification • Sediment deposited in calm vs agitated waters • Mud-bearing vs mud-free sediment • Grain vs mud support • Original components bound (biologically) • Depositional texture recognizable 38 Carbonate Rock Classification: Dunham • • Presence or absence of lime mud; is there any mud at all Calm waters allow for the accumulation of lime mud and indicates the absence of current induced agitation Grain Support: self supporting framework • • • • fluid circulation, diagenesis Grain kind: standard microfacies types Grain size, rounding, and coating: hydrologic interpretations Biogenically ppt masses bound at time of deposition: • • • • Boundstone organic framework laminations not consistent with gravity (stromatolite) roof over sediment filled cavities 39 Carbonate Depositional Systems • In the warm, clear, shallow water organisms create sediment: • Calcareous algae flourish and generate micrite • Invertebrate animal skeletons accumulate as sedimentary particles (bioclasts) • Also, particles created indirectly by biological or chemical activity • Oolitic, pelletal, and intraclastic allochems are also produced locally, depending on conditions 40 Carbonate Depositional Environments • Generic rimmed carbonate shelf platform – basin margin 41 Collaborative Activity You have two sandstones (Table, handout) A Plot the normalized proportions of Q, F, and L on the ternary diagram B For each sandstone: Classify it (give it a compositional name and indicate arenite vs wacke) Determine the most likely tectonic setting from which it originated, and give your evidence Determine the depositional environment (general - long system, short system; be more specific if you can) in which it most likely formed, and give your evidence You have three carbonates (handout) A Based on the description, for each carbonate: Give it a compositional classification under both the Folk and Dunham schemes (and indicate allochemical vs orthochemical) Describe the depositional environment as best you can and give your evidence [...]... f/q+f+r • R (or L) = r/q+f+r • 7 types of “normal” sandstones • Others = “mineral” arenite, i.e mica-arenite, magnetite-arenite 21 Siliciclastic Rocks: Sandstone • Sandstone composition is tied to source area and tectonic setting • Ternary System for Sandstone classification 22 Siliciclastic Rocks: Mudrocks • Most abundant of all sedimentary rocks • Composed of silt & clay-sized particles • Dominated... well sorted into the end member sizes of sand and clay  Sandstones at the end of the long system are mature quartz arenites 26 Terrigenous Clastic Depositional Environments • Short systems • The siliciclastic source land is proximal to (close to) the basin • Commonly observed in tectonically active regions • Sediments across the entire system are mineralogically and texturally immature • They are generally... based on proportions of: • S (sand; 0.063-2mm) - S (silt; 0.004-0.063 mm) - C (clay; 2 mm) - S (sand) - M (matrix; 30% gravel; indicates high transport energy • Further classification based on composition 18 Siliciclastic Rocks: Sandstone • Basic classification based on proportions of • Mineral grains... sorted and range in size from gravel to coarse sand 27 Carbonates • Make up 10-15% of sedimentary rocks • Excellent indicators of depositional environments; integral to study of past environments and earth history • Important reservoirs for oil and gas • Carbonates (>50% primary carbonate minerals) ▫ Limestone (CaCO3)  Chemical  biochemical ▫ Dolomite (CaMg(CO3)2)  Chemical 28 Carbonate Sediment: Origin. .. carbonate, organic matter, others • Composition modified by diagenetic processes • Variable color • Gray-black = presence of organic matter • Red-brown-yellow-green = oxidation state of Fe 23 Siliciclastic Rocks: Mudrocks 24 Siliciclastic Rocks: Conglomerates • Coarse-grained siliciclastic rock with muddy or sandy matrix • Gravel >30% of grains • Provenance easily determined by composition of clasts • Main types:... Siliciclastic Rocks: Texture • Grain size and sorting • Statistical/graphic presentation of texture • Quantitative assessment of the % of different grain sizes in a clastic rock • Mean: average particle size • Mode: most abundant class size 16 Siliciclastic Rocks: Texture • Grain size, sorting, and roundness – interpretation: • Textural Maturity • Kinetic energy during transport and reworking • Transport history... Sedimentary Basins • Basins occur in a wide range of tectonic settings  Cratonic settings:  Michigan basin  Convergent plate setting and active plate boundaries: Terrigenous Clastic Basin  Puget trough  Divergent plate boundaries:  Passive; Atlantic coast basin  Rift Basins; East African Rift Carbonate Basin 12 Sedimentary Basins and Rocks • Simple model and classification 13 Siliciclastic Rocks: ... material filling spaces between grains • Arenite = 15% matrix • “Dirty” sandstone 19 Siliciclastic Rocks: Sandstone • Many classification schemes, but most based on relative proportions of framework grains • Relative abundance a function of mineral grain’s  Availability, Chemical Stability, Mechanical Durability... • Feldspar: • K-spar (sandine, microcline), Plag (Na-Ca) • Abundant and somewhat stable (often altered) • Rock (Lithic) Fragments: • All kinds (including limestone/dolomite RF’s) • Abundant, less stable (depending on dep conditions) • Also accessory (minor abundance) “heavy” minerals 20 Siliciclastic Rocks: Sandstone • Classification based on normalized (relative proportions) of • Q = q/q+f+r • F =... (bioclasts) • whole microfossils, whole megafossils, broken shell fragments • Marine invertebrates: algae, forams, corals, bryozoans, brachiopods, gastropods, mollusks, ostracods, etc • Standard microfacies (fossil fragment type -> environment) 36 Carbonate Rock Classification • Based on depositional texture (mainly proportion of allochems) • Two main classification schemes: • Folk • % and type of allochem •
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