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Sedimentary Rocks and the Origin of Sedimentary Strata

Sedimentary Rocks and the Origin of Sedimentary Strata

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Sedimentary Rocks and the Origin of Sedimentary Strata

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  1. Sedimentary Rocks and the Origin of Sedimentary Strata Basins to Bedding

  2. 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

  3. Sedimentary Rocks • 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 • Three end-member types: IO= Impure orthochemical IA= Impure allochemical

  4. 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

  5. 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 Sedimentary Rocks: Terrigenous • Deposited as horizontal, stratified layers in sedimentary basins • Buried and lithified by • Compaction • Cementation

  6. 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: Allochemical

  7. 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 non-marine waters due to chemical changes Sedimentary Rocks: Orthochemical

  8. 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”)

  9. 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)

  11. Basins occur in a wide range of tectonic settings Cratonic settings: Michigan basin Convergent plate setting and active plate boundaries: Puget trough Divergent plate boundaries: Passive; Atlantic coast basin Rift Basins; East African Rift Sedimentary Basins Terrigenous Clastic Basin Carbonate Basin

  12. Simple model and classification Sedimentary Basins and Rocks

  13. Siliciclastic Rocks: Components • F-M-C-P • Framework Grains • >0.05 mm allogenic mineral grains, rock fragments • Residual from weathering • Detrital Matrix • <0.05 mm (clay, quartz, feldspar, carbonates, organics, oxides) • Chemical weathering products • Cement • Authigenic, post-depositional orthochemical component • Precipitated from circulating pore fluids (silica, carbonate, Fe-oxide, clay, feldspar, other oxides, zeolite, salts) • Pores; • Primary (~40%) or secondary due to leaching/dissolution • Classification based on (1) texture, (2) composition

  14. Siliciclastic Rocks: Texture • Descriptive Textural Classification • Grain Size • Uden-Wentworth grain size scale • Phi = -log2 (grain diameter in mm) • naturally occurring groups • Gravel ~ rock fragments • Sand ~ individual mineral grains (particulate residues) • Mud ~ particulate residues +/- chemical weathering products • Clay ~ chemical weathering products (clay minerals, etc.)

  15. 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 • Dispersal patterns • Beware: • Mixed sources • Biogenic reworking

  17. Descriptive textural classification based on proportions of: S (sand; 0.063-2mm) - S (silt; 0.004-0.063 mm) - C (clay; <0.004 mm) Sandstones, siltstones, and shales G (gravel; >2 mm) - S (sand) - M (matrix; <0.063 mm) Conglomerates and breccias >30% gravel; indicates high transport energy Further classification based on composition Siliciclastic Rock Classification

  18. Siliciclastic Rocks: Sandstone • Basic classification based on proportions of • Mineral grains (dominantly quartz) • Matrix (clay to silt-sized clastic material filling spaces between grains • Arenite = <5-15% matrix • “Clean” sandstone • Depositional agents that sort sediment well • Wacke = >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 • Anything Possible, most common: • Quartz : • monocrystalline, polycrystalline; ig, met, or sed source • mechanically & chemically stable, abundant • 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 = 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 by clay minerals (kaolinite, smectite, illite) • Also quartz, feldspar, 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: • Conglomerate: rounded clasts in sandy matrix • Breccia: angular clasts in sandy matrix • Diamictite: clasts in muddy matrix

  25. Long systems Complex association of depositional environments through which clastic sediment is transported and in which some sediment is deposited End product is relatively “mature” sediment Sediments are chemically and mechanically stable in composition (high temp, unstable minerals are not present) Sediments are well sorted into the end member sizes of sand and clay. Sandstones at the end of the long system are mature quartz arenites Terrigenous ClasticDepositional Environments

  26. 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 poorly sorted and range in size from gravel to coarse sand Terrigenous ClasticDepositional Environments

  27. 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 Carbonates

  28. 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 <10-20 m) Also accumulate in deep water (pelagic oozes) Inorganic precipitates from sea water also occur Can form in continental settings (lacustrine, desert, soil, springs) Carbonate Sediment: Origin

  29. Carbonate Rock Constituents • Carbonate rocks mainly composed of: • Micrite • Lime mud (<0.004 mm) • Largely fragmental algae remains, also chemical precipitate • Sparite • Crystalline carbonate material (>0.004 mm) • Forms by precipitation (often as cement) or recrystallization • Allochems • Transported chemical or biochemical precipitates (fragmental material) • Include intraclasts, ooliths, peloids, and bioclasts • Biolithic elements • Formed by organisms in situ • Bound together by precipitated material

  30. Carbonate Rock Constituents • Micrite: • Microcrystalline calcite particles of clay (<1-4 micron) size (subtranslucent matrix) formed by • Chemical or biochemical ppt • Abrasion of allochems • Can imply deposition in a low energy environment just like in terrigenous mudstone

  31. Carbonate Rock Constituents • Sparite (cement): • Clear granular (“sugary”) carbonate crystalline orthochemical material • Formed in interstitial pore spaces of carbonate sediment • Cement in pores indicates original void space • Also commonly forms during diagenesis • Recrystallized allochems or micrite

  32. Carbonate Rock Constituents • Allochems: Intraclasts • Reworked, early lithified carbonate fragments • Irregularly-shaped grains that form by syndepositional erosion of partially lithified sediment • Can indicate high energy environment or storm

  33. Carbonate Rock Constituents • Allochems: Ooliths • Concentrically laminated carbonate structures • Oolites - <2 mm in diameter • Thought to be abiogenic in origin • Layers precipitated onto a grain during wave agitation • Pisolites - same as oolites, but >2 mm • Oncolites - spheroidal stromatolites (> 1-2 cm)

  34. Allochems: Pelloids silt to fine grained, sand-sized carbonate particles with no distinctive internal structure most thought to be fecal pellets Carbonate Rock Constituents

  35. Allochems: Skeletal particles (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) Carbonate Rock Constituents

  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. In the warm, clear, shallow water organisms create sediment: Calcareous algae flourish and generate micrite Invertebrate animal skeletons accumulate as sedimentary particles (bioclasts) Carbonate Depositional Systems • Also, particles created indirectly by biological or chemical activity • Oolitic, pelletal, and intraclastic allochems are also produced locally, depending on conditions

  40. Generic rimmed carbonate shelf platform – basin margin Carbonate Depositional Environments

  41. 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) 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 Collaborative Activity