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

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Presentation on theme: "Sedimentary Rocks and the Origin of Sedimentary Strata Basins to Bedding 1."— Presentation transcript:

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

2 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 2

3 Sedimentary Rocks Three end-member types: 3 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

4 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 4

5 Sedimentary Rocks: Terrigenous Terrigenous (clastic, detrital) sediments and rocks Also called siliciclastic since most particles are silicate mineral grainssiliciclastic Grains created by weathering Transported by surface processes Water, wind, ice 5 Deposited as horizontal, stratified layers in sedimentary basins Buried and lithified by Compaction Cementation

6 Sedimentary Rocks: Allochemical Allochemical (mainly carbonate) sediments and rockscarbonate Dominantly biologic origin (shells or bones) Carbonate systems develop where siliciclastic sourcelands are low and/or very distant Carbonate systems The water is shallow marineshallow marine Climates are tropical to subtropical 6

7 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 non- marine waters due to chemical changes 7

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”) 8

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 9

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) 10

11 Sedimentary Basins 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 11 Terrigenous Clastic Basin Carbonate Basin

12 Simple model and classification 12 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 13

14 Siliciclastic Rocks: Texture Descriptive Textural Classification Grain Size Uden-Wentworth grain size scale Phi = -log 2 (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.) 14

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 15

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 16

17 Siliciclastic Rock Classification 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 17

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 18

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 19

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 20

21 Siliciclastic Rocks: Sandstone Sandstone composition is tied to source area and tectonic setting Ternary System for Sandstone classification 21

22 Siliciclastic Rocks: Mudrocks 22 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 23

24 Siliciclastic Rocks: Conglomerates 24 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 Terrigenous Clastic Depositional Environments 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 25

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 poorly sorted and range in size from gravel to coarse sand 26

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 (CaCO 3 )  Chemical  biochemical ▫ Dolomite (CaMg(CO 3 ) 2 )  Chemical 27

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; 30 o N to 30 o S 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) 28

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 29

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 Implies deposition in a low energy environment just like in terrigenous mudstone 30

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 31

32 Carbonate Rock Constituents Allochems: Intraclasts Reworked, early lithified carbonate fragments irregularly-shaped grains that form by syndepositional erosion of partially lithified sediment 32

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) 33

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

35 Carbonate Rock Constituents 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) 35

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 36

37 Carbonate Rock Classification: Dunham Dunham Classification Texture and allochem type incorporated into classification Sediment deposited in calm vs agitated waters 37 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 38

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) 39 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 40

41 Collaborative Activity 1.You have two sandstones (Table, handout) A. Plot the normalized proportions of Q, F, and L on the ternary diagram. B. For each sandstone: 1.Classify it (give it a compositional name and indicate arenite vs wacke) 2.Determine the most likely tectonic setting from which it originated, and give your evidence 3.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 2.You have three carbonates (handout) A.Based on the description, for each carbonate: 1.Give it a compositional classification under both the Folk and Dunham schemes (and indicate allochemical vs orthochemical) 2.Describe the depositional environment as best you can and give your evidence 41

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