Sedimentary Rocks and the Origin of Sedimentary Strata

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

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

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. 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

5. 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

6. 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

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

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. 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 Terrigenous Clastic Basin Carbonate Basin

12. Sedimentary Basins and Rocks Simple model and classification

13. Siliciclastic Rocks: Components • F-M-C-P • Framework Grains • >0.05 mm allogenic mineral grains, rock fragments • Residual from weathering • DetritalMatrix • <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. 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

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. 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 Siliciclastic Rocks: Mudrocks

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. TerrigenousClasticDepositional 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

26. TerrigenousClasticDepositional 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

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 <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)

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 calciteparticles 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

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

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

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)

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 intraclasticallochems 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) • 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