1 Sedimentary Rocks and the Origin of Sedimentary

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 • 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

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 nonmarine 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: 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: 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 = -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. )

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 -2 mm) - 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 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 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 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 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 (Ca. CO 3) Chemical biochemical ▫ Dolomite (Ca. Mg(CO 3)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; 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)

29 Carbonate Rock Constituents • Carbonate rocks mainly composed of: • Micrite • Sparite • Allochems • • Lime mud (<0. 004 mm) Largely fragmental algae remains, also chemical precipitate Crystalline carbonate material (>0. 004 mm) Forms by precipitation (often as cement) or recrystallization 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 • 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 intraclastic allochems are also produced locally, depending on conditions

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

41 Collaborative Activity 1. 2. 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 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