Sedimentary Rocks and the Origin of Sedimentary Strata

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

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

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 3

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

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 5

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 6

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

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

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

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

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 11

Sedimentary Basins and Rocks • Simple model and classification 12

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, Feoxide, clay, feldspar, other oxides, zeolite, salts) – Pores; • Primary (~40%) or secondary due to leaching/dissolution • Classification based on (1) texture, (2) composition 13

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

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

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

Siliciclastic Rock Classification • Descriptive textural classification based on proportions of: – S (sand; 0. 063 -2 mm) - S (silt; 0. 0040. 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

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

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

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

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

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 22

Siliciclastic Rocks: Mudrocks 23

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 24

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 25

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

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 27

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

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

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

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

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

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

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

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

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

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 37

Carbonate Rock Classification: Dunham 1. 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 2. Grain Support: self supporting framework • fluid circulation, diagenesis 3. Grain kind: standard microfacies types 4. Grain size, rounding, and coating: hydrologic interpretations 5. Biogenically ppt masses bound at time of deposition: – – Boundstone organic framework laminations not consistent with gravity (stromatolite) roof over sediment filled cavities 38

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 39

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

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 41 your evidence