Review for EAS 336(Full Year Notes).pdf

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Earth and Atmospheric Sciences
Fred Clark

Review for EAS 336 Detrial constituents: Clasts: allogenic- are epiclasts source constituted elsewhere that has been transported authogenic- are grown in place(in situ), that is they crystallize can be used to define textural maturity occur as cement or replacement -may be recognized by the following criteria: a. tend to be smaller than framework grains b. may be euhedral or have intricate outlines that wouldn‟t survive transport c. exhibit replacement textures e.g. transect grain boundaries, embayments, begin as euhedral crystals then begin to cut into previous grains d. show cement textures like syntaxial rims or drusy textures (void-filling textures) stability temperature of crystallization in Bowen‟s Reaction Series as an indicator of chemical stability of the silicates at atmospheric T/P Oxide(weathering product)/cement stability: silicate>carbonate>sulphides/halides Stability(compositional maturity):Q>F >muscovite>biotite (K15%Fe O ) 2i3erals of several structure types that oxidize to a brownish color. They tend to occur as grain coatings, peloids (occasionally oolitic), microfossil internal molds, microcrystalline micaceous texture -consensus for origin: environmental indicator most common in anoxic diagenesis of sediment deposited on oxegenated aerobic sea floor, thus the iron contente.g. foreland basin E. Iron Oxides and Hydroxides -hematite and limonite form grain coatings/stains to impart distinctive colours to the rocks A. Silica minerals -most abundant minerals in most sandstones; estimated at 65%, dominated by Q  the shape of Q grains can be disguised by syntaxial authogenic overgrowths which have optical continuity to host while hematite preserve outlines and tend to meet at straight non irregular boundaries irregular suturing exhibited by pressure solution during diagenesis(extreme interlocking supports authogenics) or possible euhedral crystal growth cements as chert and/or chalcedony, cryptocrystalline, as void filler with outlines indicating such Silica has many forms such as, chert with a different structure than Q, also quartz grains tend to be subangular, but! Smaller rounder grains are characteristic of aeolian or multicycle(beach grains) B.Feldspars Are the second most common constituent of sandstone (10-15%)They lack overgrowth(they are unstable in pore solution) while hematite preserve outlines C.Micas -platy habit(preferential settling for sphere) with one perfect cleavage, rarely comprise >2% of framework grains abundance -muscovite>biotite>chlorite -influence of shape on settling velocity, and grain size is relative to accompanying quartz and feldspar grains (hydraulic equivalence) and tend to be coarser as well D. Clay minerals 1)-occurrence in matrix rather than framework size range for sandstones; more commonly authigenic *kaolinite clusters almost certainly authigenic, as replacement of feldspar grains 2)- cant be resolved under a petrographic microscope 3) -typically <0.03 mm(below coarse silt), matrix sized for sandstones E. Heavy minerals -G>2.85 -opaque and non-opaque -zircon, tourmaline, and rutile comprise the most common and stable heavy minerals thus the ZRT index is an indicator of maturity of siliciclastic rocks F. Lithic fragments - comprise 10-20% of the average sandstone, and are denoted by L  quatz grains with tiny crystal size and or lack of crystallinity(micro- crypto) are classified as lithics, also chert, opal(occurs as secondary cement), chalcedony, volcanic rocks, mudrocks, and slates/phyllites/schists - particularly useful for provenance(source) * felsic lithics tend to be more common than mafic ones G. Organic constituents -Skeletal fragments, black-brown -characteristic low concentrations in sandstones due to coarse grain size and typical oxygenated substrate Framework grains vs matrix and cement Observation error -danger of overestimation of grain dispersion in thin section due to random relationship between section plane(t.s. cut) and grain contacts(orientation)  intact framework may appear dispersed, that is a framework intact may appear as if it is dispersed Framework supported is considered as intact, where the sediment is grain or clast supported) Matrix supported  is considered dispersed, where the sediment is mud or matrix supported -relationship to grain surfaces as a key distinguishing feature (matrix- framework) -detrial clay matrix grains likely to have faces // to host grain surfaces -relationship to grain surfaces as a key distinguishing feature(cement to framework) cements -clay cements usually with faces normal to host grain surfaces, or as boxworks (kaolinite booklets are exceptional) Framework grains -conventional definition of framework, contain all sand sized grains(>0.0625mm), also include coarse silt fraction as well (>0.03mm) -Problem(is the hydraulic incompatibility, thus current that allows framework deposition might maintain transport of matrix grains)*holmstrum curve, with possibility of infiltration by matrix grains between framework grains due to low pressure zones created in grain-grain spaces or by diagenetic alteration TEXTURES IN SILICICLASTICS CONGLOMERATES - Section 5.3, p. 135-139 Boggs 4e 1. Terminology - “rudite” as a synonym for “conglomerate”; they are siliciclastic rock with a significant proportion of clasts >2mm Dichotomy of rudite family rudite conglomerate rounded clasts orthoconglomerate clast supported diamictite matrix-supported Breccias angular clasts a) volcanic breccias – pyroclastic, volcaniclastic, autobrecciation of congealed lavas. Or quenching b) cataclastic breccias – rocks are pulverized between blocks displaced along faults c) slump breccias – produced by downslope movement of what are generally unlithified sediments d) solution/collapse breccias – dissolution(carbonate or evaporate) of part of sedimentary succession removes support for remaining material, which collapses into the voids e) impact breccias – bolide impact produces shattered rock; may fall back hence “fall-back breccias” -production of breccias is frequently non- sedimentary ex. talus piles 2. Composition -// to sandstones -effect of coarser clast size on classification of lithologies range based on types of lithic fragments, ie. a naming modifier 3. Textures -matrix comprises sand-sized or smaller -rounding of gravel-sized grains is more rapid compared to sand and finer sizes of the same material. Elongate clasts tend to orient transverse to current(long face of grain is normal to current). including imbrications where tops are tipped in direction of current, another cause can be waves -water tends to round and sort clasts(holmstrum curve), producing a clast supported conglomerate(orthoconglomerate) eg. Cadomin Fm  stelk “if the chert turns green its marine” -ice‟s effects on clasts may produce rounding, faceted striations, but doesn‟t sort due to its limitless competence; matrix-supported conglomerates (diamictites) as suggestive but not diagnostic of ice transport, need striated clasts to determine 4. Classification - Just look at the clasts and their contacts MUDROCKS/SHALES - Section 5.4, p. 139-145 Boggs 4e 1. Terminology -importance of mudrocks is that they comprise ~50% of all sedimentary rocks  desirability of “mudrock” as a term as opposed to shale because there is no textural baggage with mudrock -synonyms for “mudrock” lutite –part of the rudite-arenite-lutite trio  pelite-part of the pelite-psammite duo argillite-though this term is restricted to well-compacted and indurated(hard) mudrocks that may represent onset of metamorphism -dichotomy between shales (Boggs Figs. 5.10A, 5.11) and mudstones (Boggs Fig. 5.10B)  shales are laminated(<1cm thick) and often fissile as a result, need anoxic waters  mudstones are more thickly bedded(>1cm), thus blocky, need oxic to floc, its so floxic, whahahahahah!!!!!!!! 2. Textures -grains tend to have low sphericity and are angular except for coarse and medium silt sized grains, form doesn‟t change much in transport -distinguishing characteristics of mudrocks may be produced by fabric or related color  dispersed // grains produce fissile shale(dark color, fine grained)  flocs with random grains produce blocky mudstone(lighter color, coarse grained) they can be classified according to thickness of stratification(bulk) and development of parting(fabric) [Table 5.7 Boggs 4e] Fissility conventional explanation for development of fissil parting occurs under anoxic conditions, with individual settling of grains conventional explanation for development of blocky/slabby parting occur under oxic conditions, grains settle as flocs, with random orientation -modern perspective observes that flocculation of clays is likely in salt water; virtually all clays will flocculate; this effect may begin at concentrations as low as 0.5-1.0‰ (normal seawater is ~34.5‰)  dissolved ions balance out the repulsive forces between clay surfaces flocculation is also more frequent in smaller grains and also at increased suspended sediment concentrations(saturation) current argument that mudrocks originate as flocs; but under anoxic bottom conditions the flocs dispers to allow // settling of grains to develop fissility role of anoxia in development of fissility rather than blocky texture Note! anoxic bottom conditions also limit bioturbation  if sediment is not yet lithified, compaction can create fissility (compaction of flocs) 3. Composition and Colour -they tend to be rich in Al, K (clay constituents), and also may include Fe, whose oxidation state controls colour when present Fe Produces red  Fe produces green in the absence of significant organic carbon organic content: as sapropel, is the remains of micro-organisms or their reproductive bodies  sapropel is altered as a possible source of kerogen, the precursor to oil and natural gas Note! also oil shales, in which kerogen has not been matured or altered to produce hydrocarbons(requires immense thermal energy input to create fossil fuels) Preservation factors:  oxidation state: of the depositional environment (anoxic is best) Presence/absence of H S indicates lack of oxygen 2 Sedimentation rate: high sedimentation rate carries organic matter below the oxygen fence, before decay - rate of organic productivity and accumulation biochemical oxygen demand of accumulating organic matter depletes O , 2 allowing preservation of the remains 1 organic matter reduces iron, so high carbon correlates with black to grey shales 2 we thus see common occurrence of FeS in black2shales eg. Framboid(berry shaped pyrite accumulations generalizations: a) black shales  organic rich typically deposited in deep(abyssal plain), reducing environments(anoxic), stagnant settings, or areas of prolific organic activity(swamps) b) red shales favoured in continental to very shallow marine/supratidal settings, where sedimentation rates are very low 4. Distribution/Environments -limitations on accumulation of loess in the rock record is due to: it is deposited in non-aquatic settings, but generally lacks cement and is above erosional base level - environments favouring mudrock deposition 1) lakes(moderate extent) 2)flood plains(smallest extent) 3) some transitional marine environments(lagoon) 4) deep marine settings(most laterally extensive of deposits) Note! evidence of subaerial exposure in the form of mudcracks, rain prints, roots, and root casts DIAGENESIS OF SILICICLASTICS 1. Terminology -diagenesis: comprises all of the changes that act to modify sediment after deposition, short of metamorphism diagenetic depth range is from surface to 10km or more, depending on geothermal gradient (Boggs Fig. 5.13) 0  diagenesis Temperature limit may be the 200-300 C range (T of chlorite formation) -framework for diagenesis courtesy Choquette and Pray (1970); they recognized three stages of diagenesis (Boggs Fig. 5.14, Table 5.8): a) eogenesis (“dawn”)the time of early burial - in the depositional environment, thus influenced by  at or very near sed/water interface or sed/air non-marine settings arid and oxidizing to wet and reducing conditions marine settings marine pore waters range from oxidizing to reducing, based on oxygen fence b) mesogenesis (“middle”): time of deeper burial; begins when sediments are buried below the influence of the surface environment - increase in temperature and pressure results in modified pore fluid chemistry  T increases the reactions to accelerate chemical modification(solubility, carbonates solubility decreases)  results of large-scale, deep circulation of water can led to calcite or silica cement growth, depends on water chemistry c) telogenesis (“end/completion”): uplift and erosion, sediment exhumed; thus there is a return to near- surface T/P; causing circulation of usually oxidizing, metoric waters of low PH or the onset of metamorphism due to extensive burial 2. Biological and Physical Diagenesis - biological influence – biological influences are significant! grain sorting, burrow casts, cementation of burrow walls, destruction of fabrics(laminations), alteration of permeability….. Physical diagenesis -results of compaction – is the primary physical diagenesis  the grains are rearranged, bulk volume and porosity is reduced, and usually leads to reduced permeability resultant structures in fluid movement are sedimentary flame structures (differential compaction), escape structures Cause - increased geostatic pressure/lithostatic load(sediment pressure) Increased burial causes an increase in grain compaction and grain fracture, thus increasing number of contacts(inhibited by presence of matrix and early formations of cement)) evidence of compaction can be seen in deformation of plastic, or elastic grains(muscovite, biotite)  differential compaction in mudstone(up to 80%) >>sandstone effect- increase in porosity of sandstones due to fracture  >40% to <30%  in absence of matrix we go from point to long contacts leads to pressure solution formation of sutured contacts and styolites(soluble removed, leaving insolubles) 3. Biochemical and Chemical Diagenesis A. Reactions in the Eogenetic Zone i) Marine Sediments -the dominant process is interaction of bacteria with organic compounds -(below sediment/water interface) 1  bacterial oxidation occurs until O i2 depleted 2  anoxic reducing conditions produces sulphate reducing bacteria and low PH until the lower limit of sulphate seawater diffusion is reached~10m  this leads to fermentation that produces methane below the level of sulphate reduction 3  deeper into the zone leads to precipitation of quartz, feldspar overgrowths, and carbonate cements unstable minerals dissolve(aragonite, and HMC) ii) Non-marine Sediments -dominantly fresh/meteoric waters a)  end up with reduction of ferric oxides b) less common organic influence due to higher level of sed- water interface oxidation c) significant generation of clay minerals, iron oxides, and calcite B. Reactions in the Mesogenetic Zone -increased T usually leads to increased reaction rates and solubility except! in carbonates, thus Fe, Mg enter lattice and form dolomite, and ankerite instead -increased geostatic pressure a) porosity is reduced b) dissolution of silica with evidence of stylolites(soluble minerals are removed leaving structures composed of insoluble minerals) c)thermal maturation produces kerogen, 120 C 160 C generates CO2 C. Reactions in the Telogenetic Zone - rocks return to near surface environment; reduced T and P, which leads to common dissolution and alterations to take place, as well as a usual return to low saline, PH, and oxidizing state 4. Chemical Diagenetic Processes A. Cementation i) Carbonates - most common of quartz arenites and less so in wackes(clay dominant) -high conc. Of Ca and CO prec3pitate cement, but high CO dissolve2 the cement(balancing act) - effective carbonate cementation requires an adamant supply of Ca+-Mg+-Fe -carbonate cement textures may be mosaic, drusy, single-crystal or poikilotopic ii) Silica -most commonly as syntaxial rims on quartz grains; opal, chert, and chalcedony are more frequent in lithic arenites and wackes (Boggs Fig. 5.15C) requires 0 high silica saturation for cements(T~80 C and a few km‟s deep is ideal for rapid silica cementation)(mesogenesis) - result of upward convective movement of silica-saturated(sourced from pressure solution, weathering reactions....) basinal waters may cause precipitation of silica cement(since it decreases in T and P which makes pore fluid increasingly less silica soluble, thus leading to silica precipitation) -opal  requires extreme silica saturation (12+ppm); most common in volcanogenic sections with abundant glass -chert  unlikely to form on its own, but my form through transformation of opal iii) Others -gypsum and anhydrite as cements near evaporitic environments; gypsum dehydrates in 700m-1000m burial depth -limonite and goethite age or dehydrate to hematite as cements -feldspars and clays provide the starting materials for authigenic overgrowths of feldspar on feldspar host grains B. Dissolution -Interstitial solution affects less stable grains -forms secondary porosity reservoir C. Replacement -guest minerals replace host, thus preserving internal structure and detail, but not a cast mold(opposite) -seemingly endless possibilities Siliciclastic depositional environments Alluvial fans -convex up structures, upper fan is characterized by deeply entrenched single channel that periodically switches due to clogging; mid fan typically has anastomosing channel flow and design; the fan base are typically characterized by sheet flows -streamflow conglomerates tend to display imbrications -other potentially distinctive features may include: - lack fossils except plants and vertebrates - compositionally immature i.e. lots of feldspars, micas - low roundness of grains, i.e. angular - common oxidation Desert systems 1. Processes - arid conditions generally have two consequences: 1. lack of soil moisture or cohesion(sand castles on the beach) 2. lack vegetation, which would otherwise be a windbreak, and bind soil *a desert floor is prone to erosion -due to limited competence of air, maximum grain size is usually medium sand, but low viscosity/high turbulence causes high erosion to right grain sizes -saltation is the primary source of transportation , causing dislodging and entrainment of grains below the threshold -Quartz is the dominant, and tends to be characterized in desert settings by having frosted/etched grains with cresentric impact marks -uncommon carbonate and gypsum(only near sabkas) - deflation wind winnows fines leaving lag(desert pamement), stabalizing desert floor and leading to an erosional feature of reverse grading -sand blasting creates ventifacts and faceted clasts 2. Sand Dunes Caused by bedload movement to brink point to form flows that produce tabular features with angle of repose~30 -35 , in succession these flows produce cross beds in many dune forms [Barchan, Transverse, parabolic, longitudinal(seif), star(draas) dunes] 3. Recognition of Desert Deposits - in general, desert deposits do have: i. distinctive cross beds of large scale and unusually steep angle of repose(30 +); these are unknown in any aqueous environments (Boggs Fig. 8.21) ii. ripples of high wavelength: amplitude (ripple index) iii. distinctive frosted quartz grains with crescentic impact marks iv. characteristically fine to very fine sand, moderately to very well rounded, well sorted, almost pure quartz, and at least initially poorly cemented *may have terrestrial tracks or vertebrate remains(most commonly in playa deposits) *rare to find aqueous biota, pebbles(only as lag), clays(except loess), and channels(other than from floods) 4. Playa Lakes -ephemeral bodies of water(come and go) -we may see thin limestone, dolomite, gypsum and/or anhydrite deposits, ± mudstones ± mudcracks -freshwater aspect also a possibility; usual suspects of fossils are pelecypods, gastropods, ostracods, diatoms GLACIAL SYSTEMS 1. Ice Movement and Related Features - ice has limitless competence as a transport medium - rock flour is produced by en route milling or abraision makes fertile soil -polished bedrock and striations(indicate direction) are caused from friction of entrained sediment -chattermarks are unidirectional with ice movement, caused by the stress of large entrained clasts “bouncing” 2. Glacial Drift A. Till – this is deposited directly from ice as it melts; is poorly sorted, non- stratified, also known as diamictite/tillite composes many types of moraines B. Ice-Contact Stratified Drift – meltwater moving in contact with the glacier stratifies and sorts sediment that had been deposited directly from the ice; eskers(sub glacier ridge) and kames(conical mound from flow of super glacial water flow) C. Outwash – these are basically braid-plain deposits; have high hydraulic gradient D. Lacustrine/Glaciolacustrine Sediment – there are some marginal lacustrine fans and deltas, but the most distinctive deposits in periglacial lakes are varves - varves glaciolacustrine (c.g. sediment light{summer); f.g. sediment dark{winter}) E. Aeolian Sediment – given the enormous quantity of rock flour generated by a glacier, once that sediment is deposited, and before it is stabilized by vegetation, it may be wind-blown, as loess (e.g. China) F. Glaciomarine Sediment – lots of sediment is carried to the sea by glaciers, some of which is lost directly from the base of the ice as it moves out across marine waters (Boggs Fig. 8.30) distinctive drop stones that deform fine underlying sediment layers LACUSTRINE SYSTEMS 1. Introduction - large range of depths, and widths, that range from fresh to saline water 2. Physical Processes - primary source of energy input is wind(tidal is minimal) -rivers produce deltas and introduce sediment, as with marine deltas, water density difference causes different sed distribution -density of lake water is stratified, with sequential density inversion causing overturn, thus cycling nutrients with oxygen * varves are possible also, same goes for carbonate rich lakes -lakes differ from marine settings in the following ways: a) reduced total energy, coarse grains restricted strongly to the margins b) there are higher sedimentation rates than in marine settings, perhaps by as much as a factor of ~10X, because they are often closed deposystems (i.e. no sedimentary bypass), eventually fill and display increased energy level and coarsening upward c) lack of tides, therefore poorly developed littoral zones( between mean high tide and mean low tide) 3. Physicochemical Processes - often carbonate in humid regions(meteoric carbonate rich waters) -in arid conditions diagnostic additions of trona(Na-carbonate), borax(Na B hydoroxide), esponite(hydrated Mg-sulfate), and boloedite(Na-Mg sulfate) evaporate products 4. Organic Processes - with experience, one observes distinctive organisms, especially freshwater diatoms, pelecypods, gastropods, ostracods, even calcareous algae, and fish, plus burrowers 5. Characteristics of Lacustrine Deposits -coarse narrow margins, with much finer main body sediment -interflow waters  causing pelagic(settling) deposition of sands and finer-grained material -main body typically displays laminations - juxtaposition/interbedding of carbonates with kerogen-rich materials, that is kerogen rich sediments in marine settings tend to be deposited in deep waters, ex. Is the Eocene Green River Formation * varves are characteristic as is fine laminations as they are more laterally extensive than flood plain deposits, but less so than marine 6. Recognition of Lacustrine Deposits i) tectonic setting grabens, extensional ii) close stratigraphic associations with fluvial or other non-marine deposits (Boggs Fig. 8.24&8.25) iii) lateral continuity of fine-grained sediments >rivers,2mm Configuration: Tangential-aligned with c-axis// to laminae Radial- aligned with normal axes to C Random- no structure *recrystalization may obliterate fine structure leaving relics of concentric layering *complete dissolution my leave oomoldic porosity possible clue to oomold is presence of insoluble cortex odd-center( that is everything was removed then reset in place with foundation grain off center) ii) Oncoids -irregular shaped grains with irregular laminae coating nucleus; <2mm to >10mm(usually larger than ooid) related to activity of algae, cyanobacteria… *if produced specifically by the coating of some nucleus by red algae, are referred to as rhodoids C. Peloids -polygenetic, structureless, and variably shaped (spherical to ovoid to rod shaped grains) that are typically sand sized, dominantly micrite(thus dark in thin section) i) Fecal Pellets -invertebrate deposit feeder product, usually bound by mucous, easily deformed -prevalent in quiet, lime mud environments, and tend to be well sorted and of regular shapes ii) Micritized Grains -ooids, skeletal grains, or others may be degraded to micrite iii) Intraclasts -of course, not true peloids, but may become well rounded because they are so soft before lithification iv) Algal -may serve as nuclei for encrusting by fine (few µm) aragonite or calcite needles *these latter three types of peloids tend to be less well sorted and more irregular in shape D. Aggregate Grains -two or more carbonate grains of any type bound and cemented together; initial binding by algae and encrust foraminifera, followed by micrite infilling of the pore space -grapestone term is used when grain has appreciable surface relief as opposed to lumps which is used once surface relief is compromised by accretion of material E. Skeletal Grains -dominated mineralogically by calcite (including high-magnesian calcite [HMC], which according to most sources has >4 mol % Mg, at the other end of the spectrum is low-magnesian calcite [LMC], with <4 mol % Mg) and aragonite, and shows great structural variety as well 2. Micrite -may be referred to by its full name, microcrystalline calcite, and also lime mud, which by simple definition would be carbonate materials finer than 1/16 mm or 62 μm (Boggs Fig. 6.3) -typically aragonite in modern environment -possible sources for micrite, may be in-situ(post depositional/diagenetic); environmental significance of its presence doesn‟t necessarily imply low energy conditions -environmental significance of its absence doesn‟t imply high energy conditions, it may have been eliminated by recrystalization to neomorphic spar(coarse crystalline) -principal origins of micrite are as follows: A. Direct Precipitation -calcite and aragonite do not precipitate freely in the presence of Mg in seawater, role of organic mediation provides the limitations to precipitation of CaCO , principally the consumption of CO in photosynthesis 3 2 - it appears to form primarily as a cement(drusy calcite) or grain coating(becomes ooid...) i.e. on host grains B. Calcareous Algae - a number of taxonomic groups of algae secrete/metabolize calcite, usually HMC, or aragonite crystals in the micrite size range (<10 μm) - decomposition of tissues upon death releases disaggregated aragonite (plus other forms of CaCO ) 3eedles C. Disintegration of Skeletons - invertebrate shells, such as the lamellar and prismatic shells of molluscs, may disintegrate into their constituent small aragonite needles upon death - bioerosion: rasping and boring to reduce large skeletons to smaller particles is very effective at reducing large skeletons to micrite D. Micritization - the process tends to produce coherent aggregates, rather than loose, individual crystals/grains of micrite - involves processes A and C in combination; endolithic (boring) microbes such as algae, cyanobacteria, and fungi, attack a host grain(quartz for ex.) to produce borings, the boring membrane becomes filled with micrite cement usually HMC or aragonite, and tend to be repeated until entire outer surface is micrite - role of micritization in production of cortoids (which are therefore degraded grains, rather than coated grains) - differences between the mineralogy or microstructure of the micrite envelope and the grain interior may lead to dissolution of the interior and eventual filling by other cements, or some other divergence in diagenetic histories - peloids as micritized grains are probably completely micritized grains 3. Sparry Calcite - coarsely crystalline calcite, transparent in t.s. (Boggs Fig. 6.4) - identity as cement vs. neomorphic spar Neomorphic spar is produced in situ, recrystallization of finer calcite/ aragonite grains to coarser crystals cement, it is passively precipitated from carbonate-saturated pore fluids grows toward the center of the grain, which leads to organization and zoning which can be revealed using cathodoluminescence(Fe, Mn responsive) - collectively, micrite and sparry calcite constitute the orthochems (name means chemical sediments precipitated in place) of Folk‟s classification schemes CLASSIFICATION OF CARBONATE ROCKS  two schemes: 1. Folk (1959; 1962) - Folk‟s schemes (Boggs Fig. 6.7, Table 6.2) depend on type(s) of allochems(CG) and presence/absence and relative proportions of the orthochems(M, SC) - first of Folk‟s scheme (1959; see Boggs Fig. 6.7), a prefix derived from the dominant allochem type, followed by the dominant orthochem type(sparite, micrite) - Folk‟s “bioclasts”-skeletal grains, “dismicrite”-disrupted or bioturbated micrite , and “biolithite”-organically constructed rocks, these are not passive or mechanical accumulation of remains and sediments(reefs. Ex) - Folk (1962) very useful in thin section(micrite vs sparry calcite) grains/allochems were <1 mm pelsparite and pelmicrite >1 mm  pelsparrudite and pelmicrudite Folk used a “rud” cutoff of 1 mm Embry and Klovan used a 2 mm cutoff for their rudstones in the modified Dunham scheme Calcirudite->2mm Calcarenite1/16<2mm Calcilutite<1/16mm - Folk textural maturity classification(Boggs Fig. 6.8) sorting of carbonate grains and presence of micrite, were indicators of energy level - THE STICKING POINT: “carbonate sediments are born, not made”, i.e. organisms, as much if not more so than conditions of the environment, control carbonate sediment production, including grain size, and accumulation; exceptions are bioclastic limestone(coquina) 2. Dunham (1962); modified by Embry & Klovan (1972) grain dispersion in t.s., must interpret section accordingly - the preferred scheme (Boggs Table 6.3A), which emphasizes textual relationships i.e. grains to matrix/lime mud - reasons for and significance of the absence of micrite/lime mud diagenetic removal due to circulation of under-saturated waters - Dunham‟s scheme was eventually modified by Embry and Klovan (Boggs Table 6.3B; P&S Fig. 11.5B), to handle very coarse-grained carbonates, as well as varieties of boundstone (Folk‟s biolithite); this practical revision made sense given that they were working with carbonate reservoirs, especially reefs, of the WCSB -(>2mm) components support the sediment (=intact framework of „conglometrates‟, it‟s a rudstone -if the large grains are mud supported (dispersed framework of diamictites), it‟s a floatstone -E-K categories within dunhams boundstone reflect relationship between subtypes -bafflestone -bindstone -framestone -floatstone -rudstone Variety in boundstone types: -RELATES TO POSITION WITHIN reef complex; a powerful exploration and development tool for O&gas!!!! BIOLOGICAL INFLUENCES 1. Baffling and Trapping -two plant groups are very effective at baffling, whereby current velocity is significantly reduced, and sediment is deposited, rather than being swept elsewhere  Thalassia, or turtle grass poor preservation  blades typically host calcareous epibionts  mangroves- trees that grow along coastal regions of some carbonate environments common in marginal marine carbonate sections  respiratory system is elevated above water by dense, tangled prop root system(reduces currents, thus trapping sediment at high tide or during storm) *both groups plus calcareous algae act to bind sediment -the role of the stromatoporoid Amphipora(typified the quiet backreef lagoonal environment that baffle sediment thus growing vertically) in Devonian of the WCSB 2. Bioerosion -this may lead to a reduction in grain size, production of fecal pellets, and weakening of larger structures such as coral heads -the principal agents are: A. Parrot Fish -rasp or harvest coral head(with powerful jaws and strong teeth), to get at the symbiotic algae (zooxanthellae) in the polyp tissues; aragonite grains are ejected after the coral head is pulverized B. Echinoids - grazers, nocturnal, they exploit algae, fungi, and bacteria, and indirectly graze and rasp coral heads as wellingest aragonite and eject fecal pellets -considerable damage is caused by such echinoids as Diadema in the Caribbean damage is done by jaws, aristotles lantern C. Sponges -most prominent is Cliona, which typically invades coral and bivalve (pelecypod) hosts -hair-like structures, aided by acid secretions, penetrate the host shell; distinctive borings are the scalloped walls where chips have been removed  the chips removed and ejected by cliona have distinctive concave and convex faces that are sand sized D. Bivalves -some, such as Lithophaga, bore into solid carbonate substrates -there is a combination of acid secretion and mechanical action: attack base of coral head(severely weaken it), most excavated carbonate is put into solution and recycled into seawater (i.e. it doesn‟t appear to generate much sediment) then we may see precipitation of micrite linings, and the cavities may host other organisms such as calcareous-tubed serpulids 3. Bioturbation host of organisms burrow in carbonate sediment, for protection or for food; may destroy primary structures, alter textural parameters(pelletized grains, sort grains……), and remove organic material  it may also affect diagenesis -the ghost shrimp, Callianassa, it produces a burrow system with inhalant(much less obvious) and exhalent vents(obvious) DOLOMITES -dolomite group  dolomite- stoichiometric (Ca:Mg 1:1, perfectly ordered) is rare  ankerite  ferroan dolomite(>2 mol% FeCO ) 3 -protodolomite are examples with excess-Ca, poorly-ordered, is metastable, with greater stability as one approaches stoichiometric dolomite - seawater is saturated respective to dolomite, but Ca-carbonates crystallize more rapidly; due to the kinetic considerations (the need for ordering in dolomite) - dolomite is distinguished from calcite by XRD or stain techniques TEXTURAL CLASSIFICATION OF DOLOMITES -classification and description: original fabrics preserved- Dunham or Folk schemes may be used for naming  fabric-destructive- Sibley and Gregg 1984 would be used for naming original fabrics obliterated during diagenesis -morphologically divided:  idiotopic dolomite(planar)- planar crystal boundaries, either euhedral faces or straight, and compromise boundaries xenotopic dolomite(non-planar)- nonplanar boundaries, generally curved, lobate or serrated DOLOMITE TEXTURES 1. Descriptive - dolomite textures A. Sucrosic Dolomite-(idiotopic) -means “sugary”, with (intercrystalline porosity) between dolomite rhombs S&G equivalent- planar euhedral texture -it is frequently seen in void fillings or lining vugs, as coarse, irregular pores B. Saddle Dolomite/Baroque Dolomite-(xenotopic) -warped crystal lattice, curved crystal faces and cleavage planes, and sweeping or undulose extinction frequent with hydrocarbons(oil window 60-120 C) 0 -replacement coarse, xenotopic/ nonplanar mosaic, fabric destructive, undulose extinction -void fillercompromise boundaries are irregular, curved with scimitar-like crystal terminations into the cavity C. Limpid Dolomite-(idiotopic) -clear, euhedral(to subhedral), coarse; inclusion free (dolomite version of sparry calcite) -representative of crystallization rates and diagenetic realm with a very low crystallization rate, allowing proper lattice development, in meteoric phreatic conditions D. Zoned Dolomite - dolomite rhombs with a cloudy center(inclusions) and clear rim(inclusion free) - syntaxial overgrowths of dolomite on dolomite are also reported E. Mottled Dolomite - not strictly a dolomite texture; relates to the mottled, patchy, or irregularly dispersed relationship between dolomite and other carbonate minerals [e.g. Tyndall Formation, Tory Building] - its development may be controlled by variability in porosity, permeability, or other textural aspects of the primary rock, so one should check for burrows, bedding, fractures, fossils, or other apparent controls on its development 2. Timing of Crystal/Rhomb Formation - if dolomitization is at least in part fabric retentive, its timing relative to other diagenetic phenomena may be determined -simple principles for determination of timing  cross-cutting relationships, and displacement of grain outlines, compaction, and styolitization fronts comparing within v.s. outside rhombs EVAPORITE SETTINGS AND TEXTURES 1. Supratidal Evaporites – the Sabkha -limited in their development of carbonates, and sulphates -no demonstrable accumulation of halite or potash salts in extant sabkhas(just halite as surface crust) -distinctive anhydrite textures in this setting, where the sulphate is formed with standard supratidal features such as salt casts -selenitic gypsum curved crystal faces with lens shape in cross section; may be replaced by calcite Displacive textures: -nodular anhydrite (more common), which in the extreme occupies most of the rock as a nodular mosaic; - chickenwire anhydrite only dark stringers of non-sulfate separate the large nodules (Boggs Fig. 7.2)-like in core lab -enterolithic structure anhydrite nodules coalesce into what appears to be deformed laminae, but are just irregularily arranged nodules(represent soft sediment deformation) -as sedimentation/vertical aggradation, and progradation occur, a typical sabkha sequence(shallowing upward progression) may be developed 2. Barred Marine Basin Evaporites - anything from deep water in deep basins to shallow water in shallow, yet subsiding, or deep basin; these different conditions tend to be characterized by different textures and structures Distinctive characteristics Shallow water (bottom growth texture) especially for gypsum, prismatic, swallow-tail twins, and split crystals(may be palmate(palm-frond)) growing as vertically elongate form Deep water -laterally persistent, evenly laminated evaporates represent deep-basinal accumulation of crystals formed in near-surface conditions but settling below wave base, to preserve lamination commonly observed varves or couplets of alternating grain size and colour may represent alternation with seasonal organic blooms, or carbonate layers formed in response to seasonal fresh water input - alternative model: some lamination evaporates might form in very shallow lagoons, with high brine density inhibiting the development of waves and currents  resedimentation may occur in either model, conveying evaporates into deeper waters as debris flows or more organized turbidites 3. Lacustrine/Non-Marine Evaporites - borax, epsomite, and trona are unique to lacustrine settings -as well, these closed basins with interior drainage and no external outlets, produce a characteristic pattern for the distribution of evaporite minerals  have a bulls-eye pattern, with least soluble minerals at the lake margin CHERT DEPOSITS -chert-( SiO (silica) in various crystalline and non-crystalline forms) 2  microcrystalline to cryptocrystalline quartz  chalcedony (radiating sheaf-like bundles) -varietal names such as:  flint (typically dark grey to black)  novaculite (often whitish)  jasper (red) -opal( hydrous, amorphous form of silica, that reverts readily to chert or quartz (Phanerozoic) most commonly secreted by organisms as shells/tests/ spicules, that are protected by an organic sheath  in t.s. it appears like glass and is isotropic in xpolars, high –relief, low birefringence 1. Bedded Chert -we see laterally persistent, finely to massively laminated chert horizons - origin  silica added to oceans by stream transport/dissolved load [silicic acid at 13 p.p.m.]  halmyrolysis submarine weathering of ocean floor volcanic and detrial sediments * seawater is highly undersaturated with respect to silica -preservation:  rapid settling and accumulation to protect the materials from the corrosive effects of seawater recrystallization/alteration typically occur * this may obliterate the skeletal configuration and obscure the biological origin -siliceous skeletal groups and their stratigraphic ranges  these skeletal materials are produced by plankton such as radiolarian(Cambrian to recent) diatoms(Jurassic to recent)  silicoflagellates Benthic  glassy or siliceous sponges (Cambrian to recent, spicules cherts) diatoms(Cenozoic) * diatoms were exclusively marine through the Mesozoic 2. Nodular Chert - commonly observed as a replacement in carbonate successions, with nodules occasionally coalescing to form irregular beds (which therefore must be distinguished from true primary bedded cherts -may be fabric retentive to the point of preserving relict details of fossils and other grains - very common in Mid- to Late Paleozoic carbonates of the WCSB, thus occurs in Permian and younger siliciclastics, as chert pebbles CARBONATE SEDIMENTARY ENVIRONMENTS 1. Reefs and Mounds are structures A. Definitions and Terminology -some of the more important terms are defined as follows: Reefs: are structures formed/constructed by large, usually clonal elements (avg. >5 cm size) capable of thriving in energetic environments - classical definition of a reef is that it is a “wave resistant” organic structure, that persist through the rock record Mounds: are structures usually built by smaller, commonly “delicate” and/or solitary elements in “tranquil” settings -other commonly used terms are: Bioherm: a lens-shaped or circumscribed biogenic structure Biostrome: a tabular, biogenic carbonate rock body; e.g. crinoidal limestones of the Mississippian Carbonate Buildup: any carbonate accumulation with relief relative to the surrounding carbonates; this term is safest to use in the field, pending more data and interpretation B. Growth Form, Sediment Textures, and Reef Zonation -we observe that the growth form of reef builders controls mechanical stability, and thus relates to position within the reef or mound  delicate indicates back reef, robust indicates reef front - determining if sedimentation has kept pace with vertical growth  if it has, the margins of the colony are ragged, as growth laminae occur only on the exposed surface  if it hasn‟t, the margins of the colony are smooth, as successive growth laminae envelop the entire colony(except the base) - the balance of sedimentation versus vertical growth with reference to the enclosing sediment stability of growth forms, when sedimentation rate is sufficient, all growth forms are stable  there is a distinct windward-leeward zonation -boundstone are in the core facies of a reef  note! the predominance of encrusting forms/bindstone on the reef crest, to a maximum of 15m approximate water depth, due to high wind and wave energy -floatstone and rudstones are found in flank facies characteristic settings: -floatstone often occur in protected lagoons of the back reef, where muds will also settle -rudstone (also grainstone) are in the fore-reef talus/breccia, along with high dips -grainstones and rudstones are also found in back-reef shoals produced by storm washover -there is also a depth zonation from loss of wave energy, and diminishing light, favouring a plate-like habit in scleractinian corals C. Dynamics/Processes i. Sediment Production (the “Carbonate Factory”) -factors in growing a reef or mound: a. organic growth: upward growth of microbial and metazoan calcareous elements, in situ, and remain in place until weakened and reoriented/redistributed buy storms(storm balls) - branching forms are especially susceptible to storm activity, characteristic sediments such as bioclastic carbonates of sticks and rods(rudstone and calcirudite) -growth habit to achieve stability in reefs contrasted with mounds  reefs-stability is achieved by growth on the crest by encrusting(stromatoparoids)  mounds-use binding action of grasses(turtle grass) and algae(stromatolites) b. destruction: and not just post-mortem; we get disintegration of carbonate producing organisms, both invertebrates and calcareous green algae in particular - reefs are inhabited by borers, raspers, and grazers, generating lim
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