1 / 51

Carbonate Grains

Carbonate Grains. Introduction. Occur from Cambrian on PreC mostly dolomite w/ algae/ stromatolite Past deposition= epeiric seas Economically important Good for environmental interpretations Form in basin of deposition Skeletal grains in rks depend on age.

airlia
Télécharger la présentation

Carbonate Grains

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Carbonate Grains

  2. Introduction • Occur from Cambrian on • PreC mostly dolomite w/ algae/stromatolite • Past deposition= epeiric seas • Economically important • Good for environmental interpretations • Form in basin of deposition • Skeletal grains in rks depend on age

  3. Controls on Carbonate Deposition • Carbonate Depositon controlled by: • Shell producing organisms require warm waters • most CO3 seds in trop-subtrop belt (30-40N&S) • So/oo--require normal salinities • Water depth--shallow photic zone, agitated, deeper=CCD • siliclastic imput--dilute accumulating CO3 sed

  4. Mineralogy • MINERALOGY • recent & subrecent = 2 CO3 minerals predominate • aragonite (orthorhombic) low Mg • calcite (trigonal) • mineralogy depends on grains • Organism = specific mineralogy or mixture • Mg content in part depends on water temp • Non-CO3 mineral in LS • terrigenous quartz & clay • pyrite • hematite • chert & phosphate of diagenetic origin pengellytrust.org

  5. Ooids • Spherical-concentric lamellae around nucleus • Nucleus=CO3 particle or qtz grain • Sed composed of ooids=oolite • < 2mm diam; Pisolite=>2mm • Water depth generally <5m, may reach 10-15m • Composite ooid=>1 ooid enveloped by lamella • Most aragonitic, may get high Mg or arag/Hi Mg • Ancient ooids=lowMg, unless silicified or dolomitized • www.nhm.ac.uk/hosted_sites/quekett/island.html

  6. Ooid Place of Formation • Lakes, lagoons, rivers, tidal flats, fresh & hypersaline waters • Some ooids micritic--endolithic algae • Shallow tropical supersaturated water • Saturation, agitation, elevated temp, then prec on nuclei 031 Joulters Cay Ooids Offshore Bahamas, note Batophera algae clumps (Kendall Photo) 1 Bahamas Ooids

  7. Pisolites

  8. Peloids • Spherical, cylindrical of microxlln CO3-- no internal structure • Mostly .1-.5 mm –mostly fecal pellets • Common in ls and micriticl.s

  9. Intraclasts • CO3 rip-ups lithified or partially lithif. sed

  10. SKELETAL GRAINS • skeletal components controlled by invertebrates through time & space • controlling environmental factors: • depth • temp • S o/oo • substrate • turbulence

  11. I.D. skeletal grains • Shape/size • Internal microstructure--diagenesis may destroy • Mineralogy • Spines or spores

  12. Mollusca • Mollusca (bivalves, gastropods, cephalopod, found lower Paleozoic onward) • Bivalves • marine, fresh, brackish waters • import since Tert w/decline of brachs • mode of life • infuanal-in seds • epifauanal-attached to substrate • vagil—crawler • calcite bivalves will retain structure

  13. Gastropods • Common in shallow marine • Vast numbers • Hypersaline, brackish--tolerate fluctuations and salinity extremes • Most benthic & vagile • Look to i.d. by shape

  14. Clams

  15. Cephalopods • Nautiloids, aminoid, Paleoz & Meso, belemnoids in Meso l.s. • nektonic or nektonic-planktonic • common pelagic, deep water l.s. • shell orig. arag--lose internal stucture • large size, presence of speta & siphuncle-keys

  16. Brachiopods • Common Paleozoic & Mesozoic, less so today • shallow marine • now not prominent marine invertebrate similar to bivavles but low to high Mg calcite, • internal structure typically preserved structure blackriverfossils.org

  17. Cnidaria (especially corals) • include Anthozoa (corals) • 2 types corals: • hermatypic-contain symbiotic dinoflagellate algae (zooanthellae) in polyps • ahermatypic--do not contain symbiotic dinoflagellate • because of algae, hermatypic must be in shallow warm & clear water • hermatypic corals are reef-formers • ahermatypic corals may occur at great depth & tolerate colder water, locally • Paleozoic: Rugose & tabular corals= hi mg calcite-structure preserved • scelractinian corals (Triassic-Rec) aragonite skeleton: poorly preserved structure mesa.edu.au

  18. Corals Rugose palaeos.com Sceleractinian science.uva.nl Tabulate humboldt.edu

  19. Echinodermata • All marine, echinoids (sea urchins) crinoids (sea lillies) • Echinoids found in reef & assoc, environ. • Crinoids in deep water, not much sed. contrib. • Paleo & Meso crinoids major component • Identified by large single calcite xls • May find echinoderm spines science.jrank.org

  20. Bryzoa Small colonial marine organism • In past, helped form reef & other l.s esp in Paleozoic • Formed of string of cells (zooecia about 100 u in diam, joined by stem of calcite

  21. Foraminifera • Composed of lo Mg calcite, rarely arag • Planktonic & benthic, planktonic dominate • Many forms, but in l.s most circular to subcircular

  22. Algae • major contrib. to carb. seds • provide skeletal grains, trap sed to form laminated seds, break down particles by boring into them • many PreC carb. formed in part by algae wetwebmedia.com

  23. Algae Continued • 4 major types of algae • red algae (rhodophyta) • green algae (chlorphyta) • blue-grn algae (cyanophyta-bore into skeletal grain) • yellow green algae (chrysophyta-coccoliths)

  24. Algae Continued • Bluegreen algae & algal mats • occur in lo lat. shallow subtidal through supratidal environments (just above high tide) • also occur in hypersaline lakes and marshes • bl grn is mucilagiinous & filamentous—trap particles to form stromatolite • stromatolite occur through out but are especially important in the PreC • stromatolite=variety of growth patterns • morphologies depend on water depth, tidal & wave energy, sed rate, & frequency of exposure • small columns & dome in less agitated bay waters • low domes & planar mats in protected tidal flats AlgaeFrom capping reef deposits of the w:Tabernas basin, Spain

  25. Stromatolites & Oncalites

  26. Algae Continued • endolithic alage-borers • fine micrite due to endolithic algae then dep. at depths of 100-200 m= photic zone-- Most of the microalgal borers associated with corals are chlorophyes of the genus Ostreobium as shown below (white arrow). The red arrows indicate borings from endolithic worms or mussels to be discussed later. www2.fiu.edu

  27. Other CO3-forming organisms • sponges: spicules may be silica • arthropods • ostracods 1mm, Cam-recent • shallow marine, brakish or fresh env • shell=calcite • trilobite-Cam-Perm • entirely marine

  28. Micrite • F gr dark matrix of carb called micrite < 4 u • accumulates in tidal flats, shallow lagoons to deep ocean floor and on slopes • thought to accumulate through breakdown of blgrn algae • algae breaks down and release vast arag needles • other processes: • bioerosion-boring algae & sponges • mechanical breakdown of skeletal grains-waves/currents • biochemical precip--photosynthesis & decomposition geology.sfasu.edu atlantisonline.smfforfree2.com

  29. Aragonite Needles AAPG

  30. Micrite Continued • inorganic precipitation in some areas e.g. Arabian coast due to supersaturated water--little calcareous algae is present • cannot discount inorganic preciptin past • also, micrite may be a cement

  31. Carbonate Rocks • Carbonate rocks - limestones composed of calcite (CaCO3) or dolostones composed of dolomite (CaMgCO3). • Form through biological and biochemical processes and through inorganic precipitation from seawater • Carbonate rocks widespread and in every geological period beginning with Cambrian

  32. Environment • Most carbonate sediments form in warm waters • E.g., corals and algae common in carbonate rocks, and mostly found tropical to subtropical latitudes or from about 30o N and S latitudes • Most carbonate sediments generated in photic zone • Unlike sandstones, most carbonate grains formed in environment in which they are deposited • Detrital grains in sandstones usually derived from outside the environment in which they were deposited

  33. Environment Continued • Carbonate sediments do not usually occur below depths of about 3500 -4000 meters in the oceans • Below these depths carbonate sediments will dissolve • The depths at which carbonate sediments dissolve in the ocean is called the carbonate compensation depth or CCD

  34. Classification Scheme • Two principal classification schemes are used for limestones. One is Folk's classification scheme, named after Robert Folk, and Dunham's classification scheme, named after Robert Dunham • Folk's scheme based on presence or absence of allochems (carbonate particles or grains), matrix (limestone clay called micrite), and type of cement (spar is a cement composed of coarse) • Dunham's scheme is based on texture of rock. i.e. whether rock contains matrix, whether carbonate grains float in matrix or whether they are in contact with each other, and whether rock has carbonate grains

  35. Dunham’s Classification • Based on whether have more or less that 10% grains • Whether rock is mud or grain supported. • Also if original components were bound together like coral. • Tells about transport history.

  36. Folk Classification • Carbonate rocks consits of: • Allochems—grains • Intersitial material—micrite or spar cement • Micrite is "lime mud", the dense, dull-looking sediment made of clay sized crystals of CaCO3. • Spar—clear to translucent carbonate cement.

  37. Micrite and Spar

  38. Folk Continued • Name is built up by stringing together all the allochem names in order from least to most abundant, and then adding the interstitial material name ("matrix" below for short). For example, a rock like this:Oolites + Fossils + Spar matrix = Oo bio sparite   The name is written as one word, Oobiosparite.     Another example (again allochems from least to most abundant):Pellets + Oolites + Fossils + Micrite matrix = pel oo bio micrite   The name is written as one word, Peloobiomicrite.     But what if there is both micrite and spar matrix? The system is the same; just list them from least to most abundant.Fossils + Spar matrix + Micrite matrix = bio spar micrite

  39. Carbonate Grains • ooids--Ooids are spherical grains formed by calcite precipitation around a nucleus such as a shell fragment, a quartz grain, etc. They are less than 2mm in diameter and typically form in shallow, warm, agitated, and carbonate-saturated waters such as those near the Bahamas.

  40. pisolites--Pisolites are ooids greater than 2mm in size • Similar environments as ooids

  41. stromatolites--Stromatolites are laminated carbonate sediments composed of mats of blue-green algae and layers of sediment. The algae, which forms the mats, is a plant and plants require sunlight to survive. Therefore, stromatolites generally form in warm shallow waters. The algae is "sticky" and it grows filaments. This sticky and filamentous algae traps sediment brought in by the currents. Consequently, a layer of algae forms then a layer of sediment is deposited on top of the algae. Subsequently, the algae grows through the sediment to form another mat and the cycle begins again. Ultimately, a layered rock composed of alternating algal mats and sediments is produced. Stromatolites form in quiet, hypersaline waters with little animal life around to destroy the mats.

  42. oncolites--Oncolites are stromatolites rolled in a ball.

  43. corals--Corals have a symbiotic relationship with dinoflagellate algae called zooanthellae. Algae are plants. Therefore, in order for corals to survive they generally have to be within the photic zone (the zone of maximum light penetration in the ocean). Plants require sunlight for photosynthesis. In the process of photosynthesis, plants produce oxygen and consume CO2. Corals are major reef formers today and in the past. However, keep in mind that some corals do occur in deep waters.

  44. Intraclasts--Semi-consolidated carbonate material ripped-up and incorporated in the rock.

More Related