EOSC221 CARBONATE ROCKS

Transcription

1 EOSC221 CARBONATE ROCKS 1

2 LECTURE OUTLINE Limestones Overall Mineralogy of Calcite Geographical Limestone components Limestone Environments Terrestrial Marine Dolostones 2 hip://en.wikipedia.org/wiki/file:carmelooids.jpg

3 INTRODUCTION Limestones (CaCO3) and dolomites (CaMg(CO 3 ) 2 ) = 15% Sed. rocks Limestones 2 3x more abundant than dolomites CO 3 in carbonate rocks mostly from skeletons of organisms Recognize limestone in the field their sozness and reac@on with HCL Dolomites much reduced to no reac@on Carbonates dissolved easily KARST landscapes hip://en.wikipedia.org/wiki/file:limestone_forma@on_in_waitomo.jpg 3

4 Overall Mineralogy LIMESTONES i. ARAGONITE a polymorph of calcite. Different crystal system: (Orthorhombic): Low MgCO 3 (max 1%) precipitated by mollusks and modern corals Unstable will alter to calcite 4 hip://en.wikipedia.org/wiki/file:aragonite_salsigne_france.jpg hip://en.wikipedia.org/wiki/file:ammoniteplit.jpg

5 ii. CALCITE (Trigonal): Two forms: A) Low Mg Calcite <4% MgCO 3 B) High Mg Calcite >4% MgCO3 Low Mg Calcite more stable Calcite secre@ng organisms include: brachiopods, trilobites, echinoderms brachiopod hip://en.wikipedia.org/wiki/file:platystrophiaordovician.jpg Trilobites 5 hip://en.wikipedia.org/wiki/file:calcite jpg hip://en.wikipedia.org/wiki/file:trilobitegrowth.jpg

6 Calcite can also be deposited inorganically The type of calcite deposited will vary depending on ocean chemistry Calcite and Aragonite seas 6 hip://en.wikipedia.org/wiki/file:calcitearagonite.jpg

7 Calcite seas: High spreading rates and greenhouse High spreading rates: more cycling of sea water through crust removing Mg by of Mg rich clays High spreading rates: increased volcanism = more CO 2 = greenhouse 7 hip://en.wikipedia.org/wiki/file:mgcara@ochanges.jpg

8 Precipita9on of Calcite CO 2 + H 2 O <-> H 2 CO 3 Carbon Dioxide and Water <-> Carbonic Acid H 2 CO 3 <-> H + + HCO 3 - Carbonic acid dissociated into hydrogen ions and bicarbonate HCO 3- <-> H + + CO 3 2- Further dissocia:on of bicarbonate into hydrogen ions and carbonate Ca 2+ + CO 3 2- <-> CaCO 3 Calcium and Carbonate ions in solu:on react to form Calcite/Aragonite 8 hip://

9 Important Factors in of Calcium Carbonate: 1) of living organisms 2) DECREASES with increased pressure 3) INCREASES with increased temperture 4) Increased water INCREASES carbonate mud in Florida Bay lagoon. The presence of young mangroves helping to entrap the carbonate mud. 9 hip://en.wikipedia.org/wiki/file:fangocarb1.jpg

10 Geographical distribu9on of carbonate sediments - The Carbonate Compensa@on Depth (CCD) - Carbonate deposi@on as a factor of water depth CaCO 3 dissolves below the CCD unless covered by other sediments 10

11 - of marine carbonate will be effected by the various factors discussed: biology / temperature / water depth (pressure) / water agita@on 11 Text Book

12 - As it will for the past. Late Cretaceous 12 Text Book

13 - Limestone - Shallow Marine Late Proterozoic to Modern - Deep Marine Rare in Ancient & more common in Modern - Cave Traver@ne and Spring Tufa both Ancient & Modern - Lakes Ancient to Modern 13 hip://sepmstrata.org/carbonatepar@cles/pages/004-bahamas-andros-exumas-%26-togo.html hip://en.wikipedia.org/wiki/cave

14 Limestone components 1. Allochems: equivalent of grains in rocks 2. material (micrite and cement): equivalent to clay and cement in hip:// 14

15 4 major components: Crinoids i. Allochem: Skeletal Grains.. EG: Bivalves and other mollusks Brachiopods Echinoderms including crinoids Corals and sponges (eg stromatoporoids) Calcite algae hip://en.wikipedia.org/wiki/file:crinoids_iowa_330m.jpg Bivalve (clam) Coral 15 hip://en.wikipedia.org/wiki/file:anadarapliocene.jpg hip://en.wikipedia.org/wiki/file:syringoporid.jpg

16 hip://sepmstrata.org/index.html Drafted by Waite 99, after James 1984) 16

17 ii. Allochem: Non Skeletal Grains. a. Coated Grains: Ooids Inorganically precipitated small (<2mm) sedimentary grains 17 hip://en.wikipedia.org/wiki/file:joulterscayooids.jpg

18 Series of concentric layers around a nucleus Precipitate when satura@on of CaCO 3 increases: agita@on and/or warming Can form in both marine and lacustrine sepngs 18 hip://upload.wikimedia.org/wikipedia/commons/7/76/carmelooids.jpg

19 b. Coated Grains: Oncoids (Oncoliths) Organically precipitated concentrically around a nucleus Cyanobabteria (photosynthe@c) Warm waters within the pho@c zone Property of Author 19 hip://en.wikipedia.org/wiki/file:oncolites.jpg

20 c. Peloids Sand silt grade, no internal detail, ozen Probably fecal pellets 20 hip://en.wikipedia.org/wiki/file:peloidscarboniferousnv.jpg

21 d. Intraclasts Pieces of material (ozen carbonate mud or fragments of limestone) Common in limestones due to the rapid of carbonates e. components Quartz Clays, rock fragments. Must be <50% of the rock 21 Proprty of Author

22 iii. Matrix material ) a. Micrite Calcium carbonate mud <4um Can form from direct precipita@on from CaCO 3 supersaturated sea water of from organisms, par@cularly algae Generally indica@ve of quiet marine sepngs Trilobite fragment micrite 22 hip:// hip://

23 b. Sparite Calcium carbonate cement fills original void spaces Can be either calcite or dolomite Micrite matrix can be recrystallized into sparite sparite Please make notes on Grains and Matrix : page hip:// 23

24 Limestone Classifica9on 1. FOLK CLASSIFICATION SCHEME emphasis on the detailed of grains and (matrix) material *1 Name = allochem name combined with inters@@al name (micrite or spar) 24

25 2. DUNHAM CLASSIFICATION SCHEME The Dunham scheme focuses on textures. Were grains originally in mutual contact or characterized by the presence of frame builders and algal mats? s hip://sepmstrata.org/carbonatepar@cles/pages/009-dunham-classifica@on.html 25

26 Limestone Deposi9onal Environments Both terrestrial and Marine Many different sepngs style and character of these sepngs is quite variable in both space Bahama Banks hip://csmres.jmu.edu/geollab/fichter/sedrx/carbdepoenvir.html 26

27 TERRESTRIAL CARBONATE DEPOSITIONAL ENVIRONMENTS Includes caliche of CaCO 3 in soil horizons in arid condi@ons by evapora@ve leaching), cave precipitates and lacustrine environments Deposi:on in lakes i. Inorganic limestones: mostly micrites (lime muds) ii. Algal carbonates:- stromatolites may be present iii. Ooid and skeletal sands Stromatolites from the Eocene Green River Forma@on 27

28 MARINE CARBONATE DEPOSITIONAL ENVIRONENTS Generalized cross through some of the principle environments of of limestone. NOTE: this is a generalized model. Not every area of limestone deposi@on necessarily has to include all these features. 28 hip://csmres.jmu.edu/geollab/fichter/sedrx/carbdepoenvir.html

29 i. Near shore Environment IntermiIently covered with water, week currents, some wave Micrites. Pelmicrites with fenestrae Stromatolites / algal mates common +/- bioturba@on Penecontemporaneous dolomi@za@on 29 hip://csmres.jmu.edu/geollab/fichter/sedrx/carbdepoenvir.html

30 Tidal flat deposit, Tansill New Mexico Fenestrae Storm lags Mucracks 30

31 ii. Lagoons / Restricted Bays Behind barriers / quiet waters Wackstones / Bio-pelmicrites Salinity can vary and fauna restricted: stromatolites may be present 31 hip://csmres.jmu.edu/geollab/fichter/sedrx/carbdepoenvir.html

32 iii. Inter:dal Sub:dal Shoal Areas Agitated water Carbonate sands / ooids Bio-oo-pel sparites / grainstones Cross bedding common 32 hip://csmres.jmu.edu/geollab/fichter/sedrx/carbdepoenvir.html

33 Bimini Bahamas Note cross-bedded character hip://sepmstrata.org/index.html Ooid Shoals N Joutlers Cay Bahamas 33

34 iv. Open Shelves and PlaOorms Oceanward of barrier shoal Grainstones closer to shoreline passing into wackstones and biomicrites ocean-ward Possible thin shell lag and storm deposits common Possible patch reefs 34 hip://csmres.jmu.edu/geollab/fichter/sedrx/carbdepoenvir.html

35 v. Reef systems Have topographic relief, wave resistant structures Many different reef producing organisms Reef rocks = biolithites / boundstones organically bound together Cavi@es common filled with skeletal debris / cement Common petroleum reservoirs In front of reef (fore-reef): Bioclas@c material broken off the reef 35 hip://csmres.jmu.edu/geollab/fichter/sedrx/carbdepoenvir.html

36 Coral reef growth 25 C water Low clas@c / nutrient input Marine standard salinity Intense wave ac@on 36 hip://upload.wikimedia.org/wikipedia/commons/7/76/blue_linckia_starfish.jpg

37 Reef structure: 1. Fore Reef & reef crest: steep slope, possible carbonate turbidites 2. Reef Flat (inner reef): 1 2m water depth. Extensive coral development 3. Back Reef: Reef debris adjacent to back reef Back reef 37 hip://en.wikipedia.org/wiki/file:coral_reef_diagram.jpg hip://en.wikipedia.org/wiki/file:coral_reef_plos.jpg

38 El Capitan Reef System Major Permian Reef system Deposited in the Delaware Basin NEW MEXICO TEXAS El Paso Rio Grande Guadalupe Mountains (exposed reef) Dell City Carlsbad Carlsbad Caverns National Park Guadalupe Mountains National Park Pecos River UNITED STATES MEXICO Apache Mountains (exposed reef) buried reef Van Horn 0 50 Kilometers 0 50 Miles Alpine Glass Mountains (exposed reef) 38 hip:// hip://en.wikipedia.org/wiki/file:el_capitan_2005.jpg

39 vi. Starved Basins Pelagic basins No input OZen below depth at which benthic (boiom dwelling) organisms live Depth of controlled by CCD 39 hip://csmres.jmu.edu/geollab/fichter/sedrx/carbdepoenvir.html

40 coccolith 40 Coccolith bloom hip://en.wikipedia.org/wiki/file:gephyrocapsa_oceanica_color.jpg hip://upload.wikimedia.org/wikipedia/commons/4/48/ Coccolithophore_bloom.jpg

41 Ancient pelagic limestones can be VERY extensive Cretaceous chalk seas of NW Europe, SE USA: coccoliths White Cliffs of Dover Cretaceous Chalk Seas 41 hip://upload.wikimedia.org/wikipedia/commons/7/7e/white_cliffs_of_dover_09_2004.jpg hip://jan.ucc.nau.edu/~rcb7/090marect.jpg

42 DOLOSTONES - CaMg(CO 3 ) 2 Mg subsi@tu@ng Ca Trigonal- rhombohedral system - Does NOT dissolve rapidly in HCL - Fe in the crystal structure can give dolomite a yellow brown colour, a high Mn content produces a 42 hip://en.wikipedia.org/wiki/file:dolomit,_rumunia;_pseudomorfoza_po_kalcycie.jpg

43 - Primarily forms under processes by the of limestone - Will deal with it more in the next lesson - Limestone that has been altered to dolomite will loose many of the structures / textures of the original sediment. - Ghost textures may remain hip:// Field of view 3.5 mm. 43

44 Classifica9on of Dolomites - Use the Folk and Dunham classifica@on scheme proceded by the word dolomi@c or prefixed by dolo - EG: dolomicrite / dolobiosparite dolo dolo dolo dolo dolo 44

45 GLOSSARY All lecture material is potentially examinable. Is up to you to know unfamiliar terms / names / people. Use this space to create your own lecture glossary TERMS / NAMES DEFINITION 45

46 TERMS / NAMES DEFINITION 46