Carbonate Platforms

1. Carbonate Platforms

2. I. Intro • Limestones are biochemical rocks • Composed of CaCO3 • Calcite, Aragonite • Calcium carbonate sediments depend on: • Temperature • Pressure • Agitation • Abundance of calcareous-shelled organisms • abundant light • constant so/oo • clear, warm water • Organisms like shallow water, away from river

3. II) Platform and shelf carbonates A. Carbonate depositon mostly on platforms B. Carbonate platform is defined as: 1."a large edifice formed by the accumulation of sediment in an area of subsidence" C. Examples: rimmed platform, isolated platform, ramp

4. III. Platform Characteristics A. Most platforms: 1. have a flat top 2. steep sides 3. can be several kilometres thick 4. extend over many hundreds of square kilometres 5. can be rimmed or unrimmed

5. Platform Characteristics Continued B. Platform seds also in epeiric seas 1. distinguishing between shelf/epeiric sea carbonates difficult 2. distinction based on paleogeography C. Most carbonate production seaward 1. upwelling/nutrients i. e.g. reefs on platform margin ii. if no reef, ooid shoals--oosparite 2. landward- subtidal carbonate i. poor preservation - storms ii. seds transported offshore/onshore (tidal flats, channels) 3. nearshore protected areas i. micrite/biomicrite/pelmicrite 4. above high tide (supratidal)--dolomite

6. IV. Carbonate deposits A. Micrite mud mounds 1. 100 meter thick, 1km diameter i. deep & shallow water origin ii. lack internal structure iii. Mostly Paleozoic some Mesozoic iii. pelloid deposit? 1. skeletal component low iv. plant stabilization v. sed trapping organism--bryzoans

7. Devonian Saharan Micrite Mud Mounds Asymmetrical mud mounds at Azel Matti (Ahnet Basin, Algeria) Mud mounds (25-30 m high) at Azel Matti (Ahnet Basin, Algeria)

8. V. Reefs --Defined A. a reef - structure constructed of large elements (usually > 5 cm) capable of thriving in energetic environments

9. VI. Reef Structure & Rocks A. Low latitude, shallow water env. 1. framework organisms vary with time i. archaeocyathids, sponges, corals, algae, bryzoa, etc. B. Reef core = massive C. Forereef = talus D. Reef limestone types 1. framestone– in situ fossils form supporting framework 2. bindstone--encrusting & binding organism 3. bafflestone-- stalk-shaped fossils--reduce rate of water flow E. Reef core bounded by forereef talus 1. forereef-allocthonus seds 2. c.gr--floatstone, rudstone

10. VII. Reef Facies A. Reefs generally comprise three facies: 1. Core facies - massive unbedded carbonate with or without skeletons 2. Flank or forereef facies - bedded carbonate sand and conglomerate of in place and/or core derived material, dipping and thinning away from the core 3. Interreef or open platform facies - subtidal limestone to terrigenous clastic sediment, unrelated to reef growth.

11. Reef Facies • Miocene Llucmajor Reef Complex Components , Mallorca Spain strata.geol.sc.edu/.../049-Reef Components.jpg Miocene Reef Facies, Mallorca

12. VIII. Deep sea carbonates A. Deposition seaward of terrigenous seds 1. max depth about 5 km 2. seawater colder w/depth i. more dissolved carbon-dioxide than warm water ii. increase H2CO3= increase dissolution iii. Temp more important than P

13. B. CCD = depth below which no carbonate seds accumulate 1. deeper in equatorial water i. more carbonate production 2. shallow near margin i. More organics 3. CCD changes w/ time i. depends on CO2 content & carbonate production

14. C. Deep sea red clay or siliceous sed below CCD

15. IX. Unrimmed Shelves A. Characteristics of unrimmed shelves: 1) a 10m -300 km wide seafloor gently slopes offshore from a continental area 4) lithofacies are generally grainy 5) high-energy, carbonate sands in the wave and/or tide agitated inner shelf 6) skeletal muddy sands to muds in quiet deeper outer shelf i. periodically affected by storms 7) localized patch reefs and sand shoals, 8) no protection from onshore waves

16. XI. Rimmed Platforms A. Shallow platforms 0-30 m deep bounded at outer edges by high-energy facies (typically reefs) and a pronounced break in slope B. Reduced connection between open-ocean and shelf due to protection by reefs, sand shoals, or islands, thus dampening wave energy C. Lithofacies are generally muddy D. Shelf depth determines facies type and distribution 1. Shallow water shelves have grassy covered sands and muds on their inner parts and skeletal sands and patch reefs on the outer parts while 2. deep water shelves (lagoons with water depths < 30m) are floored by muds. E. Behind the reef are muddy carbonate sediments that contain lots of marine organisms http://wrgis.wr.usgs.gov/open-file/of01-448/images/fig18.jpg

17. Growth of Rim

18. Belize belizemodernfacies.com/?page_id=19

19. Shallow Rimmed Shelf-Florida, Deeper Rimmed Shelf--Belize • strata.geol.sc.edu/.../Sea_Level_Changes.htm

20. XII. Isolated Platform A. E.g. Bahamas B. Interior = skeletal l.s., peloid sands & mud C. Platform margins = shoals of ooid grainstone D. Talus slope & slump & gravity flows E. Platform evolution: 1. Develop on horst 2. graben = deeper water

21. Isolated Platform http://www.geologie.uni-stuttgart.de/online_kurse/virtfoss/CPR%20ordner/Figuren/Bilder%20Lec3/Fig.3-3.gif

22. X. Ramps A. gentle slope (<1o) B. shallow nearshore deposition 1. tidal flat/lagoonal facies 2. shoal water complex of bank or ooid-peloid sand shoals C. pass into deeper water 1. limestone/wackestone 2. slope & basin lime muds (from Burchette and Wright 1992)

23. Trucial Coast Ramp strata.geol.sc.edu/.../Sea_Level_Changes.htm

24. Generalized model of a Carboniferous carbonate platform (from Richards, 1989a). • http://www.ags.gov.ab.ca/publications/wcsb_atlas/A_CH14/FG14_18.html

25. XI. Intrashelf Basins on Rimmed Shelves and Ramps A. Rimmed shelf w/inshore basin B. Pass landward into coastal siliciclastics C. Seaward-basin pass to rim of skeletal or ooids D. Basin depth few 10's of m 1. below fairweather wave base 2. seds = shale, quartz sand 3. If below wave base, euxinic, dysaerobic l.s. & shale

26. Intrashelf Basin- I couldn’t find one for carbonate setting • Carbonates would occur at outer edge

27. Stop At This Slide! Succeeding Slides are extraneous.

29. Eg Trucial coast in the Persian Gulf - lithofacies include: • a back ramp with microbial intertidal flatsthat pass landward into an evaporitic basin and skeletal-pelleted sands to pelleted lime muds in protected lagoons • a shallow ramp with high energy skeletal/oolitic sand shoals, beach barrier systems and coral reefs • a deep ramp that is transitional from aggregate/skeletal sands dominated by molluscs and foraminifera to skeletal muddy sands dominated by mollusc debris • a gradual transition into bivalve rich marls of deeper water

30. Sedimentary processes • Any living reef is a balance between 4 factors: • upward growth of in-place calcareous elements • continual destruction by a host of raspers, borers and grazers • prolific sediment production by rapidly growing, short-lived, attached calcareous benthics • concurrent inorganic or organically induced cementation. • The modern reef growth window • Continuing the carbonate nomenclature of Dunham to reefs

31. Facies distribution • reef front facies • lies between about 10-100 m • diverse reef builders varying in shape from hemispherical to branching to columnar to dendroid to sheet-like (dependent on species that exist at time of reef formation) • accessory organisms and niche dwellers common • below 40 m light and wave intensity is low and corals are platy

32. fore reef facies • gravel and sand composed of skeletal debris, reef limestone blocks, reef builder skeletons • grade basinwards into muds

33. reef crest facies • down to max 15 m • receives most wind and wave energy • organisms range from encrusting to short and stubby branching types depending on wave and wind energy

34. reef flat facies • in areas of intense waves - pavement of cemented, large skeletal clasts with scattered rubble and coraline algal nodules • moderate wave energy - shoals of well washed lime sand • most material swept in from reef crest

35. back reef facies • where much of the mud formed on the reef comes out of suspension • prolific growth of sand and mud-producing bottom fauna (eg algae) • corals are stubby and dendroid or large and globular

36. Carbonate Slopes • extend from the shallow water environment of the reef and fore reef down into deep water of ocean basins • processes are similar to those of terrigenous slopes and deep water • tend to be cut by a number of parallel gullies • facies belts are parallel to the platform margin • down slope carbonate deposition controlled by the carbonate compensation depth

37. Depositional slopes • either smooth slopes that extend from shallow to deep water with the thickness of accumulated sediment decreasing seaward • or bypass slopes where the upper slope is largely bypassed with sedimentation on the lower slope

38. Erosional slopes • net removal of material due to a number of mechanisms including slumping and carbonate dissolution

39. Sediment types and facies • Slope sediments have several origins: • Pelagic sediments • accumulation of the shells of microscopic to very small marine organisms of the open ocean • Platform carbonate • material that is derived from the shallow water platform • mud to boulder-sized fragments • can be transported as much as 120 km from the platform • Hemipelagic sediment • fine-grained terrigenous clastic material • Autochthonous carbonate • faecal pellets, skeletons of organisms of the slope, carbonate cements

40. Processes operating on the slope • material settled out of suspension • sediment gravity flows such as turbidites, slumping, debris flows and creep • reworking by bottom currents • laminated mudrocks to megabreccias that indicate large scale collapse of the platform margin

41. http://www.gly.fsu.edu/~salters/GLY1000/11Seds_sedrocks/Slide35.jpghttp://www.gly.fsu.edu/~salters/GLY1000/11Seds_sedrocks/Slide35.jpg

42. USGS

43. Cambrian Paleogeography • I) Intro • A. Most of N. America along paleoequator • B. Deposition of carbonate & qtz sand, shale • C. Early Cambrian sea encroachment • D. L. Cambrian sea covered large area • E. Much carbonate deposition • F. E.g.. Sauk Sequence • 1. erosion and weathering of xln basement • 2. encroachment of marine water • 3. deposition of clean sand • 4. sandy deposition replace by carbonates • i. algae flourished • 5. sands pass westward into finer clastics & carbonates • 6. E.g. Grand Canyon • i. Cambrian Tapeats ss • ii. passes to Bright Angle Shale • iii. followed by Mauv L.S. • iv. together form transgressive sequence http://jan.ucc.nau.edu/~rcb7/Ear_Camb.jpg

44. Devonian Saharan Micrite Mud Mounds Generally, the mounds were established in a deeper subtidal environment as suggested by the lack of indications for ground-moving waves. Non-photic conditions are indicated by the absence of cyanobacterial activity (e.g. micritic envelopes) and calcareous algae. The mounds are reefs in the biological sense which means that they are autochthonous structures with a significant relief build by organisms. However, the typical Devonian reef builders, stromatoporoids and colonial rugose corals, are lacking. No rigid framework of organisms is present as that formed by the hermatypic scleractinian corals in modern reefs. The dominating faunal elements are crinoids, tabulate corals, brachiopods, trilobites, sponges, ostracods and bryozoans: thus mostly filter feeding organisms which preferred the exposed mound position because of better oxygenation and better food supply. But all these organisms are all mound dwellers, not mound builders. More than 80% of the mound volume is constituted of a fine-grained carbonate (micrite) whose origin is one of the main miracles of mud mounds. High accumulation rates (0.2-0.8 m/1000 a), purity of mound carbonates (> 95% CaCO3) and homogeneous Mg-calcite mineralogy strongly argue for autochtonous (microbial?) carbonate production. In addition, the alignment of stromatactis fabrics (characteristic open-space structures in mud mounds) parallel to the accretionary mound surfaces suggests a close relationship between stromatactis formation and carbonate production. Microbial communities could have flourished on the mound surfaces, precipitating fine-grained carbonates and consolidating the steep (35-40°) mound flanks by their mucilaginous secretions. Once embedded, these communities decayed and were successively replaced by calcite cements, finally resulting in stromatactis fabrics. But no direct evidences for calcimicrobes as the driving source of mud-mound formation have ever been found. Therefore, this theory remains purely speculative. Geochemical determination of organomolecules (if preserved) could help to solve this problem. Which factors initiate the growth of these buildups at certain localities? This is the second main question of mud-mound research. The hypothesis that they might have formed at sites of hydrothermal venting (Belka 1994, 1998) was quite innovative. There are indications, e.g. the alignment of mounds along Precambrian tectonic lineaments which may have acted as conduits for hydrothermal fluids. However, direct evidences like sulphide mineralizations or strongly depleted d13C values of mound micrites and early marine calcite cements are still lacking. Mud mounds are mostly a palaeozoic phenomenon without close recent analogues. However, the recently discovered carbonate knolls in the Porcupine Basin off western Ireland and in the Vulcan sub-basin off northwest Australia (Howland et al. 1994) seem to come near to them regarding their dimensions, geometries and also concerning their ecology. Mud mounds (25-30 m high) at Azel Matti (Ahnet Basin, Algeria) Asymmetrical mud mounds at Azel Matti (Ahnet Basin, Algeria)

46. Rimmed Shelves • shallow water shelves - grass covered sands and muds on their inner parts and skeletal sands and patch reefs on the outer parts • deep water shelves - lagoons with water depths up to 30m, floored by mud; outer reefs and patch reefs surrounded by reef talus; skeletal sands in nearshore areas.