c11_carbon_environ_part_1

c11_carbon_environ_part_1 - C11 carbonate environments...

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C11 carbonate environments
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carbonates • Roughly 1/4 sedimentary rocks • Modern environments – Primarily low latitude – Low terrigenous influx – Some cool-water shelves • Predominantly shell remains • Ancient carbonate environments – Widespread in broad epeiric seas • 100s to 1000s km wide
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Distribution of Modern carbonate systems
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http://idol.union.edu/garverj/Geo35/photos/overview.htm http://www.epa.gov/OWOW/oceans/coral/
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5 basic types of carbonate platforms or shelves • 1. Rimmed carbonate shelves – Shallow platforms with pronounced shelf break reef barrier – Reef or ooid/skeletal shoal shelf-edge rim – Low energy lagoon grades into tidal flat • 2. Unrimmed carbonate shelves – No pronounced marginal barrier @ platform edge – Leeward side of tropical bank or cool-water carbonates • 3. Carbonate ramps – Gently sloping unrimmed platform – Gradual downslope deepening from shallow water • 4. Isolated platforms – Shallow-water platform surrounded by deep water – Located offshore of shallow continental shelves, I.e., no siliciclastic input • 5. Epeiric platforms – No modern examples but common in Paleozoic – 100s to 1000s km wide, i.e., covered millions of km 2 – Connected to open sea – Minimal influx of clastic detritus
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5 principal kinds of carbonate platforms in cross section
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Barr ier reefs Patch reefs Shore line Narrow rimmed shel f - Bahamas ~ 400 m Barr ier Reef – corals - Bahamas Reef front – Red Sea Back Reef – Re d Sea http://www.eps.mcgill.ca/~courses/c186-425/carbonates.pdf. SCF!
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Subtidal Carbonate Factory… • Carbonates deposited primarily on shelf – Some landward transport to tidal flat/beach – Some seaward transport to slope/deep basin • Relatively little carbonate sediment generated in deeper basin environments – pelagic rain by calcium-carbonate secreting plankton
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Subtidal carbonate factory
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Sedimentation processes biological vs chemical processes • Relative importance of chemical vs biological production of CaCO 3 is not known • Biogenic production may be more important in modern • Carbonate-producing organisms have varied thru time – Crinoids, bryozoans, brachiopods were important during Paleozoic – Coccolithophores, planktonic foraminifers, coralline algae, green algae were important during the Cenozoic
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Various carbonate-secreting organisms through time
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Sedimentation: physical processes • Circulation brings nutrients necessary for organic growth • Breaking waves against reefs increases oxygen and decreases CO 2 because of decreased pressure • Thus, wave-agitated water stimulates carbonate production – Also breaks reef rock – Forms ooids – Aids formation of grapestones – Aids cementation by driving pore-water circulation – Winnows muds from coarser particles – Creates wave-formed bars, shoals, beaches, spits, tidal deltas
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General depositional setting • Outer shelf = highest energy environment – Characterized by lime sand or gravel sheets and shoals
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c11_carbon_environ_part_1 - C11 carbonate environments...

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