1 EES 450: Sedimentary Geology CARBONATES - Rocks with > 50% carbonate materials; most abundant biochemical/chemical rocks. - Make up 20-25% of the sedimentary.

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Presentation transcript:

1 EES 450: Sedimentary Geology CARBONATES - Rocks with > 50% carbonate materials; most abundant biochemical/chemical rocks. - Make up 20-25% of the sedimentary record from the Precambrian to the Recent. - Limestone and dolomite include > 90% of all carbonates. - The ratio of dolomite/limestone increases with age. Why? - Most dolomite is secondary or diagenetic. - CaCO 3 is polymorphic (calcite vs. aragonite). - Most carbonates are autochthonous and contain much information about their depositional environments. - Carbonates have significant industrial and agricultural uses. - 1/3 of all hydrocarbon reservoirs are in carbonates.

Limestone Chert Diatomite 2 Breccia Conglomerate Diamictite Sandstone Siltstone Shale New Clay Minerals Shale Clastic or Detrital Biochemical Evaporites Chert Limestone Ironstone Chemical Solution Plant Extraction Peat Coal Bitumen Resins Organic Mechanical Weathering Chemical Weathering Source Rocks Crustal, upper mantle melts Explosive Eruption Tuff, Bentonite Agglomerate Volcaniclastic EES 450: Sedimentary Geology CARBONATES Rudites

3 EES 450: Sedimentary Geology CARBONATES Why do we have carbonates? - Earth’s atmosphere has abundant CO 2, and CO 2 with H 2 O is a powerful weathering agent. - Feldspar (or ferromagnesian minerals) + CO 2 + H 2 O clay minerals + cations (Ca 2+, Mg 2+, etc.) + HCO silica + (Fe oxides). - Ca and Mg are provided by the weathering of some feldspars and ferromagnesian minerals. - CO 2 is provided to Earth’s atmosphere by volcanism, the decay of organic matter, the respiration of organisms and increasingly, from human activities. - Summary: We have carbonates because of the abundance of Ca 2+, Mg 2+, and CO 2.

4 EES 450: Sedimentary Geology CARBONATES Why do we have carbonates? - The production, dissolution and preservation of carbonate sediment are intimately associated with the global carbon cycle.

5 EES 450: Sedimentary Geology CARBONATES Depositional conditions. (1) Shallow, subtropical seas. (2) Warm, evaporative conditions. (3) Minimal influx of clastic sediments. H 2 O + CO 2 + CaCO 3 Ca HCO 3 -1 Modern day carbonate deposition. Modern day coral reef distribution. Why do we find reefs where we do? Heavily threatened, why?

6 EES 450: Sedimentary Geology CARBONATES Depositional conditions. H 2 O + CO 2 + CaCO 3 Ca HCO The distribution of carbonate sediments is controlled by the carbonate compensation depth (CCD) and the rate of surface production. Does the depth of the CCD change over time? What controls this? - Hypsometry = measurement of land elevation relative to sea level. - Lysocline = depth in the ocean below which the rate of calcite dissolution Increases dramatically.

7 EES 450: Sedimentary Geology CARBONATES Depositional conditions. H 2 O + CO 2 + CaCO 3 Ca HCO The distribution of carbonate sediments is controlled by the carbonate compensation depth (CCD) and the rate of surface production. NOAA Sea WIFS data, showing the surface ocean distribution of chlorophyl a; what does this tell us about the distribution of surface ocean productivity? Does this change over time? How and why?

8 EES 450: Sedimentary Geology CARBONATES Depositional conditions. - Does global climate effect carbonate deposition? Global climate belts are controlled by latitudinal variations in wind.

9 EES 450: Sedimentary Geology CARBONATES Depositional conditions. - Does global climate effect carbonate deposition?

10 EES 450: Sedimentary Geology CARBONATES Depositional conditions – Example modern day environments. Bahama platform. Joulters Cays. Persian Gulf.

11 EES 450: Sedimentary Geology CARBONATES Depositional conditions – Example modern day environments.

12 EES 450: Sedimentary Geology CARBONATES Most carbonates are biochemical in origin. BIOLOGICAL BIOLOGICALLY INDUCED PHYSICO-CHEMICAL Fossils Carbonate muds Cements, rocks Hydrophytes (carbonate encrustation). Pteropod Limacina Helicina (aragonite). Foraminifera (carbonate). Bryozoan fossils (carbonate). Stromatolites (bacterial mats). Fossilized coral (carbonate). Ooids. Travertine.

13 EES 450: Sedimentary Geology CARBONATES General carbonate facts: - Carbonates are products of specific depositional environments. - Carbonate deposition can be fast, but intermittent and “sensitive”. What does this mean? - Carbonate deposition is generally autochthonous. Carbonate particles generally produced in situ with little net transport occurring. - Carbonate formation is mostly biochemical. - Basin configuration and energy setting are the predominant controls on carbonate deposition. - Carbonates are highly susceptible to diagenetic alteration.