Chapter 4 Marine Sedimentation ©2003 Jones and Bartlett Publishers EXAM and QUIZ #1 •In 2 weeks, September 30. •Exam on chapters 1-5 in Pinet’s text. •1 - 1.5 h length Multiple choice, short answer. Study material at end of chapters: keywords Review of basic concepts questions Critical thinking essays questions Discovering with numbers questions •Quiz questions taken directly from laboratory exercises Classification of marine sediments based upon size. Sediment Type Diameter (mm) Gravel Boulder >256 Cobble 65-256 Pebble 4-64 Granule 2-4 Very coarse 1-2 Coarse 0.5-1 Medium 0.25-0.5 Fine 0.123-0.25 Very fine 0.0625-0.125 Sand Mud (silt & clay) Colloid 0.0002-0.004 <0.0002 Classification of marine sediments based upon Mode of Formation. •Terrigenous: Sands and mud produced by weathering and erosion of rocks on land. •Biogenic: CaCO3 (calcium carbonate) and SiO2 (silica) muds and oozes composed of hard parts of organisms. •Authigenic: formed by precipitation of minerals in seawater (Manganese (Mn) and Phosphorus (P) nodules). •Volcanogenic: ejected from volcanoes (ash). •Cosmogenous: pieces of meteorites that survive trip thru atmosphere. Sediment Sampling Methods Sediment Sampling Methods: CORINGpreserves deep stratigraphy, or layering. • Hjulstrom’s Diagram graphs the relationship between particle size and energy for erosion, transportation and deposition. Shelf Versus Basin Depths Worldwide distribution of recent shelf sediments by composition is strongly related to latitude and climate. • Calcareous biogenic sediments dominate tropical shelves. • River-supplied sands and muds dominate temperate shelves. • Glacial till and ice-rafted sediments dominate polar shelves. Shelf Sedimentation Model Shelf sedimentation is strongly controlled by tides, waves and currents, but their influence decreases with water depth. Sea-level rise and fall Sea-level rise and fall Geologic controls of continental shelf sedimentation must be considered in terms of a time scales. • For a time scale up to 1000 (103) years, sedimentation controlled by: – Waves – Wind-induced currents – Tidal currents (all related to water depth) Million-year time scale. • For a time scale up to 1,000,000 (106) years, sedimentation controlled by: – Glaciation and its effect on position of coastline Relict Sediment: deposited in the past under conditions that are no longer present. Shelf Sedimentation Model Hundred-million year time scale… • For a time frame up to 100,000,000 (108) years, sedimentation controlled by: – Plate tectonics and its effect on type of margin. Plate tectonics & sedimentation on shelf Carbonate Shelves If influx of terrigenous sediment is low and the water is warm, carbonate sediments and reefs will dominate. Passive (Atlantic) vs. Active (Pacific) Type Margins. • Atlantic (passive) type margin: Passive boundary Long history of sedimentation Sedimentation rate = subsidence rate Broad, smooth continental shelf • Pacific (active) type margin: Convergent boundary Sedimentary layers compressed and deformed Volcanic sediment Seismic activity causes slumps and slides of sediments to deep-sea trenches. Passive (Atlantic) vs. Active (Pacific) Type Margins. Passive (Atlantic) Type Margin. Active (Pacific) Type Margin. Deep-sea Sedimentation The Deep sea has two main sources of sediment: 1. External- terrigenous material transported to oceans via rivers and wind, 2. internal-biogenic and authigenic from the sea. River transport of sediment Wind transport of sediment Deep-sea sedimentation processes Modes of sedimentation in deep sea • Bulk emplacement: – Slumps: sediment transport by mass with little deformation or folding of layers – Slurries: debris flows and mud flows- destroy any previous bedding or layering. • Turbidity currents – Deep-Sea canyons formed by these processes. – Ice Rafting • Polar latitudes, debris from melting icebergs. • Glacial marine sediment Bulk Emplacement: Slumps and Slurries Bulk Emplacement: Ice Rafting Deep-sea sedimentation processes Modes of sedimentation in deep sea • Pelagic sedimentation: – Pelagic muds: • Inorganic red or brown clays and silt – Fine-grained (0.0002 – 0.0004 mm) – Quartz, feldspar, kaolinite & chlorite minerals – Terrigenous, windbourne, cosmogenous source – Kaolinite in tropical & subtropical waters – Chlorite in temperate & subpolar – Dominate below waters with little planktonic production. Modes of sedimentation in deep sea • Pelagic sedimentation: – Pelagic muds: • biogenic oozes – >30% of debris from planktonic organisms – Calcareous oozes (CaCO3) » Shells of foraminifera & pteropods (zooplankton) and coccolithophorids (phytoplankton). » Accumulate on seafloor above CCD. » Forms hard limestone under pressure – Siliceous oozes (SiO2) » Shells of radiolaria (zooplankton) and diatoms (phytoplankton). » Accumulate on seafloor below CCD. » Accumulate below regions of high diatom production (equator, poles, upwelling areas) Carbonate Chemistry • CaCO3 (calcite) is a solid material produced by biological or abiological processes in seawater: Ca2+ + CO32- CaCO3 – The reaction can go both ways, depending on the pH, pressure. – When the seawater is undersaturated with respect to CaCO3, calcite will dissolve: Ca2+ + CO32CaCO3 – But when seawater is saturated with respect to CaCO3, calcite will remain in its solid form and not dissolve: Carbonate Compensation Depth, CCD • Depth in ocean at which seawater becomes undersaturated with respect to calcite and rate of dissolution of CaCO3 equals its rate of delivery. – CCD ~ 4500 m (or deeper in regions of high surface productivity). – Depths below CCD: • Seawater undersaturated w.r.t. CaCO3 • Chemical properties of deep water dissolves calcite • CaCO3 oozes less common than SiO2 oozes. – Depths above CCD: • Seawater saturated w.r.t. CaCO3 • CaCO3 remains intact. • CaCO3 oozes more common than SiO2 oozes. Question: Why is the CCD sometimes referred to as the “snow-line”? Foraminifera (zooplankton with CaCO3 shell) Size ~ 1mm Diatoms (phytoplankton with SiO2 shell) Size ~ 0.01mm Deep-sea Sediment Distribution Deep-sea Sediment Distribution TYPE COMPOSITION ATLANTIC (%) PACIFIC (%) INDIAN (%) GLOBAL (%) Foram. ooze Carbonate 65 36 54 47 Pteropod ooze Carbonate 2 0.1 - 0.5 Diatom ooze Silica 7 10 20 12 Radiolarian ooze Silica - 5 0.5 3 Red clay Aluminum silicate 26 49 25 38 Authigenic deposits • Formed by chemical or biochemical reactions on ocean floor • Nodules of ferromanganese (Fe and Mn) or phosphorite (P). • Concentric layers of metal oxides accrete on particles over millions of years (1-4 mm per 106 y). • Contain economically important metals Cu, Zn, Co and Pb. (but too expensive to harvest). • Origin uncertain (biological?) Ferromanganese nodules Floor of South Pacific Ocean. Nodule size 1-5 cm diameter Ferromanganese nodules Cross-section Deep-sea stratigraphy • Broad-scale layering of sediments that cover the basaltic crust. • Strongly influenced by sea-floor spreading and direction of spreading centers with relation to latitude. Deep-sea stratigraphy The Atlantic basin contains a “two-layercake” stratigraphy–a thick basal layer of carbonate ooze overlain by a layer of mud. Pacific plate moves across latitudes… The Pacific basin contains a “five-layer-cake” stratigraphy, because unlike the Atlantic its sea floor as it spreads crosses the equator where the CCD is lowered to the ocean bottom. Geophysical3Surveying END 2-5 OF LECTURE