Reading: Boggs Chapter 6 and 7 (sections 6.1-6.3, 6.7, and 7.1-7.3)
Chemical sediments record conditions in the basin of depositon (intrabasinal), not much about source area. By contrast, siliciclastic sediments provide information about bedrock comnposition, climate, and transport history in the source area.
Two Overall Categories (how they form):
A. Inorganic (e.g. evaporites)
B. Organic (e.g. marine limestones) - by far the most common type of chemical
sediments
Most Common Compositional Classes of Chemical Sediments are:
Carbonates
Cherts
Evaporites
Ironstones
Phosphorites
Hydrocarbons
Today we talked mainly about carbonates and chert.
Table 6.5 (from Boggs, 2001; class handout): Compare
chemical composition of rivers and oceans:
What is the source of dissolved load in rivers (produced by
what processes)?
Dissolved load in rivers is derived from chemical weathering of bedrock
(and some subsurface):
(a) bicarbonate and carbonate ions from hydrolysis, dissolution of carbonate
rocks, and from carbonate equilibria reactions in water
(b) Ca2+ ions derived from hydrolysis and dissolution reactions
(c) Silicic acid derived from hydrolysis rxns.
* All the major ions in reivers are produced by hydrolysis.
Why is the composition of world rivers so different than that
of oceans? (esp. compare top 6-8 rows of Table 6.5)
Carbonate, bicarbonate, Ca, and silica are removed from ocean water by organisms to build shells (calcite and silica). Chemical sediments make up 20-25 % of all sed rocks, and most of these are organically precipitated limestone. Chert is also significant.
Therefore: Biological processes exert major control on ocean and atmospheric chemistry through time via (1) photosynthesis and (2) extraction of ions from seawater to build skeletons.
The basic reaction for dissolution and precipitation of calcite:
CaCO3 + H2O + CO2 <==> Ca2+ + 2(HCO3)-
Understand how the reaction can move to the left or right, and . . .
how certain basic factors control ppt or dissolution: pH, CO2 content,
Temperature
Most limestones are organically produced, in both shallow marine environments
and by pelgic seidmentation in deeper water. Consider conditions that favor
carbonate deposition: clear, warm, shallow water, with little or no input of
detrital sediment.
We discussed the different conditions that favor or inhibit carbonate deposition:
temperature, water depth (sunlight), salinty, turbidity (= suspended sediment
concentration), and pH.
Also can deposit carbonate on the deep sea floor by settling of pelagic organisms sych as forams and nannoplankton. They will be preserved only if they accumulate above the calcium compensation depth (CCD).
Microcrystalline Quartz (SiO2): most chert is organically produced by diatoms and radiolarians.
Deposition of thin-bedded cherts mainly by pelagic sedimentation in deep marine settings.
Chert grains can wind up in sandstone by accretion of oceanic sediments in some kind of trench or collision zone, followed by uplift, erosoin, and reworking of the accreted trench material in streams.
All are inorganically precipitated from concentrated saline solutions
Examples: gypsum, anhydrite, halite, other minerals (sulfates, chlorides, carbonates)
Reading: Boggs Chapter 2
Here's a copy of the Lecture Notes
Key Properties of Fluids: Density and Viscosity
Contrasts between water and air: Density of water is 1.0 g/cc, ~ 700-800 times that of air. This contrast is a very important factor that drives Earth's surface processes, especially flow of water downhill and ability of water to transport sedimentary particles. This is why rivers do so much work even at very low gradients.
Formal Definition of Viscosity: μ = τ / (du/dy). We worked with this in some detail.
Different Types of Fluids: Defined according to their physical behavior when forces are applied. Plotted on an x-y plot of velocity gradient (du/dy) as a function of applied shear stress (τ). Figure 2.2.
Newtonian Fluids
Non-Newtonian Fluids
Bingham Plastics
Pseudo-Plastics
Thixotroic substances
Discussed their types of behavior, definitions, and examples of each.
Important distinction between Laminar vs. Turbulent flow in moving fluids
Transition from laminar to turbulent flow depends on Reynolds Number, dimensionless parameter:
Re = U D ρ / μ (= ratio of inertial forces to viscous forces)
Flows are turbulent for Re greater than 500-2000, laminar for Re # less than that. Most natural river flows are turbulent.
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