GEOL 340 Sedimentology and Stratigraphy
Lecture 4
1. Environments of Weathering
2. Physical Weathering
3. Chemical Weathering
4. Diagenetic Products of Weathering
5. Soil Formation and Microbiology
Environments of Weathering
1. Siliciclastics versus Carbonates
2. Surficial (Seawater versus Freshwater) versus Burial Environments
3. Aqueous (Water) Environments
a. Subaqueous = under water
b. Marine = seawater
c. Submarine = under seawater
d. Meteoric = freshwater
e. Phreatic = 100% of porosity filled with water
f. Vadose = porosity filled with a mixture of water and air
3. Terrestrial Environments
a. Subaerial = above water
Physical Weathering - breakage into smaller fragments
a. abrasion by sands suspended in air, water and ice
b. frost wedging = freezing and thawing of water in fractures
in rock
c. thermal expansion = heating and cooling of rocks (minor effect?)
d. crystallization = growth of crystals wedge fractures apart
(minor effect?)
Chemical Weathering - solution and reprecipitation (Table
1.1, p. 6)
Define: Dissolution versus Solubilization
a. hydrolysis = reaction between H+ and OH- of water with
ions in mineral
CO2 + H2O Æ H2CO3 (carbonic acid) Æ H+ + HCO3-
(bicarbonate) Æ CO3
(more CO2 makes more acidic, handout)
b. hydration and dehydration = addition or removal of H2O to mineral
c. oxidation/reduction = stripping/adding electrons
d. solution = mineral dissolves
e. ion exchange = exchange of ions between water and solid mineral
f. chelation = weakly bonding metal ions to organic matter
g. rates = function of mineralogy, grain size, water volumes,
climate
(reverse Bowen's reaction series of olivine-pyroxene-amphibole-biotite;
Tab. 1.2, p. 8)
Diagenetic Products of Weathering (diagenetic
box model)
a. solid chemically altered particles or residues of the original
host rock
soils and paleosols (rootlets, cements, d13C, dessication cracks,
Tab. 1.4, p. 13)
b. diagenetic (secondary) minerals precipitated in pore spaces
(cements)
siliceous and carbonate cements (meteoric pendant cements)
c. soluble components released during water-rock interactions
seawater reacts with hot basalt = Mg and SO4 precipitated; Ca,
Mg, Si released
Soil Formation and Microbiology
Processes of Chemical Weathering (Table 2.1, p. 20)
Soil rock detritus at the surface of the earth that has been sufficiently weathered to support the growth of rooted plants
Weathering of bedrock physical, chemical, and biological
Regolith surficial layer of broken rock and soil
Soil horizons distinctive weathering zones, approximately parallel to the surface of the ground, produced by soil forming processes
Soil Horizons (diagram on handout)
a. O horizon - accumulation of OM on the ground surface beneath
vegetation cover
b. A horizon - dark upper zone of organic accumulation mixing
with mineral soil
(zone of leaching or eluviation)
c. B horizon - mostly mineral soils with little organic humus
(zone of accumulation or illuviation)
d. C horizon zone below major biologic activity
lies just above bedrock, mostly weathered bedrock, no true soil
formation
R Horizon parent sedimentary or bedrock material underlying the soil horizons
Soil Formation Controls (diagram on handout and Table 2.4,
p. 28; Fig. 2.3, p. 29)
parent material
climate (controls erosion and groundwater composition)
example of limestone soil formation of Illinois versus Jamaica
topography
time
Soil Components Seen in the Geologic Record
maturity - layered soils = mature
Paleosols (caliches) - ancient soils at subaerial exposure
unconformities
a. mineral soil (<25% Organic Matter (OM)) (diagram on handout)
b. organic humus (>25% OM)
c. red color due to Fe oxides
d. roots and alveoli
e. vadose carbonate cements
f. irregular nodules or glaebules of calcite
g. peds (Table 2.5, p. 31)
h. negative carbon isotope signature
Soil Microbiology
a. tool is ribosomal Ribonucleic Acid (rRNA) applied by Carl
Woese here at Illinois to establish our new concept of the tree
of life
b. major habitat of terrestrial microorganisms is soil (106 -
109 bacteria/gram)
c. soil-gas CO2 and the d13C signature
d. show an example of a soil-gas CO2 signature in a paleosol