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WEATHERING AND SEDIMENTARY ROCKS

Weathering - Process which acts at the earth's surface to decompose and breakdown rocks.

Erosion - The movement of weathered material from the site of weathering. Primary agent is gravity, but gravity acts in concert with running water.

Types of Weathering

  1. Mechanical or Physical - the breakdown of rock material into smaller and smaller pieces with no change in the chemical composition of the weathered material.

  2. Chemical - the breakdown of rocks by chemical agents. Obviously the chief chemical agent is water which carries dissociated carbonic acid.

Mechanical Weathering

  1. Expansion and Contraction - the thermal heating and cooling of rocks causing expansion and contraction.

  2. Frost Action - Water freezes at night and expands because the solid occupies greater volume. Action wedges the rocks apart. Requires adequate supply of moisture; moisture must be able to enter rock or soil; and temperature must move back and forth over freezing point.

  3. Exfoliation - process in which curved plates of rock are stripped from a larger rock mass. Example Half Dome. Exact mechanism uncertain but probably due to unloading.

  4. Other types - Cracking of rocks by plant roots and burrowing animals.

Chemical Weathering

Factors which effect the rate of chemical weathering are:

  • Particle size - Smaller the particle size the greater the surface area and hence the more rapid the weathering

  • Composition

  • Climate (See Figure)

  • Type and amount of vegetation

Chemical Weathering of Rocks

Show Figure and explain formation of carbonic acid

H2O + CO2 ------->> H2CO3

Acid then dissociates and the following happens:

2KAlSi3O8 (feldspar)+ 2H+ + H2O ------->> Al2Si2O5(OH)4 (clay)+ 2K+ + 4SiO2

Weathering of Igneous Minerals

Products of Weathering Figure:


  1. Quartz - slow process and largely ineffective. Quartz remains quartz. Grains are rounded.

  2. Feldspar - weathers to clay with the cations Na, Ca, and K going into solution. Clays that can form include kaolinite (pure aluminum silicate), illite and montmorillonite. Factors which dictate clay formation are (a) climate; (b) time; (c) parent material.

  3. Muscovite - Same as above

  4. Ferromagnesian minerals - weather to clay plus highly insoluble iron oxides, essentially varieties of limonite (rust).

Rates of Weathering

Studied by S.S. Goldich (Figure) and found to be inverse of Bowen's Reaction Series. Why? A function of equilibrium, the higher the temperature of formation of a mineral the more unstable it is at the earth's surface. Hence olivine weathers the most rapidly.


Soils

Soil - Surficial material that forms due to weathering. Includes an organic component. Many different soil types. Factors effecting their formation are:

1) Climate

2) Relief

3) Bedrock material

4) Time

Classification of soils varies depending on the classifier. Geologists use a very simple classification based largely on materials added or removed from the soil during its formation.

Soil consists of four major zones (horizons) (Figure).

1. O horizon - Organic layer

2. A horizon - Zone of leaching - Cations are leached from this horizon by strongly acid solutions generated in the O horizon

3. B horizon - Zone of Accumulation - Cations leached out of the A horizon accumulate here. Horizon consists of clays, iron and aluminum oxides. Deposition due to neutralization of acid solutions.

4. C horizon - Partially decomposed parent material. Lower most zone.

Soil Types

Pedalfer - Named for the abundance of Al and Fe in the B horizon. Occur in temperate, humid climates. Lie generally east of the Mississippi River, correspond with 63 cm/yr rainfall contour.

Pedocal - Named for the accumulation of calcium carbonate in the B horizon. Characteristic of temperate, dry climates. Lie generally west of Miss River. Poorly developed A horizon, B horizon is caliche (calcium carbonate).

Laterites - Tropical soils thought to represent the end products of weathering. Characterized by stark red color and abundance of iron and aluminum oxides and lesser clay minerals. Requires abundant rainfall.

SEDIMENTARY ROCKS

Sedimentary Rocks - Layered or stratified rocks formed at or near the earth's surface in response to the processes of weathering, erosion, transportation and deposition.

Rock Cycle

All rocks discussed in this class are a part of the rock cycle (Figure).

Sedimentary Cycle (Figure)

Processes

  1. Transportation - Transporting medium usually water. More rarely wind or glacial ice.

  2. Deposition - Occurs when energy necessary to transport particles is no longer available. Deposition due to the gentle settling of mineral grains. Can also be result of chemical precipitation due to changing conditions.

  3. Lithification - Involves several steps. All taken together are termed Diagenesis.

  1. Compaction - Squeezing out of water.

  2. Cementation - Precipitation of chemical cement from trapped water and circulating water.

  3. Recrystallization - Growth of grains in response to new equilibrium conditions

Single most important characteristic of sedimentary rocks is layering. Occurs in response to changes in conditions at the site of deposition. Sedimentary rocks cover 75% of the earth's surface, but amount to only 5% of the outer 10 km.

Origin of Sedimentary Material

  • Derived directly from pre-existing rocks. Ex. quartz.

  • Derived from weathered products of these rocks. Ex. clay.

  • Produced by chemical precipitation. Ex. calcite.

First two processes result in detrital or clastic rocks. Third produces nondetrital or chemical sedimentary rocks.

Minerals of Sedimentary Rocks

  1. Clay - Important constituent of mudstones and shales, but occurs in minor amounts in all sedimentary rocks.

  2. Quartz - Most abundant constituent of sandstone. In addition to detrital quartz, free silica can be chemically precipitated as opal, chalcedony and chert.

  3. Calcite - Chief constituent of limestone. Precipitates from seawater which is saturated in both Ca+2 and CO3-2. Small changes in both T and P enough to cause precipitation. Differs from most compounds in that solubility decreases with increasing temperature.

  4. Others

    1. Dolomite CaMg(CO3)2 - Most important constituent of dolostone

    2. Feldspars - Occur in sedimentary rocks formed by very quick deposition and burial allowing no time for feldspars to alter to clay.

    3. Iron oxides and sulfides - Chemical precipitates dictated by the environment at the site of deposition.

    4. Salts and gypsum - Chemical precipitates occurring in restricted sedimentary basins under arid climatic conditions. Modern analog is the Middle East (Red Sea).

    5. Volcanic Debris - Glass and other pyroclastic material incorporated into sediments.

  1. Organic Material - Forms coal and gives color to black shales.

Classification of Sedimentary Rocks

Texture - Size, shape and arrangement of particles.

  1. Clastic - Formed from broken or fragmented grains (detrital). Rock appears grainy. Basis of classification of the clastic rocks is the Wentworth Size Scale which was derived from studies of grain diameters.

Wentworth Size Scale

Boulder

>256 mm

Conglomerate

Cobble

64-256 mm

Pebble

2-64 mm

Sand

1/16-2 mm

Sandstone

Silt

1/256-1/16 mm

Siltstone

Clay

<1/256 mm

Shale

Conglomerate - Detrital rock made up of more or less rounded fragments, an appreciable percentage of which are pebble size or larger

Sandstone - Consists primarily of grains in the sand size range. Dominant mineral in sandstones is always quartz. Further subdivide sandstones based on other minerals present. Quartz sandstone is 99% quartz.  Arkose contains both quartz and feldspar. Graywacke is a garbage sandstone with quartz, feldspar, mica and rock fragments. Often has a significant fine-grained component and is poorly sorted.

Siltstone - Rare sedimentary rock composed mostly of silt sized particles. Rare because dominant mineral is quartz which does not like to get any smaller than sand size. Many siltstones thought to form by glacial grinding of sand-sized quartz grains.

Shale - Most common of the sedimentary rocks. Composed primarily of clay minerals. Often tends to split into flat sheets due to the mica-like cleavage of clay minerals.

  1. Nonclastic (chemical) - Grains are interlocked through crystallization. Has igneous appearing texture with very little open space.

Limestone - Formed by the precipitation of calcite from seawater. Most form in marine environments, but also around hot springs, as a crust in desert soils, and as cave formation.

Dolostone - Composed of the mineral dolomite. Probably starts life as limestone then is altered to dolostone by Mg-bearing solutions in arid environments.

Evaporites - Formed by partial to complete evaporation of seawater in enclosed basins. Forms salts and gypsum.

Organic Rocks - Rocks formed by the accumulation of organic material. Ex. coquina and chalk.

Coal - Rock composed of lithified plant material.

Abundance of Sedimentary Rocks (Figure)

Sedimentary Structures (Figure)


A) Structures formed during deposition

  1. Bedding - Layering of sedimentary rocks. Each bed represents a homogeneous set of conditions of sedimentation. New beds indicate new conditions. Most layering is parallel, but occasionally it is inclined. These inclined layers are cross beds. Examples of sedimentary environments in which cross beds form are dune fields and deltas.

  2. Graded beds occur when a mass of sediment is deposited rapidly. The bedding has the coarsest sediment at the bottom and finest at the top. Often found forming in submarine canyons. A collection of graded beds is termed a turbidite deposit. Well exposed in many of the sea cliffs along So. Cal. beaches.

  3. Ripple Marks - Waves of sand often seen on a beach at low tide and in stream beds.

a) Current - asymmetrical - Rivers

b) Oscillation - symmetrical - Beaches
  1. Mud Cracks - Polygonal-shaped cracks which develop in fine grained sediments as they dry out. Common in arid environments, such as a desert.

B) Structures formed after deposition

  1. Nodule - Irregular, ovoid concentration of mineral matter that differs in composition from the surrounding sedimentary rock. Long axis of the nodule usually parallels the bedding plane and seems to prefer certain layers.

  2. Concretion - Local concentration of cementing material. Generally round. Usually consist of calcite, iron oxide or silica. Can exceed 1 meter in diameter. Not understood how they form.

  3. Geode - Roughly spherical structures up to 30 cm in diameter. Outer layer consists of chalcedony. Inside lined with crystals. Calcite and quartz the most common.

C) Other features

  1. Fossils - Any direct evidence of past life. Examples are dinosaur bones, shells of marine organisms, plant impressions, etc.