University of Colorado GEOLOGY 1010 Class Note 7


Sedimentation and Sedimentary Rocks

The different minerals that form as weathering products have different characteristic grain sizes that determine the distances that they are typically transported and the environments in which they are deposited.

This produces a sequence of sedimentary rocks that represents the various weathering products of rocks and the distance that these products may be transported from their sources. Because the different grian-size fractions have different chemical compositions, the process produces a strong chemical differentiation in the various rock types.

Coarse products (boulders and cobbles) require fast moving water (mountain streams) to be moved at all, and so are not transported very far from their sources. These particles may be unweathered and retain their source mineralogy and chemistry. Intermediate-sized particles (sand) are transported by rivers and wind and deposited at coasts or in deserts. Because quartz (SiO2) is abundant and relatively resistant to chemical weathering it makes up the bulk of sand-sized particles.

Fine particles (silt and clay) are carried to regions where the water is still (off-shore environments). Clay minerals that are the weathering products of feldspars and ferro-magnesian minerals form the bulk of these particles.

Dissolved material (Ca, Na, K, CO3, SO4, Cl) is carried farthest and deposited where the ocean, sea, or lake is evaporated off. As sea water is evaporated, the sequence of minerals formed is 1) calcite (CaCO3), 2) gypsum (CaSO4.2H2O), 3) halite (NaCl), and 4) sylvite (KCl). Such rocks are called evaporites.

After sediments are deposited, they are commonly compacted by the weight of overlying sediments.

They may be lithified (solidified) by the deposit of a cement or secondary mineral that fills the pores. They may also be lithified by recrystallization of the primary minerals.

Boulders, cobbles and pebbles may be lithified to form a conglomerate if the particles are rounded, or a breccia if the particles are angular.

Sand sized particles are lithified to form a sandstone.

Silt and clay sized particles are lithified to form a mudstone if massive or a shale if fissile (fractures on fine bedding planes).

Limestone is the rock formed by calcite. Calcite is very near saturation in sea water and so is used as shells material by marine organisms. Most calcite rocks of phanerozoic age (the last 600 million years) are of biological origin. Dolomite (CaMg(CO3)2) may also be formed in this way.

Canon de Huasteco near Monterrey, Mexico. Here in the Sierra Madre Orientale, there is more than 15km of carbonate sediments. Tectonically tilted to near vertical, they are resistant to weathering in the arid climate. The large cave entrance in the middle of the cliff face is characteristic of carbonates.

Evaporites are the rocks formed by chemical precipitation during evaporation of sea water.

Shales in the Grand Canyon, Arizona, USA. Here alteration in the oxidation potential changes the color of Fe-bearing sediments from greeen to red resulting in prominent bedding visible in this outcrop.

Bedding is a series of visible layers within the rock. It is primarily due to episodic nature of sedimentation where very fine particles are laid down slowly between times of more rapid deposition.

Bedding planes are assumed to be originally horizontal or nearly horizontal for water-laid sediments.

For aeolian (wind-laid or sub-aerial) commonly show cross-bedding, because the sand can support steeper dune surfaces in air than in water.

Mud-cracks form because clay minerals may shrink by up to 15% in volume on drying out. Mud-cracks can be preserved and indicate a depositional environment that is near shore and periodically exposed to air.

Ripple marks in sandstones or mudstones are the result of shallow wave action and indicate a very near-shore environment. Asymmetric ripple marks indicate moving water.


GEOL 1010 Syllabus

Class Note 8

Class Note 6

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