Sedimentary Rocks

Introduction

Once exposed to the Earth’s surface, all rocks are subjected to processes of erosion, transportation and deposition… Thus sediment becomes sedimentary rock. Sedimentary rocks are classified on the basis of the nature of the sediment that they were formed from. Sedimentary rocks are usually divided into two main groups:

DETRITAL – named for the size of the detrital particles (e.g., sandstone, siltstone, mudstone)

CHEMICAL – named on the basis of chemical composition (limestone, chert, evaporates); sometimes biogenic rocks are classified separately…

Detrital (clastic) sediments

Involves erosion, transportation and deposition by moving water. Requires energy thresholds to transport particles of different sizes, therefore water-transported detrital rocks are often well sorted by grain size.

Conversion of unconsolidated sediment into sedimentary rock is called diagenesis or lithification – achieved by both compaction and cementation. Typical cements are calcite and quartz. Diagenesis is hastened by circulation of heated fluids, such as those given off from compaction of buried sediments in deep basins.

Weathering

Weathering forces may be either mechanical or erosional. Mechanical weathering, which produces detritus, is much less significant than chemical weathering… Chemical weathering produces dissolved material (especially alkalis and alkaline earths) and leftover rock (typically quartz, clay, kspar, garnet, zircon, rutile). Notice that both quartz and feldspar were end members in Bowen’s reaction series … thus BRS can be used to indicate the order of increasing stability of mineral’s at the Earth’s surface (with qz and fsp some of the most stable, and olivine the least stable).

Hydrolysis reactions – reactions that involve H₂O. Reactions are aided by the fact that natural rainwater is slightly acidic, thanks to the interaction of CO₂ and H₂O:

CO_2(gas) + H₂O_(liquid) çè H₂CO₃ (carbonic acid)

H₂CO₃ çè H⁺ + HCO₃^-

Similarly, sulfuric acid (H₂SO₄) can form in sulfur-rich environments .. this is a much stronger acid than carbonic acid.

Weathering of calcite

Calcite weathers by a simple dissolution reaction: CaCO₃ çè Ca²⁺ + CO₃^2-

The resulting carbonate ion dissolved in solution combines with H⁺ to form carbonic acid. As a result, the concentration of H+ is decreased, increasing the pH and “buffering” the acidity of the water.

Weathering of pyroxene and olivine

The weathering of px and ol involves the breakup of the silicate structure to release Fe, which is then oxidized.

EX: 4FeSiO₃ + O₂ + H₂O çè FeO(OH)_(solid) + 4SiO_{2 (solution)}

The hydrated Fe mineral is called limonite, a clay-like mineral that makes Fe-rich soils red.

Weathering of feldspar

Weathering of feldspar also involves the reaction of the crystal with acidic water:

EX: 2KAlSi₃O₈ + 2H⁺ çè Al₂SiO₅(OH)₄ + 4SiO₂ + 2K⁺

where the hydrous aluminous phase is the clay mineral kaolinite. Other clay minerals include smectite (a swelling clay) and illite (a mica-like clay). In general, clay minerals are sheet silicates that tend to be very fine-grained (hence the use of the term clay for both a class of minerals and a particle size). The end member of chemical weathering of feldspars is residual aluminum oxides and hydroxides, known as laterites (if not lithified) or bauxites, an important source of aluminum.

Clays are widely used in industry, arts & ceramics. They are also a fundamental component of soils (we’ll come back to this at the end of term).

Chemical sedimentary rocks

Chemical sedimentary rocks are those that precipitate from solutions. The ease of precipitation is inversely proportional to the solubility of the mineral in water. Thus silica often precipitates early in an evaporation sequence to form thick chert beds (most common in Precambrian rocks).

Carbonate minerals such as calcite are somewhat more soluble, and thus require slightly more evaporation. Carbonate precipitation may be in the form of fine-grained limestone muds, which form fine-grained limestone called micrite. Many carbonate rocks contain clastic material and/or fossils.

Salts such as gypsum (CaSO₄.2H₂O), halite and sylvite are highly soluble so that their chemical components are common dissolved species in water. These components precipitate out as evaporites in enclosed inland basins.

Other chemical sedimentary rocks include phosphorites and iron formations. the latter are mostly Precambrian in age, and composed of Fe oxides and hydroxides, magnetite, or Fe-carbonate (siderite, FeCO₃). Probably formed in shallow marine conditions (like present day Mn nodules).

Diagenesis

Diagenesis refers to chemical, mineralogical, or textural changes that occur in sediments or sedimentary rocks after deposition, but before metamorphism. Includes compaction, recrystallization, and leaching (dissolution).

Oceans – the final repository

There are both inputs to and outputs from ocean basins.

Inputs: detrital solids carried by rivers

dissolved ions in river water

Outputs: detrital sediments

chemical precipitates

biological sediments

Dissolved input to the ocean is balanced by precipitation. The average length of time that a particular element remains dissolved in sea water is the residence time, and can be calculated from

(total amt. of element in sea water) / (rate of input to oceans)

RT = A/(dA/dt)

Most common elements (Ca, Mg, Na, K, S) have residence times ~ 1-10 million years. Importance: needed to look at potential residence time for toxic chemicals, or CO₂ (related to climate change).

The chemical reactions that cause minerals to precipitate are controlled by physical parameters such as temperature and concentration of dissolved species, and by partial pressure of gaseous components involved in the reaction sequence.

The fate of CO₂

Increases in atmospheric CO₂ has a well documented impact on global climate. Increased levels of atmospheric CO₂ will also result in more acidic rainwater and will thus increase rates of chemical weathering. In turn, increased weathering rates will result in an increase in the input of dissolved materials to the oceans.

Oceanic processes that reduce atmospheric CO₂:

- photosynthesis by blue-green algae in the euphotic (light-penetrating) zone

- precipitation of CaCO₃

Thus there the oceans have a limited ability to buffer atmospheric CO₂. Which raises the question – is there evidence that the ocean has changed composition through time?

YES! The best evidence is the extent of banded iron formations in the Precambrian. (UP Michigan), formations that resulted from precipitation of Fe-bearing minerals (e.g. FeCO₃) from seawater. Currently, the ocean has very low levels of dissolved Fe. However, in the Precambrian, atmospheric O levels were much lower, meaning that Fe existed at the Earth’s surface in a reduced, rather than oxidized, state. As Fe²⁺, iron was soluble in the oceans and accumulated in considerable amount. As the atmosphere became more oxidizing, nearly all of the dissolved iron was precipitated to form the large iron formations.

The role of biological sediments:

Most important biological sediment is coal … forms from the diagenesis of peat and other organic matter.