Ch 6: Sedimentary Rocks

Sedimentary rocks

Rocks formed by surface processes including weathering transport deposition and lithification of sediments

Why sedimentary rocks are important

They record fossil evolution past climate change ancient tectonic events and host major natural resources like groundwater oil and gas

Two main sediment types

Siliciclastic sediments and chemical or biochemical sediments

Siliciclastic sediments

Physically broken fragments of preexisting rocks transported and deposited as particles such as sand mud or gravel

Examples of siliciclastic rocks

Sandstone shale conglomerate

Chemical sediments

Sediments formed by direct precipitation of minerals from solution such as halite and gypsum

Biochemical sediments

Sediments formed through biological processes such as shells and reefs composed of CaCO3

Most abundant sedimentary rock type

Clastic sedimentary rocks make up nearly 90 percent of sedimentary rocks

Most dominant chemical or biological sedimentary rock

Limestone which is about three times more common than other chemical sedimentary rocks

Weathering

Breakdown of rocks at Earth’s surface through physical and chemical processes

Mineral stability during weathering

Minerals crystallizing at high temperature in Bowen’s Reaction Series are less stable at Earth’s surface

Bowen’s Reaction Series relevance to weathering

High temperature mafic minerals like olivine weather faster than low temperature felsic minerals like quartz

Why basalt weathers faster than granite

Basalt contains unstable mafic minerals whereas granite contains more resistant quartz and feldspar

Transportation in rivers

Flow velocity decreases downstream causing sediments to become sorted by size

Strong current velocity threshold

Greater than 50 cm per second can transport gravels

Moderate current velocity range

20 to 50 cm per second deposits sand

Weak current velocity

Less than 20 cm per second carries mud

Sorting

Process by which sediments are segregated by size during transport

Conglomerate

A coarse grained clastic sedimentary rock composed of rounded gravel sized clasts often poorly sorted

Sandstone

A medium grained clastic rock composed mainly of sand sized quartz grains

Shale

A very fine grained sedimentary rock composed primarily of clay minerals deposited in low energy environments

Till

Unsorted glacial sediment containing many grain sizes transported and deposited by ice

Why glacial till is poorly sorted

Glaciers transport sediments of all sizes simultaneously without sorting

Lithification

Process by which loose sediments become solid sedimentary rock through compaction and cementation

Compaction

Reduction of pore space as sediments are buried and pressed together

Water content change during compaction

Clays may contain up to 60 percent water before compaction and about 10 percent after

Cementation

Precipitation of minerals such as silica calcite or iron oxides between grains to bind sediment

Common cementing minerals

SiO2 CaCO3 Fe2O3

Diagenesis

Physical chemical and biological changes occurring during burial that transform sediment into rock

Burial temperature gradient

Approximately 30 degrees Celsius per kilometer depth increase

Oil and gas formation depth

Organic matter may convert to oil and gas at depths around 4 km where temperatures reach about 120 degrees Celsius

Principle of Original Horizontality

Sediments are originally deposited in horizontal layers under gravity

Principle of Superposition

In undisturbed sedimentary sequences the oldest layers are at the bottom and youngest at the top

Bedding

Layering in sedimentary rocks caused by variations in particle size or composition

Cross bedding

Sedimentary structure formed by migration of ripples or dunes indicating paleocurrent direction

Ripples

Small scale sedimentary structures formed by water or wind currents indicating ancient environments

Chemical weathering

Process by which minerals dissolve or chemically transform releasing ions into solution

Major dissolved cations in seawater

Calcium magnesium sodium potassium

Major dissolved anions in seawater

Carbonate sulfate chloride

Evaporite deposits

Chemical sedimentary deposits formed by evaporation of water leaving minerals like halite and gypsum

Halite

Evaporite mineral composed of NaCl formed by precipitation from saline water

Gypsum

Evaporite mineral composed of CaSO4 2H2O formed by evaporation

Carbonate rock

Sedimentary rock composed primarily of CaCO3

Limestone

Sedimentary rock composed mainly of calcite or aragonite formed chemically or biologically

Carbonate Compensation Depth CCD

Depth in ocean typically 3.5 to 4.5 km below which carbonate shells dissolve

Why carbonate shells dissolve below CCD

High pressure low temperature and high CO2 increase solubility of CaCO3

Foraminifera

Microscopic single celled marine organisms with calcite shells contributing to limestone formation

Diatoms

Microscopic organisms with siliceous shells contributing to silica rich sediments

Why deep sea floor contains siliceous sediments instead of carbonates

Because carbonates dissolve below CCD while silica shells remain stable

Chert

Fine grained silica rich sedimentary rock formed from accumulation of microscopic silica shells

Carbonate factories

Shallow warm marine environments such as reefs where large amounts of CaCO3 are produced

Fossiliferous limestone

Limestone containing visible fossils indicating biological origin

Siliciclastic sediment source

Derived from physical breakdown of volcanic igneous and metamorphic rocks

Barnett Shale

Major shale formation in Texas with large natural gas reserves

Energy environment in deposition

High energy environments deposit coarse grains low energy environments deposit fine grains

Bioturbation

Disruption of sedimentary layers by organisms

Delta formation

Fine sediments transported by rivers accumulate at river mouths forming deltas

Relationship between tectonics and sedimentary rocks

Sedimentary basins record uplift erosion deposition and subsidence related to plate tectonics