Chapter 7: Sediments and Sedimentary Rocks:Archives of Earth’s History
Introduction: Sediments In The Rock Cycle • Sediments are transported by: • Water. • Ice. • Wind. • Gravity. • The transport and deposition of sediments are initial phases of the rock cycle. • The ultimate fate of most sediment is burial and conversion to sedimentary rock.
Sedimentation, Stratification, and Bedding • Stratification, or bedding, is the arrangement of sedimentary particles in layers. • Each stratum (plural = strata) or bed is a distinct later of sediment. • The top or bottom surface of a bed is a bedding plane.
Sediments Types And Characteristics • Three broad classes: • Clastic sediment is loose fragments of rock debris produced by physical weathering. • Sand and clay. • Chemical sediment precipitates from solution in water. • Calcium carbonate and salt. • Biogenic sediment is composed of the fossilized remains of plants or animals. • Coal, oil, and natural gas.
Characteristics of Clastic Sediment (1) • Clastic sediment is divided into four main size classes: • Gravel. • Sand. • Silt. • Clay.
Characteristics of Clastic Sediment (2) • Gravel is subdivided into: • Boulder gravel. • Cobble gravel. • Pebble gravel.
Sorting (1) • Sorting identifies sediment in terms of the variability in the size of its particles. • Poorly sorted (wide range of particle size), • Well sorted (range is small). • Changes of grain size typically result from fluctuations in the velocity of the transporting agent, such as water or wind.
Sorting (2) • The greater the speed and energy involved, the larger or heavier are the particles that can be transported. • Unusually dense minerals (e.g. gold, platinum,magnetite) are deposited first when stream velocity slows. • Lighter particles are carried onward.
Sorting (3) • Nonsorted sediment is a mixture of different sizes arranged chaotically. • Till is a nonsorted sediment of glacial origin. • Smooth and rounded particles are transported by water or air.
Varves • Rhythmic layering occurs in varves. • A varve is a pair of sedimentary layers deposited over the cycle of a single year. • In spring, the inflow of sediment laden water increases and coarse sediment is then deposited throughout the summer. • During winter, very fine sediment slowly settles to form a thinner, darker layer.
Rhythmic Lamination and Cross Bedding • Rhythmic laminations may result from a succession of large storms. • Cross bedding creates beds that are inclined with respect to a thicker stratum within which they occur. • Cross bedding is the work of turbulent flow in the transporting agency.
Graded Bedding • In graded bedding, the particles are sorted more or less according to size, grading upward from coarser to finer. • Most coarse clastic sediments consist of mineral grains and rock least susceptible to chemical and physical breakdown. • High content of quartz and potassium feldspar.
Characteristics of Chemical Sediments • Chemical sediments are formed by precipitation of minerals from solution in water. • They form in two principal ways: • Through biochemical reactions resulting from the activity of plants and animals in the water. • Inorganic reactions in the water. • When water from a hot spring cools, it may precipitate opal (a hydrated silicate) or calcite (calcium carbonate).
Salts (1) • Evaporation of sea water or lake water forms salts. • Lake waters precipitate sodium carbonate (Na2CO3), sodium sulfate Na2SO4), borax (Na2B4O7.10H2O),and trona (Na2CO3.NaHCO3.2H2O).
Salts (2) • Lake water precipitates are used in the production of: • Paper. • Soap. • Detergents. • Antiseptics. • Tanning and dyeing.
Salts (3) • The most important salts that precipitate from seawater are halite (rock salt: NaCI) and gypsum (CaSO4.2H2O). • Much of the common table salt we use daily and the gypsum used for plaster and construction materials are recovered from marine evaporate deposits.
Characteristics of Biogenic Sediments (1) • Biogenic sediments contain fossils. • If the remains are broken and scattered it is bioclastic sediment. • When floating microscopic marine organisms die, their remains settle and accumulate on the seafloor to form a muddy sediment called deep-sea ooze.
Characteristics of Biogenic Sediments (2) • Siliceous ooze resembles calcareous ooze, but is composed mainly of the siliceous remains of tiny floating protozoa (radiolarians) and algae (diatoms). • Calcareous ooze is a fine-grained, deep-sea deposit of skeletal material containing more than 30% calcium carbonate. • Siliceous ooze is made of skeletal material made mainly of silica.
Sedimentary Environments And Facies Changes • Vertical changes in strata reflect the passage of time. • Horizontal strata change over distance, but the changes occur within the same stratum. • Such a lateral change from one depositional environment to another is called a change of facies. • Each environment has distinctive physical, chemical, and biological characteristics.
Nonmarine Depositional Environments: Streams • Stream sediments. • Widespread. • The type depends on: • The energy available for doing work. • The nature of the sedimentary load.
Nonmarine Depositional Environments: Lakes • Lake sediments. • Sediments deposited in a lake accumulate both on the lakeshore and on the lake floor. • Sand and gravel form beaches, bars, and spits across the mouths of bays. • Finer sediment forms a delta.
Nonmarine Depositional Environments: Glaciers • Glacial sediments. • Sedimentary debris eroded and transported by a glacier. • Random mixture of particles that range in size from clay to boulders. • Stones in glacially deposited sediment often are angular and some are striated;
Nonmarine Depositional Environments: Wind • Eolian (windblown) sediments. • Sediments carried by the wind tends to be finer than that moved by other erosional agents. • Sand may pile up to form dunes. • When powdery dust picked up and moved by the wind is deposited, it becomes progressively thinner and finer with increasing distance downwind.
Shoreline and Continental Shelf Environments (1) • Deltaic sediments. • Lakes and marine deltas. • Estuarine sediments • Sediments trapped in an estuary. • When the stream water meets seawater, the clay particles tend to aggregate into clumps. • Beach sediments. • Quartz is typical of beach sediments.
Shoreline and Continental Shelf Environments (2) • Offshore sediments. • Fine grained sediment, carried in suspension, settle slowly to the seafloor. • Only about 10 percent of the sediment reaching the continental shelves remains in suspension long enough to arrive in the deep sea. • Carbonate shelves. • Carbonate sediments accumulate mainly on broad, flat carbonate shelves that border a continent.
Shoreline and Continental Shelf Environments (3) • Marine evaporate basins. • Ocean water occupying a basin with restricted circulation that lies in a region of very warm, dry climate will evaporate. • Deep-sea fans. • Some large submarine canyons on the continental slopes are aligned with the mouths of major rivers, like the Amazon, Congo, Ganges, and Indus. • At the base of many such canyons is a huge deep-sea fan.
Shoreline and Continental Shelf Environments (4) • Sediments drifts; • Huge bodies of sediments up to hundreds of kilometers long, tens of kilometers wide, and two kilometers high have been discovered along the continental margins bordering the North Atlantic ocean and Antarctica. • An exceptional rate of deposition, 200 cm/1000 years, has been measured for a sediment drift near Bermuda. • Cores obtained from sediment drifts can provide a record of environmental change measurable at a decadal, rather than millennial, scale.
Deep-Sea Depositional Environments (1) • Deep-sea oozes. • Calcareous ooze occurs over wide areas of ocean floor at low to middle latitudes where warm sea surface temperatures favor the growth of carbonate-secreting organisms in the surface water. • Calcareous ooze is rare where the water is deeper than 3-4 km.
Deep-Sea Depositional Environments (2) • Cold deep-ocean waters are under high pressure and contain more dissolved carbon dioxide than shallower waters. • At a level called the carbonate compensation depth, the shells are dissolved, before they can reach the seafloor.
Deep-Sea Depositional Environments (3) • Land-derived sediment. • Deep-sea strata include sediments carried to the ocean by: • Rivers. • Wave action. • Wind (fine desert dust and volcanic ash), • Floating ice.
Diagenesis: How Sediment Becomes Rock (1) • Lithification is the overall process of creating sedimentary rock. • Diagenesis is the collective term for all the chemical, physical, and biological changes that affect sediment as it goes from deposition through lithification.
Diagenesis: How Sediment Becomes Rock (2) • Processes involved include: • Compaction. • As the weight of an accumulating sediment forces the grains together. The pore space is reduced. • Cementation. • Substances dissolved in water precipitate to form a cement that binds the sediment grains together.