Sediments and Sedimentary Rocks. Sedimentary rock (a) Rock

Sediments and Sedimentary Rocks

Sedimentary rock (a) Rock resulting from the consolidation of loose sediment that has been derived from previously existing rocks and accumulated in layers (clastic) (b) Rock formed by the precipitation of minerals from solution by either organic or inorganic processes (chemical) Transport and deposition of clastic sediment Clastic particles: physically transported rock fragments produced by weathering of pre-existing rocks. Clastic sediments: accumulations of clastic particles Law: It all goes down hill

Sedimentary rocks Volume of upper crust: Area of continents: Record of geological events: e.g. Himalayas will someday be sediment Mt. Everest has fossiliferous limestones at the summit 5% 75%

Sediment stages 1. Weathering 2. Erosion 3. Transportation via water, glaciers and wind 4. Deposition 5. Burial and compaction 6. Diagenesis

Movement of sediment by wind, ice or water. Mode of transport produces distinctive deposits. Most sediments are carried by rivers: 25 billion tons/year Strong currents: faster than 50cm/s carries gravel+ finer material Moderately strong currents 20-50cm/s carries sand Weak currents: slower than 20cm/s muds and silt Transport affects the type of sediment

Sorting Sorting: measure of the variation in the range of grain sizes in a clastic rock or sediment. Well-sorted sediments indicate that they have been subjected to prolonged water or wind action. Poorly-sorted sediments are either not far-removed from their source or deposited by glaciers.

Roundness Roundness: measure of how rounded the corners are Angular grains close to its source Rounded grains transported for a great distance

Clastic sediments: Results from change in velocity of carrying medium Siliciclastic particles are mostly silicates (sandstone) Sediments are sorted: river channel has coarse sediment, banks fines sediment Chemical sediments form near place of deposition usually from seawater (carbonate rocks) Biochemical sediments: undissolved mineral remains or organisms and minerals precipitated by biological processes (coral, shells) Sedimentation in ocean: Currents are slow, only very fine material does not settle Fresh water meets saline water results in precipitation. Sedimentation Why is the ocean saline? Rivers carry dissolved matter to the ocean, evaporation concentrates the salts

Sedimentary environments I

Clastic Sedimentary environment Dominated by actions of waves, tides and currents Primary influenced by currents

Chemical and biochemical sedimentary environment

Sedimentary structures Bedding or stratification: paralel layers of grains of different size or kind. What’s up: Cross bedding: sets of bedded material deposited at angles as large as 35o. Graded bedding: coarse grains at base and progressively finer grains towards the top. Ripples: small dunes of sand or silt whose long dimension is at right angles with the current. Bioturbation: remnants of burrows and tunnels by clams worms etc

Cross bedding I

Cross bedding II Grains are deposited on the lee side of the dune

Ripples

A-Symmetrical ripples

Modern rippled sand

Ripple Marks

Making ripples

Bioturbation

Continental slope

Suspension of water, sand, and mud that moves downslope (often very rapidly) due to its greater density than that of the surrounding water (often triggered by earthquakes) Speed of turbidity currents first appreciated in 1920 — breaking of phone lines in the Atlantic; also gave indication of distance traveled by a single deposit These turbidity currents can result in Tsunamis Turbidity currents

Location of origin of turbidity currents

Graded bedding I Scale can be meters to centimeters

Graded bedding II

River sediment

Diagenetic processes I Sediments are buried compacted and lithified at shallow depths in the Earth’s crust…. …or subducted where they are subjected to high pressure and temperature Diagenesis: process- physical and chemical- that changes sediments in sedimentary rocks

Diagenetic process II Compaction squeezes out the water Compaction squeezes out the water Precipitation: addition of new minerals cements the sediment particles

Classes of clastic sediments

Relative abundance of sediments

Clastic rocks

Mineralogy of sandstone

Classification of (bio)chemical sediments

Carbonate platforms I

Carbonate platforms II Within the reef lagoon, growth of carbonate-secreting organisms, including forminefera, coral, algae and molusks, is rapid, and carbonate sediments form quickly If sealevel rises, the reef continues to grow towards the light and lagoon sedimentation outpaces sedimentation in the open ocean Eventually a carbonate platform grows with steep sides towards the sea

Coral reefs and atolls I Bora Bora atoll, South Pacifc

Coral reefs and atolls II Process first described by Charles Darwin

Chemical and biochemical sedimentsI Reefal limestone

Chemical and biochemical sediments II Evaporites: precipitations due to evaporating seawater. arid climate freshwater supply from rivers is low connections to open sea are restricted Silica sediment: chert Source of silica: diatoms or other silicate microorganisms - Diagenetic alteration Iron oxide: Banded iron formation; iron ore Older formed when the atmosphere had less oxygen Ocean contained oxygen producing microorganism oxidizing Fe Phosphorites Cold deep ocean water rising along the continental margin deep ocean water rich in phosphorous

Evaporites 1. During Miocene, the Mediterranean became a shallow evaporite basin 2. Reduced exchange with open sea 3. Evaporation removed water 4. Fresh water inflow waslimited 5. Gypsum and halite crystalize first formig evaportites