Sediment And Sedimentary Rocks - UCLA

Sediment and sedimentary rocks Sediment From sediments to sedimentary rocks(transportation, deposition,preservation and lithification) Types of sedimentary rocks (clastic,chemical and organic) Sedimentary structures (bedding,cross-bedding, graded bedding, mudcracks, ripple marks) Interpretation of sedimentary rocks

Sediment Sediment - loose,solid particlesoriginating from:– Weathering anderosion of preexisting rocks– Chemicalprecipitation fromsolution, includingsecretion byorganisms in water

Relationship to Earth’s Systems Atmosphere– Most sediments produced by weathering in air– Sand and dust transported by wind Hydrosphere– Water is a primary agent in sedimentproduction, transportation, deposition,cementation, and formation of sedimentaryrocks Biosphere– Oil, the product of partial decay of organicmaterials, is found in sedimentary rocks

Sediment Classified by particle size––––––Boulder - 256 mmCobble - 64 to 256 mmPebble - 2 to 64 mmSand - 1/16 to 2 mmSilt - 1/256 to 1/16 mmClay - 1/256 mm

From Sediment toSedimentary Rock Transportation– Movement of sediment away from its source, typically bywater, wind, or ice– Rounding of particles occurs due to abrasion during transport– Sorting occurs as sediment is separated according to grain sizeby transport agents, especially running water– Sediment size decreases with increased transport distance

From Sediment toSedimentary Rock Deposition– Settling and coming to rest of transported material– Accumulation of chemical or organic sediments,typically in water– Environment of deposition is the location in whichdeposition occurs Deep sea floorBeachDesert dunesRiver channelLake bottom

From Sediment toSedimentary Rock Preservation– Sediment must be preserved, as by burial with additionalsediments, in order to become a sedimentary rock Lithification– General term for processes converting loose sediment intosedimentary rock– Combination of compaction and cementation

Types of Sedimentary Rocks Clastic sedimentary rocks– Most common sedimentary rock type– Form from cemented sediment grainsthat come from pre-existing rocks Chemical sedimentary rocks– Have crystalline textures– Form by precipitation of minerals fromsolution Organic sedimentary rocks– Accumulate from remains of organisms

Clastic Sedimentary Rocks Breccia andConglomerate– Coarse-grained clasticsedimentary rocks– Sedimentary brecciacomposed of coarse,angular rock fragmentscemented together– Conglomerate composedof rounded gravelcemented together

Clastic Sedimentary Rocks Sandstone– Medium-grained clasticsedimentary rock– Types determined bycomposition Quartz sandstone - 90%quartz grains Arkose - mostly feldspar andquartz grains Graywacke - sand grainssurrounded by dark, finegrained matrix, often clay-rich

Clastic Sedimentary Rocks Shale– Fine-grained clasticsedimentary rock– Splits into thin layers(fissile)– Silt- and clay-sizedgrains– Sediment deposited inlake bottoms, riverdeltas, floodplains, andon deep ocean floor

Clastic Sedimentary Rocks Siltstone– Slightly coarser-grained thanshales– Lacks fissility Claystone– Predominantly clay-sizedgrains; non-fissile Mudstone– Silt- and clay-sized grains;massive/blocky

Chemical Sedimentary Rocks Carbonates– Contain CO3 as part oftheir chemical composition– Limestone is composedmainly of calcite Most are biochemical, butcan be inorganic Often contain easilyrecognizable fossils Chemical alteration oflimestone in Mg-rich watersolutions can producedolomite

Chemical Sedimentary Rocks Chert– Hard, compact,fine-grained,formed almostentirely of silica– Can occur aslayers or as lumpynodules withinothersedimentaryrocks, especiallylimestones

Chemical Sedimentary RocksEvaporites– Form fromevaporating salinewaters (lake,ocean)– Common examplesare rock gypsum,rock salt

Organics in Sedimentary Rocks Coal– Sedimentary rock forming fromcompactionof partially decayed plant material– Organic material deposited in waterwith low oxygen content (i.e.,stagnant) Oil and natural gas– Originate from organic matter inmarine sediment– Subsurface “cooking” can changeorganic solids to oil and natural gas– Can accumulate in porous overlyingrocks

Sedimentary Structures Sedimentary structures– Features within sedimentary rocksproduced during or just aftersediment deposition– Provide clues to how and wheredeposition of sediments occurred– Bedding– Cross-bedding– Graded bedding– Mud cracks– Ripple marks

Sedimentary Structures Bedding– Series of visible layerswithin a rock– Most commonsedimentary structure Cross-bedding– Series of thin, inclinedlayers within a horizontalbed of rock– Common in sandstones– Indicative of depositionin ripples, bars, dunes,deltas

Sedimentary Structures Graded bedding– Progressivechange in grainsize frombottom to topof a bed Mud cracks– Polygonal cracksformed indrying mud

Sedimentary Structures Ripple marks– Small ridges formedon surface ofsediment layer bymoving wind or water Fossils– Traces of plants oranimals preservedin rock– Hard parts (shells,bones) more easilypreserved as fossils

Sedimentary Rock Interpretation Sedimentary rocks give importantclues to geologic history of an area Source area– Locality that eroded and providedsediment– Sediment composition, shape, size andsorting are indicators of source rocktype and relative location

Sedimentary Rock InterpretationSediment deposits often become thinner away fromthe source area, and sediment grains usuallybecome finer and more rounded

Sedimentary Rock Interpretation Depositional environment– Location where sediment came torest– Sediment characteristics andsedimentary structures (includingfossils) are indicators

Sedimentary Rock Interpretation Depositional environment– Examples: glacial valleys, alluvial fans, river channelsand floodplains, lakes, deltas, beaches, dunes, shallowmarine, reefs, deep marine

Plate Tectonics andSedimentary Rocks Tectonic setting plays key role in thedistribution of sedimentary rocks Occurrence of specific sedimentary rock typescan be used to reconstruct past plate-tectonicsettings Erosion rates and depositional characteristicsgive clues to each type of tectonic plateboundary

Plate Tectonics andSedimentary Rocks Convergent boundary: Rapid erosion: coarse-grained clasticsediments are transported by streams and turbidity currentsand are deposited in basins near mountains.

Plate Tectonics andSedimentary Rocks Divergent boundary: thick wedges of gravel and coarse sand alongfault-bounded margins of developing rift valley. Lake bed depositsand evaporate rocks are located on the floor of the rift valley.

Sediment and sedimentary rocks Sediment From sediments to sedimentary rocks(transportation, deposition,preservation and lithification) Types of sedimentary rocks (clastic,chemical and organic) Sedimentary structures (bedding,cross-bedding, graded bedding, mudcracks, ripple marks) Interpretation of sedimentary rocks

Clastic Sedimentary Rocks Sandstone – Medium-grained clastic sedimentary rock – Types determined by composition Quartz sandstone - 90% quartz grains Arkose - mostly feldspar and quartz grains Graywacke - sand grains surrounded by