Sedimentary RocksSedimentary RocksGeology 200Geology for Environmental ScientistsGeology for Environmental ScientistsMajor Concepts• Sedimentary rocks form by the processes of weathering, erosion, transportation, and g, , p ,deposition, or by chemical precipitation.•Two main typesTwo main types– clastic: pieces of older rocks (clasts)hil ii df dil–chemical: precipitated or formed in place• Sedimentary structures are critical to interpreting sedimentary rocks.The Rock CycleThe Rock CycleMajor Concepts•Major sedimentary environments:•Major sedimentary environments:– fluvial and alluvialli–eolian– glacial–delta– shoreline–organic reefs– shallow marine– deep marineMajor ConceptsSedimentary rock layers are hierarchically grouped or subdividedhierarchically grouped or subdivided into both:k i hi i–rock stratigraphic units–time stratigraphic unitsgpWe’ll cover this in the section on tti hi ltistratigraphic correlation.Clastic Sedimentary Rocks• Produced by weathering of rocks.•Breccialarge angular grains•Breccia-large, angular grains• Conglomerate - large, rounded grains• Sandstone - sand sized grains•Siltstonesilt sized grains•Siltstone -silt sized grains• Shale and Mudstone - silt and claysized grainsGrain Size• Grain size can be an indicator of the energy of the environmentenergy of the environment.• Generally speaking, higher energy id i dwater or wind currents are required to move larger grain sizes.Grain Size cont’dGrain Size, cont dSize ranges: small to largeSeages:sa oage• boulders, >256 mm (>1 ft.)bbl >64 (>3 i )•cobbles, >64mm (>3 in.)• pebbles, >4mm• granules, 2-4mm•sand 1/8-2mmsand, 1/82 mm• silt, >1/256 mml1/26•clay, <1/256 mmFig. 5.4. Grain size ranges for classification ofcommon clastic sedimentary rocksGrain SortingGrain Sorting•Grain sorting can also be an indicatorGrain sorting can also be an indicator of the energy of the environment.Wllddi d i d i•Well-sortedsediments are deposited in high energy environments. Currents sort the grains by size.•Poorly-sortedsediments may indicatePoorlysortedsediments may indicate weak currents, or transport by glaciers.Well sorted sand grains and an outcrop of gpwell-sorted sandstone.Poorly sorted glacial deposit ranging from cobbles to mud-sized particlescobbles to mud-sized particles.ConglomerateConglomerateConglomerate deposited as an alluvial fCt fUthfan, Cretaceous of UtahSandstoneSandstoneSh lShaleChemical Sedimentary RocksChemical Sedimentary Rocks• Produced by chemical precipitation.• Evaporites - formed by evaporation of seawaterseawater–Salt, NaCl–Gypsum, CaSO4•Carbonates•Carbonates–Limestone, CaCO3–Dolostone, CaMg(CO3)2GypsumypLimestoneF ilifFossiliferous LimestoneCarbonates• Typically, carbonates form in warm, clear water free of clastic sediment.•Carbonate grainstonecomposed of•Carbonate grainstone -composed of sand-sized grains from invertebrate k l t litskeletons or oolites.• Carbonate mudstone -clay and silt-ysized grains from pellets and calcareous algae.calcareous algae.Carbonate Grainstone: OoliticOolitic LimestoneOolite ShoalsCarbonate Mudstone: Micritic LimestoneCarbonate Mud Banks in Florida BayInvestigating Carbonate Mud Banks in Fl id BFlorida BayInvestigating Carbonate Mud Banks in Fl id BFlorida BayCoal• Classified as a chemical rock by most textbooks But it is not a chemicaltextbooks. But it is not a chemical precipitate. Instead it forms from plant debris not consumed by bacteria anddebris not consumed by bacteria and other organisms. It is an organic rock.• Takes about 10 ft. of peat to form 1 ft. of bituminous coal.o b u ous co .Types of coal:peatlignitebituminousth itanthraciteUpper Freeport Coal, Preston County, WVUpper Freeport Coal, Preston County, WVExamples of SedimentaryExamples of Sedimentary StructuresSedimentary Structures• Provide clues to depositional environments Some examples:environments. Some examples:• Cross bedding - rivers, dunes, tidal hlchannels• Graded bedding - storms and turbiditesg• Ripple marks - lower energy• Mud cracks - subaerial exposureBedding planes are primary sedimentary structuressedimentary structures.Modern Sedimentary Structures: Mudcracks on a flood plainMudcracks on a flood plain.Ancient Sedimentary Structures: mudcracks in rocks 1 billion years old.yModern Sedimentary Structures: Current ripples on a modern beachripples on a modern beach.Beach in Ireland at low tideSedimentary Structures: Current ripples on an ancient beach, 400 MY old.,The processes by which symmetrical pyyand current ripples are formed.Geometry of sand dunes, both eolian and aquatic.TOPBOTTOMFig. 5.9. Large scale cross-bedding formed by eolian dune migration. Flow direction and top of bed indicated.Wind blown dune deposits.pMississippian cross beds near Altoona, PAMississippian ppcross beds near Altoona, PAJurassic Navaho Sandstone, Zion National Park, wind-blown cross-beddingwind-blown cross-bedding.Marine Cross Beds in Ste. Genevieve Ls., IndianaTidal Cross Beds in Ste. Genevieve Ls., IndianaSmaller cross beds deposited in aSmaller cross beds deposited in a river or stream.A meandering stream with a sandy point bar on the inside curve, and a cut bank on the outside curve. The point bar dips toward the ppstream channel.St Ch lStream ChannelUd dd Fl id KUnderwater sand dunes, Florida KeysGraded Bedding –formed byformed by storms or turbidite depositiondepositionAnother example of a graded bedSedimentary Environments or Environments of DepositionColor of Sedimentary Rocksy• Color can be useful in the interpretation of depositional environments.• Black color - indicates deposition in the absence of oxygen in either thethe absence of oxygen in either the ocean, lakes, or swamps.• Red color - indicates deposition in the presence of abundant oxygen in a pygwarm, humid terrestrial environment.Black Shale, Anoxic EnvironmentSideling Hill Outcrop, I-68. What is the depositional environment of the black layer?depositional environment of the black layer?Red Beds, Oxygenated EnvironmentEnvironments of Deposition• 3 Major EnvironmentsCi lT il•Continental or Terrestrial•MarineMarine• TransitionalContinental or TerrestrialContinental or Terrestrial Environments• Glacial - both Alpine and Continental•Alluvial Fansat the base of mountains•Alluvial Fans -at the base of mountains• Lakes• Rivers and Floodplains = Fluvial and alluvial depositsalluvial