Interpreting the Sedimentary Rocks at

Interpreting the Sedimentary Rocks at ____________________Step 1: Describe the texture and composition of the clastic (detrital) rocks.A. Grain/clast sizeRounded grainsConglomerateGravel Grains >2 mmAngular grainsBrecciaGeneral termSandstoneSandGrains

Step 2: Look for these physical features to try to figure out the depositional environment.A. Rock ColorOxygen-poor, stagnant settings and high organicBlack Enriched in organic matter & pyrite (FeS 2 ) productivityCoal swamp or restricted basinOxidizing conditions, often associated withRed Enriched in iron oxides such as hematite (Fe 2 O 3 ) subaerial exposure & hot arid climates.Terrestrial or shallow marineB. Surface markingsRipple Small-scale ridge of sandEnvironment was affected by flowing water, waveaction, or wind motione.g., river, estuary, sand dunePolygonal-shaped cracks formed in mud that has Environment affected by alternating wet and dryMudcrackdried out in a terrestrial environmentconditionsTidal flat, lake shore, desertC. Internal bedding featuresSettling of sediment in suspensionLaminationLake: Seasonal deposition of fine (winter) andVery fine layering composed of discrete layerscoarse (summer) sedimentof sediment a millimeter or so in thicknessMarine: Indicates lack of bioturbation andtherefore stressed conditions, e.g., low oxygen.GradedbeddingEnvironment was affected by flowing water, waveaction, or wind motione.g., river, estuary, sand dune (large-scale cross-beds)Deposition in environment where current flowvaries considerablyTidal flat, estuaryCrossbeddingHeterolithicbeddingConvolutebeddingUpward gradation in grain size from coarseror finer materialFormed by the migration of the slip-faces ofripped bedforms or dunesClosely interbedded deposit of sand and mudFolds whose intensity dies out both upwardsand downwards within a single bedFining-upward - deposition from a waning current(turbidity flow)Coarsening-upward – deposition underincreasingly high energy conditions (environmentis shallowing)Subjection of water-rich sediments to an exteshock (earthquake, large waves)Step 3: Check yourself by rea ding about the depositional environmentyou inferred.EnvironmentCommonLithologiesSedimentary StructuresFossilsTerrestrial Sandstone Unidirectional ripples and/or RareRivercross-bedding; channel forms incross-sectionTerrestrial Mudrock, shale, None or lamination; evidence for Plant rootlets; coaly seams,Floodplain siltstonesoil developmentplant debrisTerrestrial Mudrock, shale, None or laminationRare; microscopic fossilsLake siltstone, limestone Bioturbation on bed surfaces such as diatomsTerrestrialRipples; large-scale crossbeddingSandstoneSand duneRareRipples and/or cross-bedding; Often rare; organisms thatCoastalchannel forms in cross-section can withstand brackishSandstoneEstuaryEvidence for tidal influence;conditionsBioturbationMudrock, shale,Common; organismsthatOffshore/shelf siltstone; sandstoneinterbeds possibleCommon bioturbationrequire normal marineconditions to livernal