SPRING 2005GEOLOGY 285:INTRO. PETROLOGYDr. Helen LangDept. of Geology & GeographyWest Virginia UniversitySedimentary Structuresand what they can tell usSome sedimentary structures tell which way was UP during depositionGraded BeddingUP(fining upward sequence)Graded bedding is typical of turbidites**Turbulent mixtures of sand, gravel, mud and water that are produced by submarine landslides, deposits fine upward and basinward, submarine mudflows/debris flowsDistal=far from sourceProximal=close to sourceBouma SequenceDistal Turbidites (Washington coast)Scour surfaceWhite sand, coarserGray shale, finerUPLoad casts or ball-and-pillow structures tell WAY UPMud cracks are concave UPCross-beds tell current direction and sometimes UP directionCaused by avalanching sand down lee slope of wave-like structuresDOWNSTREAMNote that cross-beds are cut off on top, asymptotic at bottomCross-beds in wind-blown dunes, Supai Sandstone, Sedona AZCross-beds in unlithified sandsCross-beds and laminar beds in beach rockOther Current direction indicatorsSteeper slope is downstreamDOWNSTREAMAsymmetric Ripple-marksElongate clasts dip upstream (pushed like falling dominoes)DOWNSTREAMImbricated clastsFlute Casts: Spoon-shaped sole markings on bottom of beds, commonly caused by turbidity currentsTop viewSide viewDOWNSTREAMDOWNSTREAMdeepestFlute casts are preserved on the bottom of overlying sand bedsExamples of flute casts: note flute shape(?)Trace fossils=tracks of organismsDinosaur Tracks, Tazones, Spain“dinoturbation”Soft-sediment deformationThis is just the beginning of what one can learn from sedimentary structures (more in
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