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Sedimentary structures

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Sedimentary structures

  1. 1. Sedimentary Structures Click to edit Master subtitle style 6/14/12
  2. 2. Primary Sedimentary Structures Mechanical structures resulting from sediment deposition. v Under unidirectional flow, ripples begin under critical entrainment velocity is reached. v 6/14/12 v
  3. 3. Stratification & Bedforms • • • Beds are tabular or lenticular layers of sedimentary rock with lithologic, textural, or structural unity. Distinguishable from subjacent and suprajacent layers Upper/lower surfaces are bedding 6/14/12 planes or bounding planes
  4. 4. Terminology of Bedsets 6/14/12
  5. 5. Plane Bedding Structures Simple, horizontal beds > 1 cm vResults from suspension sedimentation, vHorizontal accretion, vEncroachment into lee side of obstacle vLamination < 1 cm thick vAbsence of lamination may be due to flocculation v 6/14/12
  6. 6. Low Flow Regime Sedimentary Structures • • • Ripple Index – ratio of ripple length : ripple height Out-of-phase wave propagation with bedforms Ripples – smallest bedform with RI ~8 (coarse) to 20 (fine sand), forming in sand and silt. 6/14/12
  7. 7. Low Flow Regime Sedimentary Structures Dune – larger bedform with RI ~5 (fine sand) to 50 (gravel) • 2D Dune – straight/sinuous and long crested; 10 cm 100s m • 3D Dune – curved faces, irregular and short crested; 10 cm • 6/14/12
  8. 8. Upper Flow Regime Sedimentary Structures In-phase wave propagation with bedforms §Plane-bed flow stage with transport over a relatively flat bed §Internal planar lamination (mm - cm laminae) §Highest flow velocities may create antidunes §Antidunes – low, undulating with RI ~7 to 100 §Low angle cross beds directed upstream § 6/14/12
  9. 9. Multidirectional Flow- Generated Bedforms Oscillation (wave) ripples generally symmetrical to slightly asymmetrical due to eddies •Orbital velocity difference < 1 cm/s = symmetrical bedforms •Orbital velocity difference > 5 cm/s = asymmetrical bedforms •Crests are straight to sinuous, bifurcate •Herringbone cross-beds; interference ripples; lenticular beds; flaser beds • CURRENT RIPPLES CURRENT-DOMINATED OSCILLATION RIPPLES 6/14/12 WAVE-DOMINATED
  10. 10. Geometries of Beds Planar stratification – internal layers and laminae that are parallel to bedding planes •Cross strata – internal layers or laminae that are at an angle to bedding planes •Beds composed of cross-laminated or cross-stratified units are cross beds •Bedsets comprised of similar beds or cross beds • 6/14/12
  11. 11. Cross-Beds Result of ripple and/or dune migration; filling of scour pits & channels •Foreset laminae develop as avalanche or suspension settling phenomena; lee side of ripple with steep and straight laminae •Bottomset laminae from suspension load nearly at the angle of repose •Topset laminae rarely preserved (sigmoidal cross beds) •Occur in cross-bed sets – small scale bedsets < 5 cm; large scale bedsets > 5 cm • 6/14/12
  12. 12. Cross-Bed Geometries McKee & Weir (1953) – Tabular & Trough Cross bedding •Tabular Cross beds – units broad in lateral dimensions with respect to set thickness with planar bounding surfaces. •Migration of large-scale ripples and dunes; lower flow regime •Trough Cross beds – units whose bounding surfaces are curved, consist of elongate scour filled with curved laminae •Migration of small-scale or large-scale ripples •Paleocurrent measured in dip direction of foreset laminae • 6/14/12
  13. 13. Hummocky Cross Stratification Undulating sets of cross laminae both concave-up (swales) and convex-up (hummocks) •Common in 15 - 50 cm thick sets; wavy erosional bases and rippled, bioturbated tops •Fine sandstone to coarse siltstone, micaceous with dispersed plant debris • 6/14/12
  14. 14. Turbidites Density current in ocean and/or lake flowing downslope •Initiated by short-lived catastrophic events (earthquake trigger; storm) •Flow divided into: Head – 2x thick as remainder of flow with turbulent flow; Body – uniform thickness with uniform flow; Tail – flow thins and becomes dilute •Thick-bedded (high density flow) and thin-bedded (dilute •density flow) turbidites • 6/14/12
  15. 15. Idealized turbidite sequence recording decay of flow strength §Subdivided into Units A through E §A – Massive graded bed (coarsest settled); B – plane laminated bed §(high flow); C – lower flow ripples and wavy lamination; D – laminated §silt; E – laminated mud §Hsü believes can be divided into only 2 units §Lower, horizontally laminated unit; Upper, cross-laminated unit 6/14/12 § Bouma Sequences
  16. 16. Graded Bedding GRADEDSTRATIFIED INVERSE TO NORMAL Vertical gradations in grain size within a bed •Basal coarse particles that grade upwards to finer particles at top is Normal (common). •Basal fine particles that grade upwards to coarser particles at top is Reverse (rare) •Basal contacts are sharp; Attributed to turbidites • 6/14/12
  17. 17. Massive Bedding Bed appears homogenous and lacking internal structure vTurbidite and/or bioturbation generated 6/14/12 v v
  18. 18. Soft-Sediment Deformation Structures Convolute bedding & lamination due to liquefaction processes vComplex folding or crumpling of semiconsolidated vBeds vFlame structures – wavy or flame-shaped tongues injected into overlying layers vBall and Pillow – hemispherical or kidney-shaped masses 6/14/12 vinto underlying mudstone v
  19. 19. Bedding-Plane Markings Underside of beds as positive-relief casts and irregular markings – “Sole Markings” v Current-formed structures include: flute casts (elongate ridges, bulbous at one end and flare in direction); current crescents (obstacle scours) v Tool-formed structures include: groove casts (object dragged across surface); bounce, brush, prod, roll, and skip marks (intermittent object contact with bottom) vLoad Casts – irregularly shaped without current indicators v 6/14/12

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