Presentation on theme: "GE0-3112 Sedimentary processes and products Lecture 3. Sedimentary structures I – fluid flows Geoff Corner Department of Geology University of Tromsø 2006."— Presentation transcript:
GE0-3112 Sedimentary processes and products Lecture 3. Sedimentary structures I – fluid flows Geoff Corner Department of Geology University of Tromsø 2006 Literature: - Leeder 1999. Ch. 7, 8 & 9. Sediment transport and structures. Sediment transport and structures.
Contents ► 3.1 Introduction ► 3.2 Unidirectional water flows ► 3.3 Atmospheric flows ► 3.4 Combined flows and tides ► Further reading
3.1 Introduction ► Bedforms and structures (definition) ► Plane bed, ripples and dunes ► Bed shape changes with flow strength ► Feedback: bedforms modify flow
Classification of primary sedimentary structures
Plane bed, ripples, dunes Ripples Dunes Plane bed
3.2 Unidirectional water flows ► Current ripples ► Lower-stage plane bed ► Dunes ► Upper stage plane beds ► Antidunes ► Bedform relationships
Current ripples ► Are stable bedforms at low flow strength in fine sand. ► Do not form in sand coarser than 0.7 mm (c.s.). ► Asymmetric profile parallel to flow: gentle stoss, steep (c. 35 o ) lee. ► Height (h): <4 cm; wavelength (λ): <0,5 m. ► Ripple index (λ/h): 10-40. ► Ripple size varies clearly with grain size (λ ≈ 1000d) but not with flow strength or water depth.
Ripple shapes ► Ripple crests are straight, sinuous or linguoid (tongue-shaped). ► Straight- and sinuous ripples are metastable and change to linuoid with time.
Flow over a rippled bed Flow separation Flow separation and re-attachment Flow re-attachment
Dunes ► Similar to ripples in general shape but distinctly different because: ripple and dune form indices do not overlap. ripples occur on the backs of dunes in apparent equilibrium. ► Height: 5 cm - 10 m; wavelength: 0,6 – 100’s m. ► Modification during stage variation may produce ’reactivation’ surfaces.
Upper-stage plane beds ► Bed and water surface in phase; rapid flow. ► Plane bed actually comprises very low amplitude (c. 1 – 10 med mer) bedwaves that move downstream. ► Each bedwaves may deposit a thin lamina some few grains thick. ► The bed surface shows primary current lineation (parallel heavy-mineral streaks, etc.)
Froude number and flow regime ► Froude number: ratio of inertial to gravity forces in water flow having free surface ► Fr < 1: Tranquil flow Lower flow regime; water surface and bed out of phase. ► Fr > 1: Rapid slow Upper flow regime: water surface and bed in phase. (NB. Upper and lower flow-regime concept not as clear cut as previously thought.)
3.3 Atmospheric flows ► Differences between air and water flows ► Ripples ► Dunes
Comparison of air and water ► Low shear stresses in air limits maximum bedload grain size to v.coarse sand/v.f.pebble. ► Collision effects and saltation more important in air. ► Energetic kollisions promote abrasion of grains and substrate. (NB. Snow particle abrasion is effective in periglacial regions). ► Suspension transport of sand is more difficult in air than in water because of lower buoyancy. ►
Aeolian sediments ► Gravel transport by rolling and saltation (< 4 mm) gravel normally forms protective lag ► Sand median typically (fine sand) aeolian sand ideally better sorted than beach sand sorting varies bedforms: ripples and dunes ► Silt typically coarse silt (loess)
Aeolian bedforms ► Two major groups: ripples and dunes. ► Draas are large composite bedforms made up of smaller dunes. Previous classification acc. to size (Wilson 1972): ► draas20-450 m high ► dunes0.1-100 m " ► ripples0.005-0.1 m high Ripples Dunes
Ripples (ballistic ripples) ► Asymmetic profile parallel to flow: gentle, slightly convex stoss, steep (c.20 o ) lee. ► Height (h): few mm-10 cm; wavelength (λ): 2- 200 cm. ► Ripple index (λ/h): 8 – 50. ► Wavelength increases with grain size and wind strength.
Ripple shapes ► Persistent sinuous crests common. ► Barchanoid shapes form where sediment is sparse.
Ripple variability ► Wavelength increases with increasing grain size and wind strength.
Formation of wind ripples ► Ballistic collisions due to saltation cause up to 25% transport as ’creepload’. ► Lee slopes migrate more from effects of saltation bombardment than avalanching (hence lower angle than in water ripples) ► Crests contain coarser grains more resistent to bombardment (gives inverse grading in structures)
Internal structure of wind ripples ► No clear internal structure. ► Parallel bedding shows inverse frading
Internal structure of wind ripples ► Climbing ripples form where net accumulation of sand
Wave ripple formation ► Shallow-water waves (d=λ/20) cause horisontal bottom motion. ► Above threshold of motion movement occurs rolling and saltation. ► Initial ripple crests are low (< c. 20 grain diameters high) with broad troughs. ► Increased shear stress gives flow separation vortices on either side of symmetrical ripples.
Wave ripples ► Wavelength: c. 0.9 cm – 2 m. ► Height: c. 0.3 – 25 cm. ► RI (L/H): c. 4 – 13. ► Wavelength increases with increasing wave period. ► Bifurcation common
Further reading ► Allen, J.R.L. 1970. Physical processes of sedimentation. Chapter 1 covers the same ground as Leeder and explains clearly the principles involved; good supplementary reading for aquiring a sound grasp of the physics of fluid dynamics and sedimentation. Alternatively consult the more encyclopedic: ► Allen, J.R.L 1984. Sedimentary structures: their character and physical basis. A more encyclopedic alternative to the above if it is unavailable.