GEOL 440 Sedimentology and stratigraphy: processes, environments and deposits Lectures 6 & 7: Flow, bedforms and sedimentary structures in oscillatory and multidirectional flows
Today, aim to examine: Main flow features in oscillatory boundary layers Sediment transport under waves Bedforms and sedimentary structures under oscillatory flows Combined flows (oscillation and superimposed unidirectional flow) Multidirectional flows (linked to shelf storms sometimes?)
Pure oscillatory flow: wave parameters Wave Parameters h Deep water: h> λ/2 ; Transitional waves: λ/20 <h< λ/2; Shallow water: h< λ/20
(h >λ/2) y = λ/2 Rick Cheel Brock U
(λ/20 < h < λ/2) h < λ/2 Rick Cheel Brock U
Waves in Shallow Water as waves approach the shoreline, the water shallows and they change from deepwater - transitional - shallow water waves, when h < L/20.
Wave Deep water wave Orbits (h> λ/2) C=1.25 λ Shallow water wave C= gd where C is celerity (speed) Stoke s Drift
Unidirectional Component A unidirectional component
Boggs, Fig. 10.6
What bedforms are generated under short-period waves?? Bedform Phase Diagram 0.15-0.22 mm sand Wave ripples T = time taken for a wave to travel 1 wavelength i.e., T= λ/c
Types of Oscillation Ripple
Plots of ripple spacing (l) versus the wavelength of oscillation (λ) forming the ripples delineate three ripple types: Orbital ripples; one orbital per ripple with the relationship: l = 0.8d o Suborbital ripples: intermediate between orbital and anorbital ripples. Anorbital ripples; where there are several ripples per orbital and l << 0.8d o
Wave Ripples
Wave Ripples 2
Interference Ripples
Cross-lamination types In cross-section some wave ripples can look very similar to unidirectional ripples BUT. draping & undulating boundaries crestlines frequently bifurcate in planform Up to 25 cm high Ripple index ~6-7 rather than ~8-15
As always, need to account for aggradation rate and bedform migration
Oscillation period Rick Cheel Brock U
With longer period oscillations. Reversing bedforms.. herring bone cross-stratification http://www.depauw.edu/acad/geosciences/tcope/sedstruct/hires/herringbone.jpg
With longer period oscillations. Reversing crest / reversing ripples
Flaser Bedding
Flaser and Lenticular Bedding
Lenticular Bedding & Bioturbation Bruce Molnia, AGI ImageBank
With even longer period oscillations. Tidal bundles Higher velocities and longer duration of sand transport during Spring tides
Deposits thus reflect changes in: bed shear stress settling of grains transport direction
Tidal bundles In well-preserved cases these provide a precise geological clock down to a monthly/daily/diurnal scale!
Combined flow: oscillation and a unidirectional component Rick Cheel, Brock U
Multidirectional flows: oscillation and a transverse component HCS Formation
Multidirectional flows: oscillation and a transverse component HCS Hummocky Schematic Cross-Stratification HCS View
Cross-stratification formed by 3-D ripples includes hummocky crossstratification (HCS) and swaley cross-stratification (SCS) Hummocky cross-stratification is characterized by: 1. Convex upward (hummocks) and concave upward (swales) laminae and internal bounding surfaces; spacing of hummocks and swales is commonly large, in excess of 1m. 2. Low angle (generally less than 10 but up to 15 ), erosional bounding surfaces. 3. Internal laminae that are approximately parallel to the lower bounding surfaces. 4. Individual laminae that vary systematically in thickness laterally and their angle of dip diminishes regularly. 5. Internal laminae and bounding surfaces dip equally in all directions (i.e., they are isotropic). Rick Cheel, Brock U
Summary Deep and shallow water waves Sequence of bedforms in a pure oscillatory flow Oscillatory bedform phase diagram (remember it!) Differences in bedforms as oscillation period chnages Oscillatory, combined and multidirectional flows Reading: B&D: Chapter 7 Boggs, relevant parts of Chapters 4 & 10 Leeder, Chapter 9 Collinson and Thompson, Chapter 6