GEOL 340 Sedimentology and Stratigraphy
Lecture 7
1. Current, Wind and Wave Bedforms
2. Sediment Loads and Entrainment
3. Sediment Gravity Flows
Current, Wind and Wave Bedforms
Bedforms = stratification and sculpting of the bed surface
cross-bedding = successions of inclined bedding patterns
Current Bedform Successions for Sands (Fig. 2.7, p. 38)
a. Lower-Flow Regime = water surface no disturbance or out-of-phase
w/ bedform
migrate downstream due to erosion and deposition on ripples and
dunes
ripples = 10 - 20 cm (structure Fig. 2.8, p. 38)
wavelength, lee and stoss sides, crest, trough, brink point,
eddy
dunes = 10's cm to 100's m
b. Upper-Flow Regime = water surface in-phase with bedforms (Fr
> 0.8)
migrate upstream due to erosion and deposition on ripples and
dunes
lower and upper plane beds
antidunes = 10's cm to m's
chutes and pools
Wave Motion and Transport
a. deep water - orbital paths, no net forward motion (diagram)
dies out at a depth of 1/2 the wavelength
b. shallow water - eliptical paths, assymetry causes forward
motion (diagram)
impinges on the bottom (strongest at < 1/2 the wavelength)
c. bedforms -
oscillation ripples to current ripples
symmetrical (V < 1 cm/s) or assymmetrical (V > 5 cm/s)
offshore (symmetrical) to onshore (assymetrical) sequences
Sediment Loads and Entrainment
Bedload = coarse grains moving close to bed (Fig. 2.6,
p. 36)
a. traction - grains are in continuous contact with bed
b. saltation - grains bounce along bed, not always in contact
Suspended Load = finer grains moving in the water above the
bed
a. intermitant suspension
b. continuous suspension
washload - suspended sediments transported in by water
dustload - suspended sediments transported in by air
ice-transport load - suspended in ice (Fig. 3.4, p. 64)
balance of accumulation (head) and melting (snout)
c. Nepheloid Layers - hundreds meters thick layers of suspended
sediment in the
deep ocean, stay in suspension for long periods of time
Sediment Gravity Flows
Gravity Flow Mechanisms (Fig. 2.14, p. 48)
a. turbidity current - turbulence supports grains
up to 67 km/hr covering 300 km seafloor
sediment dislodgment trigger events (earthquakes, storms, etc.)
grading - vertical succession of grain sizes
normal grading - fining upward
reversed grading - coarsening upward
b. liquified sediment flow - upward escape of intergranular fluids
support grains
c. grain flow - grain interaction (dispersive pressure) supports
grains
d. debris and mud flows - matrix strength supports grains
lahars - debris flows composed of volcanic particles
Bouma Sequences (Figs. 2.15 and 2.18, pages 49 and 52)
two general types (grade laterally into each other)
thick deposits from high density flows - poor grading
thin deposits from lower density flows - finer grains at base
a. turbidity current
b. liquified sediment
c. grain flow
d. debris and mud flows