Wave-Dominated Coasts
In open water, waves are purely an
oscillatory motion
Where water depth < 0.5 wavelength,
water interacts with bed (=wave base)
Typical ocean waves have l = 10-30 m: fair-weather wave base (FWWB) is
about 5-15 m depth
Large storm waves can have l < 400 m: storm wave base (SWB) is anywhere
between 30-125+ m depth
Shallow marine (“littoral”) zone can be subdivided based on wave energy regimes
Gradient ~ 0.3°
MHW = mean high water
MLW = mean low water
Gradient ~ 0.03°
Offshore facies: typically fine-grained (mudstone/siltstone) with occasional
distal sandy storm deposit (“tempestite”)
Hummocky Cross-Stratification: deposition by storm waves
Forms only in medium or finer sand, indicates deposition below FWWB
HCS is the most distal influence of storm waves and usually occurs in
alternating sandstone / mudstone successions
Base of HCS beds can contain erosive sole marks from turbulent storm scouring
(e.g., gutter casts)
Gutter Cast
Hummocky cross-stratification (HCS)
Gutter casts on bed base
HCS often grades upwards into Swaley Cross-Stratification (SCS) in which
hummocks and intercalated mudstone are rare/absent (deposited on storminfluenced shelves above or near FWWB)
Hummock
Swale
Wave ripples
Form in oscillatory water motion created by fair-weather waves
Symmetrical profile, often sharp crest with “tuning-fork” bifurcations
Jurassic, Utah
Oblique incidence of waves at the shoreline creates a longshore
current from the shore-parallel component of wave energy
If longshore current is strong (and storms
are weak), shoreface deposits can include
migrating subaqueous dunes with foresets
perpendicular to shoreline
Hummocky cross-stratification (HCS)
Gutter casts on bed base
Not storm-dominated: Wave ripples
Storm-dominated: Swaley cross-stratification (SCS)
Longshore-dominated: shore-parallel planar (maybe trough) x-beds
Beach sediment deposited/reworked by successive wave swashes
Fast velocity, shallow flow, and typically fine grain size = upper plane bed
Berm
Foreshore
Backshore
Low angle (<5°), seaward-dipping upper plane bed stratification possibly with
discordant scour surfaces, usually well-sorted mature sand
When beaches supplied by coarser sediment, upper plane bed is not developed
Low angle (<5°), seaward-dipping planar pebbly layers, possibly with scours
Low angle (upper plane bed if sand) stratification
Hummocky cross-stratification (HCS)
Gutter casts on bed base
Not storm-dominated: Wave ripples
Storm-dominated: Swaley cross-stratification (SCS)
Longshore-dominated: shore-parallel trough/planar x-beds
Regressive surface of marine erosion can form in shoreface settings during FSST
Regressive surface of marine erosion
In wave-dominated settings, coastal profile set by wave energy
Equilibrium profile is concave-up, so erodes during base level fall
Basal Surface of Forced Regression
Regressive Surface of Marine Erosion
Swaley cross-stratified FSST shoreface
sandstone abruptly overlying ?HST
offshore sandstone/mudstone
Swaley cross-stratified FSST inner ramp limestone abruptly overlying ?HST
outer ramp limestone, separated by scoured surface with gutter casts