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 = m: fair-weather wave base (FWWB) is about 5-15 m depth Large storm waves can have < 400 m: storm wave base (SWB) is anywhere between m depth

Gradient ~ 0.3° MHW = mean high water MLW = mean low water Shallow marine (“littoral”) zone can be subdivided based on wave energy regimes Gradient ~ 0.03°

Offshore facies: typically fine-grained (mudstone/siltstone) with occasional distal sandy storm deposit (“tempestite”)

Forms only in medium or finer sand, indicates deposition below FWWB Hummocky Cross-Stratification: deposition by storm waves

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

Hummock Swale HCS often grades upwards into Swaley Cross-Stratification (SCS) in which hummocks and intercalated mudstone are rare/absent (deposited on storm-influenced shelves above or near FWWB)

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 Foreshore BermBackshore

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

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 Low angle (upper plane bed if sand) stratification

Regressive surface of marine erosion Regressive surface of marine erosion can form in shoreface settings during FSST

In wave-dominated settings, coastal profile set by wave energy Regressive Surface of Marine Erosion Basal Surface of Forced Regression Equilibrium profile is concave-up, so erodes during base level fall

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