I. Introduction

A. Complicated Picture

1. Interaction of varialble Tides & Waves that supply energy to system

2. Sea Level Fluctuations that move environments laterally across shelf

3. Deposition of Sediments in environments that vary along strike

B. Important Depocenter

1. Most sediment - erosion of continents & carried by rivers to ocean (less carried by wind & glaciers)

2. 80% of sediments on Earth dumped at continental margins, including coastal zone & continental shelf

a. 25% carried by 4 rivers (draining Himalayas) - Huang-Ho (2.1x109 tons/year), Ganges (1.6x109), Bramaputra (0.8x109), Yangtze (0.55x109); 33% by 7 rivers - Indus (0.48x109), Amazon (0.4x109), Mississippi (0.34x109) [plus Irrawaddy (0.33x109), Mekong (0.19x109), Colorado (0.154x109)]

b. mostly into marginal seas, except Ganges-Bramaputra, Indus, Amazon

C. Sediment Disequilibrium

1. The glacial to interglacial sea level rise has been rapid

2. Modern environments are in a disequilibrium situation

a. the coast is drowned - drowned river valleys & irregular shorelines

b. the shelf is covered with relict sediments - coarse beach sands & fine lagoonal muds

3. Trend is toward equilibrium - estuaries are filling & shorelines are being straightened

II. The Nearshore Zone

A. There are a number of nearshore environments - deltas, estuaries, lagoons, barriers, & beaches

B. Deltas

1. Generally associated with larger rivers & regressive shorelines - sediment is delivered to the basin faster than basinal processes (tides, waves & oceanic currents) can redistribute it

2. Delta morphology

a. controlled by interaction of:

i. sediment supply - River process

ii. depth of water in basin, wave energy in basin, tidal energy in basin - Basin processes

b. delta morphology models

i. high-constructive vs. high-destructive model (4 types)

ii. sand distribution model (6 types)

3. Delta facies

a. delta plain environments - distributary channels, distributary mouth bars, & levees, plus interdistributary bays, floodplains, lakes, tidal flats, marshes, swamps & salinas

b. prodelta

4. Overlapping lobes

C. Estuaries

1. Semienclosed basins, open to the ocean, where freshwater & seawater are mixed

a. either the tidally influenced lower stretches of rivers, or drowned river valleys

b. most developed where tidal range is >2 m

2. Widespread now due to Holocene drowning - probably less important in the past (replaced by deltas)

3. Sediment Trappers

a. many rivers are undersized with respect to their adjacent estuaries

b. river flow decelerates upon entering the estuary & sediment settles out

4. Controls on circulation

a. size of river, size of estuary, & interaction of tides & waves

5. Types of estuaries

a. Type A = Salt Wedge = well stratified (low tidal range, large river inflow)

b. Type B = Partially Mixed (larger tidal range, smaller river flow)

6. Controls on sedimentation

a. deceleration of river flow for coarser grain sizes

b. flocculation for finer grain sizes - aggregation of clay particles by electrolytic attraction by salt ions in seawater

i. highest sedimentation rates are at the tip of the salt wedge in type A estuaries, & just downstream from the furthest seawater intrusion in type B estuaries

c. transport of adjacent continental shelf sediment by shoreward moving currents

D. Lagoons

1. Lagoons vs. estuaries

a. similarities: semienclosed basins that are open to ocean

i. lagoons develop behind a barrier & are open to the ocean through tidal inlets

b. differences: lagoons generally have limited river inflow & tidal exchange (so freshwater & seawater do not mix)

i. Also lagoons are generally elongate parallel to the shoreline, while estuaries are elongate normal to the shoreline

2. Lagoon sediment is generally fine grained

a. sand may be transported into the lagoon from the adjacent barrier as washover fans (which generally develop into backbarrier marshes)

b. flood tidal deltas develop on the lagoon side of tidal inlets as well

E. Barriers

1. Long, straight features parallel to the shore; separated from the mainland by lagoons, bays, & marshes

a. mostly islands (few km to 200 km long, a few to 10 km wide) with associated beaches

b. can also be connected to mainland as barrier spits

c. some are regressive, some are transgressive

2. Barrier geomorphology

a. the island & associated beach; the backbarrier flat & lagoon; & tidal inlets (which may migrate) & associated well developed ebb tidal deltas which may form

3. Origin of modern barriers - controversial

a. a steady supply of sand is necessary (from offshore erosion, rivers, headland erosion, or in situ )

b. real controversy is steady state migration vs. drowning

c. steady state migration - formed at shelf edge & migrated across shelf during transgression

i. equilibrium profile with shoreface erosion

d. drowning - barriers tied to sand source - as sea level rises, barriers are drowned

e. look to shelf

F. Beaches

1. Dynamic features dominated by wave action

2. Morphology - 3 parts

a. backshore - supratidal (berm crest & berm top); foreshore - intertidal (beach face); & inshore - subtidal (longshore trough & bar)

b. steepness of foreshore depends on grain size - steep = coarse grained; gentle = fine grained

3. Sediment source - same as barriers

4. Waves

a. wave terminology

i. wavelength (l) - distance between 2 adjacent crests or troughs (or other similar part of a wave)

ii. wave height (h) - vertical distance from trough to crest - waves break when h> l/7

iii. wave base - depth to which wave motion extends = l/2

iv. wave speed (C) = l/T = &Atildeg*l/2p in deep water (depth [D] >l/2)

= &Atildeg*D/p in shallow water (D<l/2)

v. frequency (f ) - number of waves to pass a point in a given amount time (usually a second)

vi. period (T) - the time for 1 wave to pass a point (f = 1/T)

b. changes in wave behavior as waves travel from deep water (D>l/2) into shallow water (D<l/2) & approach a beach

i. T remains the same

ii. C decreases because D decreases

iii. l decreases because C decreases

iv. h increases slightly, & because l is decreasing, close to shore h becomes >l/7 & waves break

v. if the waves are approaching a beach at an angle, they will change direction (refract) to become more parallel to the shore

vi. all of the above happens because, in shallow water, a wave "feels" the bottom; that is, wave motion does not dissipate in the water column but interacts with the bottom

c. longshore transport - movement of beach sand parallel to the shore

i. onshore movement of sand is by SWASH & BACKWASH

inshore movement of sand is by LONGSHORE CURRENTS

ii. both phenomena occur because most waves approach a beach at angle

iii. SWASH & BACKWASH are the upward & downward movement of water on beach face

iv. sand grains entrained by swash & backwash are moved down the beach

v. LONGSHORE CURRENTS develop because water is trapped between the beach & approaching waves

d. seasonal sand movement - smaller summer waves move sand onshore to the berm, larger winter waves move sand offshore to the longshore bar

III. Continental Shelves

A. Topography

1. Very complex interplay of erosion & deposition

a. exposed during last glacial maximum

i. eroded by rivers & glaciers, and deltas & moraines were deposited by rivers & glaciers

b. transgressed during glacial to interglacial transition

i. shorelines developed during pauses & slowdowns in rate of transgression

ii. erosional & depositional as well

c. redeposition of sediments by modern waves & tides

2. Varies geographically

a. high latitudes

i. mid-shelf low - glacial erosion

ii. outer-shelf high - glacial deposition

iii. also eastern North America north of 40_N & western Europe north of 45_N

b. temperate latitudes - continuous deepening

c. tropical latitudes - continuous deepening with drowned shelf edge reefs, or mid-shelf lagoon & marginal reef

B. Sediments

1. For clastic shelves - a nearshore modern sand prism, a modern mud blanket, & a middle & outer shelf relict sediment blanket (50-70%)

a. modern sand prism

i. dominantly sand brought to the shore by rivers & dispersed by longshore transport

ii. coarser onshore, finer offshore

iii. 5-10 km wide

b. modern mud blanket - mostly found only off large rivers & in shelf depressions & generally not continous with the modern sand prism

c. relict sediment blanket

i. relict sand is coarser than modern sand, stained with iron & exhibits dissolution pitting

ii. relict mud is associated with freshwater peat, oyster shells & terrestrial animal skeletal material

2. Carbonate shelves are covered mostly by modern carbonate sands & muds generated in situ

C. Processes Redistributing Shelf Sediments

1. Although much of the shelf is covered with relict sediments that were deposited when sea level was much lower, these relict sediments are being redistributed by tides & waves

a. reworked relict sediments are more properly called PALIMPSEST sediments

2. Wave reworking

a. wave base for ocean waves is 20 to 100 m - shallower for fair weather waves, deeper for storm waves

b. thus waves (particularly storm waves) are capable of moving sediments on the entire shelf surface

c. storm currents are helical - generate shoreface-attached ridges & are part of drowned barriers controvesy

3. Tidal reworking

a. tides are shallow water waves - interact with bottom everywhere & effects are amplified in elongate basins

b. form tidal sand ridges

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