Ch. 9: Learning objectives • Marginal marine environments are the transitional areas between terrestrial and fully marine realms • Understand the sedimentary facies of deltas, beaches and barrier islands, estuaries, and tidal flats – types of sediments; sedimentary structures; lateral and vertical facies associations; and all other characteristics Strat & Sed, Ch. 9
1
Deltas • Delta = a discrete deposit formed by fluvial sediments building into a standing body of water – River deltas (geologically most significant) • Fluvial-dominated • Tide-dominated • Wave-dominated
Strat & Sed, Ch. 9
2
Fluvial-dominated deltas • River processes are more dominant than either tidal or wave processes • Usually characterized by high sediment and water discharge • Elongate to lobate in plan shape • Muddy bulk composition (interdistributary areas), but with sandy channel facies, distributary mouth bar facies, and bar front facies Strat & Sed, Ch. 9
3
Strat & Sed, Ch. 9
4
Strat & Sed, Ch. 9
5
Fluvial-dominated deltas
Mississippi delta Strat & Sed, Ch. 9
6
Fluvialdominated deltas
Strat & Sed, Ch. 9
7
Tide-dominated deltas • Tidal currents may overwhelm river flow in areas with high tidal range • River mouth sediments may be redistributed by tidal action • Estuarine to irregular in plan shape • Mixed sediment composition – Muddy estuarine areas – Sand-filled channels – Sand ridges Strat & Sed, Ch. 9
8
Strat & Sed, Ch. 9
9
Strat & Sed, Ch. 9
10
Tide-dominated deltas Ganges-Brahmaputra delta
Strat & Sed, Ch. 9
11
Wave-dominated deltas • In areas of strong wave activity, river mouth sediments may be reworked and redistributed to form wave-built beaches, barrier bars, spits, beach ridges • Overall shape is smooth, arcuate to cuspate • Composition is mainly sand Strat & Sed, Ch. 9
12
Strat & Sed, Ch. 9
13
Strat & Sed, Ch. 9
14
Wavedominated deltas
São Francisco delta, Brazil Strat & Sed, Ch. 9
15
Delta facies • Upper delta plain (above high tide) – River channel deposits, lacustrine delta-fill deposits, floodplain deposits
• Lower delta plain (between high- and low tide) – Interdistributary bay deposits, crevasse splay, natural levee, abandoned distributary deposits
• Delta front (from low tide to ~ 10m subsea) – Distributary mouth bar deposits, pro-delta distal bar deposits
• Prodelta (seaward of subaqueous delta) – Fine-grained marine sediments Strat & Sed, Ch. 9
16
Delta facies
Strat & Sed, Ch. 9
17
Constructional vs. destructional phases • Constructive phase occurs when input of sediment is sufficient for progradation – Prodelta fine sediment is overlain progressively by delta-front silts and sands, distributary mouth sands, and upper delta plain deposits
• Destructive phase occurs when a delta lobe is abandoned or during major transgression – Erosion and redistribution of sediments rather than seaward growth Strat & Sed, Ch. 9
18
Delta progradation Delta plain: Non-marine fluvial to distributary channel to finer marsh-, lake-, or interdistributary bay
Delta front: Distributary mouth Bar to distal bar to silts
Strat & Sed, Ch. 9
Prodelta: Marine silts and muds 19
Vertical facies associations • Delta progradation results in an overall coarsening-upward sequence (usually 50150m thick) overlain by finer delta plain deposits (2-15m thick) • Progradational cycles may be stacked, depending on history of lobe abandonment
Strat & Sed, Ch. 9
20
Finer-grained delta plain deposits
Coarsening-upward sequence
Strat & Sed, Ch. 9
21
Delta lobe abandonment
Strat & Sed, Ch. 9
22
Beaches and barrier islands • Beach and barrier island complexes are best developed on wave-dominated coastlines with small tidal range – Beach = linear sand body attached to mainland – Strand-plain = broader beach ridge system consisting of multiple parallel ridges – Barrier island = linear sand body separated from mainland by a lagoon or marsh Strat & Sed, Ch. 9
23
Beach-barrier island anatomy
Strat & Sed, Ch. 9
24
Strat & Sed, Ch. 9
25
Strat & Sed, Ch. 9
26
Beach morphology • Beach morphology is same for beaches and seaward coast of barrier islands – Backshore = above high tide level (including beach dunes) – Foreshore = swash zone between high and low tide – Shoreface = low tide to fair-weather wave base (~10-15m) Strat & Sed, Ch. 9
27
Beach morphology
Strat & Sed, Ch. 9
28
Wave processes • In breaker zone and surf zone, breaking waves translate into bi-directional currents – High turbulence throws sediment into suspension – Net landward transport of bedload and suspended load (often with a longshore component) – Repeated surf action produced a winnowed sediment (well sorted, positively skewed) Strat & Sed, Ch. 9
29
Current processes • Longshore currents are caused by breaking waves that approach the shoreline at an angle – Some translational wave energy is deflected to produce a unidirection current shoreline
current
wave crests Strat & Sed, Ch. 9
30
Longshore drift & longshore current
Strat & Sed, Ch. 9
31
Beach and barrier island facies • Overall depositional system is relatively narrow and produces an elongate set of deposits that are parallel to coastline • Sediment is dominantly sand – 10’s to 100’s meters wide – 10’s to 100’s km long – 10-20 m thick Strat & Sed, Ch. 9
32
Beach and barrier island facies • Beach deposits are: – Fine to medium grained sand – Well sorted – Subhorizontal, parallel laminations dominant, but also low-angle landward- and seawarddipping cross-beds
Strat & Sed, Ch. 9
33
Beach and barrier island facies
Strat & Sed, Ch. 9
34
Laterally adjacent facies • Eolian facies may be developed landward of beach • Other facies include: – Sandy washover deposits from back of barrier island – Coarser tidal channel deposits (bi-directional paleocurrent) – Fine-grained lagoon and marsh deposits – Tidal flat sands, silts, and muds (often flaser-bedded and lenticular bedded) – Offshore finer sediments Strat & Sed, Ch. 9
35
Ch. 9 Marginal marine environments (part 2) • Estuaries & Tidals flats – Know types of sediments; sedimentary structures; lateral and vertical facies associations; and all other characteristics
Strat & Sed, Ch. 9
36
Estuaries • “Estuary” is an inlet of the sea that reaches up a river valley as far as the upper limit of the tidal range – Interaction of both fluvial and marine systems – Progradation of an estuary may change it into a delta
Strat & Sed, Ch. 9
37
Estuary types
Strat & Sed, Ch. 9
38
Strat & Sed, Ch. 9
39
Strat & Sed, Ch. 9
40
Estuary environmental setting • Influence by river, tides, and waves • Salinity may vary within an estuary at any given time, or seasonally • Proximal parts of estuaries are riverdominated • Distal parts of estuaries may be: – Wave-dominated – Tide-dominated Strat & Sed, Ch. 9
41
Wave-dominated estuary • High wave energy at mouth of estuary – Estuary mouth sand bar – Quiet water in central part of estuary (muds)
• Water is partially mixed to well-stratified – Fresh vs. marine density stratification
• High river energy at head of estuary – Bay-head delta sediments Strat & Sed, Ch. 9
42
Wavedominated estuary
Strat & Sed, Ch. 9
43
Tide-dominated estuary • High tidal energy at mouth of estuary, all the way to tidal-fluvial transition – Estuary mouth tidal sand bars – Generally higher overall energy than wave-dominated estuaries
• Water is well mixed – no density stratification
• Ripple and dune bedforms common – Foresets may dip in both directions Strat & Sed, Ch. 9
44
Tidedominated estuary
Strat & Sed, Ch. 9
45
Estuarine sedimentary facies • Cross-bedded, bioturbated sands form near estuary mouth and in fluvial-tidal channels • Laminated, bioturbated muds form in nonchannel middle and upper parts of estuary • Fauna is typically low diversity, but possibly high abundance – typically dominated by mollusks • Oysters, mussels, gastropods
Strat & Sed, Ch. 9
46
Vertical succession of estuarine facies • Depends on whether tide- or wavedominated • Transgressive sequence will produce landward migration of environments – Estuarine facies above fluvial facies
• Regressive sequence will produce seaward migration of facies – Fluvial facies above estuarine facies Strat & Sed, Ch. 9
47
Transgressive estuarine facies
Strat & Sed, Ch. 9
48
Tidal flats • Environmental setting – Mesotidal to macrotidal (4 to 15 m tidal range), low-relief coastlines where wave energy is minimal – Also, behind barriers (barrier islands, spits, reefs) – Characterized by twice-daily flood and retreat of marine water Strat & Sed, Ch. 9
49
Strat & Sed, Ch. 9
50
Strat & Sed, Ch. 9
51
Strat & Sed, Ch. 9
52
Strat & Sed, Ch. 9
53
Tidal flat facies
Strat & Sed, Ch. 9
54
Tidal flat facies • Subtidal zone – Below mean low tide – High tidal current velocities – Characterized by bedload transport and sand deposition
• Intertidal zone – Between mean high and low tide; no significant vegetation – Dissected by tidal channels – Both suspended and bedload deposition (mixed mud and sand)
• Supratidal zone – Above mean high tide – Heavily vegetated and incised by tidal channels – Sedimentation from suspension during storm tides Strat & Sed, Ch. 9
55
Tidal flat sediments • Sediments are mostly sand and muds • Supratidal zone characterized by muds with evidence of plant roots • Intertidal zone characterized by mixed mud and sand, with sand in channels • Subtidal zone characterized by sand, with channels and bars Strat & Sed, Ch. 9
56
Tidal flat sedimentary structures • Channels may have dunes and cross-bedding with bimodal paleocurrent directions • Mixed sandy and muddy sediments may exhibit small-scale ripple cross-stratification, flaser bedding, wavy bedding, lenticular bedding, or finely laminated bedding, bioturbation (Skolithos) • Supratidal deposits may be thinly laminated, but with plant bioturbation and evidence of subaerial exposure Strat & Sed, Ch. 9
57
Tidal flat sedimentary structures
Strat & Sed, Ch. 9
58
Vertical facies associations • Tidal flat progradation (regression) will produce a fining-upward depositional sequence – Subtidal sands overlain by mixed intertidal sediments, overlain by supratidal muds
• Transgression may produce a coarseningupward sequence, or it may destroy intertidal and supratidal deposits through reworking Strat & Sed, Ch. 9
59
Tidal flat progradational sequence
Strat & Sed, Ch. 9
60
Vertical facies associations Coarsening-upward sequence is produced by prograding beach complex
Strat & Sed, Ch. 9
61
