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Depositional Coastal Landforms

Accretional (Depositional) Coastal Features – Cuspate Forelands / Capes

World class example: Outer Banks, NC

Mechanism of formation is debated:

1. Form at points of LST convergence from 2 directions?

2. May have a geologic control?

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Accretional Coastal Features – Captured Capes?

Accretional (Depositional) Coastal Features - Spits

Netarts Spit, Oregon

Tied to the coast at one end

Fundamental in smoothing an irregular coastline

Often exhibit a “recurved” hook at the downdrift end

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Examples of Spit Growth

Older hooks reflect paleo- spit ends

Geomorphic history of Fire Island, NY - from aerial photography - growth in direction of L.S.T.

Spit growth can deflect the course of a river

Google Earth Exercise - Go Spit Hunting

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Accretional (Depositional) Coastal Features - Tombolos

Occur where spit growth extends to an offshore island

Island often influences wave field - refraction

Italian term, where they are numerous and well-developed

see Figure 2.16 in Komar

Barrier Islands

Occur along 13 % of the world’s shorelines

Prevalent in areas of small tidal range = 1-2 meters

Santa Rosa island, FL: A new breach was formed across Santa Rosa Island by Hurricane Ivan west of Pensacola Beach. A continuous sheet of storm-generated overwash deposits is observed in this low-lying and uninhabited portion of the barrier island. Route 399 was cut by the new breach, making Fort Pickens section of the Gulf Islands National Seashore reachable only by boat.

Courtesy USGS: http://coastal.er.usgs.gov/hurricanes/ivan/photos/florida.html

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Barrier islands – Theories of Formation

DeBeaumont (1845) presented a theory based on cross shore sediment transport, whereby continued deposition of a submarine bar (originating from wave breaking) eventually accreted to an elevation above sea level, creating a barrier island.

In the famous USGS Monograph 1 by G.K. Gilbert (1890) on Lake Bonneville, a theory is presented which offers an explanation that barrier islands were the result spit growth from longshore sediment transport.

McGee (1890) argued that the drowned river valleys along the east coast of North America testified to the submergence of the shoreline, which he claimed explained the formation of barrier islands unconnected to the mainland. Hoyt (1967) also supported this theory.

Washover of Barrier Islands

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Regressive vs. Transgressive Sequences

Rooted in Walther’s Law

Regressive Sequence - Landward sediments are deposited on top of seaward seds, reflect a seaward shift of shoreline.

Transgressive Sequence - Seaward sediments are deposited on top of landward seds, reflect a landward shift of shoreline.

Not necessarily diagnostic of SL rise or fall - think about sediment supply.

Inlet Dynamics: Spit Growth to Equilibrium Length

Thought to be controlled, in part, by tidal prism:

1.  Within each tidal cycle, a “prism” of water must be moved into, and out of, the bay.

2.  Spit extends freely until tidal currents, through narrowing inlet, disallow longshore transport - tidal currents generate shear stress sufficient to block longshore sediment transport.

3.  Sediment “sinks” are the flood tidal and ebb tidal shoals (deltas)

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Inlet Spacing Related to sediment supply / distance from source

Near the sediment source (eroding headland, river mouth, etc.), inlet spacing is wide.

Flood and ebb-tidal shoals are a net sink for sediment, LST rate decreases downdrift.

At the distal end of the barrier island system (relative to source), inlet spacing becomes close, due to decreased LST rate

Beach Ridge Generation by Changes in Wave Climate & Sediment Supply

St. Vincent Island, Apalachicola Delta, FL

Ubiquitous Landforms, without a clear explanation of their formation, preservation, and evolution.

One clear requirement – sediment supply.

Numerous beach ridge sets visible with Google Earth.

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Beach Ridge Sets – Cape Canaveral and Merritt Island

Rink & Forrest (2005) combined OSL dating with Brooks (1972) RC dates to establish land accretion rate from beach ridges to be ~135 m/century