Importance of Understanding Coastal Landforms

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1 Importance of Understanding Coastal Landforms Costa Concordia Shipwreck, Isola del Giglio, Italy Depositional Coastal Landforms Can interpret landforms in light of geomorphic processes, both terrestrial and marine, operating on a tectonic canvas. Note that depositional vs. erosion features do not strictly represent tectonically active vs. passive settings, respectively! 1

2 Accretional (Depositional) Coastal Features Cuspate Forelands, Capes, Beach Cusps 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? Accretional Coastal Features Captured Capes? 2

3 Accretional (Depositional) Coastal Features - Spits Tied to the coast at one end Fundamental in smoothing an irregular coastline Often exhibit a recurved hook at the downdrift end Can have a limit to it s growth inlet formation* Netarts Spit, Oregon Examples of Spit Growth Geomorphic history of Fire Island, NY - from aerial photography - growth in direction of L.S.T.* Older hooks reflect paleospit ends Spit growth can deflect the course of a river 3

4 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) Google Earth Exercise - Go Spit Hunting 4

5 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 Try a Google Earth flyover the rock of Gibraltar 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 formed across Santa Rosa Island during Hurricane Ivan west of Pensacola Beach. A continuous sheet of stormgenerated overwash deposits is visible in this uninhabited portion of the barrier island. Route 399 was cut by the breach, making Fort Pickens section of the Gulf Islands National Seashore reachable only by boat. Courtesy USGS: 5

6 Washover of Barrier Islands 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. 6

7 Beach Ridge Generation by Changes in Wave Climate & Sediment Supply 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. St. Vincent Island, Apalachicola Delta, FL 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 7

8 Erosional Features (Typical of Active Margins) Sea cliffs Sea stack Originate in settings where there is a relative sea level fall - where land surface is rising at a faster rate than sea level is rising. West Cliff Drive, Santa Cruz, CA Pocket beach between headlands Bonny Doon Beach, Central California Sea Arch, Kachemak Bay, Alaska 8

9 Sea Arch Santa Barbara, CA Features associated with sea cliff retreat Formation of erosional coast features 9

10 Control on Cliff Slope* relative influences of terrestrial vs. marine processes Cliff slope is controlled, largely, by the relative influences of terrestrial vs. marine processes. When terrestrial processes dominate, significant talus piles (low slope - angle of repose) develop. When marine processes dominate, steep bedrock walls are exposed. Sea Cliff Retreat Rates in California ~30 cm/yr Goleta beach, Santa Barbara, CA 10

11 Sea Cliff Erosion feedbacks* Laboratory experiments reveal positive and negative feedbacks in the abrasion of the basal notch of sea cliffs. With enhanced retreat, more comminuted material is available as tools for further abrasion accelerating retreat, but as notch grows (and beach grows), more energy is dissipated slowing retreat. grain entrainment - abrasion - positive feedback beach development - wave energy dissipation - negative feedback Pocket Beaches Form between headlands, where there is a fluvial source of terrestrial sediment (a river mouth). Sediment in a pocket beach is trapped by bounding headlands, which restrict longshore transport. Typically, adjusts its planform to accommodate the dominant incident wave direction. Equilibrium" planform shape of the pocket beach can be described as logarithmic spiral. 11

12 Variability in Wave Cut Platform Continental Shelf Features at Active Margins Bathymetry at Mavericks At active margins, shelf can be an erosional feature 12

13 Erosional Geomorphic Features - Marine Terraces Note sharp edges of the modern (actively forming) sea cliffs and diffuse, scraps of terrace treads high in the landscape Highway 1 near Santa Cruz, California Correspondence of Terrace Elevation with Age/SL highstand From Merritts and Bull, 1989, Geology 13