Eighth Edition
by Harold L. Levin

Chapter 5 - page 3

The Sedimentary Archives

Transitional Depositional Environments

Transitional environments are those environments at or near the transition between the land and the sea.

1. Deltas
   1. Fan-shaped accumulations of sediment
   2. Formed where a river flows into a standing body of water, such as a lake or the sea
   3. Coarser sediment (sand) tends to be deposited near the mouth of the river; finer sediment is carried seaward and deposited in deeper water.
   4. The delta builds seaward (or progrades) as sediment is deposited at the river mouth.

   Examples: Mississippi River delta, Nile River delta, Niger River delta


A = Map of the Mississippi River delta
B = Map of the Niger River delta

---

2. Beaches and Barrier Islands
   1. Shoreline deposits
   2. Exposed to wave energy
   3. Dominated by sand
   4. Marine fauna
   5. A few km or less in width but may be more than 100 km long
   6. Separated from the mainland by a lagoon (or salt marsh)
   7. Commonly associated with tidal flat deposits

Examples: Outer Banks (NC), Chincoteague and Assateague Islands (MD, VA), Hilton Head (SC), Tybee, St. Simons and Jekyll Islands (GA), Padre Island and Galveston Island (TX)


Outer Banks of North Carolina, a barrier island. Kitty Hawk, NC. Note lagoon (Pamlico Sound) in the background at top of image. Mainland is barely visible.
Photo courtesy of Pamela Gore.

3. Lagoons
   1. Bodies of water on the landward side of barrier islands
   2. Protected from the pounding of the ocean waves by barrier islands
   3. Contain finer sediment than the beaches (usually silt and clay)
   4. Lagoons are also present behind reefs, or in the center of atolls.

Examples: Pamlico Sound and Currituck Sound (NC), Barnegat Bay (NJ), Chincoteague Bay (MD, VA),

4. Tidal flats
   1. Nearly flat, low relief areas that border lagoons, shorelines, and estuaries
   2. Periodically flooded and exposed by tides (usually twice each day)
   3. May be cut by meandering tidal channels
   4. May be marshy, muddy, sandy or mixed sediment types (either terrigenous or carbonate)
   5. Laminations and ripples are common
   6. Sediments are intensely burrowed
   7. Stromatolites may be present (if conditions are appropriate)


High tide and low tide at Five Islands Provincial Park, Bay of Fundy, Nova Scotia, Canada. Photos courtesy of Pamela Gore.

Sandy tidal flat, Georgia coast. Photo courtesy of Pamela Gore.	Salt marsh at high tide, near Tybee Island, Georgia. Photo courtesy of Pamela Gore.

Examples: Bay of Fundy (Nova Scotia, Canada), area around Mont Saint Michel (France), Georgia coast, Fujimae Tidal Flat (Japan)

5. Estuaries
   1. Mouth of a river drowned by the sea
   2. Brackish water (mixture of fresh and salt water)
   3. May trap large volumes of sediment
   4. Sand, silt, and clay may be deposited depending on energy level
   5. Many estuaries formed due to sea level rise as glaciers melted at end of last Ice Age
   6. Some formed due to tectonic subsidence, allowing sea water to migrate upstream

Examples: Chesapeake Bay (MD, VA), Delaware Bay (DE, NJ), Lake Ponchartrain (LA), Galveston Bay (TX), San Francisco Bay (CA), Tampa Bay (FL)