GEOL 342 Sedimentation and Stratigraphy

Fall Semester 2021

Marginal Marine Environments:

Transitional Environments: Depositional environments that are influenced both by sea water, fresh, water, and often subaerial exposure. Effectively includes everything terrestrial along a coast and the shallowest portion of the marine shelf. These intergrade, but for simplicity we recognize four major transitional environments:

• Deltas: The interface of fluvial and marine environments.
• Peritidal: Dominated by tide action, subaerially exposed fine sediments at low tide, shallow and marshy at high tide
• Shoreface: Where sediment is deposited and transported by wave action and subjected to intermittent subaerial exposure.
• Barrier complexes: Long sandy islands or peninsulas that parallel shore but are separated from it by lagoons.

Some of their primary characteristics are indicated by the table below.

Peritidal environments

What if a stream met the ocean without transporting in copious sediment. This yields a tide dominated environment of tidal estuaries and wetlands. The key features of this environment are:

• Extreme range of salinity
• Alternation between submergence and subaerial exposure.
• Low net circulation yielding poor oxygenation.
• Thus, only a small cast of organisms can successfully colonize it, typically marsh grasses.
• Sediments tend to occupy reducing environments, so dark, organic rich.
• Muds interfingered with sands and silts delivered by incoming tides or rivers.

Tidal Flats: If a peritidal environment experiences enough tidal current flow that vegetation can't become established, the result is a tidal flat. (E.G. the Turnagain Arm of the Cook Inlet, AK.) Diagnostic features include:

• Experience daily reversals in flow direction
• Herringbone cross beds (from alternating current flow)
• Flaser, wavy, and lenticular beds form as flows of differing strength alternately deposit sands and muds
• Mudcracks and algal mats on upper-most reach of the flat

Shoreface/Beach:

A deposit of unconsolidated sediment extending from low tide to a topographic break such as a line of dunes or cliffs.

• Requirements for beach formation:
  • loose sediment
  • Wave energy strong enough to:
    • remove clay and silt sized particles
    • transport sand by saltation, actually moving sand up onto shore, covering heavier particles
    • Separates sand by energy level creating a beach profile

  Proverbial truths of the beach:

  • On a small scale, beach deposition is driven by wave action.
  • On a large scale, by longshore currents - the summation of the action of many waves - dominate
  • Beach processes occur above normal wave base.

  
  

• Beach regions:
  
  
  • Offshore: below wave base. Accumulates finer grained sediment from water column.
    
    
  • Lower shoreface: Shoaling zone. Wave action intensely affects lower shoreface only during storm events. Primarily finer sands with layers of mud. Planar lamination present if not disrupted by bioturbation.
    
    
    From Thinklink
  • Upper shoreface: Influenced by regular wave energy. Breaker/surf zone. Sediments:
    • Medium-fine sands, well sorted
    • Dominated by longshore currents
    • waves (moving perpendicular to shore) deposit sediments in bars parallel to shore
    • Trough cross bedding common as a result of unidirectional current flow in longshore troughs.
    • Surf zone dominated by multi-directional trough cross beds, low angle planar cross beds.
    
    
    
    Bartolome Island foreshore.
  • Foreshore: The swash zone. Between high and low tide lines. Strong shallow flows. Upper flow regime planar beds dominate. Sometimes slightly cross bedded.
    
    
    Berm at Puerto Morelos, Quintana Roo, Mexico
  • Backshore: Between normal high tide and margin of beach. Characterized by berms, shelf-like storm deposits. Except during storms or exceptionally high tides, the wind is the primary agent of transport. Expect sand ripples, possibly with mud drapes or intraclasts. Beyond the berm, eolian dunes are common.

Barrier complexes:

Barrier: elongate sandy islands or peninsulas that run parallel to shoreline. Their formation requires:

• Separation of beach deposits from shore by lagoons and marshes
• Abundant sediment supply
• Longshore currents and wave action dominant over tidal action

The seaward side of barriers have typical shoreface environments

Tombolo. Sometimes, a sea stack or island may form a wave shadow. When that happens, sediment can accumulate behind it, eventually connecting it to the mainland.

Spits: Imagine that a beach ends at an embayment, and that a longshore current is carring sediment into the embayment. Voila! A spit! A projection of a beach into a body of water.

Baymouth bar: If the spit continues to develop, it may completely enclose the embayment, forming a baymouth bar.

Barrier islands: Of course, a baymouth bar does form a barrier between two bodies of water. If this is broken, tidal inlets or passes can form. The separated segments become barrier islands.

Lagoon Body of quiet water isolated between the barrier and mainland.

• Sediment: Fine silts and clays settle, often laminated
• Usually rich in microbial activity and/or vegetation
• Water can be stagnant and anaerobic, forming organic rich muds
• Can be part of the tidal range and exhibit tidal flat deposits

A final note: Whereas fresh water is acidic, sea water is just slightly alkaline. Thus, calcareous cements (usually aragonite) can cement beach rock in situ.