Marginal-Marine Environments Deltas, Beach and Barrier Island systems,
Marginal Marine
• Transitional
• Where land meets the sea
• Dominated by rivers (fluvial), wave, and tidal processes.
• High-energy – waves and currents
• Quiet water – lagoonal, and estuarine
Deltas – Deltaic system
• Alluvial Delta
• Subaerial, subaqueous
• Sediment in a standing body of water
Ancient Delta Deposits
• Found in all ages of stratigraphic sequences
• Important petroleum and natural gas deposits, coal, and other mineral resources, uranium is one.
• Excellent reservoir rocks, cap rocks and traps in this sequence of rocks, stratigraphic traps
Modern deltas
• Table 9.1
• Largest delta plain area = Ganges –Bramaputra
• Large water discharge = G-P, Irrawaddy, Mississippi
Sediment Characteristics of Deltas
• Deltaic plain – subaerial component
• Subaqueous Deltaic plaine – Delta front– prodelta
Flow and Discharge
• Jet – discharge of sediment• Homopycnal flow – equal density water of river
entering basin of equal density water, rapid mixing and abrupt deposition of sediment.
• Hyperpycnal flow – river water higher density than basin water, flows beneath the basin water, vertically oriented plane-jet flow, forms turbidites on gentle slope of delta.
• Hypopycnal flow – river water is less dense than the basin water, river flow into marine or saline lake flow on top of basin as a horizontal plane-jet,
Delta Cycles
• Progradation of delta seaward – construction phase– Coarsening-upward sequence
• Fine prodelta muds overlain by delta front silt and sand
• Distributary-mouth sands on top of these• Marsh and fluvial deposits
• Transgression of sea– Destruction phase
Ancient Delta system -
Mississippian
What tectonic event would these deltas be associated with of the Appalachian Mountains?
What do you know of the sea?
• Ocean currents?– Rotation?– Hurricanes rotate in what direction?
• What about tides?– How many low tides per day?– Spring tide?– Neap tide?
Beach and Barrier Island Systems
• Beach is the dominant marginal marine deposit
• What is the dominant Wentworth’s clast size found on the beach?
Depositional Settings
• Tidal ranges– Microtidal = 0 – 2 m tidal range, barrier
islands– Mesotidal = 2 – 4 m tidal range– Macrotidal = > 4 m tidal range
Beach to Barrier Island
• Single Beach – attached to the mainland• Strand Plain – broad beach-ridge system,
multiple parallel beach ridges and parallel swales, lack well-developed lagoons or marshes– Chenier Plain – sandy ridges elongated along
the coast and separated by coastal mudflat
• Barrier Island – separated wholly or partly form mainland by a lagoon or marsh
Characteristics
• Beach deposits– Fine to medium Sand– Heavy-mineral laminae
• Backshore– Eolian sand deposits– Storm-wave deposits
• Shoreface deposits– Upper shoreface – bidirectional cross-bedding sets, Skolithos
burrows, – Middle shoreface – fine to medium sand, shell material,
landward and seaward dipping cross-beds, Skolithos and ophiomorpha
– Lower shoreface – fine to very fine sand, intercalated layers of silt and mud, small scale cross-stratification, horizontal laminations, hummocky cross-stratification
Deposits
• Back-barrier– Washover deposits – sandy deposits in the mud-rich
lagoon– Tidal-channel deposits – sand and lag deposits, fluvial
marine deposit, cross-bedding– Tidal delta deposits – sandy deposits, parabolic
shape in cross-section– Tidal Flat deposits – sand lenses and mud deposit– Lagoonal – fine grained muds, organic rich muds– Marsh – sandy to silts to muds, peat deposits,
bioturbated a lot.