Sedimentation and Stratigraphy Geology 5142 Dr' Thieme

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Sedimentation and StratigraphyGeology 5142Dr.
Thieme

• Lecture 15 Tidal Flats, Estuaries, and Deltas

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Tide

• cyclical rise and fall of the level of the sea
  due to the gravitational attraction by the Moon,
  and to a lesser extent by the Sun
• spring tide (the highest high tide) occurs when
  the gravitational effects of Sun and Moon
  coincide
• neap tide (lowest high tide) occurs when solar
  and lunar gravity are working in opposition

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Tide

• Each coast's tidal cycle is affected by
• the shape and orientation of the coast
• the gradient of the coast
• the topography offshore
• wind speed and wind direction
• weather and storm patterns

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Tidal Range

• Vertical distance between high and low tide,
  represented statistically as the difference in
  elevation between "mean high water" (MHW) and
  "mean low water" (MLW)
• The greatest tidal range occurs in tidal basins
  whose natural "resonance period" corresponds to
  that of the principal tide-producing force

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Tidal Range

• The world's largest tidal range, approximately 15
  m, is found in the Bay of Fundy
• The resonance period of 12.58 hours closely
  corresponds to the lunar semi-diurnal period of
  12.42 hours

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Tidal Effects

• tidal range determines the degree of mixing of
  fresh and salt water
• stratification and circulation may develop with
  less dense fresh water extending over a salt
  water "wedge"
• such circulation will be inhibited by larger
  tidal range, resulting in less freshwater
  dilution
• tidal currents transport sediment, particularly
  silt and clay, resulting in pulsed or "rhythmite"
  deposition

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• Flaser bedding - ripple mark bedforms in which
  fine muds have settled out along the top of each
  set of sand laminae
• periods of current activity must alternate with
  periods of quiet energy in a regular cycle

Lenticular bedding - ripples or sand lenses
isolated within planar or hummocky
cross-stratified mud
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Tidal Flat

• "flask" shaped drainage basin, often linked in
  series to other small basins
• each basin is connected to the open sea by a
  narrow channel
• barrier islands typically adjoin each narrow
  tidal inlet channel

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Estuary

• from the Latin word "aestus," meaning tide
• lower course of a river which is open to the sea
• inlet of the sea reaching into a river valley as
  far as the upper limit of tidal rise (Fairbridge)
• some net landward movement of sediment derived
  from outside the estuary mouth (Dalrymple and
  others, 1992)
• "drowned" river valley (e.g. Delaware Bay)

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• Tidal Estuary
• tidal channel and tidal flat subenvironments are
  most distinct where tidal range is large (gt 3 m)
• tidal channel bedload may include shells and
  other large clasts but is typically sand with
  large-scale current ripples or dunes
• tidal flat deposits are bioturbated or laminar
  mud with thin sand sheets

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Tidal flat muds can be distinguished from other
muddy facies by marsh plant roots, other plant
macro-fossils, and certain species of foraminifera
Inclined heterolithic stratification
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Estuary Types

• fjord (drowned high-relief)
• fjard (drowned low-relief)
• ria (drowned bedrock meanders)
• coastal plain (funnel-shaped)
• bar-built (coast-parallel lower reach)
• blind (seasonally blocked)
• deltaic (tidally inundated delta front)
• tectonic (subsiding)

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Most modern estuary examples formed during sea
level rise (fjord, fjard, and drowned meander
types). Coastal plain, bar-built, and blind
estuaries are subtypes due to local shoreline
controls Tectonic or climatic controls may also
result in tidal inundation of river deltas, but
examples are less common.
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Delta

• ? - The word delta was first used by Herodotus
  about 490 B.C. to describe the triangular-shaped
  alluvial plain formed at the mouth of the Nile
  River
• any deposit, subaerial or subaqueous, formed by
  fluvial sediments that build into a standing body
  of water (Boggs)

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Deltas and Rivers

• not all rivers form deltas
• some have an estuary at the mouth
• others have no distinctive sediment body where
  the river meets the sea

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Deltas and Rivers

• the delta begins where the river channel
  transforms itself into the delta channel or into
  a system of distributary channels
• sand and sometimes gravel continue to be
  transported as bedload across the delta top to
  the delta plain - delta channel facies
• the delta top is dominated by channel and
  overbank ("delta plain facies") deposits

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• River-dominated Delta
• delta channel and delta plain (overbank) facies
  are basically fluvial deposits, similar to those
  found in the meandering stream facies
• unidirectional fluvial current continues into the
  sea across the delta front
• distributaries are spread over wide land area as
  channels avulse and form a birds foot" pattern

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The "bird's foot" pattern of the Mississippi
River delta is due to the abundant sediment
supply and large river discharge relative to the
much lower wave energy for sediment transport in
the Gulf of Mexico
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• Tide-influenced Delta
• form where the river meets a coast with a large
  tidal range (gt 3 m)
• tidal channels on the delta front experience flow
  reversal as the flood tide balances the
  downstream discharge
• mud drapes
• flaser or lenticular bedding
• tidal flat deposits may or may not occur at top
  of section
• sandbars are aligned perpendicular to the
  shoreline

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• Wave-influenced Delta
• form where strong and persistent wave energy
  exceeds river or tidal energy
• bedload sand or gravel is reworked by waves as
  quickly as it is deposited, inhibiting the
  formation of "natural levee" features
• longshore currents also wash material along the
  coast to form beach spits
• delta front sandbars and beach ridges are aligned
  perpendicular to the delta river channel
  (parallel to shore)

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Fan Deltas

• form where a braided river or alluvial fan
  supplies coarse gravelly detritus to a body of
  still or slower-moving water
• offshore slope and depth of water result in three
  distinct subtypes
• Gilbert-type fan delta where water reaches to the
  top of a steeply-sloping shore
• Shelf-type delta where the shoreline slopes
  gradually and shallow waves rework the fan
• Slope-type delta with a distinct increase in
  slope offshore

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distinct topset, foreset, and bottomset beds are
most clearly developed in a Gilbert-type
delta G. K. Gilbert first described this
stratigraphy where Pleistocene Lake Bonneville
filled the steep slopes of the Rocky Mountains in
the western United States
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• a Gilbert-type fan delta builds up to a stable
  base level, defined by the height of its topset
  beds
• the topset/foreset contact may be controlled by
  eustatic sea level or by a bedrock nickpoint
  through which meltwater begins to drain in a
  system of glacial lakes

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• a Shelf-type fan delta occurs in relatively
  shallow water with a gentle slope
• pebbly and sandy deposits of the delta front are
  separated laterally rather than vertically from
  the "foresets" of the fan
• clast size increases gradually offshore in
  proportion to the decrease in wave energy

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• a Slope-type fan delta will form on a coast
  there is a distinct increase in slope offshore
• sediment transport beyond the shelf break is by
  debris flows and turbidity currents
• turbidite beds replace the "foresets" of the
  Gilbert model and may be stacked vertically over
  fine-grained bottomset beds