Lacustrine Environments

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Lacustrine Environments
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What is a lake?

• A lake is a very slowly flowing body of water in
  a depression of ground not in contact with the
  ocean.
• Definition of aquatic habitats difficult because
  these characteristics exist as a continuum, not a
  discrete set of definitions.
• High precipitation high retention permanent
  lakes
• Dry areas have more intermittent lakes

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Geology and Formation of Lakes

• 1.- Tectonic Movements
• Movement of tectonic plates has created some of
  the oldest, deepest lakes on earth
• Graben lakes form where fault allows a block to
  slip down, causing a massive depression that
  fills with water.
• Horst lakes form where a block tilts (instead of
  completely slipping down)

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Geology and Formation of Lakes

• 2.- Damming
• Natural processes that form dams can create
  lakes from flowing water
• - Landslides
• - lava flows
• - animals that construct dams
• - thick vegetative growth

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Geology and Formation of Lakes

• 3.- Glacial activity
• Very common mode of lake formation as glaciers
  move through landscape they scour out
  depressions.
• Many different characteristic patterns of lakes
  formed by this process
• - The deepest scour at the head of the valley
  forms a cirque lake
• - As the glacier retreats down the valley, it
  leaves a chain of paternoster lakes

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Paternoster Lakes
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Fjord Lakes

• Long glacial lakes formed in very steep valleys

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Glacial Till

• As the glacier moves along the landscape it
  entrains rocks, sediments, etc. As it melts at
  its edge, it leaves this glacial till behind.
• If large blocks of ice are left in this till,
  they will melt and leaves lakes/wetlands (e.g.
  prairie potholes)
• When forward flow backward melt rate, a
  terminal moraine is left that forms a dam.

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Glacial Ice Dams

• A large block of glacial ice can form a dam,
  holding a large amount of water behind it.
• If this ice dam breaks, this massive amount of
  water is released downstream.
• This severe flooding event can scour out existing
  lakes and leave new, larger lakes in its wake.

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Geology and Formation of Lakes

• 4.- Volcanic activities can form lakes
• A notable example is Crater Lake in Oregon.
  This was formed after Mt. Mazama erupted 7700
  yrs ago. The resulting collapse of the mountain
  walls left a deep caldera that became filled with
  water.

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Geology and Formation of Lakes

• 5.- Dissolution of bedrock can create sinkhole
  (typical in karst topography)
• 6.- River / stream flow dynamics (meanders, etc.)
  can create fluvial lakes
• - Oxbow lakes
• - Levee lakes

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Lake Habitat Terms

• Lakes are referred to as lacustrine or lentic
  environments.

Pelagic Zone
Littoral Zone
Profundal Zone
Benthic Zone
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Lake Morphometry

• Lakes are often compared / classified according
  to their morphometry
• Bathymetry (spatial depth profile) can be
  indicative of geological formation process
• Lakes with low mean depth are generally more
  productive
• Ease of wind mixing
• Photosynthesis for primary production

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Bathymetric Map of Lake Erie
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Retention Time

• The retention time or water residence time is the
  time is takes for water to flush through the
  system
• (Lake volume) / (rate of inflow)
• Can range from hours to thousands of years (e.g.
  Lake Tahoe retention time 700 yrs)

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Reservoirs
Fig. 6.9
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Lake Stratification

• Two most important factors influencing
  stratification of lakes
• Temperature
• Salinity

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• The lake deposits can be divided into three major
  groups
• 1 - Clastic lake deposits The sediments grain
  size distribution of the lakes is eccentric
  (concentric rings), with grain size increasing
  outwards (FIGURE).
• 2- Chemical lake deposits depend mainly on the
  dissolved and suspended material brought into the
  lake by rivers and/or brines (TABLE).
• 3 Biogenic lake deposits this can be either a
  strong concentration of organic substance, or
  development of stromatolites, or swamp and peat,
  bog Developments.

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Schematic distrubution of sediments in a lake.
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The lacustrine evaporites.
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• Types of lakes
• 1- Terrigenous permenent lakes Those are
  infilled by terrigenous sediments occur in
  mountainous terrain where the high precipitation
  and sediments run-off are combined.
• 2- Autochthonus permenent lakes occur in
  low-lying terrain in temperate and warm humid
  climates, where carbonate sedimentation occurs
  away from the river mouths, both around the lake
  shores and in the deeper part of the lake.
• 3- Ephemeral lakes (playa) lakes of short
  duration, consisting a red-brown mudstone
  containing varing amount of clay, silt, and
  disseminated carbonate, and can also deposit
  evaporite minerals.

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How to recognized the ancient lakes (lacustrine
deposits)?

• 1- Laminated-fine grained sediments of either
  argillaceous, calcareous and evaporitic, and
  marginal of lakes may be very coarse and
  conglomeritic.
• 2- Ancient lakes showing cycalicity in the form
  of varves which may be capped by mud crackes
  indicating regular fluctuations and evaporations
  of lake water.
• 3- fauna is low in density compared to a marine
  fauna, it consist cheifly of fresh water bivalves
  and gastropods together with fish. Burrows are
  generally rare because the sediments is poorly
  oxygenated, plants are especially abundant close
  to the banks.
• 4 Ideal model showing an upward coarsening grain
  size profile ( because there are three major
  facies within a lake environments lake fades,
  delta fades, and river fades).

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SHEMATIC SECTION FOR LITHOFACIIES OF MARGINAL,
MUDFLAT AND LACUSTRINE ENVIRONMENTS OF THE GREEN
RIVER FORMATION.
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• THE END