Environmental Science Geology 1401410 Instructor: Don Thieme

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Environmental ScienceGeology 1401-410Instructor
Don Thieme

• Lecture 20 Glacial and Lacustrine Deposits

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Snow - hexagonal crystal lattice with density
typically 0.07-0.18 g cm-3
Firn - more spherical granules with density
between 0.4-0.8 g cm-3
Ice - density is between 0.8-0.9 g cm-3, just
slightly less than the density of Water
Firn actually forms more slowly at extremely cold
temperatures, and hundreds to thousands of years
are required in cold polar regions.
Ice crystals are anisotropic, and the c-axes
become progressively more aligned as a glacier
accumulates, giving glacial ice a fabric.
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Glacier Mass Balance

• the difference between the accumulation of snow
  and ice (mostly in winter) and the ice melted or
  ablated (mostly in summer)
• calculated from topographic measurements over a
  complete water year or series of water years
• positive mass balance for net accumulation
• negative mass balance for net ablation

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Glacial Till

• material directly emplaced by glacial ice
• "lodgement" till is material deposited at the
  base of an ice sheet
• "ablation" till is material let down in place as
  the ice melts back
• "flow" till cascades off of the front and sides
  of a glacier, grading down-ice into "proglacial"
  deposits
• poorly sorted sediment consisting of both very
  large and very small clasts
• sometimes referred to as "diamicton" which does
  not imply genesis

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Boulder Till in the Old Saybrook moraine,
Connecticut
Boulder and Cobble Till in sandy matrix from
southern Connecticut
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Glacial Moraines

• the term "moraine" as used by American geologists
  usually refers to landforms created by glaciers
• end moraines or terminal moraines formed at the
  outermost edge of an ablating ice sheet
• lateral moraines form along valley walls or at
  the edge of an advancing ice sheet
• medial moraines form in the center of an ice
  sheet, typically where two glaciers have merged
  together

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Most modern glaciers are valley glaciers. The
only remaining continental glacier is the
Antarctic ice sheet.
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Glacial Striations
Pleistocene age striations on Llewellyn sandstone
near Wilkes-Barre, Pennsylvania

• made by sand and gravel clasts adhering to the
  ice at the base of the glacier
• one of the best indicators for the direction of
  ice advance

Proterozoic glacial striations on sandstones in
the Kimberly region, Australia
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Ancient Glacial Deposits

• tillites - lithified tills identified by
  diamict, poorly sorted texture
• erratic clasts derived from distant outcrops
• dropstones large boulder, cobble, or pebble
  clasts set in a fine carbonate or clay matrix

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• glaciation of Antarctica during the Tertiary
• a key player in global changes in
• climate
• sea level
• ocean circulation
• alpine glaciers also formed at other times and
  places during the Tertiary, particularly in the
  Andes, western China and the western United
  States

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• glacial and proglacial deposits are now mapped
  together as a "morphosequence" by geologists
  working in the eastern United States.
• proglacial deposits are supplied by the "dirt
  machine" that occurs where live ice rides up over
  stagnant ice.
• the morphosequence includes all deposits laid
  down as the ice stood at a mappable location on
  the Earth's surface.

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Proglacial rivers or lakes may begin directly in
front of the stagnant ice (A fluvial
ice-contact C lacustrine ice-contact). Proglaci
al lakes may be fed by meltwater rivers (D
Fluvial lacustrine ice-contact)
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Rivers may be fed by water from a proglacial lake
through a spillway (F Lacustrine fluvial
ice-contact)
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Glacial Lakes

• glaciers dam pre-existing rivers
• glaciers depress the land surface, forming new
  lake basins
• glaciers produce moderate to large volumes of
  water during spring thaw
• continental glaciers produced particularly large
  volumes of water when they melted back during the
  late Pleistocene

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Climbing ripples from an ice-contact delta in
Rhode Island
Varves in a Swedish glacial lake
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• Lake environments can be categorized as
• profundal - deepest parts of the basin where
  sedimentation is nearly all pelagic
• littoral - at shallow depths, wave ripples are
  created by winds.
• Palustrine environments occur where lake margin
  sediments are subaerially exposed

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Thermal Stratification
Thermocline separates warm surface water and
colder deep water in tropical lakes Temperate
lakes "turn over" every fall and every spring.
They are stratified in the summer.
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Lacustrine sediments often contain fossil fish,
plants, pollen that indicate their shorelines and
changes in their shorelines.
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The Green River formation (Eocene) from Wyoming
is one of the best known lacustrine deposits from
the rock record.
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Pluvial lakes filled most of the large basins in
the western United States during the early
portion of the Holocene epoch. Some were filled
by meltwater, others by heavy monsoonal rains.