Sedimentary Rocks Continued

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Sedimentary Rocks Continued
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Sedimentary Rocks

• Geologists divide sedimentary rocks into two main
  groups
• Clastic ( detrital) sediments
• Chemical (and biochemical) sediments

Clastic Sedimentary Rock
Chemical Sedimentary Rock
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Tools for Environmental Reconstruction

• Sed rocks are one of the most accurate picture of
  past environments
• Composition
• Texture
• Fossils
• Sedimentary structures

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Composition

• Qtz and feldspar is more resistant to weathering
  than mafic minerals, therefore ends up in "clean"
  or mature sandstones. Often found in continental
  settings where transport times are high. One
  indication of maturity or amount of time the
  grains have spent being actively worked in the
  sedimentary environment.

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Texture

• Grain Size is a measure of the energy of the
  system. Beaches deposit sands or small cobbles
  high-energy rivers can deposit large cobbles or
  boulders. Glaciers and landslides, probably the
  highest energy environments on Earth, can deposit

Conglomerate/Breccia ---- sandstone
-------siltstone ---shale --cobbles---------------
------------sand------------ silt/mud ----clays,
mud
In general, the smaller the grains the lower the
energy of the depositional system
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Sorting

• Sorting is a measure of the range of partical
  sizes.Poor sorting reflects sudden changes in
  the energy of the system--according to specific
  gravity -- gold placer deposits--according to
  size -- most common minerals are the same
  specific gravityPoor sorting in tills, landslide
  deposits.Good sorting in beach sands deposits.

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Grain shape

• grain shape indicates length of transport.round
  vs. angularround means repeated abrasion during
  transportangular means short transportation
  distance

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Fossils

• Tell us about the paleo-environment if we know
  something about the living habits of similar
  present-day organisms.

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Bedding and Sedimentary Structures

• Cross-bedding is formed from dunes (wind,
  water)-- indicates down-stream or down-wind
  direction. -draw pictures of process,
  steep-sided ripples, symetric ripples. -Ripples
  small, low velocity
• -Dunes big, higher velocity
• -Graded -- heavy comes out first -- turbidites,
  mudslides, -- indicates which way is up in old
  rocks -- gradational vs sharp contacts.
• -Mud Cracks-- seen from the top. Like columnar
  jointing

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Sediment Facies

• Facies refers to a distinctive set of
  characteristics that differs, as a group, from
  those elsewhere in the same rock unit.Use
  barrier islands as an example.In the rock
  record, we see vertical records of various
  facies. How can this happen?-Rising or falling
  sea level

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Facies

• Transgressive Sequence or Transgression is when
  the sea-level rises and the shore-line moves away
  from the center of the basin, leaving its
  sedimentary recordRegressive Sequence or
  Regression is when the sea-level falls and the
  shore-line moves towards the center of the basin,
  also leaving its sedimentary recordSuccession
  of Facies
• Superimposed facies are those that can be
  observed beside each other in present-day
  environments.

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Biogenic Sediments

• LimestoneSiliceous ooze in the deep sea --
  ChertFossil Fuels -- compressed plant
  matterPeat accumulation of plant material in a
  reducing (no Oxygen) environment turns into coal

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Classifying sediments
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Clastic sediments

• composed of fragments or grains derived from
  existing rocks, by weathering, erosion,
  transportation and deposition.
• include clays, silts, sands and gravels, which
  when lithified to form hard rocks become shales,
  siltstone, sandstone and conglomerate,
  respectively.
• Clastic sediments are deposited by water (e.g.,
  rivers, oceans), wind (e.g., sand dunes) and ice
  (e.g., glacial till).

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Volcaniclastic sediments

• One groups of clastic sediments, volcaniclastic
  sediments, the products of explosive volcanic
  eruptions into the atmosphere -- volcanic ash,
  pumice, etc., also accumulate on the seafloor.

Iceland Summer 2003
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Chemical and biochemical sediments

• formed in place, usually by precipitation or
  crystallization directly from a fluid.
• Organisms (particularly microbes) may assist in
  forming some of the sediments.
• They include such rocks as most limestones,
  cherts (silica), phosphates, and evaporites
  (salts deposits such as potash or gypsum).
• Unlike clastic sediments, chemical sediments
  normally have not be transported but formed where
  they are found.

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Evaporites
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Iran's Salt Glaciers
http//earthobservatory.nasa.gov
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Stratigraphy the whole story