Middle Ordovician to Late Silurian Geology

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Middle Ordovician to Late Silurian Geology

• Eastern United States

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Middle Ordovician

• Shallow water limestones and dolomites of Great
  American Bank change
• Mudcracks stromatolites disappear
• Beds become thinner
• Fauna increases in abundance diversity
• Brachs, bryos, trilos, corals, molluscs, etc.
• Volcanic ashes appear

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Middle Ordovician

• Limestones abruptly overlain by black shale (10s
  to 100s meters)
• Shale is black from organic matter
• Fauna is very restricted
• Graptolites, few cephalopods trilos

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Middle Ordovician

• Black shales succeeded by 100s of meters of
  interbedded shale and immature litharenites
  (Flysch)
• Sandstones have very distinctive sedimentary
  structures

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Structures in Sandstones

• Sharp, erosional base
• Contact with underlying shale is abrupt
• Tops are commonly gradational into overlying
  shales

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Structures in Sandstones

• Basal erosion surfaces commonly show Flute Casts
• Flutes show direction of current from East!
• Flutes widen in downcurrent direction

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Structures in Sandstones

• Graded Bedding within sandstone beds
• Graded interval may be overlain by Ripples
  Small Scale Cross Stratification
• Tops of beds grade into overlying shale

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Structures in Sandstones

• Sharp, erosional base /- Flute Casts
• Internally graded
• Graded interval may have ripples /- small scale
  cross stratification near top
• Sequence repeats countless times
• Bouma Sequence

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Middle to Late Ordovician Flysch

• Thick sequences of interbedded dark, graptolitic
  shales and immature, litharenites
• Commonly deformed
• Deformation may have been syndepositional

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Late Ordovician to Middle Silurian Molasse

• Molasse 100s to 1000s of meters thick
• Red shales
• Red shales with red sandstones
• Quartz arenites and quartz pebble conglomerates
• Follows Deformation

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Molasse

• Interbedded red shales and sandstones
• Sandstones are thicker than in flysch
• Sandstones show different sedimentary structures
• Shales may have mudcracks

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Molasse Sandstones

• More quartz-rich sublitharenites
• 10s of centimeters to several meters thick,
  each.
• Bases are erosional grain size fines upward, but
  not as single graded beds
• Large small scale cross stratification planar
  stratification

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Upper Molasse

• Red Molasse passes upward into White Molasse
• Quartz arenites and quartz pebble conglomerates
• May have marine fossils

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An Example of Facies in the White Molasse
East quartz pebble conglomerates Shawangunk
Conglomerate
Central Quartz arenites Tuscarora Sandstone
West Interbedded quartz arenites dark grey
shales Tuscarora Sandstone
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Review of Sedimentary Sequence

• Molasse (top) red sandstones and shales
• White Molasse (top)
• Red Molasse
• Late Ordovician to Silurian
• Flysch immature sandstones and dark shales
• Middle to Late Ordovician
• Flysch Molasse Clastic Wedge
• Black Shales starved basin
• Middle Ordovician
• Limestones (base) Great American Bank
• Cambrian to Middle Ordovician

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Tectonic Interpretations of the Sedimentary
Sequence

• Great American Bank Passive Margin (Base)
  tectonically quiet Pre-orogenic
• Black Shales Starved Basin early orogenic
  subsidence
• Flysch Synorogenic
• Molasse (Top) Post Orogenic
• Red Molasse erosion of orogen
• White Molasse transgression and reworking of
  Red Molasse

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But, what does it mean?

• Great American Bank
• Passive Margin Opening of Iapetus
• Middle Ordovician Limestone
• Facies indicate increased depth
• Transgression?
• Increased Subsidence?
• Volcanic ashes indicate proximity of volcanic
  source
• Passive Margin replaced by Starved Basin
  increased subsidence

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But, what does it mean?

• Flysch indicates new sediment source to the east
  high relief, low grade metamorphics volcanics
• Syndepositional deformation indicates Orogeny
• Taconic Orogeny
• Molasse post-orogenic destruction of Taconic
  Mountains
• Queenston Clastic Wedge

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What is a Clastic Wedge?

• Wedge-shaped mass of sediment shed from mountains
• Wedge is thicker and coarser grained close to
  source
• Wedge gets thinner and finer grained away from
  source
• Usually made of flysch molasse

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Taconic Orogeny

• Subduction begins somewhere in Iapetus
• Subduction creates island arc Taconia
• Subduction closes part of Iapetus
• Taconia is accreted to North America
• Subduction begins beneath new eastern margin of
  North America (?) Andean-type convergent margin
  (?)

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Sedimentary Responses to Tectonic Processes

• For every tectonic event, there is a sedimentary
  response
• Passive Margin/Thermal Subsidence thick wedge
  of limestones, quartz arenites, other quartz-rich
  sandstones
• Sinking of passive margin black shales
• Approach of island arc or other landmass
  immature sandstones
• Erosional destruction of orogen - molasse

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Top Ten Signs You Are Experiencing Orogeny

• 10) You get that sinking feeling.
• Apparent transgression caused by increased
  subsidence
• 9) The sky is falling, and it isnt snow!
• Volcanic ashes appear in sequence
• 8) You get the munchies for more sediment.
• Formation of Starved Basin
• 7) People say your sediments are so immature!
• 6) You feel the need to dump your garbage on
  others.
• Formation of Clastic Wedge

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Top Ten Signs You Are Experiencing Orogeny

• 5) You definitely get bent out of shape.
• Folding
• 4) You cant deny that you have faults.
• Thrust faulting
• 3) You intrude where you are not welcome.
• Igneous Intrusions (/- lava flows)
• 2) You get all hot and bothered.
• Regional Metamorphism
• 1) Youre caught in a cover-up, trying to hide
  your true inclinations.
• Formation of Unconformities (especially angular)