Transcript Case Study in Sedimentary Tectonics: The Taconic Orogeny

Middle Ordovician to
Late Silurian Geology
Eastern United States
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
Middle Ordovician
Limestones abruptly
overlain by black shale
(10’s to 100’s meters)
Shale is black from
organic matter
Fauna is very restricted
Graptolites, few
cephalopods & trilos
Middle Ordovician
Black shales
succeeded by 100’s
of meters of
interbedded shale
and immature
litharenites (Flysch)
Sandstones have
very distinctive
sedimentary
structures
Structures in Sandstones
Sharp, erosional base
Contact with
underlying shale is
abrupt
Tops are commonly
gradational into
overlying shales
Structures in Sandstones
Basal erosion
surfaces commonly
show Flute Casts
Flutes show direction
of current – from
East!
Flutes widen in
downcurrent direction
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
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
Middle to Late Ordovician Flysch
Thick sequences of
interbedded dark,
graptolitic shales and
immature, litharenites
Commonly deformed
Deformation may
have been
syndepositional
Late Ordovician to Middle Silurian
Molasse
Molasse – 100’s to
1000’s of meters thick
Red shales
Red shales with red
sandstones
Quartz arenites and
quartz pebble
conglomerates
Follows Deformation
Molasse
Interbedded red
shales and
sandstones
Sandstones are
thicker than in flysch
Sandstones show
different sedimentary
structures
Shales may have
mudcracks
Molasse Sandstones
More quartz-rich; sublitharenites
10’s 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
Upper Molasse
Red Molasse passes
upward into White
Molasse
Quartz arenites and
quartz pebble
conglomerates
May have marine
fossils
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
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
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
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
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
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
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 (?)
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
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 isn’t 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
Top Ten Signs You Are
Experiencing Orogeny
5) You definitely get bent out of shape.

Folding
4) You can’t 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) You’re caught in a cover-up, trying to hide your true
inclinations.

Formation of Unconformities (especially angular)