Middle Ordovician to Middle Ordovician to Late Silurian GeologyLate Silurian Geology

Eastern United StatesEastern United States

Middle OrdovicianMiddle OrdovicianShallow water Shallow water limestones and limestones and dolomites of Great dolomites of Great American Bank changeAmerican Bank changeMudcracks & Mudcracks & stromatolites disappearstromatolites disappearBeds become thinnerBeds become thinnerFauna increases in Fauna increases in abundance & diversityabundance & diversityBrachs, bryos, trilos, Brachs, bryos, trilos, corals, molluscs, etc.corals, molluscs, etc.Volcanic ashes appearVolcanic ashes appear

Middle OrdovicianMiddle Ordovician

Limestones abruptly Limestones abruptly overlain by black shale overlain by black shale (10’s to 100’s meters)(10’s to 100’s meters)

Shale is black from Shale is black from organic matterorganic matter

Fauna is very restrictedFauna is very restricted

Graptolites, few Graptolites, few cephalopods & triloscephalopods & trilos

Middle OrdovicianMiddle Ordovician

Black shales Black shales succeeded by 100’s succeeded by 100’s of meters of of meters of interbedded shale interbedded shale and immature and immature litharenites (litharenites (FlyschFlysch))

Sandstones have Sandstones have very distinctive very distinctive sedimentary sedimentary structuresstructures

Structures in SandstonesStructures in Sandstones

Sharp, erosional baseSharp, erosional base

Contact with Contact with underlying shale is underlying shale is abruptabrupt

Tops are commonly Tops are commonly gradational into gradational into overlying shalesoverlying shales

Structures in SandstonesStructures in Sandstones

Basal erosion Basal erosion surfaces commonly surfaces commonly show show Flute CastsFlute Casts

Flutes show direction Flutes show direction of current – from of current – from East!East!

Flutes widen in Flutes widen in downcurrent directiondowncurrent direction

Structures in SandstonesStructures in SandstonesGraded BeddingGraded Bedding within within sandstone bedssandstone beds

Graded interval may be Graded interval may be overlain by overlain by RipplesRipples & & Small Scale Cross Small Scale Cross StratificationStratification

Tops of beds grade into Tops of beds grade into overlying shaleoverlying shale

Structures in SandstonesStructures in Sandstones

Sharp, erosional base +/- Flute CastsSharp, erosional base +/- Flute Casts

Internally gradedInternally graded

Graded interval may have ripples +/- small Graded interval may have ripples +/- small scale cross stratification near topscale cross stratification near top

Sequence repeats countless timesSequence repeats countless times

Bouma SequenceBouma Sequence

Middle to Late Ordovician FlyschMiddle to Late Ordovician Flysch

Thick sequences of Thick sequences of interbedded dark, interbedded dark, graptolitic shales and graptolitic shales and immature, litharenitesimmature, litharenites

Commonly deformedCommonly deformed

Deformation may Deformation may have been have been syndepositionalsyndepositional

Late Ordovician to Middle Silurian Late Ordovician to Middle Silurian MolasseMolasse

Molasse – 100’s to Molasse – 100’s to 1000’s of meters thick1000’s of meters thick

Red shalesRed shales

Red shales with red Red shales with red sandstonessandstones

Quartz arenites and Quartz arenites and quartz pebble quartz pebble conglomeratesconglomerates

Follows DeformationFollows Deformation

MolasseMolasse

Interbedded red Interbedded red shales and shales and sandstonessandstonesSandstones are Sandstones are thicker than in flyschthicker than in flyschSandstones show Sandstones show different sedimentary different sedimentary structuresstructuresShales may have Shales may have mudcracksmudcracks

Molasse SandstonesMolasse Sandstones

More quartz-rich; sublitharenitesMore quartz-rich; sublitharenites

10’s of centimeters to several meters thick, each.10’s of centimeters to several meters thick, each.

Bases are erosional; grain size fines upward, but not Bases are erosional; grain size fines upward, but not as single graded bedsas single graded beds

Large & small scale Large & small scale cross stratification; planar cross stratification; planar stratificationstratification

Upper MolasseUpper Molasse

Red Molasse passes Red Molasse passes upward into White upward into White MolasseMolasse

Quartz arenites and Quartz arenites and quartz pebble quartz pebble conglomeratesconglomerates

May have marine May have marine fossilsfossils

An Example of Facies in the White An Example of Facies in the White MolasseMolasse

East – quartz pebble conglomerates =Shawangunk Conglomerate

Central – Quartz arenites =Tuscarora Sandstone

West – Interbedded quartz arenites &dark grey shales = Tuscarora Sandstone

Review of Sedimentary SequenceReview of Sedimentary Sequence

Molasse (top) – red sandstones and shalesMolasse (top) – red sandstones and shales White Molasse (top)White Molasse (top) Red MolasseRed Molasse Late Ordovician to SilurianLate Ordovician to Silurian

Flysch – immature sandstones and dark shalesFlysch – immature sandstones and dark shales Middle to Late OrdovicianMiddle to Late Ordovician

Flysch + Molasse = Clastic WedgeFlysch + Molasse = Clastic WedgeBlack Shales – starved basinBlack Shales – starved basin Middle OrdovicianMiddle Ordovician

Limestones (base) – Great American BankLimestones (base) – Great American Bank Cambrian to Middle OrdovicianCambrian to Middle Ordovician

Tectonic Interpretations of the Tectonic Interpretations of the Sedimentary SequenceSedimentary Sequence

Great American Bank = Passive Margin Great American Bank = Passive Margin (Base) – tectonically quiet – Pre-orogenic(Base) – tectonically quiet – Pre-orogenicBlack Shales = Starved Basin – early Black Shales = Starved Basin – early orogenic subsidenceorogenic subsidenceFlysch – SynorogenicFlysch – SynorogenicMolasse (Top) – Post OrogenicMolasse (Top) – Post Orogenic Red Molasse = erosion of orogenRed Molasse = erosion of orogen White Molasse = transgression and reworking White Molasse = transgression and reworking

of Red Molasseof Red Molasse

But, what does it mean?But, what does it mean?

Great American BankGreat American Bank Passive Margin – Opening of IapetusPassive Margin – Opening of Iapetus

Middle Ordovician LimestoneMiddle Ordovician Limestone Facies indicate increased depthFacies indicate increased depth Transgression?Transgression? Increased Subsidence?Increased Subsidence?

Volcanic ashes indicate proximity of volcanic Volcanic ashes indicate proximity of volcanic sourcesourcePassive Margin replaced by Starved Basin = Passive Margin replaced by Starved Basin = increased subsidenceincreased subsidence

But, what does it mean?But, what does it mean?

Flysch indicates new sediment source to the Flysch indicates new sediment source to the east – high relief, low grade metamorphics & east – high relief, low grade metamorphics & volcanicsvolcanics

Syndepositional deformation indicates OrogenySyndepositional deformation indicates Orogeny

Taconic OrogenyTaconic OrogenyMolasse – post-orogenic destruction of Taconic Molasse – post-orogenic destruction of Taconic MountainsMountains

Queenston Clastic WedgeQueenston Clastic Wedge

What is a Clastic Wedge?What is a Clastic Wedge?

Wedge-shaped mass of sediment shed Wedge-shaped mass of sediment shed from mountainsfrom mountains

Wedge is thicker and coarser grained Wedge is thicker and coarser grained close to sourceclose to source

Wedge gets thinner and finer grained Wedge gets thinner and finer grained away from sourceaway from source

Usually made of flysch + molasseUsually made of flysch + molasse

Taconic OrogenyTaconic OrogenySubduction begins Subduction begins somewhere in Iapetussomewhere in Iapetus

Subduction creates island Subduction creates island arc – “Taconia”arc – “Taconia”

Subduction closes part of Subduction closes part of IapetusIapetus

Taconia is accreted to Taconia is accreted to North AmericaNorth America

Subduction begins beneath Subduction begins beneath new eastern margin of new eastern margin of North America (?) – North America (?) – Andean-type convergent Andean-type convergent margin (?)margin (?)

Sedimentary Responses to Sedimentary Responses to Tectonic ProcessesTectonic Processes

For every tectonic event, there is a For every tectonic event, there is a sedimentary responsesedimentary responsePassive Margin/Thermal Subsidence – Passive Margin/Thermal Subsidence – thick wedge of limestones, quartz arenites, thick wedge of limestones, quartz arenites, other quartz-rich sandstonesother quartz-rich sandstonesSinking of passive margin – black shalesSinking of passive margin – black shalesApproach of island arc or other landmass Approach of island arc or other landmass – immature sandstones– immature sandstonesErosional destruction of orogen - molasseErosional destruction of orogen - molasse

Top Ten Signs You Are Top Ten Signs You Are Experiencing OrogenyExperiencing Orogeny

10) You get that sinking feeling.10) You get that sinking feeling. Apparent transgression caused by increased subsidenceApparent transgression caused by increased subsidence

9) The sky is falling, and it isn’t snow!9) The sky is falling, and it isn’t snow! Volcanic ashes appear in sequenceVolcanic ashes appear in sequence

8) You get the munchies for more sediment.8) You get the munchies for more sediment. Formation of Starved BasinFormation of Starved Basin

7) People say your sediments are so immature!7) People say your sediments are so immature!

6) You feel the need to dump your garbage on others.6) You feel the need to dump your garbage on others. Formation of Clastic WedgeFormation of Clastic Wedge

Top Ten Signs You Are Top Ten Signs You Are Experiencing OrogenyExperiencing Orogeny

5) You definitely get bent out of shape.5) You definitely get bent out of shape. FoldingFolding

4) You can’t deny that you have faults.4) You can’t deny that you have faults. Thrust faultingThrust faulting

3) You intrude where you are not welcome.3) You intrude where you are not welcome. Igneous Intrusions (+/- lava flows)Igneous Intrusions (+/- lava flows)

2) You get all hot and bothered.2) You get all hot and bothered. Regional MetamorphismRegional Metamorphism

1) You’re caught in a cover-up, trying to hide your true 1) You’re caught in a cover-up, trying to hide your true inclinations.inclinations.

Formation of Unconformities (especially angular)Formation of Unconformities (especially angular)