Middle Ordovician to Late Silurian Geology Eastern United

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 – 100’s to 1000’s of meters thick 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 n n n White Molasse (top) Red Molasse Late Ordovician to Silurian Flysch – immature sandstones and dark shales n Middle to Late Ordovician Flysch + Molasse = Clastic Wedge Black Shales – starved basin n Middle Ordovician Limestones (base) – Great American Bank n 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 n n Red Molasse = erosion of orogen White Molasse = transgression and reworking of Red Molasse

But, what does it mean? Great American Bank n Passive Margin – Opening of Iapetus Middle Ordovician Limestone n n n 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. n Apparent transgression caused by increased subsidence 9) The sky is falling, and it isn’t snow! n Volcanic ashes appear in sequence 8) You get the munchies for more sediment. n Formation of Starved Basin 7) People say your sediments are so immature! 6) You feel the need to dump your garbage on others. n Formation of Clastic Wedge

Top Ten Signs You Are Experiencing Orogeny 5) You definitely get bent out of shape. n Folding 4) You can’t deny that you have faults. n Thrust faulting 3) You intrude where you are not welcome. n Igneous Intrusions (+/- lava flows) 2) You get all hot and bothered. n Regional Metamorphism 1) You’re caught in a cover-up, trying to hide your true inclinations. n Formation of Unconformities (especially angular)