Name: From Ordovices an ancient tribe of Wales; a compromise period proposed by Lapworth (1879) for overlap between Sedgewick’s Cambrian and Murchison’s Silurian  systems; type area is in UK; Dobb’s Lynn;graptolitic dark shales typical of British sections
Commences at major faunal change from Cambrian; terminates with a very major mass extinction.

Biotic diversification:  Age of rapid diversification of marine invertebrates, especially brachiopods, bryozoans, echinoderms; a few vertebrates; beginnngs of terrestrial life

In terms of faunas, as well as separation by a major unconformity, Ordovician should probably be divided into two systems: Lower Ordovician or "Canadian " and Middle-Upper Ordovician.
Mostly are separated by major unconformity-up to 30 million years, Knox Unconformity in North America; MAY be related to lithospheric upwarp associated with Taconic Orogeny

Sequence Stratigraphy: Parts of two major depositional sequences: end of Sauk terminated with Knox Unconformity and then first (Creek) phase of Sloss’s Tippecanoe Supersequence: global regression at end of period produced by glacio-eustatic sea-level fall: Cherokee Unconformity

PlateTectonics: Laurentia- Baltica at maximum separation as Iapetus opened and then began to close; Baltica moving northward shows gradual climatic change from cold water clastics to subtropical oolitic limestones; Laurentia still straddling paleo- equator stayed more or less fixed in position

Major Orogeny in North America: Taconic in 2 tectophases due to colllision of eastern Laurentia with small terranes, island arcs; created Taconic thrusts (allochthons) and major Queenston clastic wedge

Climate: Mainly greenhouse climate regime, but late in Ordovician global cooling led to renewed continental glaciation in present-day Saharan Africa; brief icehouse interlude, but why??

Very widespread Middle-Late Ordovician black shales; perhaps major burial of organi matter drew down atmospheric CO2; icehouse effect; also Gondwanaland shifted over South Pole; snow, ice began to accumulate in present Saharan Africa

Early-Middle Ordovician: Great American Carbonate Bank: Eastern North America:

Lower Ordovician carbonates;  Sauk sequence continues: Knox-Beekmantown massive, sparsely fossiliferous (abiotic) carbonates; similar to Cambrian; Great American carbonate bank" continued; some evaporites; vast carbonate banks on western side of Laurentia as well

Ends with major relative sea level drop; major exposure from Transcontinental arch downward; karstification major relief along Knox Unconformity; in places unconformity amounted to 30 million years of erosion

Tippecanoe (Creek) Supersequence commences with a renewed transgressive sandstone-carbonate succession analogous to Tapeats-Muav or Potsdam- carbonates succession in Cambrian Sauk transgression but then changes upward to siliciclastics and finally major shallowing and regression

Tectono-Sedimentary Cycle: Record of the Taconic Orogeny

1) Basal transgressive sands Most famous basal sandstone is St. Peter in mid-continent area; supermature, recycled sand almost pure quartz, glasss-making sand of Mississippi Valley; farther east minor clastics at base of Middle Ordovician (Pamelia)

2) Clean Carbonates: Clastics give way upward into Chazy (locally) and Black River (High Bridge) Group clean carbonates; extremely extensive peritidal environments

3) Muddy Carbonates: In northern Appalachians the Black River Group is followed by shallow to deeper shelf muddy, highly fossiliferous carbonates of Trenton Group; some show evidence of seafloor deformation; slumping, sliding: seismites

4) Volcanic ashes: Both Black River and Trenton show distinct metabentonite beds, including the most extensive bentonite in Earth history; Milbrig-Big Ash bed; reflect colossal eruption in volcanic island arc off the coast of Laurentia found over much of eastern North America and in Scandinavia

5)  Black Shales: Muddy carbonates are followed abrupt shift to black shales (Utica) with bentonites or laterally equivalent gray shales (Kope): early deep basinal phase;

6) Flysch: These pass upward into thick successions of shales and turbidites Martinsburg: flysch some wildflysch; basin filling with sediment

7) Molasse: Finally see coarser sands and Queenston-Juniata red beds (laterally equivalent to Richmond Group of Cinn. Arch)  molasse; basin overfilling; delta progradation

This is the classic sedimentary succession of tectonic cycle in a foreland basin, and may be interpreted as follows:
:
1) pre-orogenic: mature sands and carbonate platform

2) synorogenic carbonate shelf subsidence and collapse -initial thrusting

3) synorogenic underfilled basin phase: black shale to flysch

4) late orogenc to post-orogenic overfilled basin

In southern Appalachians there is a more complex pattern as follows :

       1) Ls: Black River carbonates: tectonic quiescence
Middle Ordovician   2) Flysch: Athens-Sevier: black shale underfilled basin
       3) Molasse: Moccasin-Bays red beds: overfilled basin

       4) Ls.Eggleston Ls (bentonites): tectonic quiescence
Upper Ordovician    5) Flysch Martinsburg shale, flysch:underfilled basin
       6) Molasse: Juniata red beds: overfilled basin

Thus, in south, two complete tectono-sedimentary cycles indicates two orogenic phases of thrust-loading, foreland subsidence, erosion of uplfts and basin filling: two tectophases of Taconic Orogeny: early Blountian Tectophase and later Vermontian Tectophase Taconic
Allochthon; in  north we see evidence for only a single, Vermontian

Orogenies are not always single simple events; may involve several phases:
 
 
 
 

Taconic Orogeny: Case Study of an Island Arc-Continent Collision

What was the Taconic Orogeny??

It is now clear that at least in Vermontian phase the basin subsidence and sedimentary wedges are related to overthrusting  and loading of the Laurentian cratonic edge by a series of thrust slices termed the; rocks seen in eastern New York and east-central Pennsylvania

These are weakly metamorphosed (slaty) deep water sediments of Early Cambrian to Middle Ordovician age: series of green, black, purple and red slates (seen on many roofs and patios around the country) some turbidite sandstones, thin carbonate turbidites and debris flows;

These rocks originated in deep oceanic environments on the slope and rise to abyssal plains of the Iapetus Ocean-east (modern direction) of the Laurentian craton; carboante debris flows indicate proximity to steep carboante bank edge-like Bahamas Escarpment

East of the Taconic area rocks are highly metamorphosed but give further indications of what caused the thrusting:

1a) In north: Green Mountains, Berkshires, Manhatten Prong; Baltimore=
Reading Prong;  Uplifted basement blocks backthrusted;  PC Grenville basement pushed up and west over the Taconic rocks

1b) In south Blue: Ridge lies east of Taconic basins; these mountains appear to be late Proterozoic (Hadrynian) metavolcanics and sediments that had rifted fromLaurentian and then been pushed back

East of Green-Berkshires-Manhatten-Blue Ridge lies the Piedmont area;complex metamorphic-igneous terrane, includes:

2) Staten Island (NY) and a belt of rock in central New England composed of serpentine belt (asbestos mines in Quebec- Vermont);  suggest a thrust ophiolite slice

3) A chain of granitic an granodiorite domes (Highlandcroft plutons) appear to be the cores of volcanoes of an island  arc (Ammonoosuc Arc)

Model:  Island Arc-Continent Colllision: Peripheral Foreland Basin

A) Taconic Orogeny represents collision of series of small blocks
a) :Blue Ridge microcontinent in south and b) Amonoosuc island arc with east edge of Laurentia;

B) Eastern Laurentia partially subducted into trench  along east-dipping subduction zone; Normanskill flysch deposited in this trench

C) Accretionary wedge continental slope and rise and ocean floor sediments offscraped and underplated to form stack of slabs: subducxtion complex this was pushed up and west onto craton to form Taconic Allochthons

D) The slaty Taconic rocks were highly deformed and thrust 40-50 km westward (modern direction)  from where deposited  over shelf sediments;

E) As this thrust mass pushed downward on lithosphere it caused reactivation of old fault blocks and subsidence of Taconic foreland basin; this is a peripheral type foreland basin; i.e. a very low angle trench on a craton the load implaced on the crust is an accretionary prism (rather than a retroarc fold and thrust belt)

F) As basin pused down it may have caused isostatically compensating peripheral bulge or forebulge on inner side of foreland; Lexington platform ?

Loading may also have reactivated old Hadrynian faults in Pa-Ohio- Kentucky-Tennessee to form Sebree Trough;

F) The erosion of the Taconic Mountains (hinterland) spewed masses of sediments and some volcanic ashes into the subsiding foreland; evewntually the basin was overfilled as sedimentation oustripped subsidence; Queenston clastic wedge prograded westward

Ordovician Life History

In life history: a new phase of adaptive radiation led to establishment of "Paleozoic Fauna"; beginning of "plateau phase" of family diversity;

Sepkoski et al. postulated that more archaic "Cambrian fauna" of primitive trilobites, hyoliths, simple mollusks, etc. was displaced offshore as new "Paleozoic fauna" dominated by articulate brachiopods, bryozoans,and crinoids diversified in shallow water

Furthermore, (Arnie) Miller and Mau suggested that diversification and evolutionary innovation occurred preferentially in active foreland asins such as the Taconic basin

Stromatolites decline: perhaps due to rise of grazing and burrowing organisms in offshore environments

Bioturbation depth and intensity increases

Many new forms appear:

 Receptaculitids ("sunflower" green algae); armored to protect from grazing?

Stromatoporoids: skeletonized sponges

Cnidaria, major tabulate and rugose corals, first "coral reefs" (Isle Lamotte, L. Champlain)

Bryozoa the latest phylum to evolve diversified rapidly with five orders by end of Ordovician; note great abundance in Cincinnatian

Brachiopods: four new orders of brachiopods;  dominance of orthids and strophomenids, formed many  classic Cincinnatian shell beds

Mollusks: diverse Gastropods; including platyceratid gastropods on crinoids; Bivalves diversify  especially in nearsore environments; Cephalopods "monsters" of the Ordovician; huge nautiloids, endoceroids, huge predators, up to 10 m long!

Trilobites: not as diverse as in Cambrian at species level but more disparate, greater variety of basic forms; enrollment evolves (Flexicalymene etc), burrowing forms (Isotelus), blind Cryptolithus,  pelagic forms,

Echinoderms: about ten new classes of echinoderms, Rhombifera, (cystoids), Blastoids, Crinoids, Starfish, Brittle Stars, Echinoids plus others;

Graptolites: first planktonic graptolites, major index fossils; widespread, rapidy evolving ; hit by major extinctions at end Ordovician

Coniodonts: diverse and more complex than simple cones of Cambrian: another key index fossil group; appear to have been simple eel-like vertebrates with specialized teeth appararatuses for grasoing prey and ingesting it

Vertebrates: jawless fish scavengers and perhaps first sharks, acanthodians

Terrestrial life:  still meager but probably first land plants, small psilophytes;  oldest terrestrial (millepede burrows)
 

Major Late Ordovician Extinction: post-Cincinnatian;  probably 2nd largest in life history ~ 22% of families of marine organisms;  many trilobites, graptolites; families of brachiopod;  two waves: first decimated tropical forms; second hit cooler adapted forms

Probably related to rapid change in climate; cooling associated with Gondwana glaciation;  global regression; anoxia; long period of lowered diversity in Early Silurian