Name: From Silures another ancient warlike tribe in the Welsh Borderlands (old border between Wales and England); period proposed by Murchison (1834) for fossiliferous succession  area. Type area near Wenlock and Ludlow in the Welsh Borderlands (Note: overlap with Sedgewick's definition of "Cambrian").  Commences with recovery from major Late Ordovician extinction  and ends with minor extinction before Devonian (there is really not a major Silurian-Devonian break in most areas

Climate:  A retrun to greenhouse conditions following Ordovician glaciation; but some lingering glaciation in Paleoandes Mountains of South America through Early Silurian; mid and Late Silurian climates appear to have been quite equable world-wide lots of shallow carbonate depostion, some return to stagnation and balck shale development in deeper basins

Plate Tectonics:  Final rumbles of Taconic Orogeny in eastern Laurentia Ettensohn and Brett recognize a third and final tectophase of Taconic Orogeny (after the Blountian, and Vermontian phases- this might be called the Tuscarora phase) in Early Silurian; produced new clastic wedge and pulse of westward subsidence.

Baltica had moved northward through Ordovician; now collides with northern Laurentia to form the widespread Caledonian Orogeny and creats Euroamerica ("Old Red Continent" straddling the paleoequator. At the same time the microcontinent of Avalonia was converging on (present) eastern North America, making initial contact in the northern Maritime region in late Silurian or Early Devonian;  some orogenesis in mid Silurian of eastern North America (Laurentia) termed the "Salinic Orogeny"' see renewed pulse of subsidence and westward migration of the foreland basin;
Gondwanaland remained in southern near polar position

Sequence Stratigraphy: Sea-level rise following lowstand that produced Cherokee Unconformity' Silurian-Early Devonian encompassed in Tutelo (second) Phase of Sloss's Tippecanoe Supersequence; sea-lvel quite high through mid Silurian, beginning to fall in later Silurian; excellent 3rd and 4th order sequences in Silurian of eastern North America, as follows:

Early Silurian siliciclastics; Tuscaroora-Shawangunk-Medina clastic wedge; thich quartz-rich sandstones overlies angular Taconic Unconformity in Appalachians; seem to record renewed uplift and   sedimentation; in Niagara Region these show basal transgressive sandstones, highstand marine shales, and late HST (regressive) redbeds; in Ohio these give way to marine carbonates and shales of Brassfield (rests on Cherokee Unconformity)

Mid Silurian mixed siliciclastics-carbonates and ironstones; records wearing down of Taconic source terranes, shift back to open marine intrabasinal sediments; distinctive feature is widespread "Clinton iron ores"; these are famous fossil rich and oolitic hematites;  mined extensively near Birmingham, Alabama to NY for steel ans red paint oxides; probably associated with Fe enrichment of sediments due to deep weathering of Taconic uplifts; Fe-coatings promoted by sediment starvaion; hence ironstones commonly associated with phosphatic nodules: flooding surfaces

Mid Silurian carbonate bank; very widespread crinoidal sands and tabulate stromatoporoid reefs in Great Lakes area (Thornton reef near Chicago is part of reefal complex around Michigan Basin;  patch reefs in Indiana, Ohio, NY; will see; show good succession of pioneer thicket formers to climax communities dominated by head corals and stromatoporoids; important petroleum reserves

Upper Silurian; more arid climate and landlocking of seas: in Michigan basin by barrier reef complex; in eastrn Appalachian region by new siliciclastic wedge-Vernon-Bloomsburg delta issuing from Salinic Orogenic uplift;

these restrictive barriers prevented open circulation enabled salinity to build up to point of precipitating evaporites; cyclic process of influshing sea-water then boiling off enabled anhydrite, gypsum and halite deposits to accumulate to considerable thivkness; over 1 km under Detroit;  (largest salt mine in Western Hemisphere was in Silurian salts in western NY until shut down by roof collapse of 1994)
 

Biotic Events: Rediversification to Mid Ordovician levels of family diversity; continuations of "Paleozoic Fauna" dominated by brachiopods, bryozoans, corals, stromatoporoids, crinoids, etc., diversification of reef builders- appearance of widespread large tabulate coral-stromatoporid reefs in tropics to warm temperate areas; low provinciality suggests convergence of landmasses and shelf seas;

Stromatolites have declined in normal marine environments but are still common in hypersaline shallow water

Stromatoporoids: common in subtropical shallows; major reef-builder

Sponges: common in slightly deeper shelf areas; astreosponges typical of early reef stages

Tabulates: favositids, and halysitid chain corals common also form both biostromes and bioherms (reefs)

Rugose corals: solitary and colonial

Bryozoans ramose (twiggy; fistuliporoid (crustose; fenestrates

Brachiopods: highly diverse esp. sprifierids and pentamerides (typical of shoal margins); classic benthic assemblage concept BA-1 (lingulid); BA-2 (Eocoelia); BA-3 (pentamerid); BA-4 (stricklandiid); BA-5 (Dicoelosia)

Mollusca: bivalves nearshore; gastropods (include platyceratids) and diverse nautiloid cephalopds

Echinoderms: crinoids very diverse and abundant (first "age of crinoids") rhombiferan cystoids, blastoids continue relatively abundant; changes upward in reefs: deep water fragile small crinoids; riugh water stage: rigid box-like camerate crinoids and cystoids

Trilobites: rediversify from Ordovician extinctions but do not attain Ordovician levels

Graptolites barely survived Ordovician extinctions then underwent new radiation monograptids evolved rapidly key index fossils in deeper water settingss;

Conodonts also critical index fossils

Vertebrates also show origin of jawed fishes; jaws appear to have arisen as an ezaptation of gill arches probably the second gills arch;  gill arches already had two cartilage bars united by muscular hinge; teeth may have been derived from skin denticles (in sharks the denticles have same basic configuration as teeth: pulp cavity, etc.)

Shark may date to Middle Ordovician, but first definite jawed vertebrate fossils are acanthodians (small fish with diamond like enameloid scales and fin spines )

Terrestrial life shows appearance of first certain vascular land plants plus terrestrial animals (myriapod group)
 
 

DEVONIAN PERIOD (415-365 Ma)

Name:  Named by Murchison and Sedgewick jointly in 1839 for exposures of rock in Devonshire (Devon County) England with fossils intermediate between Silurian and Carboniferous (source of the Great Devonian Controversy).

 A relatively long period with very many important events in Earth and life history: major orogenies in Europe (end of Caldedonian), eastern North America (Acadian), and for the first time in the Phanerozoic, western North America (Antler)

Climate: Much of the Devonian was characterized by warm, mild "greenhouse" type climates and a strong tendency for stagnation in deeper sea environments (very widespread anoxic black shale deposits); however, toward the end of the period there is evidence for climatic stress associated with renewed glaciation in Gondwana (esp. South America)

Sequence Stratigraphy; During Early Devonian, the final, regressive phaser of Sloss's Tippecanoe (Tutelo phase) Supersequence were seen in the Early Devonian (Helderberg carboantes of eastern NY-Pa)
Major Wallbridge Unconformity truncates these sediments and separates from overlying Kaskaskia Supersequence (also divided into two parts)

Early phase created basal transgressive quartz sandstones (Oriskany) analogous to Potsdam  (Sauk Ss.) and St. Peter (Tippecanoe Ss.) and equivalent limestones : an orthoquartzite-carbonate suite; but tthese give way to mixed siliciclastics and carbonates

Plate Tectonics some uncertainties about positions of continents; where was Gondwanaland; apparently getting close to Euroamerica' Laurentia stil on paleoequator though rotated from earlier position; present east side was south of equator; NW in doldrums (bauxites) Alaska and Arctic Canada have reefs and evaporites

1) Caledonian Orogeny continued from Silurian caused major folding and thrusting in Scandinavia (Norway esp.) parts of northern Britain; famed Siccar Point angular unconformity shows folding of older Silurian flysch sediments, uplift, erosional beveling and then inundation by Devonian red  sediments

2) Major theme for eastern North America is convergence of Avalonia with Laurentia for produce Acadian Orogeny in a series of 3-4 tectophases; each follows a similar pattern (see below)

Avalonia attached to outer side of Laurentia; suture in central England (S England is Avalonia, Scotland is Laurentia); eastern Newfoundland (Avalon Peninsula) welded to western Newf.; Boston area sutures on (Cambrian Paradoxides trilobites common to England found here)

Effects of Acadioan Orogeny are evident throughout New England and maritime provinces;  subduction of Avolonian plate westward under the old Taconic land of New England created a new magmatic arc; many of the granite and gneiss domes  of the "Granite State" (NH) plus those of the Acadia National Park were formed in Devonian; sediments of forearc and backarc basins heavily folded, thrusted and high grade metamorphosed in New England (originally thought to be PreCambrian until Devonian fossils were found in some quartzites)

Acadian fold and thrust belt propagated from east to west, reaching west as far as the Hudson Valley region of eastern New York (Hudson Valley fold and thrust belt evident along Thruway Albany-Kingston)

Further west a foreland basin developed then migrated westward in repsonse to episodes of thrust loading back to the east;

Acadian Orogeny a cotinent to mcrocontinent collision; created a magmatic arc and retroarc fold and thrust belt (HVTB); this loaded acraton and produced a retroarc foreland basin that filled with clastic sedimednts

Series of pulses or tectophases; each shows simialr pattern:  a) clean sandstone- limestone—darks shales—gray mudstones and sandstones—redbeds; reflect tectonic quiescence changing to abrupt subsidence in foreland basin (flysch);  filling of basin with clastic seds (molasse),

Tectophase I:  Early Devonian  seen primarily in north;  sedimentary Tristates Group: basal orthoquartize-carbonate (Oriskany-Glenerie); abrupt change  to black shales with K-bentonite; shallowing upward to siltstones, ss, then shift to carbonate-shale (do not go all the way to red beds

Tectophase II: Middle Devonian: basal minor ss,-limestone (Onondaga);  K-bentonites in upper beds; sudden shift to black shale (Marcellus); then shallowing upward through siltstones, sandstones into red beds

Tectophase III. Middle-Early Late Devonian; basal carbonate (Tully Ls.), sudden shift to black shales (Genesee facies); general progrdation of siltstone, sandstone (prodelta, Portage facies; deltaic platform; Chemung facies), redbeds (subaerial delta platform; Catskill facies)

Basin shifted southwestward to progressively younger black shales spread into Midwest; Cleveland-New Albany-Chattanooga;

Why did discrete tectophases occur and why did locus of basins shift southwestward?

Thie Laurentia-Avalonia collision was mainly completed by  Middle to early Late Devonian; rotational scissors-like closure[ and different portions of the Avalon terranes hit different promontories of Laurentian margin

This pattern was recognized in mid 1900s as the pattern of basin filling-overfilling by deltaic progradation of the great Catskill Delta; the flysch-molasse of the Acadian Orogeny eventually it was sorted out that these dacies were diachronous, older to the east

Comparable facies seen elsewhere; e.g. Old Red Delta in Europe; shallow marine to red non-marine facies prograded southesrd over deeper marine sediments in central Europe; Early Devonian Hunsruck Shale (famous fossil bearing unit) shallows up into redbeds

In westrern North America see evidence of very Late Devonian tectonic activity; Antler Orogeny;
Late Devonian volcanics and remant island arc plutons in Klamath Mtns., California
Major thrust sheets of dep water Cambrian to Devonian sediment (Roberts Mountain allochthon
Shift from carbonates to very Late Devonian-Mississippian black shales and coarser siliciclastics

Antler Orogeny appears analogous to Taconic: collision of an island arc (Klamath arc) and accretionary wedge (Roberts Mountain allochthon) with western Laurentia—formation of peripheral foreland basin

In other areas carbonate banks persist through Devonian
 Morocco get condensed goniatite rich limestone; southern Europe

Major Mid to Late Devonian reef development in several areas: Alberta area major barrier reefs surrounding a hypersaline Williston-West Canada basin; the latter accumulkated major evaporites; the reefs are Canada’s major petroleum producers

Australia; huge barrier reef complex in the Canning Basin; even atolls
But reefs ;argely terminated at Famennian extinctions
 
 

Biotic Events:  Marine ecosystems shows great diversity of invertebrates; reef building at an all:time high but collapses toward end of period;

Reef ecosystem: age of diverse  stromatoporoids, rugose and tabulate corals; bryozoans prolific, together form reefs

Brachiopods:  especially spiriferids at a maximum; include abundant strophomenids esp. chonetids and first spiny productids

Mollusks :well represented by gastropods, bivalves (including first fresh water forms); nautiloids, and first ammonoids (goniatites: simple sutures)

Echinoderms: crinoids and blastoids diverse in shallow water; form much of shoal type sediment; starfish show first evidence of predatory behavior

Arthropods: trilobites are still very abundant  show some bizarre, large and spinose forms (anti predatory adaptation? ) but show decline in diversity and were very diminished by end of period; beginnings of decapod crustaceans

 Index fossil groups: conodonts extremely important over 40 conodont zones in Devonian; extinction of graptoloid graptolites in Early Devonian but first ammonoids appear just slightly later typified by rapid evolution, much turnover but widespread; make excellent index fossils in deeper water facies

Adaptive radiation of jawed fishes,  first  (and last) placoderms; first  sharks; bony fishes, ray fins and lobe-fins and their descendents:  first amphibians

Marine ecosystems shows intervals of relative stability punctuated by abrupt turnovers at several parently global events, each associated with widespread anoxia and possible climatic change: Early Devonian Dalje and Chotec events,  Middle Devonian Kacak and Taghanic events:

Upper Devonian Kellwasser events; near Frasnian-Famennian boundary these resulted in one of the greatest mass extinctions in Earth history; 25% of marine familires; decimated reef-building corals and stromatoporoids; caused extinction of cystoids, pentamerid  and atrypid (orders) brachiopods; most ammonoids extinct; terrestrial system not so drastically affected; causes unclear; like other episodes seem to involve sea-level fluctuatio; fall and rise; widespread hypoxia; climate stress; perhaps threshold effect

 Terrestrial ecosystem: greatly expanded with  fresh water plants, clams, arthropods and fishes; adaptive radiation of vascular land plants: lycopsids, sphenopsids; ferns, progymnosperms, first seed plants; first spiders, insects; and finally, nearly at end of period, first terrestiral amphibians;  derived from group of crossopterygian lobe finned fish