Pagetia Clytioides

Baltagnostus

angustilobus

Baltagnostus

stockportensis

Bathyuriscus

eboracensis

Bolaspidella

fisheri

Corynexochides?

expansus

Hypagnostus parvifrons

Olenoidcs stockportensis Pagetia erratica Ptychagnostus punctuosus

UPPER CAMBRIAN Prosaukia briarcliffcnsis

Ordovician Period

Pagetia clytioides Ptychagnostus gibbus

Plethomctopus knopfi

Corynexochides?

expansus

Hypagnostus parvifrons

Unlike the Cambrian, the Ordovician Period was a relatively long interval, spanning about from 489 to 438 million years ago. During this long span, North America continued to straddle the paleoequator, and New York lay in the southern subtropics (Figure 4.5). Early Ordovician saw a continuation of the passive Great American Tidal Flat environment. But during the Middle Ordovician time the eastern edge of Laurentia began to encounter an offshore volcanic island arc (chain) (Figure 4.12). Ultimately this collision pushed (thrust) a great mass of deep-sea sediments up onto the present eastern edge of Laurentia, creating the Taconic Mountains and causing the continental edge to collapse into a foreland basin.

FIGURE 4.12. Stratigraphie chart of the Ordovician exposures in New York. Modified from Isachson et al. (1991). Printed with permission of the New York State Museum, Albany, N.Y

Early Ordovician

The Lower Ordovician interval in eastern North America is locally referred to as the Canadian Series (Figures 4.12 and 4.13). This interval, some 20 million years in duration, is set off from the higher Ordovician rocks in New York State and in most parts of North America by the major Knox Unconformity, a 25- to 30 million-year gap in the geological record. Moreover, the type of sediments and the style of stratigraphy change markedly between the Lower Ordovician and the late Middle Ordovician rocks that overlie the unconformity. Lower Ordovician rocks contain a sparse and rather poorly documented fossil fauna, dominated by certain small mollusks, such as various gastropods and nautiloid cephalopods and rare trilobites. This restricted Early Ordovician fauna suggests that somewhat unusual, perhaps slightly hypersaline, conditions continued in the North American interior seas during this interval ot geologic time.

The Lower Ordovician strata in New York generally are assigned to the upper part of the Beekmantown Group that takes its name from an area near Lake Champlain. In the outcrop belt the Lower Ordovician rocks are best exposed and most complete in areas around Lake Champlain and southward to about Lake George. In this vicinity at least four distinct formations, each bounded by a minor unconformity, are represented in the Canadian Series. South of the Adirondacks, in the central Mohawk River Valley, the Lower Ordovician rocks are relatively thin (about 35 to 50m) and are assigned to a single formation, the Tribes Hill (Figure 4.12).

The Lower Ordovician (Canadian) Series rocks crop out in a roughly concentric belt around the Adirondack region. To a large extent, this outcrop belt is controlled by the rather recent (late Cenozoic) uplift of the Adirondacks. However, it should be noted that Canadian or Lower Ordovician rocks are thin to absent in a belt running northward from Utica, New York, to an area north of Watertown. This suggests that although most of North America was covered by shallow seas during the Early Ordovi-cian, a low peninsular area of land extended eastward off the Canadian Shield roughly in the area of the Thousand Islands and Adirondacks of the present day. This peninsula probably has nothing to do with the present expression of the Adirondacks. It represents an ancient arch that was present in the continent and probably developed during the time of rifting of North America from another continent in the late Proterozoic (late Precam-brian). This region is referred to as the Frontenac Arch. The Ordovician carbonates thickened regularly to the south away from this area. Isopach maps (maps showing variations in thickness of a particular rock unit) for the Lower Ordovician reveal a pattern comparable to that seen in the Cambrian in which the

FIGURE 4.13. New York during the Early and early Middle Ordovician. A. Early Ordovician, 495 million years before present. B. Early Ordovician, 475 million years before present C. Cross section of plate movement during the Early Ordovician showing the Taconic Orogeny. D. Chazyan time. E. Black River times. F. Cross section of the plates during the Middle Ordovician. From Isachson et al. (1991). Printed with permission of the New York State Museum, Albany, NY

FIGURE 4.13. New York during the Early and early Middle Ordovician. A. Early Ordovician, 495 million years before present. B. Early Ordovician, 475 million years before present C. Cross section of plate movement during the Early Ordovician showing the Taconic Orogeny. D. Chazyan time. E. Black River times. F. Cross section of the plates during the Middle Ordovician. From Isachson et al. (1991). Printed with permission of the New York State Museum, Albany, NY

FIGURE 4.13. Continued.

rocks thicken dramatically southward from the approximate zero line (pinch-out of the Lower Ordovician strata) that coincides approximately with the present position of the south Lake Ontario shoreline. Ordovician or Canadian age strata attain a thickness of nearly 1500 m in the subsurface near the New York State-Pennsylvania border and thicken still more into central Pennsylvania, where they may exceed 3000 m in thickness. The rocks also thicken eastward from the area of the eastern Adiron-dacks into New England where they are represented as metamorphosed carbonates (marble), including the Stockbridge Marble, about 1000m thick. North of the Frontenac Arch, relatively thin sandy dolostones of the uppermost Theresa and Ogdensburg formations represent the Lower Ordovician carbonates. The high sand content in these carbonates and in some thin intervals within the Lower Ordovician of the Mohawk Valley suggests that some terrigenous sediment continued to be swept from the now deeply weathered and eroding highlands of the Frontenac Arch into adjacent shallow seas.

The upper units of the Lower Ordovician, particularly the Fort Cassin Formation (Figures 4.12 and 4.13), are restricted to areas east of the Adirondack Mountains and the Champlain Valley. No trace of these units is found in the Mohawk Valley farther west. This suggests that toward the end of the Early Ordovician, the area of deposition was restricted to a relatively narrow, north-south trending basin lying close to the present eastern New York State line and into New England. This major change from widespread shallow seas over much of eastern North America to a narrow eastern basin is not fully understood. In part, it may reflect major regression (shallowing or lowering of sea level) associated with the end of the Sauk Supersequence (Figure 4.13B). Another factor may be tectonic disturbance of the eastern margin of North America. Such a disturbance may have produced a subsiding (deepening due to crust deformation) trough in the region east of New York and New England, while at the same time the former shelf area to the west was uplifted in a broadly upwarped archlike feature. It is notable that the Fort Cassin Formation bears a number of faults that are truncated by the overlying unconformity. These faults must have occurred following the deposition of the Fort Cassin Formation but before the deposition of the overlying Middle Ordovician strata. Evidently, the eastern edge of the continent was undergoing some stresses, perhaps associated with an initial encounter of eastern North America with a trench and the development of an offshore volcanic island arc complex. Some geologists have argued that minor volcanic ash input was already coming into the basin during the time of the late Canadian Epoch.

Early Ordovician deposition was terminated throughout eastern North America by a major fall in sea level, and an ero-sional unconformity, commonly referred to as the Knox Unconformity, was initiated (Figure 4.13A-D). During the middle portion of the Ordovician, in places such as Mohawk Valley, the older carbonates that had been deposited during the Early

Ordovician were exposed to the atmosphere for some 25 to 30 million years and became deeply eroded. Because they had a relatively low siliciclastic content when exposed to rainwater, they mainly underwent solution, although some thin residues of sili-ciclastic mud may have been developed on the karstic or solution surface. Major solution features, such as sink holes and collapse breccias, developed at this time owing to karstification or dissolution and cave formation and collapse of the older Ordovician and Cambrian carbonates. In places, the unconformable surface has a relief of up to tens of meters.

Lower Ordovician Strata and Trilobites

The oldest of the Lower Ordovician Canadian Series of rocks are represented by the Whitehall Formation (Figures 4.12 and 4.13) that is well exposed in the region north of Lake George near the town of Whitehall. This formation technically spans the Cambrian-Ordovician boundary, but an upper unit within the Whitehall appears to be set off by a minor unconformity that occurs close to that boundary. An erosion surface and overlying sandstone and siltstone unit, the Winchell Creek Member, appears to be a signature of a drop and initial rise in sea level. This may represent a widespread regression that occurred near the end of the Cambrian but still within the overall Sauk Super-sequence. The Winchell Creek Siltstone is overlain by somewhat fossiliferous limestone that has yielded occasional fragments of trilobites as well as other marine fauna, suggesting partially normal marine conditions.

The next higher and somewhat better-known interval, the Tribes Hill Formation (0 to 3 0m), in the Mohawk Valley, again commences with a sandy or silty carbonate in New York State, the Palatine Bridge Member. Again, this silt and sandstone unit overlies an unconformity that may represent an interval of minor sea-level drop. The Palatine Bridge is overlain by burrow-mottled and somewhat fossiliferous Wolf Hollow Member that represents shallow marine shelf deposition. The most fossiliferous unit within the Tribes Hill is the Fonda Member. The Fonda is a fossil-rich limestone. Some beds display reddish to greenish color due to the presence of iron mineralization, especially the clay mineral glauconite. This latter mineral is believed to form in open marine environments during times of relative sediment starvation that enables mineral precipitate to become concentrated around decaying organic matter, especially fecal pellets. The glauconite granules of the Fonda Member are also associated with hashy, finely broken down, skeletal remains of many types of organisms. Particularly prevalent are several species of small gastropods, cephalopods, and a bivalve-like organism referred to as a ribeiroid rostroconch. The rostroconchs represent a nonhinged bivalve mollusk that possibly was ancestral to the bivalves. The Fonda Member also contains rare, disarticulated fragments of trilobites.

Finally the upper portion of the Tribes Hill is represented again by stromatolitic dolostones assigned to the Chuctununda Creek Member. This unit contains large, but very poorly pre served stromatolites referred to as "hippo backs" where they crop out, especially along Canajoharie Creek, Montgomery County.

The two higher packages of predominantly dolomitic but mollusk-containing carbonates, the Rochdale and the Fort Cassin Formation, make up the remainder of the Beekmantown Group of Lower Ordovician in New York (Figure 4.12). The Fort Cassin shows a repeat of the same pattern observed in the lower units, particularly the Tribes Hill; that is, it commences with a widespread silt and sandstone unit, the Ward Siltstone Member, and this in turn is overlain by a fossiliferous condensed limestone that may record maximum marine flooding of the craton on another higher cycle. The upper part of the Fort Cassin, the Bridport Member, consists mostly of thin-bedded to massive stromatolitic dolostones and records the final regression in the late part of the Canadian Series. Brett and Westrop (1996) recently reviewed the trilobites from the Fort Cassin Formation.

Trilobites reported, in total, from the Lower Ordovician are as follows:

Acidiphorus whittingtoni Bathyurus? perkinsi Bellefontia gyracanthus Benthamaspis striata CJeJandia parabola Strigigenalis caudatus Hystricurus conicus Hystricurus cf. H.

oculilunatus Isotcloidcs canalis Isoteloidcs whitfieldi Robergiella cf. R. brevilingua Shumardia pusilla Symphysurina sp. Symphysurus convexus

Bathyurus levis (?)

Bathyurellus platypus

Bellefontia sp.

Bolbocephalus seelyi

Eoharpes cassinensis

Grinnellaspis cf. G. marginiata

Hystricurus crotalifrons

Hystricurus ellipticus

Isoteloides peri Paraplethopeltis sp. Scotoharpes cassinensis Strigigenalis cassinensis Symphysurina cf. S. woosteri

Allochthonous Rocks of Early Ordovician Age

Lower Ordovician allochthonous rocks exposed in the western part of the Taconic Mountains closely resemble those of the Upper Cambrian. Lowest Ordovician strata are represented by beds of the upper Hatch Hill Formation, as indicated by the presence of distinctive index fossils including conodonts. Graptolites first become common in the strata of the Taconic Mountains in the earliest Ordovician, where they are represented by dendroid graptolites such as the genus Dictyonema. Hatch Hill Shales, now metamorphosed in places to slates, continue upward from the Cambrian to the Ordovician. Thin carbonates, sandstones, and carbonate breccias, occurring at or near this level as well, represent sediments that were washed off the shallow platform of North America at the end of the Cambrian and earliest Ordovi-cian. The Hatch Hill dark shales give way upward in the strati-graphic succession to olive-greenish, slaty shales with some thin limestones but no debris flow breccias. These strata have been assigned to the Poulteney or Deep Kill Formation (Figure 4.10). Although predominantly green, some thin dark shale partings occur within these strata, particularly in the vicinity of Deep Kill, a small creek near Melrose, Rensselaer County. These Deep Kill black beds have yielded a highly diverse and well-preserved assemblage of graptoloid graptolites, typically preserved as silvery carbonized remnants on the dark slaty bedding planes. These graptolites have been used to correlate the Deep Kill rocks with portions of the Lower Ordovician in other parts of the world. However, the bulk of the predominantly green Poulteney or Deep Kill Formation is sparsely fossiliferous. In the Taconic allochthon succession as on the craton, the Sauk Unconformity or the Knox Unconformity appears to be present as a gap or break in sedimentation. Just why this should be so in deeper water is unclear. In fact, one might anticipate the occurrence of an increase in the influx of sediments, at least if shales were uplifted and exposed to erosion during the long span of the Knox Unconformity. However, the continental slope and rise area of New England appear to have been relatively sediment starved during this interval of time. Siliceous layers within the Poulteney represent probable volcanic ash beds that were being implaced within the Iapetus Ocean. Also, some cherty beds represent some of the world's oldest radiolarian-based, deep-sea silica ooze deposits. The Poulteney is overlain, probably with an unconformity, by the Indian River red slates. Trilobites are not present in these deposits.

In the Early to Middle Ordovician, the Iapetus Ocean no longer continued to widen, and a portion of seafloor attached to the east edge of Laurentia began underthrusting the rest of the seafloor (Figures 4.5B and 4.13C). This action produced a deep trench on the Iapetus Ocean floor and a so-called subduction zone, wherein the western plate or slab was forced downward beneath the eastern or overriding slab. This interaction produced an offshore island arc —the Taconic or Ammonoosuc Arc, a chain of volcanoes that formed on the overriding plate of proto-Atlantic seafloor. The magmas (melted rock) were generated by frictional heating and partial melting of the downgoing slab and broke through to the surface, forming the volcanic chain.

As subduction of the proto-Atlantic seafloor continued, the eastern edge of Laurentia itself eventually was brought close to the subduction zone. Ultimately, the sediments that bordered Laurentia along the continental slope and deep ocean floor were scraped off from the downgoing slab, forming a somewhat chaotically deformed series of slabs of strata — an accretionary wedge. This mass would become the rocks of the Taconic Allochthon (a mass of rock ultimately displaced some 80 km west of its site of origin into the area of present-day eastern New York). The Taconic allochthon was thrust or pushed up onto the continental shelf of Laurentia, or one might say that the east edge of the continental shelf was subducted beneath this mass of deformed sediments. The latter area also was being compressed and thrust westward by collision of the Ammonoosuc Arc with the accre-tionary wedge and Laurentia.

Middle Ordovician

The Knox Unconformity is manifested as the sharp upper contact of the Beekmantown Group or the underlying Cambrian units. The Knox Unconformity, with a relief of up to several meters, due to karstification, is one of North America's major stratal breaks and forms the subdivision between two huge packages of strata, referred to as SJoss supersedences: the Sauk Supersequence below and the base of the Creek phase of the Tippecanoe Supersequence above (Sloss 1963; Figure 4.1).

Middle Ordovician Chazy Group

The oldest Middle Ordovician strata to accumulate above the Knox Unconformity in New York are the sandstones and carbonates of the Lake Champlain area, assigned to the Chazy Group (Figure 4.13D). These strata are of middle Middle Ordovician or Chazyan age (the Llandeilo Series of the British terminology) (Figures 4.13D, 4.14, 4.15). The Chazy Group strata evidently accumulated in a relatively narrow, restricted trough or foreland basin that existed in present-day northeastern New York State and once extended farther to the north and east into central Quebec and Vermont. Laterally equivalent sediments of the Youngman and Carmen formations of western Vermont consist of thin-bedded, ribbon-like limestones and interbedded dark shales that represent more basinal accumulations (Figure 4.13D). The localized nature of the Chazy basin probably reflects additional subsidence of the outer portion of the continental margin of Laurentia, which, during the Middle Ordovician, was beginning to encounter the trench or subduction zone associated with the collision of a volcanic island arc.

Chazy strata typically commence with cross-bedded sandstones containing sparse fossils, but including lingulid bra-chiopods, referred to as the Day Point Formation. These siliciclastic sands apparently were recycled from older sandstones that had been eroded during the long span of the Knox Unconformity. They were deposited in shallow subtidal to intertidal environments. The overlying beds of the Crown Point and Valcour in the Chazy Group are carbonates that display generally deepening upward trends. Coarse-grained limestones representing cross-bedded shoals of crinoidal and other skeletal debris occur low in the Crown Point Formation but are inter-bedded with and succeeded by nodular to wavy-bedded finegrained limestones containing abundant fossil fragments. Locally, within the Crown Point, small bioherms or reeflets were developed on skeletal shoals (Figure 4.14). These were composed of sponges and bryozoans, with some primitive rugose and tabulate corals. A small patch reef or bioherm in a cow pasture on Isle Lamotte, Vermont, is often said to be the world's oldest coral reef, although most of these bioherms were not composed of corals.

Trilobites as disarticulated elements are rather common and highly diverse in some of the Chazy nodular limestone beds. The trilobite fauna of the Chazy (Shaw 1968) includes over 60 species of trilobites:

Acanthoparypha? sp. Apianurus narrawayi

Bumastoides apJatus Bumastoides gardenensis CaJyptauJax annuJata CcratocephaJa triacantheis

Ceraurus granuJosa CybeJoides prima Dinieropyge dintoncnsis

Eobronteus sp. Gabriceraurus hudsoni GJaphurus pustuJosus Hemiarges turneri amicuJus Hibbertia vaJcourensis IJJaenus crassicauda IsoteJus beta IsoteJus giganteus Kawina? chazy ensis Kawina vuJcanus Lonchodomas haJJi NaniJJaenus? raymondi Nieszkowskia? saJyrus Otarion spinicaudatum

Physemataspis insuJaris

PJatiJJacnus Jimbatus Proetus cJeJandi

Pseudosphaerexochus vuJcanus Sphaercxochus parvus ThaJeops arctura Uromystrum brevispinum

Vogdesia bearsi

AmphiJichas minganensis

BasiJicas (BasiJieJJa)

whittingtoni Bumastoides comes Bumastoides gJobosus Carrikia setoni CeraurineJJa Jatipyga Chcirurus (Nieszkowskia) Cyrtometopinid sp. DoJichoharpes sp. Eoharpes antiquatus GJaphurina Jamottensis HeJiotneroides akacephaJa

Hibbertia sp. Hyboaspis depressa IsoteJus angusticaudum IsoteJus canaJis IsoteJus harrisi Kawina? sp.

Lonchodomas chaziensis

NaniJJaenus? punctatus Nieszko wskia biJJi) Jgs i NiJeoides perkinsi Paraceraurus ruedemanni

PJatiJJacnus erastusi PJiomerops canadensis Pseudosphaerexochus?

approximus RemopJeurides canadensis

Sphaerocoryphe goodnovi

ThaJeops Jongispina Uromystrum minor Vogdesia? obtusus

Middle Ordovician Allochthonous Rocks

As previously mentioned, the Ammonoosuc island arc, lay outboard of North America in the proto-Atlantic in the region that today would be occupied by central New England. The old cratonic edge of Laurentia may have been uplifted to the east of the Chazy trough. This relatively narrow trough area lay to the east, between the old continental margin (present-day central Vermont and Massachusetts) and an accretionary prism, consisting of the sedimentary rocks and some ocean-floor volcanics, which were being obducted off of the downgoing oceanic-floor plates (Figure 4.13B, part E, F). The uppermost or youngest sediments of the Taconic allochthon succession accumulated in this trench during the Middle Ordovician and are approximately the mars

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