Lipp Alian

FIGURE 4.1. Continued.

Grenville mountain belt, fully formed by a billion years ago, was exposed in a life-less continental interior to the forces of weathering and erosion. We know that by about 550 million years ago, an entire thickness of continental crust had been removed and erosion had exposed the roots of the ancient Grenville Mountains.

Laurentia is the term geologists apply to the ancestral Paleozoic core of North America, lacking certain areas such as the present eastern seaboard region (eastern Massachusetts, eastern Nova Scotia, eastern Newfoundland), Florida, California, and other marginal areas that were added later. Laurentia was isolated as a separate continent during rifting and opening of new ocean basins that began approximately 700 to 600 million years ago during the interval of time referred to as late Proterozoic (or Neo-proterozoic). Notably, on the present east side of Laurentia,

Metamorphic Rocks Precambrian Age
FIGURE 4.2. Precambrian Grenville (1.0 BP) metamorphic/igneous rocks. Note the granitic pegmatite (a) and smaller folded dikes (b) cutting gneiss. Rte. 12, near Alexandria Bay, Jefferson County.

rifting or fracturing of the crust began over 600 million years ago — some 400 million years after the Grenville Orogeny. The fractures and faults ultimately tore the supercontinent of Rodinia apart. Evidently, ancestral South America, which had collided with proto-Laurentia to form the Grenville Mountains, now pulled back away. A new ocean, the Iapetus, or Proto-atlantic, began to open along a line that would pass through present-day central New England and southward to the east-central Carolinas, a bit east of the Blue Ridge Mountains in the Appalachian chain (these mountains formed later).

At the beginning of the Paleozoic Era, about 545 million years ago, a narrow, but widening Iapetus Ocean lay slightly to the east of present-day New York State (Figure 4.5A). East of the present Berkshire (Massachusetts) and Green (Vermont) Mountains lay the edge of Laurentia, which, much like the present eastern edge of modern North America, formed a continental shelf bordered eastward (southward at that time) by a relatively steep drop-off along a continental slope into the deep water of the Iapetus Ocean.

Seawater was displaced upward out of the Iapetus Ocean, partly because of the expanding midocean ridge (or spreading center, an area of hot upwelling magma). This seawater spread out onto the craton of Laurentia and caused a major rise of the shoreline (transgression) up onto the old weathered remnants of the Grenville rocks.

Initially, a large volume of sediment that eroded from the ancient Grenville terrane was shed off the old weathered craton and into the narrow Iapetus basin. However, as the sea level continued to rise during the latest Proterozoic and Early Cambrian, the old land was finally flooded and the source of sediments cut off (Figure 4.5A). The entire eastern border of Laurentia had become a passive continental edge; a sea with a very broad continental shelf ultimately extended from the area of central New England and the mid Atlantic states region westward to the present Mississippi Valley during the Cambrian to Early Ordovi-cian (Figure 4.5A, B). As spreading ensued in the Iapetus mido-cean rift, the ocean basin grew wider, at least up to the Early Ordovician time, about 480 million years ago, before the process began to reverse and the Iapetus basin began to shrink and ultimately close.

Cambrian Period

The Cambrian Period, as it is now dated, spans approximately 54 million years from about 543 to 489 million years before present. Yet this was one of the most significant times in the history of life, for it was during this time that nearly all phyla or major groups of animals appeared in the rock record.

The onset of the Cambrian was marked by a time of low-diversity fossil assemblages typified by "small shelly" skeletons

FIGURE 4.3. Map showing the extent of the Grenville belt in eastern North America. Rocks in this area were deformed and metamorphosed about 1 billion years ago. The dotted pattern shows regions where Grenville rocks are buried beneath younger strata; the slanted lines indicate areas where Grenville rocks are exposed; and cross-hatching shows areas where the Grenville rocks are deformed by later orogenies. Modern Adirondack Mountains lie just to the east of the Frontenac Arch (labeled). From Isachson et al. (1991). Printed with permission of the New York State Museum, Albany, N.Y.

FIGURE 4.3. Map showing the extent of the Grenville belt in eastern North America. Rocks in this area were deformed and metamorphosed about 1 billion years ago. The dotted pattern shows regions where Grenville rocks are buried beneath younger strata; the slanted lines indicate areas where Grenville rocks are exposed; and cross-hatching shows areas where the Grenville rocks are deformed by later orogenies. Modern Adirondack Mountains lie just to the east of the Frontenac Arch (labeled). From Isachson et al. (1991). Printed with permission of the New York State Museum, Albany, N.Y.

Laurentia Mid Proterozoic Reconstruction

FIGURE 4.4. Paleomagnetic reconstruction of the supercontinent Rodinia as it existed in the late Proterozoic, about 700 million years ago. Note the position of Laurentia, near the center. The dark belt shows the position of the Grenville Orogenic belt. After Dalziel (1997), reproduced with permission.

FIGURE 4.4. Paleomagnetic reconstruction of the supercontinent Rodinia as it existed in the late Proterozoic, about 700 million years ago. Note the position of Laurentia, near the center. The dark belt shows the position of the Grenville Orogenic belt. After Dalziel (1997), reproduced with permission.

including a variety of calcareous phosphatic tubes, rods, and plates. Only in later Early Cambrian time (-519 millon years ago) did trilobites first appear as body fossils, although fossil trackways (traces of walking on the bottom) and resting pits (Ruso-phycus) suggest that unpreserved soft-bodied trilobites, or similar arthropods, occurred earlier.

The Cambrian rocks of New York State are generally sub divided into two great packages of strata (Figure 4.7). The autochthonous (nontransported) rocks, which are in the site of original deposition, a shallow-shelf environment, and the Taconic allochthonous (displaced) rocks, mostly deep-water shales (now often slates) that were originally deposited to the east of New York in oceanic environments. The Taconic rocks were transported westward 80km during the Middle Ordovician

lapetus Ocean ___X

Taconic island arcs

Avalonia And Baltica Collision

FIGURE 4.5. Position of the Laurentian plate and neighboring Baltica and Avalonia terranes from the Cambrian to the Devonian. Areas of exposed land are dark, areas covered by water are white, and New York is highlighted in black. A. Middle Cambrian as most of what is the United States was beginning to be flooded by shallow seas resulting in carbonate depositions in the Late Cambrian (note Taconic island arcs). B. Middle Ordovician with almost complete flooding of Laurentia and the beginning of the Taconic Orogeny with collision in the southeast. C. Silurian, with Baltica colliding with Laurentia from the east. Also note the smaller "islands of Avalonia." D. Middle Devonian, with the Acadian Orogeny fully developed due to collision of the Avalon terranes; note heavy sedimentation from the eastern mountains. Modified from Witzke (1990). Reproduced with permission.

FIGURE 4.5. Position of the Laurentian plate and neighboring Baltica and Avalonia terranes from the Cambrian to the Devonian. Areas of exposed land are dark, areas covered by water are white, and New York is highlighted in black. A. Middle Cambrian as most of what is the United States was beginning to be flooded by shallow seas resulting in carbonate depositions in the Late Cambrian (note Taconic island arcs). B. Middle Ordovician with almost complete flooding of Laurentia and the beginning of the Taconic Orogeny with collision in the southeast. C. Silurian, with Baltica colliding with Laurentia from the east. Also note the smaller "islands of Avalonia." D. Middle Devonian, with the Acadian Orogeny fully developed due to collision of the Avalon terranes; note heavy sedimentation from the eastern mountains. Modified from Witzke (1990). Reproduced with permission.

Taconic Orogeny (a major mountain-building event) along major thrust faults. This deformation apparently was related to the convergence of an island arc complex with eastern Laurentia (or ancestral North America).

Cambrian Autochthon

The autochthonous rocks of the central Appalachians, which include basal sandstone and limestone or dolostone, represent extensive shallow, subtropical platform seas, sometimes referred to as the Great American Tidal Flat (Figures 4.5, 4.7). In southeastern New York and southern Pennsylvania these beds may be up to 5 km thick and were mainly deposited in very shallow, tide-influenced environments. The immense thickness is accounted for by active subsidence, or downward sinking, of the Cambrian continental shelf. Following the interval of rifting that produced the Protoatlantic or Iapetus Ocean, the continental shelf, which is represented by the region from New England westward into central New York, underwent rapid subsidence due to cooling. Lime mud and silt probably were produced by organisms such as algae, the growth of which was evidently able to keep up with the rate of subsidence so that these shelf environments remained in very shallow water, above the normal wave-base. Cambrian to Early Ordovician rocks form a package of sandstones and carbonates — the Sauk Sequence — bounded above and below by major unconformities.

During Cambrian and Early Ordovician time, eastern North America lay in a subtropical position, perhaps 25° south of the paleoequator (Figure 4.5). Today this zone is known as the "subtropical desert belt" because it is here that some of the driest

FIGURE 4.6. Cambrian rocks in New York. A. Upper Cambrian Potsdam Sandstone at Ausable Chasm near Plattsburg, Clinton County. B. Tilted Precambrian-Cambrian (Lippalian) nonconformity. Approximately 500 million years of nondeposition and erosion separate dark amphibolites (a) (metamorphic rocks) from the light gray, Upper Cambrian Little Falls Formation (b). Cut along U.S. Rte. 5S near Fonda, Montgomery County.

FIGURE 4.6. Cambrian rocks in New York. A. Upper Cambrian Potsdam Sandstone at Ausable Chasm near Plattsburg, Clinton County. B. Tilted Precambrian-Cambrian (Lippalian) nonconformity. Approximately 500 million years of nondeposition and erosion separate dark amphibolites (a) (metamorphic rocks) from the light gray, Upper Cambrian Little Falls Formation (b). Cut along U.S. Rte. 5S near Fonda, Montgomery County.

conditions on Earth develop. There is some evidence that ancestral North America in the Cambrian was relatively arid in the (present) eastern regions. There is also evidence for the buildup of slight hypersalinity in the Cambrian waters. Cambrian dolo-stones contain vugs and sometimes molds of evaporite crystals such as gypsum, anhydrite, and halite. The occurrence of ooids (small spherical, concentrically ringed calcium carbonate grains) and stromatolites (moundlike structures formed of sediment trapped by cyanobacteria or blue-green algae) also is typical of hypersaline (elevated-salinity) waters in the subtropics today. Whatever the case, much of the Cambrian seafloor in the New York area was relatively low in shelly organisms.

The broad continental shelf of the Cambrian sea was bordered to the east, in what are today central Vermont, western Massachusetts, and Connecticut, by an abrupt slope into deeper water. In the region flanking the continental shelf of North America, sediments accumulated very gradually (Figures 4.7 and 4.9).

Cambrian Trilobite-Bearing Autochthonous Rocks

The lowest and shallowest-water deposit of the Cambrian autochthonous rocks belong to the Potsdam Sandstone or equivalent sandy Little Falls Formation (Figures 4.6 and 4.8). The Potsdam Formation consists of up to 140 m of clean quartz aren-ites or quartzose sandstones that display cross-stratification, ripple marks, and other features indicative of deposition in shallow-wave and sometimes tide-dominated environments (Figure 4.8). The Potsdam sediments represent sand eroded from deeply weathered areas of the North American craton. The presence of herringbone cross-stratification (inclined bedding formed by alternate migration of ripples in opposite directions) is an indication of the oscillatory currents associated with tidal action. Some portions display large-scale trough cross-bedding and may represent wind-formed sand dunes in coastal areas.

The Potsdam is generally sparse in body fossils, although trace fossils (burrows, tracks, and trails) of a variety of forms are present, including the bizarre and huge (by Cambrian standards) CJimactichnites. This elongate trail up to 30cm (or more than a foot) wide closely resembles marks made by a tractor tire in soft sand. This trace is found in flat-bedded sands that may represent the upper foreshore or beach. Just what large organism in the Cambrian was able to come out into very shallow water is quite unclear. Traces may have been preserved by the sun drying initially wet sands of coastal areas. Yochelson and Fedonkin (1993) argued that it might have been a large gastropod-like mollusk. In slightly more offshore Potsdam fades, trace fossils such as vertical shafts (SkoJithos) and U-shaped burrows (DipJocraterion) are quite abundant. Lingulid brachiopods and small fragments of trilobites also have been obtained in a few levels, but in general body fossils are rare.

The higher beds of the Upper Cambrian and those straddling the Ordovician boundary are carbonates. The Little Falls Dolo-

stone of the Mohawk Valley, the Hoyt Limestone of the Saratoga area, the Whitehall Formation of Lake George, and the Theresa Dolostone of the Saint Lawrence region are carbonates: limestones, and dolostones, formed very late in the Cambrian or in earliest Ordovician time (Figures 4.7 and 4.9). They display an abundance of stromatolites. Some of the most famous stromatolites in the northeastern part of North America occur in the Petrified Gardens within the Hoyt Limestones near Saratoga Springs, New York (Figure 4.9).

The Little Falls Formation stromatolitic dolostones (up to 30 m thick) are only very sparsely fossiliferous. Only a single free cheek of the trilobite EJvinia has been found from the Cambrian Little Falls Formation, but this is sufficient to bracket its age within the second to last or Franconian Stage of the Cambrian. (The Little Falls is noted for its characteristic vugs or cavities that contain beautiful, doubly terminated quartz crystals referred to as Herkimer Diamonds; such crystals formed much later during deep burial of the Little Falls sediments.) Just why most Upper Cambrian carbonates are so poor in body fossils, including trilo-bites, but rich in stromatolites is as yet unclear. It may be that the seas were somewhat hypersaline. However, the Hoyt Limestone contains not only the famous stromatolites but also beds of oolitic limestone and some fossiliferous limestone in which sclerites (skeletal pieces) of trilobites belonging to a number of Late Cambrian species are found; Ludvigsen and Westrop (1983) recently described these species from the Hoyt and Galway limestones.

These trilobites are associated with other fossils, including brachiopods, molluscan fragments, and even plates of the world's oldest chitons. This diverse assemblage indicates relatively favorable, normal-salinity conditions in the area of Saratoga Springs during this period. However, relative high energy due to wave action and slow deposition prevented the easily disarticulated trilobites from being preserved whole. The trilobites of the Galway and Hoyt are:

GALWAY FORMATION CaJocephaJites cf. C. minimus DeJJea saratogensis Drabia cf. D. menusa

HOYT LIMESTONE Delicti? Jandingi KeithieJJa depressa Pie tho peJt is sara togensis Prosaukici tribuJis

Cameraspis convexa Drabia cf. D. curtoccipita EJvinia granulata

Hoytaspis speciosa PJetJwpeJtis granuJosa

Prosaukia hartti

Saratogia (Saratogia) caJcifera

Cambrian of the Taconic Allochthon

Initially, a substantial amount of silt, sands, and muds was swept from the Grenville basement of Laurentia, which had been exposed to weathering and erosion for over 400 million years.

Trilobites Body Parts
Prolo North Amcrica lapeius Ocean Taconic Island Are
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