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ABSTRACT The Neoproterozoic–Early Devonian platformal succession of the North Slope subterrane, northeastern Brooks Range, Alaska, represents a carbonate-dominated peri-Laurentian continental fragment within the composite Arctic Alaska–Chukotka microplate. The basal ca. 760–720 Ma Mount Weller Group consists of an ~400 m thick mixed siliciclastic and carbonate succession that records the onset of regional extensional tectonism associated with the separation of southeastern Siberia from northern Laurentia during the break-up of Rodinia. These strata are overlain by ca. 720 Ma continental flood basalts of the Kikiktat volcanic rocks, which provide a link between the northeast Brooks Range platformal succession and the ca. 723–717 Ma Franklin large igneous province (LIP) of northern Laurentia. The overlying Sturtian Hula Hula diamictite and Cryogenian–Ediacaran Katakturuk Dolomite record abbreviated thermal subsidence of the northeast Brooks Range platformal succession prior to renewed Ediacaran–early Cambrian extensional tectonism and deposition of the overlying lower Paleozoic Nanook Group (new name). Equivalent strata of the deep-water Cryogenian–lower Cambrian(?) Ikiakpuk Group (new name) are identified herein with new δ 13 C carb and 87 Sr/ 86 Sr isotopic data from the Fourth Range of the northeastern Brooks Range. The Nanook Group is formally divided herein into the Black Dog and Sunset Pass formations, which record isolated peri-Laurentian platformal carbonate sedimentation along the northern margin of Laurentia, in an analogous tectonic position to the modern Bahama Banks. A profound Late Ordovician(?)-Early Devonian unconformity within the platformal succession is marked by subaerial exposure, paleokarst development, and tilting of the northeast Brooks Range peri-Laurentian platformal fragment prior to deposition of the overlying Lower Devonian Mount Copleston Limestone.

A new tectonic map of the southern and central Appalachians incorporates modern field and structural-stratigraphic, geochronologic (mostly sensitive high-resolution ion microprobe–reverse geometry, SHRIMP–RG, and Sm-Nd), geochemical, and geophysical data to identify crustal boundaries and blocks. Major tectonic units include the ∼735 Ma Laurentian failed rift, ∼565 Ma rifted margin sedimentary-volcanic assemblage deposited on Grenvillian and pre-Grenvillian crust, the Laurentian platform, and a series of distal Laurentian terranes (Cowrock, Cartoogechaye, Tugaloo-Chopawamsic-Potomac) accreted to Laurentia during the Taconian (Ordovician) or Neoacadian (Late Devonian–early Mississippian) orogenies. The Dahlonega gold belt consists of more proximal metasandstone and pelitic schist; it also contains Ordovician arc volcanic rocks, and a mixed detrital zircon suite of Laurentian and Gondwanan, or Penokean, affinity. The newly recognized Cat Square terrane contains Laurentian, Avalonian, and 430 Ma detrital zircons, and is considered a remnant ocean basin that closed during Acadian-Neoacadian accretion of the Carolina superterrane. The Pine Mountain terrane (southernmost exposed Appalachians) consists of Grenvillian basement and a cover sequence bearing Gondwanan or Penokean detrital zircons. The Carolina superterrane contains numerous peri-Gondwanan terranes that were deformed, metamorphosed, and amalgamated prior to 530 Ma, then accreted to Laurentia during the Neoacadian along the central Piedmont suture. The Raleigh-Goochland terrane contains blocks of Laurentian basement and cover that moved SW (dextrally) out of the collision zone to the north as the Theic ocean closed north to south during the early Alleghanian orogeny. This event also produced the Kiokee-Raleigh belt high-grade metamorphic core in the eastern Piedmont, and includes faults of the Pine Mountain window. The latter is framed by Alleghanian thrust and dextral faults formed at different crustal depths (times?). Subsurface components of the southern and central Appalachians are recognized in potential field and limited drill data. The Carolina superterrane extends beneath the Coastal Plain—possibly eastward to the East Coast magnetic anomaly. South of the Carolina superterrane and north of the Wiggins suture is the Brunswick (Charleston) terrane, another peri-Gondwanan terrane. The east–west Alleghanian Wiggins suture with the Suwannee terrane is recognizable to the south beneath Georgia and Alabama in potential field data, truncating all Appalachian structures and older crustal blocks west of the Appalachians. South of the suture, African basement and cover lie in the eastern Florida subsurface, while to the west are other Gondwanan or peri-Gondwanan components that may have originally connected with Yucatan.

Abstract During the 1970s, geologists considered that the Upper Ordovician Taconic Orogeny represented the collision of Laurentia with the Ammonoosuc arc, now largely exposed on the Bronson Hill anticlinorium. Subsequently, several researchers noted that magmatic rocks which intrude and overlie the Ammonoosuc arc are younger than the c. 455–451 Ma Taconic Orogeny. This led them to hypothesize that a Middle Ordovician collision was followed by westward-dipping subduction beneath the amalgamated Laurentian–Ammonoosuc zone to produce the younger arc rocks. In this model, the Taconic allochthons and foredeep were produced later in a retro-arc setting above westward-dipping subduction. However, those models prove inadequate due to the lack of ash beds, foredeep sedimentation and deformation on the Laurentian platform prior to the Upper Ordovician Taconic Orogeny. Here, we resolve the dilemma by recognizing that the magmatic rocks, which post-date the 455–451 Ma Taconic Orogeny, are not arc rocks but, instead, typical post-collisional slab-failure rocks as old as 450 Ma, with Sr/Y > 10, Sm/Yb > 2.5, Nb/Y > 0.4 and La/Yb > 10. Thus, in New England and western New York, the Upper Ordovician Taconic Orogeny represents the collision of the Ammonoosuc arc with Laurentia followed by slab failure of the descending plate.

Abstract The carbonate strata of the great American carbonate bank (GACB) have been subdivided and correlated with ever-increasing precision and accuracy during the past half century through use of the dominant organisms that evolved on the Laurentian platform through the Cambrian and the Ordovician. Trilobites and conodonts remain the primary groups used for this purpose, although brachiopods, both calcareous and phosphatic, and graptolites are very important in certain facies and intervals. A series of charts show the chronostratigraphic units (series and stages) currently in use for deposits of the GACB and the biostratigraphic units (zones, subzones, and biomeres) whose boundaries delineate them. Older and, in some cases obsolete, stages and faunal units are included in the figures to allow users to relate information from previous publications and/or industry databases to modern units. This chapter also provides a brief discussion on the use of biostratigraphy in the recognition and interregional correlation of supersequence boundaries within the Sauk and Tippecanoe megasequences, and the varied perspectives on the nature of biostratigraphic units and their defining taxa during the past half century. Also included are a concise update on the biomere concept, and an explanation of the biostratigraphic consequences of a profound change in the dynamics of extinction and replacement that occurred on the GACB in the Early Ordovician when the factors responsible for platformwide biomere-type extinctions faded and ultimately disappeared. A final section addresses recent and pending refinements in the genus and species taxonomy of biostrat-igraphically significant fossil groups, the potential they hold for greatly improved correlation, and the obstacles to be overcome for that potential to be realized.

Abstract Cambrian–Ordovician shelf-margin deposits of the great American carbonate bank (eastern North America) experienced significant regional dolomitization and/or metamorphism, but the Middle Cambrian Ledger Formation in south-central Pennsylvania contains a shelf-margin facies complex that includes exceptionally well-preserved microbialite sheet reefs riddled with centimeter- to meter-scale submarine cavities. The reefs and associated sands, composed of reef-related allochems, interfinger with ooid shoals, forming a high-energy shelf-margin facies association located near the seaward margin of the Middle Cambrian Laurentian platform. The Ledger Formation’s ooid shoal complex, exposed in the Magnesita Refractories quarry in York County, Pennsylvania, is pervasively dolomitized. Forthcoming research documents multiple stages of dolomitization and dedolomitization in the ooid dolostone; therefore, the ooid dolo-stone is not discussed here. In contrast to the ooid dolostone, most of the Ledger reef facies remains limestone. This has facilitated detailed interpretation of the reef depositional and diagenetic history, including new information presented here. Previous publications describe the Ledger reef geologic setting, mechanisms for generating the cavities, and petrographic and geochemical analyses of radiaxial fibrous and herringbone calcite fibrous submarine cements within the cavities. This chapter provides new information on the microbial reef sheet facies, describes a previously undocumented type of cryptic microbial morphology (endolite), and interprets a 1-m (3.3-ft)-thick intraclastic grainstone bed. Modern reefs in high-energy settings adapt by building robust coral frameworks that can withstand normal current activity and wave action. In The Middle Cambrian, coral framebuilders were absent, so to exploit high-energy ecological niches, organosedimentary constructers, primarily cyanobacteria (±algae and bacteria), had to develop a similarly robust morphology. We propose that low-growing, thick, cohesive microbial sheets, such as documented here from the Ledger Formation, provided minimal wave resistance and, therefore, outcompeted stromatolites and thrombolites to form subtidal wave-resistant structures in such high-energy settings. Similartomodern reefs, these microbial sheets contain cavities across arange of scales from millimeter-size fenestrae to meter-size stromatactis-type voids capable of sheltering and supporting delicate shrubs of Epiphyton -like dendrites and cryptic endolites, as detailed later in this chapter. Microbial processes dominated all ecological niches, forming the substrate, colonizing cryptic spaces, and coating and encrusting other microbes. The reef microbialite consists of weakly bedded sheets composed of shrubs and stubby strands of calcified Epiphyton - and Angulocellularia -like elements. Centimeter-scale domal stromatolites, thrombolites, oncolites, dendrolites, and oval multiple-layered organosedimen-tary cryptic structures, termed “endolites,” form lenses and distinctive structures. Petro-graphically, the microbialite is expressed as clots, stringers, arborescent garlands, and dendritic shrubs. Stromatactislike and fenestral cavities within the microbialite formed primarily through processes of gas and water escape, although syndepositional slumping and channel undercutting produced other types of cavities and void spaces. Grainstone, composed of microbial clasts and fragments, accumulated as cross-bedded intrareef channel sands. Large stromatactislike cavities were stabilized with multiple generations of microdolomite-bearing calcite radiaxial fibrous and herringbone calcite cements and intercalated internal sediment. Cement morphology, internal sediment associations, stable isotopes, and trace element geochemistry suggest that the cements precipitated from marine fluids as magnesium calcite and subsequently stabilized to calcite during diagenesis. The Ledger microbial assemblage closely resembles living cryptic, mat, and domal cyano-bacterial forms reported from the Tikehau Atoll, French Polynesia. Detailed descriptions of the cyanobacteria involved in creating the modern structures provide useful analogies for enigmatic Middle Cambrian fossil morphologies.

Abstract Ordovician rocks extensively border and cover Laurentia or the North American Craton in Canada. These rocks represent diverse and significant successions spread across a variety of depositional and palaeogeographic settings in the Canadian Arctic Islands, Eastern Canada, Western Canada and the Canadian Interior. During much of the Ordovician, Laurentia straddled the palaeoequator and was flooded by extensive epicontinental seas, experiencing high temperatures and high faunal diversity. The central and western parts of the Laurentian Craton remained relatively stable during the Ordovician, but there was substantial tectonic activity with the Taconic Orogeny affecting the Appalachian area and the Pearya composite terrane affecting the Franklinian Margin along its eastern and northern margins, respectively. The large Hudson Bay and Williston basins and smaller satellite basins cover the cratonic interior in Canada. These shallow intracratonic basins, dominated by marine carbonates with some evaporites, are the erosional remnant of a more extensive sea episodically connected with the adjacent platformal areas during the Ordovician. The Global Boundary Stratotype Section and Point (GSSP) for the base of the Ordovician System is exposed in Green Point, western Newfoundland while the Ordovician–Silurian boundary interval is well exposed on Anticosti Island, Québec but is also present in Ontario, Manitoba, Yukon and Arctic Canada.