Abstract The Cambrian–Ordovician carbonate platform units on the New York promontory of eastern Laurentia reflect the south tropical location of the area. The slow subsidence of the region through much of the Cambrian–Ordovician meant that strong eustatic rises and falls defined unconformity-bound carbonate formations. These depositional sequences aid in paleoocean-ographic reconstruction as they correlate with organic-rich dysoxic–anoxic mudstones on the Laurentian continental slope. Eustatic rise increased insolation as epeiric seas covered the platform and produced climate maximums with reduced deep-water circulation. The oldest carbonate platform unit (Forestdale Marble and equivalents, upper Lower Cambrian) overlies rift facies deposited with the Rodinia breakup and origin of the Iapetus Ocean and marks the transition to a passive margin. Drowning of the Forestdale platform occurred, and the overlying anoxic black mudstone (Moosalamoo Phyllite) abruptly shoals up into tidalite sandstone (Cheshire Formation). This depositional history records a decreased rate of sea level rise as the Cheshire Formation continued to onlap middle Proterozoic basement. Super-Cheshire Cambrian carbonate platform units in the northern Appalachian are mostly hydrothermally dolomitized, record eustatic highs (Dunham, Winooski, and Little Falls Formations), and correlate with black mudstone macroscale units on the slope (Browns Pond and Hatch Hill dysoxic– anoxic intervals). The latest Early Cambrian Hawke Bay regression ended carbonate platform deposition of the Dunham Formation, led to quartz arenite or red shaly dolostone of flap or shoaling deposits on the platform, and was coeval with oxic green mudstone on the continental slope (Hawke Bay oxic interval in Taconian allochthons). Subsequent Middle Cambrian eustatic rise is recorded by dolostone (Winooski and upper Stissing), but carbonate deposition was again suppressed as quartz sand swept toward the shelf margin (Danby Formation) coincident with cratonic transgression by the upper Potsdam Formation (uppermost Middle Cambrian–lower Upper Cambrian). Post-Potsdam depositionwas carbonate dominated through the middle Late Ordovician and included the Beekmantown, Chazy, Black River, and Trenton Groups. The Cambrian-Ordovician boundary is an unconformity between platform carbonates (Little Falls and Tribes Hill Formations of the Beekmantown Group). The Lower Ordovician–lower Upper Ordovician is a series of unconformity-bound platform depositional sequences (Tribes Hill, Rochdale, Fort Cassin, and Providence Island Formations of the upper Saukmegasequence and Chazy Group of the lower Tippecanoe megasequence). The Ordovician depositional sequences coincide with eustatic highs and show a repeated depositional motif (lower transgressive sandstone, upper highstand carbonate). The Ordovician eustatic highs also correlate with thin (as much as 10 m [33 ft] thick) macroscale dysoxic–anoxic black mudstones on the slope. The black mudstones alternate with oxic greenish mudstones, locally with debris flows with giant carbonate blocks on the upper slope (Levis conglomerates), which indicate platform-margin caving during eustatic falls. Ordovician green mudstones are composed of mesoscale redox-carbonate mudstone cycles (Logan cycles) on the upper slope. A major development was the abrupt formation of the latest Early Cambrian–Early Ordovician Franklin Basin in northwestern Vermont. The dysoxic–anoxic Franklin Basin resulted from fault-driven foundering of part of the carbonate platform that overlay the failed arm of the Ediacaran triple junction. This faulting is coeval with the oldest (late Early Cambrian) onlap in the Ottawa-Bonnechere aulocogen. Late Ordovician collision with the Ammonusuc arc ended carbonate platform deposition in the New York promontory region, as sands and muds eroded from the Taconic orogen filled a fore-arc basin and extinguished carbonate deposition across eastern Laurentia.

Abstract Rodinia break-up with late Ediacaran rifting defined a NE Laurentia triple junction (New York Promontory–Ottawa–Bonnechere aulacogen (OBA)–Quebec Reentrant). Rifting persisted to c. 510 Ma. The oldest passive-margin shelf units (Forestdale Marble and Moosalamoo Phyllite) underlie a sandstone (Cheshire) commonly regarded as the oldest passive unit. Late Dyeran–Middle Cambrian rifting led to the oldest OBa sedimentation and formed the Franklin Basin (NW Vermont). Cambrian–Darriwillian shelf–slope facies are linked eustatically – not Taconic Orogeny onset. Onlap and shelf carbonates are coeval with black slope mud; and lowstand shelf unconformities with green, oxic slope mud. Early–middle Dyeran eustatic change defined slope units: (1) Browns Pond Formation dysoxic–anoxic (d–a) interval with debrite cap (Holcombville Member, new); (2) Middle Granville Formation Oxic Interval (new); and (3) lower Hatch Hill Formation d–a interval. Our analysis leads to two controversial conclusions: (i) the existence of the Dashwoods and other micro-continental blocks due to hyperextension is not supported by cover sequences linking Laurentia to proposed Dashwoods areas (i.e. Green Mountains) and an arc origin of the type Dashwoods; and (ii) ‘Hawke Bay Event(s)’, widely interpreted as Cambrian global regressive event(s), is a local highstand systems tract facies with shelf sand bypass onto the Hatch Hill Formation slope in its NE Laurentia type region.

ABSTRACT The Ottawa aulacogen/graben on the NE US—Canadian (SW Quebec and eastern Ontario) border is a long ENE-trending structure formed with initial late Neo proterozoic rifting of the Rodinia supercontinent. This rifting formed the active spreading arms (New York Promontory and Quebec Reentrant) along the (presently) NE margin of the new Laurentia paleocontinent, with the Ottawa aulacogen commonly regarded as a failed arm of the rifting. However, no sediment accumulation in the aulacogen is recorded until the late early Cambrian subsidence of a SE- trending belt that includes the aulacogen and its extension, the Franklin Basin, in NW Vermont. Late early Cambrian marine onlap (Altona Formation) followed by more rapid late middle Cambrian subsidence and deposition of fluviatile arkoses (Covey Hill Formation of SW Quebec and Ausable Formation/Member of eastern New York) record rapid foundering of this “failed arm.” Subsequent deposition (latest middle Cambrian–Early Ordovician) in the Ottawa aulacogen produced a vertical succession of lithofacies that are fully comparable with those of the shelf of the New York Promontory. One of the greatest challenges in summarizing the geological history of the Ottawa aulacogen is the presence of a duplicate stratigraphic nomenclature with lithostratigraphic names changing as state and provincial borders are crossed. RÉSUMÉ L’aulacogène/graben d’Ottawa, situé sur la frontière entre le NE des États-Unis et le Canada (SW du Québec et est de l’Ontario), est une longue structure d’orientation ENE formée au Néoprotérozoïque tardif durant le rifting initial du supercontinent Rodinia. Ce rifting a aussi mené à la formation de segments à expansion active (promontoire de New York et réentrant de Québec) le long de la marge NE (coordonnées actuelles) du nouveau paléo-continent Laurentia, avec l’aulacogène d’Ottawa qui est généralement considéré comme un segment de rift avorté. Toutefois, aucune accumulation de sediments n’est documentée au sein de l’aulacogène avant la fin du Cambrien précoce, période durant laquelle une ceinture d’orientation SE, representée par l’aulacogène et son prolongement dans le bassin de Franklin vers le NW du Vermont, a subi une subsidence. La sedimentation marine de la fin du Cambrien précoce (Formation d’Altona) a été suivie d’une subsidence rapide à la fin du Cambrien moyen et de la déposition d’arkoses fluviatiles (Formation de Covey Hill dans le SW du Québec et la Formation/Membre d’Ausable dans l’est de l’état de New York) qui ont enregistré un affaissement rapide de ce “bras avorté.” La sédimentation subséquente (Cambrien moyen tardif–Ordovicien inférieur) au sein de l’aulacogène d’Ottawa a produit une succession verticale de lithofaciès qui sont comparables à ceux de la plate-forme du promontoire de New York. Un des principaux défis dans la synthèse de l’histoire géologique de l’aulacogène d’Ottawa demeure la duplication des termes stratigraphiques de part et d’autre des frontières interprovinciales et entre les différents états.

Abstract The Cambrian–Ordovician Sauk megasequence of the great American carbonate bank (GACB) comprises a succession of mixed lithologies, but dominantly carbonate rocks, whose thickness, stratigraphy, and lithofacies distribution reflect the presence of a complex of intrabank platforms and basins, aulacogens, and tectonically active margins that together make up the major part of the paleocontinent Laurentia. The stratigraphy of the Sauk megasequence can be subdivided and correlated across the GACB through the recognition of major unconformities, marine flooding events, and stratigraphic stacking patterns, documented within a robust biostratigraphic framework. The base of the Sauk megasequence is typically defined as the contact of Cambrian or sub-Tippecanoe-megasequence Ordovician rocks with Precambrian, mostly igneous, basement. The Sauk megasequence is overlain (commonly unconformably) by the Middle Ordovician Tippe-canoe megasequence, the age of which varies across the GACB. Where subsequent erosion has occurred, the Sauk megasequence may be overlain by rocks younger than the Tippecanoe megasequence. Palmer’s (1981b) subdivision of the Sauk megasequence into Sauk I, II, and III subsequences (now referred to as supersequences) is widely, but not universally, recognized. Across many areas of the GACB, the Sauk III supersequence of Palmer can be subdivided into two supersequences (defined as “Sauk IIIA” and “Sauk IIIB” in this chapter), based on an unconformity and/or biostratigraphic changes near the Cambrian-Ordovician boundary. Additional significant unconformities and marine flooding events that can be correlated across much of the GACB are summarily described in this chapter. The recognition of correlatable surfaces across the GACB has been challenging because of local syndepositional tectonics and paleotopography, and lithofacies heterogeneity. However, confidence in correlation across the GACB has been greatly enhanced by an increasingly refined biostratigraphic framework.