Abstract The Early Cambrian palaeogeographical enigma arises when tectonic reconstructions are made using palaeoclimatic v. palaeomagnetic data that result in possibly contradictory tropical, mid-latitude, and south polar locations for major continents. For example, NW Africa and Cadomia may have lain in a tropical zone (0° to ±30° latitude) based on the presence of archaeocyath reefs, minor evaporites, and carbonate platforms at c. 520 Ma ± 5 Ma or, alternatively, NW Africa and Cadomia may have lain in a south polar zone (90° to 60° south latitude) based on palaeomagnetic constraints. Greater Avalonia may have evolved independently from NW Africa if a dropstone constraint implying polar latitudes at c. 530 Ma and a palaeomagnetic constraint implying c. 50° latitude at c. 505 Ma are accommodated. We show here how counterclockwise rotation of Gondwana during the Cambrian about an interior axis may solve the enigma. Gondwanan apparent polar wander becomes consistent with tropical conditions inferred for NW Africa when adjusted to accommodate constraints placing the south pole near Peru for c. 540–520 Ma. Concurrent counterclockwise rotation of Baltica and Gondwana during the Middle Cambrian may have facilitated separation of Greater Avalonia from Baltica across dextral shear zones.

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 was originally defined as a rock interval with a trilobite-dominated fauna that overlay a presumed biologically barren Precambrian epoch. Work to formally define the Cambrian base arose after the discovery of Precambrian macrofossils in South Australia. The Working Group on the Precambrian–Cambrian Boundary (set up in 1972) promoted an emphasis on an extended pre-trilobitic interval with mineralized skeletal elements (small shelly fossils or early skeletal fossils). The study of early skeletal fossils made the Ulakhan-Sulugur (Siberia) and Meishucun (South China) sections candidates for the basal Cambrian Global Stratotype Section and Point (GSSP), but difficulties in correlation and the taxonomy of early skeletal fossils did not allow the selection of a reliably correlative horizon. Bilaterian ichnofaunas below the first diverse early skeletal fossils suggested the definition of a Cambrian base GSSP below the early skeletal fossils. Work on the thick, stratigraphically continuous and fairly unifacial sections in the Burin Peninsula, southeastern Newfoundland led in 1992 to an ichnofossil-defined GSSP at Fortune Head. Despite arguments for a revision and redefinition of the lower boundary of the Cambrian System, the best definition of the basal Cambrian GSSP is at Fortune Head and does not rely on the Treptichnus/Trichophycus pedum (abbreviated below as T. pedum ) first appearance datum, but rather on the base of the T. pedum Assemblage Zone at the highest occurrence of Ediacaran taxa and in the lower range of T. pedum .