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.

ABSTRACT We have identified 57 large-magnitude, sequence boundaries in the Phanerozoic succession of the Canadian High Arctic. The characteristics of the boundaries, which include angular unconformities and significant changes in depositional and tectonic regimes across the boundaries, indicate that they were primarily generated by tectonics rather than by eustasy. Boundary frequency averages 10 million years throughout the Phanerozoic and there is no notable variation in this frequency. It is interpreted that each boundary was generated during a tectonic episode that lasted two million years or less. Each episode began with uplift of the basin margins and pronounced regression. This was followed by a rapid subsidence and the flooding of the basin margins. Each tectonic episode was terminated by a return to slow, long-term subsidence related to basin forming mechanisms such as thermal decay. The tectonic episodes were separated by longer intervals of tectonic quiescence characterized by slow subsidence and basin filling. The tectonic episodes are interpreted to be the product of changes in lithospheric stress fields with uplift being related to increased, compressional horizontal stress and the following time of rapid subsidence reflecting a decrease in such stresses or an increase in tensional stresses. Conversely, the longer intervals of tectonic quiescence would reflect relatively stable, horizontal stress fields. The episodic changes in stress fields affecting the Canadian High Arctic throughout the Phanerozoic may be a product of intermittent, plate tectonic reorganizations that involved changes in the speed and directions of plate movements. The longer intervals of tectonic quiescence would occur during times of quasi-equilibrium in the plate tectonic mosaic. The tectonic episodes that generated the sequence boundaries were governed by nonlinear dynamics and chaotic behavior, and there is a one-in-10-million chance that a tectonic episode will be initiated in the Canadian High Arctic in any given year. Thus, the major transgression associated with each episode can be referred to as a “10-million-year flood.”

Abstract Five stratigraphic successions are recognized in the Sauk megasequence in the Canadian Arctic Islands. The Lower Cambrian succession was deposited on a distally steepened carbonate ramp that overlaid thick Lower Cambrian siliciclastics. The second Middle Cambrian succession is composed of oolitic grainstone, microbial boundstone, lime mudstone, and mixed carbonate-siliciclastic facies assemblages that developed on a platform. The second and third successions are separated by an unconformity that spanned most of the Steptoean. The third succession includes mixed carbonate-siliciclastic strata and spans the Sunwaptan Prosaukia Biozone to the Cordylodus proavus Biozone. This succession was terminated by the Cambrian–Ordovician unconformity. The shelf had a ramplike configuration during this time. The fourth succession starts above the Cambrian-Ordovician unconformity with a widespread shelf sandstone that spans the C. proavus to C. lindstromi Zones. This was followed by a rapid deepening in the earliest Ordovician (Iopetognathus fluctivagus Zone) marked by the deposition of open-marine carbonates. A progressive shallowing culminated in evaporite units in the Stairsian. A marked change in basin architecture occured during this fourth succession. Distinctive shelf-margin units appeared consisting of fenestral mudstone, shoal deposits, and common karst breccias. The shelf margin during this interval was very steep, and carbonate was not transported into the deep water. The platform also changed configuration during this time, with the development of an intraplat-formal basin. Evaporites accumulated in this silled basin. Strata in the intraplatformal basin are thicker than those at the shelf margin. The fifth succession (Tulean–Blackhillsian) consists of shallow subtidal carbonates. The first two sequences in the Arctic Islands correspond closely to Sauk I and II elsewhere in Laurentia. Strata in the Arctic that are equivalent to the standard Sauk III supersequence contain three unconformity-bounded stratigraphic assemblages. This reflects local tectonic conditions that resulted from the change from a passive-margin setting in the Early Cambrian– earliest Ordovician to convergence later in the Early Ordovician. The downgoing slab, interpreted to be dipping below Laurentia, affected carbonate sedimentation along northwestern Laurentia during this time.

Abstract Analysis of a large thermal maturity dataset indicates that the Carboniferous to Eocene Sverdrup Basin in the Canadian Arctic had a uniform response to thermal stress with depth for Mesozoic strata. Thermal maturity was established at the level of the widespread Upper Triassic Gore Point Member; a good seismic reflector, occurring in close vertical proximity to the two main oil-prone source rocks in the basin. The Gore Point Member is in the gas window ( R o >1.35%) in the northeastern part of the Sverdrup Basin, whereas in the western Sverdrup Basin its maturity does not exceed 1.2% R o . This would support the hypothesis that large quantities of gas found at the Drake, Hecla and Whitefish fields have derived from a deeper source, probably in Permian or lower Palaeozoic strata. A normal burial curve is established using boreholes drilled in areas with no structural complexity at time of maximum burial. Low-amplitude structures, including the Drake, Hecla and Whitefish fields, show little or no uplift following maximum burial in the Paleocene, indicating that these structures formed prior to the Eocene folding related to the Eurekan Orogeny. Because they were present at the time of maximum burial, they were available to be charged during hydrocarbon migration. In contrast, high-amplitude structures show evidence of large uplifts following maximum burial. They formed in the Eocene and hence post-date most hydrocarbon migration.