The Scotian Basin contains Mesozoic-Cenozoic sedimentary rocks, up to 16 km thick in places, that were deposited during the rifting of Pangea and the opening of the North Atlantic. The earliest fill in the basin, deposited during Triassic rifting, consists of red continental clastics and evaporites. With the transition to seafloor spreading in the Early Jurassic, the rift basins were gradually filled by clastics and carbonates. Fully marine conditions developed by Mid Jurassic, leading to an array of alluvial plain, deltaic, and carbonate facies. Late Cretaceous / Early Cretaceous sedimentation was dominated by transgressive shale, limestone, and chalk units. Relative sea level fluctuations during the Paleogene and Neogene created a mix of marine sandstones and shales interspersed with coarse clastics and marine carbonates. These are overlain by unconsolidated glacial till, glaciomarine silts, and marine sediments that were deposited during the Quaternary.

Generalized stratigraphy of the Central Scotian Shelf. Click on the formation names for a more detailed description of each unit (excerpted and modified from an internal report written by John Wade, 2004)

The oldest of the synrift sequences related to the opening of the Atlantic is a thick series of Triassic/lowermost Jurassic red sandstones, siltstones and shales named Eurydice Formation. Wells have encountered Eurydice Formation beneath both the Grand Banks and the Scotian Shelf. In Orpheus Graben, where the type section of the Eurydice Formation is defined, almost 600 m of the formation were drilled and seismic data indicates a total formation thickness of over 3 km. More than 1.5 km of Eurydice Formation were drilled in the Naskapi Graben complex on the LaHave Platform and seismic data indicates that it also is over 3 km thick there.

Overlying Eurydice Formation, and interfingering with it on the basin margins, is the Argo Formation which consists primarily of salt. The distribution of salt within Scotian Basin suggests that the major grabens were loci of early deposition accumulating thick sequences of evaporites and associated redbeds which thinned across intervening highs. The salt has flowed extensively due to subsequent sediment loading and, possibly, to periodic reactivation of the rift fault system during later stages of continental breakup. Salt pillows, diapirs and canopies are common in areas of thick salt, particularly in a major zone of diapiric structures which trends beneath the continental slope from eastern Georges Bank to the western Grand Banks

Figure 1: Major tectonic features of the Scotian Basin. larger image [GIF, 67.7 kb, 598 X 363, notice]

Figure 2: Seismic section 88-1a illustrating salt toungue canopies beneath the Scotian Slope. larger image [GIF, 179.4 kb, 600 X 442, notice]

A significant unconformity, the breakup unconformity, occurs between synrift and post rift sequences in the Scotian Basin. It is dated earliest Jurassic. It is readily identifiable across the shallow grabens on the LaHave Platform and distally beyond the zones of salt diapirism beneath the lower slope.

Beneath the Scotian Shelf, the Iroquois and Mohican formations overlie the breakup unconformity. Iroquois Formation, which consists primarily of dolostones, is coeval with the lower part of the Mohican Formation on LaHave Platform where it attains a maximum thickness of about 800 m. It may be represented by a series of thin dolomitic beds in the lower part of wells drilled on the western Grand Banks.

Sandstones and shales of the Mohican Formation form a very thick lower to middle Jurassic molasse sequence deposited into actively subsiding subbasins adjacent to the hinge zone. The formation is widespread on Scotian Shelf and has been encountered in a number of wells. The thickest Mohican section drilled is just over 400 m on LaHave Platform but seismic data indicate that the formation is more than 4 km thick south of the hinge zone in Abenaki Subbasin and as much as 5.5 km thick over the synrift sequences beneath the eastern Scotian Shelf. The Mohican Formation thins dramatically at the hinge zone and either pinches out against it or is truncated by the post-Jurassic Avalon Unconformity.

Mohican Formation is overlain by a second very thick, predominantly clastic, post-rift sequence, the Mic Mac Formation. In Scotian Basin the Mic Mac Formation, and its equivalents the Abenaki, Mohawk and Verrill Canyon formations, span the late-Middle and Late Jurassic.

The Mic Mac Formation ranges in thickness from 6 km in the Laurentian Subbasin to an erosional or depositional edge along the LaHave and Burin platforms and Avalon Uplift. Southeast of Sable Island it consists of 4 to 5 km of interbedded sandstones, shales and limestones. To the north and west of Sable Island, along the hinge zone, there is a prominent carbonate bank facies, the Abenaki Formation. The more continental facies, landward of the bank is designated Mohawk Formation; it is comprised of texturally mature, felspathic sandstone and siltstone with interbeds of shale and limetstone. Distally, the carbonate and clastic facies grade into fine grained, basinal facies, Verrill Canyon Formation.

Some variations in the Upper Jurassic facies can be interpreted from seismic data. On the northern flank of Laurentian Subbasin, the seismic expression is one of semi-continuous parallel reflections which are interpreted to be interbedded sandstones and shales deposited in an alluvial plain environment. Adjacent to the hinge zone of the eastern part of Abenaki Subbasin, the Mic Mac Formation consists of shales with thin to thick interbeds of sandstone and siltstone, Further south, where only the upper part of the formation has been drilled, 40 to 50% of the drilled section consists of thick intervals of limestone which cap prograding sequences of Mic Mac clastics. Further seaward, the seismic expression is one of discontinuous and weak reflections where sandstones and limestones of the Mic Mac Formation are replaced by marine shales of the Jurassic Verrill Canyon Formation.

Spanning the Middle Jurassic to Early Cretaceous, the Verrill Canyon Formation is the deep water facies equivalent of the Mohawk, Abenaki, Mic Mac, and Missisauga formations. It consists primarily of grey to brown calcareous shale with thin beds of limestone, siltstone, and sandstone. The Verrill Canyon Formation was deposited in the prodelta, outer shelf, and continental slope settings and ranges in thickness from 360 metres in the southwestern Scotian Basin to more than 915 metres in the northeast.

Sandstones of the Logan Canyon Formation grade distally to the deep water shale facies of the Shortland Shale which was deposited in prodelta, outer shelf, and continental slope settings.

The Laurentian Formation is a progradational wedge of Quaternary and uppermost Pliocene sediments. At its thickest point, along the outer shelf and upper slope, there are over 1500 metres of glaciomarine and marine sands, silts, and clays.

The Abenaki Formation has been subdivided into four members: the Scatarie, Misaine, Baccaro, and Artimon. It forms a distinctive limestone bank complex and a prominent seismic sequence. It is best developed at the hinge zone between LaHave Platform and Shelburne and Sable subbasins. The maximum drilled thickness of this formation is 1644 m.

During the latest Jurassic the eastern margin of Canada was affected by the breakup of Iberia from North America. Most strongly impacted was the margin south of Newfoundland where there was uplift, deformation and extensive erosion of Jurassic and older strata. This breakup unconformity, the Avalon Unconformity, is observed from the Avalon Uplift westward into the eastern Scotian Basin. During this event there was a corresponding shift in depocentres westward from Laurentian Subbasin to Sable Subbasin.

The Missisauga Formation is widespread in Scotian Basin where it varies considerably in both facies and thickness. Across the LaHave and Burin platforms and Canso Ridge it is less than 1000 m thick and consists of 60-80% sandstone with some significant local limestone facies in the southwest. In the Sable Subbasin, more than 2770 m have been drilled in the Sable Island area and total thickness is estimated at more than 3 km with 30-50% sandstone/siltstone content. Basinward, the Missisauga Formation grades into turbidites and shales of the Cretaceous portion of the Verrill Canyon Formation.

The Logan Canyon is an upward fining succession with a similar distribution to the Missisauga Formation but with a maximum expected thickness of about 2.5 km. It has been subdivided into four members two of which are shale dominated. This formation also has a distal turbidite/shale equivalent, the Shortland Shale.

Marine shales, chalks and minor limestones were deposited across the Scotian Basin during the Late Cretaceous. The first of these transgressive units was Dawson Canyon Formation which varies in thickness from more than 700 m in South Whale Subbasin and parts of Scotian Shelf to about 200 m on Canso Ridge and approximately 100 m in the more sediment starved outer part of Sable Subbasin. Because of its transgressive nature, the Dawson Canyon may be the most extensive Cretaceous formation on Burin Platform. Distally, it is often missing due to Tertiary erosion.

Wyandot Formation is composed of chalks, chalky mudstones, marls and minor limestones. Its thickness ranges from less than 50 m in some wells in the Sable Island area to about 400 m in the southeastern corner of the Scotian Shelf but it is missing over extensive portions of the basin due to Tertiary erosion. Beneath the outer shelf and slope, the top of the Wyandot Formation is often marked by an unconformity which is overlain by Tertiary sediments.

The sedimentary succession between the top Wyandot and the top Cenozoic is included in the Banquereau Formation. It is coarsening-upward, predominantly Tertiary in age and consists of a series of downlapping or prograding sequences with the result that in the northern (proximal) areas of the basin Paleocene or Eocene sediments overlie Cretaceous rocks whereas distally, Miocene or younger sediments do. Thicknesses range from a zero edge along the basin margin to more than 4 km associated with areas of salt withdrawl beneath the continental slope.