Jean‑Philippe Gobeil

On the Scotian Shelf, Lower Cretaceous deltaic sandstone reservoirs are being exploited for gas and explored for oil. Correlative strata on land, the Chaswood Formation, consisting of silica sand and quartzrich gravel associated with kaoliniterich clay beds outcrop in a few pits from central Nova Scotia to Cape Breton Island. The best exposure is the West Indian Road pit, Shubenacadie, where silica sand and quartzrich gravel intercalated with three major clay units occur in a faultbounded sedimentary basin.

Clay Unit 1, at the base of the pit, consists principally of a dark grey clay, rich in organic matter, showing a few zones with charcoal and/or lignite fragments and pyrite nodules. This suggests a calm, closed and oxygenreduced environment of deposition. The occurrence of charcoal within the clay and the lack of marine fossils are indicators of a subaerial environment. All the clay units show color changes and mottling. This suggests the development of a paleosoil and therefore a subaerial environment. These factors combined suggest a lacustrine or marsh environment as the first stage of deposition of the Chaswood Formation in the study area.

Silicarich sands and gravels within the West Indian Road pit typically show a light grey color, locally masked by strong orange, pink or purple staining. Grain size varies from very fine to very coarse sand to pebble gravel. These sands show a few percent of muscovite and traces of heavy minerals. A great variety of sedimentary structures such as trough crossbedding, planar crossbedding, parallel lamination, ripples and grading were observed in the sand and gravel units. The observation of lithofacies successions shows a general fining upward sequence in the sediments, composed of several superposed small fining upward sequences with a mean paleoflow direction to the SE. These sand and gravel units are interpreted to have been deposited in a braided river environment.

Two major fault systems define an overall graben type structure for the West Indian Road pit deposit. Synsedimentary deformation features such as major angular unconformities, bed thickness changes and coarsening of correlative facies across faults are preserved in the sand and gravel units, and suggest underlying bedrock block movements. Although they show evidence of syndepositional motions, the lack of basement clasts in the sediments and the mean paleoflow direction at high angle to the fault systems suggest postdepositional motions along the major faults. Faulting activity could be related to the CobequidChedabucto Fault.

A provenance study of the sand and gravel units was conducted using heavy minerals assemblages, pebble petrography and ArAr dating of detrital muscovite grains. The most abundant heavy minerals are rutile, altered ilmenite, zircon, and tourmaline, and in lesser amount staurolite and andalusite. The concentration of these resistant minerals suggests recycling. The gravel fraction of the sand and gravel units is mainly vein quartz and vein quartz with tourmaline. However, it presents a low percentage of exotic pebbles such as granite, diorite, and rhyolite,. Petrographic study of these pebbles has suggested a Cobequid Highland source for the granite, diorite and rhyolite pebbles, and a Horton and Pictou Groups sedimentary source rocks for the quartzarenite, subarkose, arkose, mudstone to muddy sandstone, and conglomerate pebbles. The 374 Ma ArAr age for the muscovite grains suggest a primary South Mountain Batholith origin. The grains are interpreted to be recycled from the surrounding Horton Group muscovitebearing sandstones.

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Supervisor: Georgia Pe-Piper