Preliminary results from a drilling programme aimed at evaluating the oil shale potential of the Whitby Formation (Upper Ordovician) of Ontario showed that the Craigleith Member of this unit possessed the greatest promise. However, some preliminary lithostratigraphic observations contradicted those of earlier workers, prompting a detailed analysis of the lithostratigraphy, log response, and organic geochemistry of this unit of organic-rich interbedded shale and limestone, also known informally as the "Collingwood shales." Previously grouped with overlying non-calcareous shales, these strata are found to be in gradational contact with underlying limestones of the Lindsay Formation. This, together with a sharp decrease in carbonate and organic carbon contents at their top, leads to their redefinition as the Collingwood Member of the Lindsay Formation. Analysis of subsurface data shows that the unit now termed the "Collingwood Formation" in well records does not contain any of the Collingwood strata as originally defined, and that the potential oil shales of the newly defined Collingwood Member have a significantly restricted distribution. The non-calcareous blue-grey shales previously referred to the upper part of the Whitby Formation are redefined as the Blue Mountain Formation, which is dominated by the brown and grey slightly organic and calcareous Rouge River Member only in the area east of Toronto.

Summary Weathering of Oxford Clay has been studied in five profiles from different areas. The well-known weathering reaction (pyrite + calcite→gypsum) is the major compositional change. The calcite distribution in vertical profiles is distorted by the introduction of calcite-rich solifluction material above in situ weathered clay. Although the correlation of calcite with depth is low, that of calcite with undrained shear strength is high. It is concluded that, although interparticle bond weakening by calcite solution is occurring, the calcite introduced from overlying material may be acting as a cement. Variations in bulk clay mineralogy are small and do not follow consistent patterns. However, the crystallinity of illite in the clay fraction decreases towards the surface; this may reflect the degradation of the crystal structure at the particle margin, where bonding occurs. The weakening of overconsolidated clays is interpreted in terms of bond weakening following oxidation of pyrite; most weathering changes are concentrated near the base of the weathered zone, where this reaction occurs.

The slake durability test has been proposed as a means of predicting the engineering performance of shales. Testing of sections in the two major shale units of Ontario show that the test is capable of making distinctions between apparently similar shale samples. Queenston Shale has generally lower durability than the Georgian Bay Formation samples. Inspection of the shale fabric shows that this is due partly to inefficient cementing by calcite in Queenston Shale, but is primarily because the microcracks in Queenston Shale are more curved than those in Georgian Bay Formation. Variation in shale durability within the shales is controlled by mineralogy. In Queenston Shale, it is controlled almost entirely by calcite cementation. In Georgian Bay, where hard bands (shaly limestone) are present, these dominate the test, giving a high durability. However, pure shale durability is controlled by clay content.Comments on the performance of the test are made that aim to make the testing process and interpretation of the results more reliable. The method appears suitable for shale index testing and should be used as widely as possible in order to establish reliable correlations.

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