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Samples of Mesozoic and Cenozoic strata from four exploration wells on the Labrador Shelf were analysed by x-ray diffraction to determine the relative abundance of smectite and illite/smectite mixed-layer clay, illite, chlorite and kaolinite. Interpretation of the trends in clay distribution shows that only limited diagenesis occurred. Weathering was the primary process by which clay minerals evolved. Lithology of the source rock, climate and relief were the primary controls affecting the weathering process.

High smectite abundance in Early Cretaceous volcanic strata is the result of alteration of rift-associated volcanics in a warm, humid climate. Coeval siliclastic rocks with elevated kaolinite abundance, suggest the subareal weathering of Precambrian and Paleozoic igneous and metamorphic bedrock under similar climatic conditions. A change in the clay mineralogy of Late Cretaceous strata suggests an enlarged drainage basin which homogenized clays from many sources prior to their final dispersal in the spreading and deepening Labrador Sea. Eocene and later strata contain chlorite as a major component, indicating a shift in emphasis from chemical to mechanical weathering, and the onset climatic cooling.

A diagenetic trend of increasing illite abundance at the expense of smectite is absent from most wells on the Labrador Shelf. Iron-rich smectite, formed during the Late Cretaceous and Paleogene by alteration of mafic rocks in hot, humid climates or by precipitation of vent fluids at submarine spreading centres, may have interfered with the illitization, even when physical constraints for the reaction had been satisfied.

The persistence of smectite at depth may have catalyzed the formation of hydrocarbons at low thermal maturation indices but their migration would not have been assisted by interlayer water normally expressed during diagenesis.

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