ABSTRACT

The Albian Harmon Member is a marine mudstone that was deposited in a restricted basin beneath anoxic to dysaerobic bottom waters about 102 Ma. Petrographic observations by back-scattered electron scanning electron microscope (BSE-SEM), in conjunction with stable isotopic data, indicate the following paragenetic sequence of diagenetic minerals: pyrite, kaolinite, siderite concretions and calcite cement, dolomite, disseminated siderite, kaolinite cement, illite, and quartz.

Pyrite, siderite concretions, and early calcite cement formed under conditions of low water-rock ratios. The ¹⁸O values of the early authigenic carbonates indicate mineral formation from mixed meteoric-marine fluids. The isotopic composition of later, but still early disseminated dolomite and siderite indicates formation from fluids of dominantly meteoric origin. This suggests that compacting Harmon sediments were recharged by meteorically-derived fluids during early compaction.

Kaolinite cement occludes up to 30 per cent pore space in silt laminae that, in conjunction with porosity-loss data, indicates cementation at less than 400 m burial depth. The precipitation of kaolinite and the isotopic data from authigenic carbonates are interpreted as indicating that early diagenesis of the marine Harmon Member was dominated by meteoric fluids.

Meteoric recharge was most likely induced by the large freshwater hydraulic head of the rising Cordillera to the west-southwest and the relative drop in sea level during deposition of the overlying Cadotte and Paddy members.

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