COMPOSITIONAL TRENDS OF A CRETACEOUS FORELAND BASIN SHALE (BELLE FOURCHE FORMATION, WESTERN CANADA SEDIMENTARY BASIN) - DIAGENETIC AND DEPOSITIONAL CONTROLS

Compositional trends of the Cenomanian Belle Fourche Formation, a mari ne shale unit in the Western Canada Sedimentary Basin, have been inves tigated on a regional scale using bulk-rock geochemistry and mineralog y, clay mineral compositions and oxygen isotope geochemistry of shale and bentonite core samples. Smectitic illite-smectite found in the mat rix of immature, hemipelagic samples is compositionally and isotopical ly consistent with an origin from low-temperature alteration of volcan ic ash in the central Western Interior Seaway, where the basin receive d minimal detrital input. The origin of the more illitic matrix in the deeply buried, western, pro-deltaic shales can be interpreted in term s of either diagenetic 'illitization' of a smectitic precursor, or dep ositional mixing of abundant, detrital, illitic material with minor am ounts of ashfall-derived smectite. It is concluded that: (1) documente d silicate mineral reactions during deep diagenesis of the Belle Fourc he Formation took place in a relatively closed system, with no signifi cant import or export of mobile species at the formation scale; and (2 ) diagenesis and depositional mixing can have similar effects in terms of bulk-rock and oxygen isotope geochemistry, and mineral composition s and assemblages.