DELINEATING NONMARINE PARASEQUENCE BOUNDARIES USING GEOCHEMISTRY OF EARLY DIAGENETIC CARBONATE CEMENTS IN FLUVIAL DEPOSITS: ALBIAN BOULDER CREEK FORMATION, NE BRITISH COLUMBIA

The Late Albian Boulder Creek Formation of northeastern British Columbia is one of several nonmarine to marine clastic wedges deposited along the margins of the Cretaceous Western Interior Seaway. The formation consists of alluvial deposits, some with extensive pedogenic modification. Coal exploration cores, MDD 80-08, Gulf Trefi 78-17, and Pacific MUD 78-17, were used for this study. Variable stacking patterns within the cored sections thin basinward and are correlated by changes in elemental (Fe, Ca, Mg, and Mn) and stable isotope (d¹⁸O) geochemistry of early diagenetic carbonate cements. Systematic variations of elemental and stable isotope geochemistry are interpreted to be in response to minor (parasequence scale) changes in sea level.

The d¹⁸O values of early diagenetic siderite cements range from -8‰ to -17.5‰ (VPDB). d¹⁸O compositions of sphaerosiderites in interstratified paleosol horizons are less variable, ranging between -8‰ and -13.5‰ (VPDB). Later diagenetic phases, including burial and syntectonic vein calcite cements, have significantly lighter d¹⁸O values (-19‰ to -21‰ (VPDB)). The d¹⁸O compositions of early siderite cements and sphaerosiderites are interpreted to be proxies for paleoprecipitation, and are, therefore, useful for quantifying changes in the local hydrologic cycle. d¹⁸O values of early diagenetic carbonate cements indicate meteoric groundwater compositions ranged between -11‰ and -19‰ (SMOW).

The d¹⁸O excursions recorded through the Boulder Creek Formation sections coincide with changes in Mg/Fe and Mg/(Ca+Mg) cation ratios. Progressive ¹⁸O depletion and decrease in cation ratios with increasing distance from the coastline sections reflect changes in relative sea level and marine influences on local groundwater compositions. Measured geochemical variations coincide with minor changes in relative sea level and have the potential to better delineate parasequence boundaries in coastal plain successions.