Stratigraphic implications of meteoric sphaerosiderite delta O-18 values in paleosols of the Cretaceous (Albian) Boulder Creek Formation, NE British Columbia foothills, Canada

The MD 80-08 coal exploration core from the Albian Boulder Creek Formation of northeastern British Columbia contains 90 ra of alluvial strata that und erwent extensive pedogenic modification. Gleyed colors, pedogenic slickensi des, clay coatings, mottling, sphaerosiderite, and Sedimentary pyrite chara cterize the paleosols. The isotopic compositions of 30 sphaerosiderite-bearing horizons commonly s how trends of invariant delta O-18 and variable delta C-13. values. Equal-a rea plots of delta O-18 vs. delta C-13 were used to construct meteoric spha erosiderite lines (MSLs). The MSLs indicate that meteoric groundwater compo sitions ranged from approximately -11 parts per thousand to -16 parts per t housand (SMOW). The sphaerosiderite delta O-18 values indicate formation in shallow groundwaters predominantly recharged by local precipitation. The s phaerosiderite isotopic compositions are a proxy record of mid-Cretaceous p recipitation, and are useful for quantifying changes in the hydrologic cycl e during humid "greenhouse" periods. Micromorphology of a 1.15 meter interval (69.75-68.6 m), with a +1.31 parts per thousand increase in the average delta O-18 values, records a polygene tic history of paleosol development. The pedocomplex has a four-stage devel opmental history characterized by: (1) an initial well-drained phase, with wet-dry cycles and relatively low baselevel; (2) erosion, baselevel rise an d coarser-grained sedimentation; (3) saturated, reducing soil conditions, h igh baselevel, with some marine influence on pedogenesis; and (4) meteoric- water-dominated hydromorphic soil conditions, high baselevel, and sphaerosi derite precipitation. The delta O-18 excursion recorded through this interv al reflects minor mixing of marine and meteoric groundwaters. The modal abu ndance of pyrite, and increased Mg/(Ca + Mg) and Mg/Fe cation ratios in the sphaerosiderites further substantiate marine influence on pedogenesis. The alluvial section of the Boulder Creek Formation is aggradational, and s everal changes in baselevel recorded in the paleosols are interpreted to ha ve been driven by minor (parasequence-scale) changes in relative sea level.