ABSTRACT

The MD 80-08 coal exploration core from the Albian Boulder Creek Formation of northeastern British Columbia contains 90 m of alluvial strata that underwent extensive pedogenic modification. Gleyed colors, pedogenic slickensides, clay coatings, mottling, sphaerosiderite, and sedimentary pyrite characterize the paleosols.

The isotopic compositions of 30 sphaerosiderite-bearing horizons commonly show trends of invariant ¹⁸O and variable ¹³C values. Equal-area plots of ¹⁸O vs. ¹³C were used to construct meteoric sphaerosiderite lines (MSLs). The MSLs indicate that meteoric groundwater compositions ranged from approximately -11o/oo to -16o/oo (SMOW). The sphaerosiderite ¹⁸O values indicate formation in shallow groundwaters predominantly recharged by local precipitation. The sphaerosiderite isotopic compositions are a proxy record of mid-Cretaceous precipitation, and are useful for quantifying changes in the hydrologic cycle during humid "greenhouse" periods.

Micromorphology of a 1.15 meter interval (69.75-68.6 m), with a +1.31o/oo increase in the average ¹⁸O values, records a polygenetic history of paleosol development. The pedocomplex has a four-stage developmental history characterized by: (1) an initial well-drained phase, with wet-dry cycles and relatively low baselevel; (2) erosion, baselevel rise and coarser-grained sedimentation; (3) saturated, reducing soil conditions, high baselevel, with some marine influence on pedogenesis; and (4) meteoric-water-dominated hydromorphic soil conditions, high baselevel, and sphaerosiderite precipitation. The ¹⁸O excursion recorded through this interval reflects minor mixing of marine and meteoric groundwaters. The modal abundance 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 several changes in baselevel recorded in the paleosols are interpreted to have been driven by minor (parasequence-scale) changes in relative sea level.