Glauconitic Sediments on the Continental Shelf off Vancouver Island, British Columbia, Canada

Abstract

ABSTRACT The tectonically active Vancouver Island continental margin is characterized by a very narrow (< 80 km) continental shelf and an extremely steep continental slope. The shelf is marked by high wave and current energies on the inner, nondepositional shelf and very slow rates of sediment accumulation on the outer shelf. Within the muddy sands of the outer shelf and slope, glauconitic grains are frequently an important constituent of the sand fraction, occurring in four principal forms: (a) locally abundant, pale green infillings of benthic foraminifera; (b) very abundant, pale to moderately dark green, dull grains displaying varying degrees of fracturing of the external surfaces; (c) scarce, moderately dark green, altered and expanded mica grains; and (d) scarce, very dark green, small (< 200 µm) grains with polished, highly fractured surfaces. Mineralogically, the grains mainly reflect their origins as detrital sediments. They consist most commonly of quartz, plagioclase minerals, chlorite, smectite, and illite. Also, a mixed-layer mineral, smectite-illite, occurs in significant amounts in glauconitic grams but is absent in unaltered mud fragments and in the clay fraction of sediments from the outer shelf. The appearance of this mineral reflects the first stages of glauconite authigenesis. Bulk analyses of glauconitic grains yielded K2O values of less than 2%, reflecting their very early stage of evolution, but 19-28% Fe2O3, several times the level in the surrounding matrix sediments. Microprobe analyses and scanning electron microscopy show that glauconitic smectite forms in the pores and along fractures in the host grains, commonly as a boxwork arrangement or rosette clusters of blades. Fe2O3 and K2O values at these sites can reach 32.5% and 7.5%, respectively. Several lines of evidence, including radiocarbon age determinations on glauconitized foraminifera tests, indicate a Recent age for the Vancouver Island shelf glauconitic grains.