Abstract

Modern sands and gravels from Barkley Sound, Vancouver Island, are mainly composed of igneous lithic fragments, present in sediments coarser than medium sand, and of andesine plagioclase, present in finer size fractions. Heavy mineral assemblages are dominated by hornblende, epidote, opaque minerals, and chlorite. This sediment mineralogy is mainly controlled by the composition of the source area, and to a lesser extent, by hydraulic processes of the depositional environment; effects of weathering at source and river transport are minimal. Sources include Mesozoic, eugeosynclinal basalts and andesites, late Mesozoic, post-geosynclinal granodiorite and diorite plutons, and Cretaceous to (?) late Pleistocene sediments which themselves were ultimately derived from the aforementioned igneous sources.Relict sands and gravels on the continental shelf adjacent to Barkley Sound, are similar in composition to the modern sediments and have been derived from similar igneous and sedimentary source rocks. However, relative to modern sands, relict sands have a greater variety of minor constituents, particularly heavy minerals, which in addition to hornblende, epidote, and opaque minerals, include garnet, orthopyroxene, and clinopyroxene. It is suggested the more varied mineralogy is due to: (1) a more extensive source area; and (2) introduction of additional sediment from outside the study area by modern bottom currents.

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