The shells of molluscan megafauna from Lower Cretaceous coarse-clastic strata of Queen Charlotte Islands, British Columbia, principally ammonites and inoceramid bivalves, commonly show partial replacement alteration to fibrous, Mg- and Fe-rich chlorite. Although Cretaceous volcanic strata are very rare on Queen Charlotte Islands, strata containing the altered shells are generally rich in mafic volcanic detritus, derived from older and widespread Triassic basalts; these basalts are considered to be the source of the Mg and Fe cations present in the chloritic shells. Differences in degree of alteration to chlorite are attributed to local variations in Cretaceous sandstone permeability.

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