Selective replacement of mollusk shell by chlorite, Lower Cretaceous Longarm Formation, Queen Charlotte Islands, British Columbia

The shells of molluscan megafauna from Lower Cretaceous coarse-elastic stra ta of Queen Charlotte Islands, British Columbia, principally ammonites and inoceramid bivalves, commonly show partial replacement alteration to fibrou s, Mg- and Fe-rich chlorite. Although Cretaceous volcanic strata are very r are on Queen Charlotte Islands, strata containing the altered shells are ge nerally rich in mafic volcanic detritus, derived from older and widespread Triassic basalts; these basalts are considered to be the source of the Mg a nd Fe cations present in the chloritic shells. Differences in degree of alt eration to chlorite are attributed to local variations in Cretaceous sandst one permeability.