Geochemical mapping in Hokuriku, Japan: influence of surface geology, mineral occurrences and mass movement from terrestrial to marine environments

The authors have created terrestrial and marine geochemical maps of the Hokuriku region, Japan, and examined the background distribution of elemental concentrations. The terrestrial geochemical maps strongly reflect the surface geology and mineral occurrences. An analysis of variance (ANOVA) reveals the relationships between the distribution of surface geology and geochemical maps. Granite, metamorphic and felsic volcanic rocks, and sedimentary rock in accretionary complexes give rise to high concentrations of alkali metal, Be, Y, Ba, REE (except Eu), Tl, Th and U. The distributions of MgO, Al2O3, P2O5, CaO, 3d transition metals (except Cu), Ga, Sr and Eu are controlled by mafic volcanic and ultramafic rocks. The watersheds with above threshold levels for Cu, Zn, As, Cd, Mo, Sn, Sb, Hg, Pb and Bi strongly relate to the mineral occurrences, but ANOVA demonstrated that they are partly affected by surface geology. The marine geochemical maps exhibit control by 3 factors: calcareous sediments on the continental shelf, clay minerals on the continental slope and ocean floor, and sediments supplied from rivers. Many elemental concentrations are quite low in continental shelf sediment because of dilution by calcareous materials. The continental slope and deep valley sediments are rich in most elements except for MgO, CaO and Sr. Coarse sediments supplied from large rivers contribute to local enrichments of several elements such as K2O, Cr and Zn. The spatial distribution patterns of Cr and Zn concentrations show that gravity moves the river sediments over 60 km offshore along a deep-valley.