Discriminating depositional environments of sands from modern source terranes using modal analysis

Composition of modem beach, dune and fluvial sands from the Gulf of Mexico coast, and the Kapiti and Foxton Coasts, New Zealand, were compared in order to establish petrological criteria that discriminate between depositional environments and subenvironments. The three areas were chosen because of their different sand compositions. Sand from the Gulf of Mexico is quartzofeldspathic to feldspatholithic. Sand from the Kapiti Coast sands is feldspatholithic and sand from the Foxton is lithofeldspathic. Using a confidence level of 95%, petrographic data were plotted as bar charts for each environment (beach, dune, river) and for each subenvironment (inshore, foreshore, backshore of the beaches, stoss, crest, lee face of eolian dunes, upstream and mouths of the river). The best discrimination of beach, dune and river environments was achieved for the Gulf of Mexico sands using Q-F-L and Qm-K-P plots. Terranes with multiple source rock types such as the Gulf of Mexico enhance petrological discrimination between sands from beach, dune and river subenvironments and environments. In terranes with few rock source types such as the Kapiti Coast, only the river sands can be discriminated from those of other environments. The single rock source of the Foxton Coast allows little petrological discrimination between the beach, dune and river sands. This study examines compositional differences between beach, dune and river subenvironments and environments, and the degree to which these differences are dependent on the variation of rock types in the source terrane. The study also shows the usefulness of Q-F-L, Qm-K-P and Lv-Lm-Ls bar charts. The discrimination among beach, dune and river sands is partly due to grain-size variation. (C) 2001 Elsevier Science B.V. All rights reserved.