Geochemical compositional changes at the Pliocene-Pleistocene transition in fluviodeltaic deposits in the Tegelen-Reuver area (southeastern Netherlands)

At the Pliocene-Pleistocene transition, major changes occurred in the Rhine system: Climatic cooling, extension of the drainage area into the Alps and a marine regression. We studied the geochemistry, clay mineralogy and micr omorphology of several sections with Pliocene and Early Pleistocene Rhine d eposits to determine the effects of this change on the bulk geochemical rec ord. We found a general increase in Na2O contents, which can be attributed to the Alpine source supplying fresh, sodic plagioclase-rich material inste ad of the local, strongly weathered sediments. Increasing K2O/Al2O3 can be attributed to a similar decrease in degree of weathering. However, this tre nd is disturbed by the loss of K from clay minerals during weathering in or ganic-rich layers. Local high TiO2 anomalies, caused by preferential sortin g and concentration, are found in most Pliocene sections, but they are abse nt in the Upper Pliocene and Lower Pleistocene Alpine-derived deposits. Thi s change is probably due to a change in the energy of the fluvial system. F inally, (pyrite-) S contents drop (siderite-) Fe contents rise. Micromorpho logical observations indicate that the Pliocene pyrite was formed when fres hwater deposits were flooded with seawater during short-term events. The de crease in S, and the increase in siderite-Fe, can be attributed to decreasi ng marine influence, as a result of the marine regression at the Pliocene-P leistocene transition.