Changes in environment and provenance within the Changjiang (Yangtze River) Delta during Pliocene to Pleistocene transition

Abstract

This study investigates heavy mineral assemblages in sediments of the modern Changjiang Delta (Yangtze River) and the Quaternary/Pliocene (Q/N) boundary strata within the delta area with the aim of deciphering changes in sediment provenance and the depositional environment during the Pliocene to Pleistocene transition. Major heavy mineral assemblages in the modern Changjiang sediments originate from their provenance rocks, while the distribution of unstable (amphibole), ultrastable (zircon, tourmaline, and rutile), and altered minerals (limonite, leucoxene) are closely related to the climate and the hydrodynamic environment within the catchment. The ratio of a certain unstable mineral (amphibole) to an altered mineral (limonite) is considered to be a sensitive indicator of the chemical weathering intensity that is largely affected by climate conditions. Heavy mineral data of core sediments reveal distinctly different environments and provenances between the upper (Quaternary) and lower (Pliocene) sedimentary strata in the Changjiang Delta. The Pliocene sediments are characterized by abnormally high contents of limonite (42%), leucoxene (8%) and zircon (15%), but an extremely low amphibole content (5%). Enrichment of zircon is indicated from the mid-lower Changjiang catchment, where granitoid rocks are widely distributed. The strong enrichment of altered minerals and the considerable great loss of unstable minerals suggest that Pliocene weathering was much stronger than it is nowadays. In contrast, high contents of amphibole, garnet, and pyroxene, and the low stable mineral content in the Quaternary strata indicate that sediment provenances may have extended to the upper Changjiang catchment, which underwent weak chemical weathering. These results imply that Changjiang as a large river system might have drained the East China's continental margin prior to the Q/N transition. The distinct heavy mineral assemblages found at the Q/N boundary in the Changjiang Delta are indicative of a drastic environment with provenance changes in response to intensive neotectonics. It is thus inferred that sensitive heavy mineral indices can be used to indicate the Q/N boundary in the Changjiang Delta, where reliable geochronological proxies have rarely reached an agreement.