Chapter 24 Interpretation of Quaternary Tectonic and Environmental Change Using Heavy Minerals of the Yangtze Delta Plain

Abstract

A 20 m long core drilled into the southern Yangtze delta plain reveals temporal variations, and variations in the distribution of heavy minerals in late Quaternary stratigraphic horizons. The most abundant transparent heavy minerals observed in the core are amphibole, epidote, iron-titanium oxides, metamorphic minerals (garnet, sillimanite, kyanite, staurolite, chloritoid), pyroxene, zircon, tourmaline, rutile and apatite. Heavy minerals found in lower quantities are sphene, monazite, allanite and siderite. Also present in the heavy mineral separates are rock fragments, authigenic framboidal pyrite and pyritised foraminifera. Relative abundances of heavy minerals suggest one main source of sediment to the delta plain during the late Pleistocene to early Holocene and the addition of another source in the late Holocene. Proximal sources west of the southern Yangtze delta plain, including bedrock of the southern delta plain itself (Jurassic to Cretaceous igneous rocks, Precambrian medium-grade metamorphic rocks and Palaeozoic to Mesozoic carbonate sequences) and the western Mao-shan and Tian-mu-shan highlands (Palaeozoic to Mesozoic clastics and carbonate rocks), most likely were the source of late Pleistocene to early Holocene fluvial sediments. These sediments were carried eastward in river channels originating in the highlands. A complex of fluvial channels that migrated southward across the delta plain during the late Holocene added an additional source of sediment to the delta plain. They originated from western and northern distal source areas similar to those exposed in the modern Yangtze drainage basin. Evidence from heavy minerals supports previous studies of the core that document the tectonic and palaeoenvironmental history of the southern Yangtze delta plain. In particular, prior to 10,000 years B.P., the southern Yangtze delta plain supported the formation of an oxidising soil when sea level was low and heavy mineral influx came from proximal sources. During the period from 10,000 to 7000 years B.P., an open coastal setting characterised the southern delta plain region. Pyrite in heavy mineral separates, representing the period between 7000 and 4000 years B.P., supports evidence from other studies indicating that lagoons and freshwater marshes created reducing conditions during this time period. As sea level approached its present level 4000 years B.P., tectonically induced differential subsidence of the Yangtze delta plain progressed from north to south. Heavy mineral input to the delta plain reflects the resultant southward migration of the Yangtze river channel and deltaic depocentre to its present position.