HEAVY MINERALS AND RIVER CHANNEL MIGRATION IN THE YANGTZE DELTA PLAIN, EASTERN CHINA

A twenty meter long core collected from the southern Yangtze delta plain reveals variations in the distribution of heavy minerals from the late Pleistocene through the Holocene stratigraphic horizons. The most abundant heavy minerals in the core are: amphibole, epidote, iron-titanium oxides, metamorphic minerals (garnet, sillimanite, kyanite, staurolite, chloritoid), pyroxene, zircon, tourmaline, rutile, and apatite. Other heavy minerals in lesser abundance are sphene, monazite, allanite, and siderite. Also present in heavy mineral separates are rock fragments, authigenic framboidal pyrite, and pyritized foraminifera.

The lower section of the core (>=14.9 m) spans the late Pleistocene to early Holocene transition. Heavy mineral assemblages from this portion of the core contain amphibole, epidote, iron-titanium oxides, metamorphic minerals, zircon, tourmaline, rutile, apatite (ZTRA), and rock fragments. Heavy mineral separates from the middle layers of the core (middle to late Holocene; 14.9 to 2.7 m) contain mostly framboidal pyrite and pyritized foraminfera though the zone from 10.5 to 14.9 m contains some amphibole, epidote, iron-titanium oxides, metamorphic minerals, ZTRA, and rock fragments. The upper portion of the core (late Holocene; <=2.7 m) contains the same heavy minerals as the lowest layer but also contains noticeable amounts of pyroxene. These differences in mineral assemblages 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.

Late Pleistocene to early Holocene fluvial sediments are likely derived from proximal sources west of the southern delta plain and bedrock of the southern delta plain itself, as river channels originating in the highlands flowed eastward. Late Holocene deposits include these sediments as well but also those sediments transported by a complex of fluvial channels that migrated southward across the delta plain and originated from distal source areas similar to the modern Yangtze drainage basin.

Our investigation provides heavy mineral evidence to support previous studies showing that differential subsidence of the Yangtze delta plain has progressed from north to south and has resulted in the southward shift of the Yangtze river channel and deltaic depocenter.