SUMMARY

Some interpretations of upper Miocene sediments in the Hanoi depression

Mai Thành Tân, Đinh Văn Thuận, Nguyễn Văn Sơn, Ngô Thị Đào, Nguyễn Văn Tạo, Lê Đức Lương

Upper Miocene sediments in the Hanoi depression are interpreted in terms of depositional characteristics, environments and paleoclimate, based on the analyses of grain-size, thin section, chemistry, X-ray diffraction, foraminifera and palynology from samples of 3 boreholes. Study result had revealed that these Upper Miocene sedimentswere deposited mainly in coastal areas including:marshes, tidal flats, estuaries, neritic shallow waters. They could be divided into lower and upper parts. The lower is composed mainlyof medium-grained sandstones with fine-or-coarse-grained sandstonesof gray and ash-gray colour, siltstones andclaystones in similar color andcoal; weakly to moderatelyconsolidated; formed in environments of agitated shallow water with influence of turbidity currents, estuaries, tidal flats and coastal marshes. The upper is characterized mainly by fine-to-medium-grained sandstones with lesser coarse-grained sandstones in gray and ash-gray interbedded with thin layers of gray, gray-brown and blue-gray siltstones, claystones and coal; weakly consolidated; deposited in environments of aggitated shallow water with influences of turbidite currents or rivers, estuaries, tidal flats and coastal marshes. The paleoclimate in late Miocene washot and humid tropical or subtropical, however, the hot and humid degree had temporally changed.

Keywords: Upper Miocene, depositional environments, paleoclimate, Hanoi depression.