HOLOCENE EXTREME FLOODS AND ITS SEDIMENTARY CHARACTERISTIC IN THE UPPER REACHES OF THE YELLOW RIVER
Over the past two decades, analysis of slackwater sediments has become a widely accepted tool in palaeoflood research. This paper reports a period of enhanced palaeoflood activity during Holocene that recorded in a suit of overbank slackwater sediments found on the second terrace of the upper reaches of the Yellow River at the Guanting basin, and the sedimentary characteristic of slackwater deposits. Since each flood units in slackwater sediments occur as couplets, we used a portable Bartington susceptibility meter to discern the number of couplets as the organic rich deposits cause a magnetic susceptibility variation. 14C dating samples collected from archaeological sites relative to palaeofloods determine the timing of the period, and grain size analysis was undertaken to examine the sedimentary characteristic of slackwater deposits. There are abrupt changes in color, grain size, and sedimentary type between the slackwater deposits and the underlaid floodplain sediments. Floodplain sediment is uniform brownish-yellow fine silt-sand, significant different from slakcwater sediments that alternate between red clayed-silt and grayish-black silty-clay with horizontal bedding. The curve of magnetic susceptibility for slackwater sediments has 14 fluctuations. The wave crests correspond to the grayish-black layers due to organic matter, while the wave troughs correspond to the brownish-red layers. 14 fluctuations imply at least 14 extreme floods because the profile only records the floods whose backwaters are capable of overtopping previously existing slackwater sediments. Two samples of wood recovered from two late Qijia refuse pits immediately under slackwater sediments at the Lajia Site, which yielded two radiocarbon ages of 3 678 ±75aB. P. ( cal. ) and 3 875 ±70aB. P. ( cal. ) suggest the start of slackwater deposits. In the upper of the slackwater sediments, an organic matter sample was sampled from the ruins of the Xindian culture buried in the slackwater sediment, Lamafeng Site, including fragments of Xindian potteries and bones in a large amount of ashes. 14C dating of ashes produced an age of 2 775 ± 50aB. P. ( cal. ) . And settlements of the mid-late Xindian sites have been found on top of the slackwater sediment. Radiocarbon dating of a single piece of animal bone in a securely stratified archaeological context at one of these settlement sites provided a radiocarbon date of 2 685 ±50aB. P. (cal.) and provides an upper age limit for the end of slackwater deposits, although it is acceptable that there could be a prolonged hiatus between the date provided by the animal bone and the final deposition of the sediments. Thus we inferred that there was an enhanced extreme flood activities during ca. 3 800 ~ 3 700aB. P. The grain size analysis reveals that the slackwater deposits are almost composed of suspended loads. The average grain diameter is ca. 6φ, the sorting coefficient is less than 2 and the coefficient of skewness is less than zero. Compared with the sedimentary characteristics of slackwater deposits in the middle reaches of the Yellow River, the average grain diameter and the coefficient of skewness are smaller in the upper reaches.
the upper reaches of the Yellow RiverGuanting Basinslackwater depositsHolocene extreme floodsedimentary characteristic
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万方数据
维普
