华南古大陆边缘湘北九溪下奥陶统碳酸盐等深岩丘

A LOWER ORDOVICIAN CARBONATE CONTOURITE DRIFT ON THE MARGIN OF THE SOUTH CHINA PALAEOCONTINENT IN JIUXI, NORTHERN HUNAN

湘北桃源九溪碳酸盐等深岩丘发育于早奥陶世华南古大陆边缘湘北深水区。该区早奥世层系厚逾350m;其中极为发育的等深岩可分为灰泥等深岩、砂屑等深岩、粉屑等深岩、细砾屑等深岩和生物屑等深岩等5种。前3类等深岩常构成完整或不完整等深岩层序,砂屑等深岩作为夹层,近于均匀地分布于整个层系中,使等深岩丘显示单调的韵律性剖面结构。该层系空间上的展布型式表明该等深岩丘是沿古斜坡走向延伸的巨大脊状沉积体。众多的流向标志指示了沿斜坡自西而东的古流向。

The lower part of the Lower Ordovician in Jiuxi, northern Hunan, is represented by a succession of deep-water carbonate sediments deposited on the margin of the South China palaeocontinent. In this succession, welldeveloped contourites may be classified into 1) calcilutitic contourite, 2) calcarenitic contourite, 3) calcisihitic contourite, 4) fine calcisiltitic contourite, and 5) bioclastic contourite. Calcilutitic contourite is characterized by calcilutite with frequent calcisiltite stripes, in which sometimes large-scale mud-wave stratification and longitudinal crossstratification formed by lateral shift of a submarine furrow. Calcarenitic contourite is characterized by development of irregular horizontal stratification, abundant erosive surfaces, sparitecemented mottles, stripes or even thin layers rich in grains and a common geopetal structure of trilobite fragments. Calcisihitic contourite is a transitional type between the two above-mentioned types, in which ripple bedding is sometimes recognized. Fine calciruditic contourite is composed of calcilutite clasts and matrix of carbonate fine sands and silts. Bioclastic contourite occurs as centimeter-sized lenses arranged in ranks, consisting mainly of trilobite fragments. The first three types show strong bioturbation, yielding the trace fossil assemblage of Glockeria and 'Scribbling grazing trace'. Usually from below upwards, calcilutitic contourite, calcisiltitic contourite, calcarenitic contourite, calcisiltitic contourite and calcilutitic contourite display a reverse- and normal-grading symmetrial arrangement, forming a contourite succession 10 to 200 cm thick. The succession of the Jiuxi carbonate contourite drift, over 350 m thick, is composed mainly of various types of contourite mentioned above, with part of autochthonous deposits and minor gravity flow deposits. Structural feature of the section of this succession is that oearser calcarenitic contourite beds are separated by thicker, fine calcsihitic and calcilutitic contourites, and especially they are almost uniformly distributed in the whole succession at only 3—4m intervals. The distribution of the succession indicates that the Jiuxi contourite drift is a huge ridge-shaped sedimentary body extending parallelly to the trend of the palaeoslope. Ripples, mud waves, longitudinal cross stratification and oriented fabrics of fragments have generally revealed the eastward palaeoflow direction along the slope.