Abstract:

Sediment association of the Guanjiagou Formation is characterized by increasingly coarsening upwards sequence.The lower of the Guanjiagou Formation is typical turbidity current deposit, which consists mainly of mudstone and sandstone. Its grain-size changes coarse upwards and the conglomerate stratum becomes thick in the middle part.The upper part is thicker conglomerate with lamella massive sandstone.The content of gravel in conglomerate and granularity increases and augments respectively. By study, the Guanjiagou Formation is marine sediments which were showed by the fossil.Therefore, it is deduced that the Guanjiagou Formation is submarine fan.The geochemistry characteristics of siltstone and slate show that the values of La, Ce, ΣREE, δ（Eu）of the Guanjiagou Formation are similar to those of island arc. By the tilted lay of stratum, the excellent trend of gravel and the change of gravel in orientation ,the current of the Guanjiagou Formation is analyzed synthetically and it is deduced that the direction of paleo-current is about 230° to 356°, and most is west-north; while the gravel excellent trend is 228° to 338°, most north-northwest. From south to north, the size of gravel becomes small gradually, gravel round increasing, and gravel excellent trend is from disorder to order by field statistic.With previous results, the Guanjiagou Formation is in Bikou forearc basin and it is presumed that the sediments were from east-southern island arc.

Key words: the Guanjiagou Formation, marine fan, island arc, forearc basin