摘要:
对于具有较宽陆架的下刚果盆地,当相对海平面下降时期,陆架区沉积中心向外陆架迁移,形成陆架边缘三角洲。其为深水区提供大量陆源碎屑物质,使低位体系域发育碎屑流沉积、浊流水道及前端扇体系;当相对海平面上升时期,沉积中心后退至内/中陆架,使深水区海侵+高位体系域以深海、半深海原地泥质沉积及泥质碎屑流沉积为主。深水层序以凝缩层段为界,层序界面之上为泥质碎屑流沉积,或者为浊流沉积;界面之下以深海、半深海原地沉降泥质沉积为主。测井曲线界面多为岩性突变面,GR曲线由"微齿状"转变为退积的"钟形"或加积的"箱形"。根据上述沉积旋回特征,将下刚果盆地中新统地层划分了7个三级层序、2个二级层序。海平面的周期性变化决定陆架区沉积中心的位置,从而决定了对深水区的沉积物供给,同时沉积物供给量主要受构造和气候的控制,潮湿的气候(冰室气候)及构造抬升有利于陆上沉积物的剥蚀,沉积物供给相对增加,从而决定了深水层序发育的类型和分布特征。
Abstract:
The Lower Congo Basin is a basin with a wide continental shelf. In such a basin, when the relative sea-level falls, depocenters on the inner shelf will shift to the outer to form shelf-margin deltas. It will result in the acceleration of sediment supply to the deep-water area, and then lowstand systems tract, such as debris-flow deposits, turbidites and frontal fan deposits will develop. By contrast, when the relative sea-level rises, depocenters will shift towards the middle and inner shelf, then transgressive systems tract and highstand systems tract characterized by hemipelagic and pelagic sediments and muddy debris-flow deposits will form. The deepwater sequence is bounded by condensed layers. There are muddy debris-flow deposits or turbidites above, and hemipelagic and pelagic sediments below the sequence boundary. The sequence boundaries in the well logs usually are lithological discontinuities. In GR logs,they turn from "micro-tooth-like" to retrogradated "bell-like" or aggradated "box-like" in succession. According to the depositional cycles mentioned above, we divided the Miocene in the basin into seven third-order sequences and two second-order sequences. The cyclic change in sea level decided the location of the depocenter as well as the sediment supply to the deep water area, in addition to the tectonic and climatic influences. Humid climate(ice house climate) plus tectonic uplifting will accelerate the erosion of land and increase the sediment supply and therefore influence the development of deep water sequences and their distribution.
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