南美被动大陆边缘A地区上白垩统马斯特里赫特阶周期性阶坎特征及主控因素

Characteristics and main controlling factors of Upper Cretaceous Maastrichtian cyclic steps in A region of the passive continental margin of South America

周期性阶坎是近年深水沉积研究的重要内容之一。利用测井及地震资料对南美A地区上白垩统马斯特里赫特阶周期性阶坎的形态、展布及垂向演化等开展了研究。研究结果表明:a)研究区周期性阶坎外形为波状,不对称,地震反射特征为连续性好、中强反射。整体顺水道向北延伸,规模逐渐扩大,主要发育在海底扇水道朵叶过渡带。b)周期性阶坎可进一步分为早、晚两期。早期发育在马斯特里赫特阶低位体系域,典型周期性阶坎(①~⑦号)波长6.75~15.67 km,平均波高85.38~182.90 m;晚期分布在海侵及高位体系域中,典型周期性阶坎(①~⑦号)波长5.28~11.34 km,平均波高为83.67~127.04 m。两期波形皆不对称,顺水道分布,垂直于水道延伸方向,向北展布,且具有继承性发育特征,但早期周期性阶坎规模大于晚期。c)超临界流产生的水跃和坡度的变化是周期性阶坎形成的主控因素,相对海平面升降和重力流能量变化进一步控制两期周期性阶坎的形态及规模等。对周期性阶坎的研究不仅可以进一步深化对深水重力流沉积过程的认识,又可为深水重力流砂体分布预测提供依据,进而为油气勘探服务。

Cyclic steps are one of the important contents of deep-water sediment research in recent years.The shape,distribution and vertical evolution of the Upper Cretaceous Maastrichtian cyclic steps in A region of South America were studied by using logging and seismic data.The results show that:a)the shape of cyclic steps in the study area is wavy and asymmetric,and the seismic reflection is characterized by good continuity and medium strong reflection.As a whole,they extend northward along the channel,and gradually expand in scale,mainly developed in the submarine fan channel lobe transition zone.b)The cyclic steps can be further divided into early and late stages.The early cyclic steps are developed in the Maastrichtian lowstand system tract,with typical cyclic steps(①~⑦)having a wavelength of 6.75~15.67 km and an average wave height of 85.38~182.90 m;The late cyclic steps are distributed in transgressive and highstand system tracts,with typical cyclic steps(①~⑦)having a wavelength of 5.28~11.34 km and an average wave height of 83.67~127.04 m.The waveforms of the two stages of cyclic steps are asymmetric,distributed along the channel,perpendicular to the extension direction of the channel,distributed northward,and have the inherited development characteristics,but the scale of the early cyclic steps is larger than that of the late stage.c)The hydraulic jump generated by supercritical gravity flow and the change of slope are the main controlling factors for the formation of cyclic steps,while the relative sea level rise and fall and the changes of gravity flow energy further control the shape and scale of the two stages of cyclic steps.The study of cyclic steps can not only further deepen the understanding of deep-water gravity flow sedimentation process,but also provide a basis for the prediction of deep-water gravity flow sand body distribution,thus serving for oil and gas exploration.