Isopach and borehole data as indicators of rifted margin evolution in southwestern Africa
Isopach and borehole data as indicators of rifted margin evolution in southwestern Africa
复制标题
等厚线和钻孔数据作为西南非洲裂谷边缘演化的指标
DOI：
10.1016/0264-8172(90)90005-2
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发表时间：
1990
影响因子：
4.2
通讯作者：
M. Summerfield
中科院分区：
文献类型：
--
作者：
D. Rust;M. Summerfield
等厚和钻孔数据被用来记录南部非洲西部边缘裂谷后的沉积物堆积。等厚资料显示，到始新世末，泥沙堆积速率一直在增加，但钻孔资料普遍显示出相反的趋势。然而，对这两类数据的综合分析与裂谷后排水发展的一般模式是一致的，在该模式中，在沿边缘的线性上隆起被突破后，提供泥沙的区域显著扩大，从而形成了主导的水系网络。钻孔资料表明，这种来自次大陆内部(古橙色)的主要沉积出口是在晚白垩世(科尼阿西期)在现今奥兰治河河口附近建立的。在转换为等效岩石体积后，等厚数据可以与一般排水模型相结合，以推断南部非洲在四个近海沉积序列累积期间可能的侵蚀时空模式，从与裂谷有关的沉积开始，一直延伸到现在。这一分析使用一般模型的三种不同形式，以涵盖广泛的排水发展可能性，西缘集水区的平均侵蚀速率从第一次沉积序列(−15 2~∼113 Ma)的41~82m Ma∼1下降到最后一次沉积序列(−37~0 Ma)的约7~9m Ma∼1。裂谷后侵蚀的最大深度为几公里，似乎发生在与边缘接壤的狭窄地带，主要是在其早期开发期间，而一些内陆地区的侵蚀深度可能要小一个数量级。据估计，裂谷后整个流域的平均侵蚀深度约为1.8千米，这一时期的侵蚀速率平均约为12m Ma−1。
Isopach and borehole data are used to document post-rifting sediment accumulation along the western margin of southern Africa. The isopach data reveal an increasing rate of sediment accumulation up to the end of the Eocene, but the borehole data generally show the opposite trend. An integrated analysis of both types of data, however, is consistent with a general model of post-rifting drainage development in which the area supplying sediment expanded significantly after breaching of a linear upwarp along the margin allowed a dominant drainage network to develop. The borehole data suggest that such a major sediment outlet from the interior of the sub-continent (the palaeo-Orange) became established in the Late Cretaceous (Coniacian) in the vicinity of the present Orange River mouth. After conversion to equivalent rock volumes, the isopach data can be combined with the general drainage model to infer the likely spatial and temporal patterns of erosion over southern Africa during the accumulation of four offshore sedimentary sequences, beginning with the onset of rift-related sedimentation and extending up to the present.This analysis, using three variant forms of the general model to encompass a wide range of drainage development possibilities, indicates that the average rate of erosion for catchment areas draining to the western margin has declined from 41 – 82 m Ma−1during the first sedimentary sequence (∼152 to ∼113 Ma) to about 7–9 m Ma−1during the last sequence (∼37 to 0 Ma). Maximum depths of post-rifting erosion of several kilometres appear to have occurred in a narrow zone bordering the margin, primarily during its early development, while depths of erosion in some interior areas have probably been an order of magnitude less. The average depth of post-rifting erosion for the present-day catchment as a whole is estimated to be about 1.8 km, and the erosion rate over this period has averaged about 12 m Ma−1.