Timing and origin of the South Atlantic pre-salt sag basins and their capping evaporites
Timing and origin of the South Atlantic pre-salt sag basins and their capping evaporites
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DOI：
10.1144/sp285.2
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发表时间：
2007-01-01
期刊：
影响因子：
--
通讯作者：
Gamboa, L. A. P.
中科院分区：
文献类型：
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作者：
Karner, G. D.;Gamboa, L. A. P.
西非和巴西之间的大陆伸展直接或间接地导致了盐下凹陷盆地和南大西洋盐盆地蒸发岩的发育。解释这些盆地及其盖顶蒸发岩的沉降机制包括：（1）沉积在巴列姆纪时代的洋壳上； (2) 裂谷从东向西扩展穿过西非边缘，使得东部开始裂谷后沉降，而西部仍在发生裂谷； (3) 与深度相关的岩石圈伸展。预测的洋壳或裂谷岩石圈的热沉降不足以为蒸发岩提供足够的容纳空间。在桑托斯盆地内，盐下凹陷盆地内的伸展断层发生在蒸发岩底部；延伸显然持续到阿普第晚期。横跨西非边缘的陆上和近海盐下剖面的时间相当，如果这些剖面被认为是裂谷后的，则无法产生足够的沉降，因此从东向西的裂谷传播不符合观测到的盐下盆地和蒸发岩的机制。巴列姆期-阿普第期依赖于深度的伸展作用最好地解释了西非和巴西边缘的一般裂谷和裂谷后发育以及同裂谷断层的缺乏，应变平衡是通过下地壳和大陆地幔从邻近大陆岩石圈下横向就位来实现的。顶部盐下凹陷部分的区域暴露和截断证明了阿普第中期期间气候引起的湖泊水位下降，并提供了一种简单的机制来为西非-巴西裂谷系统的蒸发降水产生浅水环境。在随后的海侵中，加蓬和安哥拉的盐类以及蒸发岩沉积在卡马穆-阿尔马达、热基蒂尼奥尼亚和库穆鲁沙蒂巴盆地内。桑托斯盆地和坎波斯盆地的蒸发岩较年轻。南大西洋海洋入侵的障碍以及桑托斯和坎波斯蒸发岩沉积可能的延迟与原始沃尔维斯海岭的岩浆构造以及巴西东南部高地长期存在的异常地形有关；坎波斯和桑托斯盆地的延伸必然叠加在广阔、高地势的高原上。
Continental extension between West Africa and Brazil was responsible, directly or indirectly, for the development of the pre-salt sag basins and the evaporites of the South Atlantic salt basin. Subsidence mechanisms to explain these basins and their capping evaporites include: (1) deposition on Barremian-aged ocean crust; (2) rift propagation from east to west across the West African margin such that post-rift subsidence commenced in the east while rifting was still occurring to the west; and (3) depth-dependent lithospheric extension. Predicted thermal subsidence of oceanic crust or rifted lithosphere is inadequate to generate sufficient accommodation for the evaporites. Within the Santos Basin, extensional faulting within the pre-salt sag basin occurs up to the base of the evaporites; extension clearly continued to the late Aptian. Time-equivalent onshore and offshore pre-salt sections across the West African margin, and the inability to generate sufficient subsidence if the sections are considered to be post-rift, disqualifies east to west rift propagation as a mechanism for the observed pre-salt basins and evaporites. Barremian-Aptian depth-dependent extension best explains the general rift and post-rift development of the West African and Brazilian margins and the paucity of syn-rift faulting, the strain balance being achieved by the lateral emplacement of lower crust and continental mantle out from under the adjacent continental lithosphere. Regional exposure and truncation of the top pre-salt sag section attests to a climate-induced lake level drawdown during the mid Aptian, and offers a simple mechanism to generate the shallow water environments for evaporite precipitation across the West African-Brazilian rift system. In the subsequent marine transgression the Gabon and Angolan salts and the evaporites within the conjugate Camamu-Almada, Jequitinhonha and Cumuruxatiba basins were deposited. Santos and Campos basin evaporites are younger. The barrier to southern Atlantic marine incursions and the possible delay in Santos and Campos evaporite deposition relates to the magmatic constructions of a proto-Walvis Ridge and the long-lived anomalous topography of the southeastern Brazilian highlands; Campos and Santos basin extension was necessarily superimposed on a broad, high-relief plateau.