Seismo-stratigraphy and numerical basin modeling of the southern Brazilian continental margin
Seismo-stratigraphy and numerical basin modeling of the southern Brazilian continental margin
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巴西南部大陆边缘的地震地层学和盆地数值模拟
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发表时间：
2011
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通讯作者：
J. Contreras
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作者：
J. Contreras
在巴西南部从裂谷边缘向“被动”边缘演化的过程中，近海坎波斯盆地(CB)、桑托斯盆地(SB)和佩洛塔斯盆地(PB)发展了截然不同的构造，无论它们的同裂成因如何。在二维地震反射剖面和井的基础上，对这三个盆地进行了二维地震地层学和盆地数值分析。在巴雷棉-全新世盆地填充区(130-0 Ma)内，利用现有井的生物/年代地层年龄绘制了12至14个区域地震层位。它们定义了地震地层沉积序列，根据容纳/沉积和边界地层面对其进行了分类。随后，应用反盆地模拟对可容纳空间的演化和控制因素(隆升、沉陷、泥沙供应)进行了定性解释。这一程序允许在每一年代地层中恢复盆地几何形状和古水深度。此外，对总下沉的成因成分--热构造、挠曲和压实引起的下沉--的综合分析提供了每个下沉成分在时间上的相互作用和对盆地形成的单独贡献。获得的沉陷数据和泥沙供给史与地震地层学相结合，代表了正演地层建模的输入信息。该方法旨在通过一系列模拟相互依赖的构造和沉积过程的模拟变量来模拟地层演化。模拟参数的敏感性分析揭示了盆地发育的最重要控制因素，并为建立与现今盆地形态相适应的构造-地层模型提供了最佳适配值。这种综合方法使我们能够从层序地层解释中评估固有的模型不确定性，并评估各种可能的地质情景的可靠性。结果表明，该区存在6个沉降/隆升趋势(ST1-ST6)，每个趋势为7~51m.y。持续时间，控制了巴雷米期至全新世盆地的发育。这些趋势及其沿陆架-盆地过渡的分布与巴西东南部边缘同裂带-漂移带的地球动力学演化直接相关。同裂谷伸展引发了较高的热构造沉降(趋势ST1)，并控制了巴雷米期三个盆地可容纳空间的形成。然而，结果显示了单一盆地独特的地壳演化：巴雷米亚同裂谷大陆延伸横跨佩洛塔斯盆地，包括广泛的岩浆活动和自阿普提亚早期以来受裂谷后热收缩影响的厚火山壳(趋势ST2)。随着大陆裂解从PB向北扩展到CB，岩石圈变形和沉降模式在盆地之间发生了变化。在Sb和CB中，巴雷米世早、中期的少量岩浆活动和脆性变形在巴雷米世晚期的同裂谷早期演化为随深度变化的大陆伸展作用(趋势ST2)。变薄的大陆地壳以横向恒定的下沉速率为特征，在桑托斯-坎波斯边缘段产生了广泛的凹陷-盐岩沉积中心。在阿尔卑斯后裂谷和新生代-全新世漂移演化(ST3-ST6)期间，沉积物供给和挠曲载荷的变化是由于陆上构造-岩浆事件造成的。这些关键因素是导致所分析的三个盆地在结构和油气远景上存在差异的原因。根据模拟过程中建立的过程/响应关系，陆架-盆地沉积系统最重要的地层-构造过程包括：(1)源区的构造抬升引发进积和深水浊积流增加沉积物供应；(2)陆架区域的挠曲诱导反弹有利于沉积物绕行和灾难性的陆架斜坡破坏；(3)盐盆和盐的再动员，增强了陆架边缘的不稳定性，改变了弯曲载荷分布；(4)底流重新分布深海沉积物填充。对巴西大陆边缘的这项研究表明：(I)大陆边缘的沉降和沉积物供应的变化在三到二级时间间隔内变化，不受海平面变化的影响；(Ii)挠曲和压实引起的沉降是成熟漂移阶段对可容纳空间的最重要控制；(Iii)不能从大陆边缘的定性层序地层格架直接推断构造和板块构造重构；(Iv)沉积体系受控于一系列相互关联的构造和沉积参数，这些参数超出了层序地层学的定性观察。大陆边缘演化的定量模型必须考虑广泛的变量。地壳变形和大陆边缘地貌演变以及局部海平面波动与容纳空间和岩性同样重要。这一方法提供了对古代构造-沉积系统的更准确的解释，这对于更好地理解大陆边缘沉积盆地的复杂地球动力学和构造-地层演化具有关键贡献。
During the evolution of southern Brazil from rift to “passive” margin, the offshore Campos (CB), Santos (SB), and Pelotas (PB) basins developed highly differing architectures regardless of their associated syn-rift origin. Based on 2D seismic reflection profiles and wells, the three basins have been investigated in terms of 2D seismo-stratigraphy and numerical basin analysis. Within the Barremian-Holocene basin infill (130-0 Ma), twelve to fourteen regional seismic horizons have been mapped and calibrated with bio-/chronostratigraphic ages from the available wells. They define seismo-stratigraphic depositional sequences, which have been classified in terms of accommodation/sedimentation and bounding stratigraphic surfaces. Qualitative interpretations of the evolution of accommodation space and the controlling factors (eustasy, subsidence, sediment supply) have been subsequently quantified by applying inverse-basin modeling. This procedure allowed the restoration of the basin geometry and paleowater depths during each of the chronostratigraphic layers. Furthermore, a comprehensive analysis of the genetic components of total subsidence – thermo-tectonic, flexural, and compaction-induced subsidence – provided the temporal interactions and individual contribution of each subsidence component to basin formation. The obtained subsidence data and sediment supply history, combined with seismo-stratigraphy, represent the input information for forward stratigraphic modeling. This method aims to simulate the stratigraphic evolution in terms of a wide range of modeling variables that simulate interdependent tectonic and sedimentary processes. Sensitivity analysis of modeling parameters revealed the most important controls on the basin development, and provided the best-fit values to construct plausible tectono-stratigraphic models to the present-day basin configuration. This integrated approach allows us to assess the inherent model uncertainties from the sequence-stratigraphic interpretation, and evaluate the reliability of various possible geological scenarios. The results show that six subsidence/uplift trends (ST1-ST6), each of 7 to 51 m.y. duration, controlled the Barremian to Holocene basin development. These trends and their distribution along the shelf-to-basin transition are directly linked to the syn-rift-to-drift geodynamic evolution of the southeastern Brazilian margin. Syn-rift extension triggered high thermo-tectonic subsidence (trend ST1) and controlled the creation of accommodation space in all three basins during the Barremian. However, the results show a distinctive crustal evolution of the single basins; Barremian syn-rift continental extension across the Pelotas Basin involved extensive magmatism and a thick volcanic crust affected by post-rift thermal contraction since Early Aptian times (trend ST2). As continental break-up propagated northward from the PB to the CB, the mechanisms of lithospheric deformation and subsidence patterns changed between the basins. In the SB and CB, minor magmatism and brittle deformation during the Early-Middle Barremian evolved to depth-dependent continental stretching during the incipient syn-rift phase in the Late Barremian (trend ST2). The thinned continental crust featured laterally constant subsidence rates, generating wide sag-salt depocenters across the Santos-Campos margin segment. During the Albian post-rift and Cenomanian-Holocene drift evolution (ST3-ST6) changes in the sediment supply and the flexural loading were due to onshore tectono-magmatic events. These key factors were responsible for the differences in architecture and hydrocarbon prospectivity between the three basins analyzed. Based on the process/response relationship established during modeling, the most important stratigraphic-structural processes on the shelf-to-basin sedimentary systems include: (i) tectonic uplift of the source areas triggering increased sediment supply with progradation and deep-water turbidite flows; (ii) flexurally-induced rebound of the shelf realm facilitates sediment bypass and catastrophic shelf-slope failure; (iii) formation of salt basins and salt remobilization, enhancing the instability of the shelf-edge and modifying the flexural loading distribution; (iv) bottom-currents redistributing the deep-marine sediment infill. This study on the Brazilian continental margin shows that: (i) changes in subsidence and sediment supply on the continental margin varied over 3rd to 2nd order time intervals, and were not subordinate to eustatic sea-level changes; (ii) flexural and compaction-induced subsidence represent the most important controls on accommodation space during the mature drift stage; (iii) structural and plate-tectonic reconfigurations cannot be directly inferred from qualitative sequence stratigraphic frameworks of continental margins; (iv) depositional systems are controlled by a wide range of interconnected tectonic and sedimentary parameters that go beyond qualitative observations from sequence stratigraphy. Quantitative modeling of the evolution of continental margins must consider a wide range of variables. Crustal deformation and the geomorphological evolution of the continental margin, as well as local-scale sea-level fluctuations are as important as accommodation space and lithologies. This approach provides more accurate interpretation of the ancient tectono-sedimentary systems, which represent key contributions to a better understanding of the complex geodynamic and tectono-stratigraphic evolution of continental margin sedimentary basins.