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Palaeogeography and tectono‐stratigraphic evolution of the Aptian Ezanga‐Loémé evaporites along the proximal domain of the south Gabon‐Congo‐Cabinda margin
Basin Research ( IF 2.8 ) Pub Date : 2024-08-13 , DOI: 10.1111/bre.12893 Alexandre Pichat 1, 2 , Vincent Delhaye‐Prat 1 , Michel Guiraud 3 , Laurent Gindre‐Chanu 4 , Eric C. Gaucher 5
Basin Research ( IF 2.8 ) Pub Date : 2024-08-13 , DOI: 10.1111/bre.12893 Alexandre Pichat 1, 2 , Vincent Delhaye‐Prat 1 , Michel Guiraud 3 , Laurent Gindre‐Chanu 4 , Eric C. Gaucher 5
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During the Early Cretaceous, massive evaporite accumulations formed in the opening South Atlantic. However, the depositional model of these salts is still poorly constrained at the scale of the West African margin. The present study focuses along the proximal domain of the south Gabon‐Congo‐Cabinda margin and is based on (i) log interpretations of 246 wells crossing undeformed to weakly deformed evaporite intervals and (ii) a structural characterization of the basement. The evaporites show 11 regional evaporite depositional cycles (CI–CXI) bounded by meter‐thick shale beds. The cycles display alternating meter‐scale carnallite‐halite beds that can be correlated over several hundred kilometres, and CVI, CVII, CVIIIa and CX culminate in localized tachyhydrite accumulations. Cross section correlations and isopach maps help to understand the palaeogeographical evolution of each cycle and depositional environments that evolved from relatively deep at the base of cycles, to very shallow at their top. CI formed a mosaic of halite‐prone depocenters deposited in pre‐salt topographic relief. CII and CIII were deposited uniformly over a flattened basin in a highly extended brine pond. From CIV to CVIIIa, the stratigraphic architecture of the salts was shaped by freshwater inflow sourced from the north and possible basement movements. This setting, together with an increased confinement of the proximal domain from the distal one with basin drawdown, favoured the development of depocenters with perennial subaqueous conditions and extreme salinities, in which more than 70 m of tachyhydrite accumulation could locally be preserved. From CVIIIb to CXI, the basin returned to a flat depositional setting without well‐developed depocenters and with increasing subsidence westwards. Marine influx increased starting from CX, allowing the deposition of sulphate beds. The salt section is capped by anhydrite deposits interbedded with clastic and dolomite, before the final marine invasion of the basin. For the first time, this study provides a large‐scale depositional tectonostratigraphic setting of the Aptian salts in the proximal domain of the West African margin. The results are of interest for K‐Mg salts exploration resources in the Aptian and pave the way for further investigation of the salt depositional environment in the distal domain of the margin.
中文翻译：
加蓬-刚果-卡宾达南部边缘近端地区阿普第安埃桑加-洛埃姆蒸发岩的古地理和构造地层演化
在早白垩世期间，在南大西洋开口处形成了大量的蒸发岩堆积。然而，这些盐类的沉积模式在西非边缘的尺度上仍然很难受到约束。本研究重点关注加蓬-刚果-卡宾达南部边缘的近端区域，基于 (i) 对穿越未变形至弱变形蒸发岩层段的 246 口井的测井解释和 (ii) 基底的结构特征。蒸发岩显示 11 个以米厚页岩层为边界的区域蒸发岩沉积旋回 (CI-CXI)。这些循环显示了交替的米级光卤石-盐石床，可以在数百公里范围内相互关联，并且 CVI、CVII、CVIIIa 和 CX 在局部速水石堆积中达到顶峰。横截面相关性和等厚图有助于了解每个旋回的古地理演化和从旋回底部相对较深到顶部非常浅的沉积环境。 CI 形成了沉积在盐下地形起伏中的易石盐沉积中心的马赛克。 CII 和 CIII 均匀沉积在高度延伸的盐水池中的平坦盆地上。从 CIV 到 CVIIIa，盐的地层结构是由来自北部的淡水流入和可能的基底运动塑造的。这种环境，加上盆地下降对近端区域的限制增加，有利于具有常年水下条件和极端盐度的沉积中心的发育，其中可以局部保存超过 70 m 的速水岩堆积。从CVIIIb到CXI，盆地恢复到平坦的沉积环境，没有发育良好的沉积中心，并且向西沉降不断增加。 从 CX 开始，海洋流入量增加，导致硫酸盐床沉积。在海洋最终入侵盆地之前，盐区被硬石膏沉积物覆盖，其中夹有碎屑和白云石。这项研究首次提供了西非边缘近端区域阿普第盐的大规模沉积构造地层背景。研究结果对阿普第阶钾镁盐资源勘探具有重要意义，为进一步研究边缘远端区域盐沉积环境奠定了基础。
更新日期：2024-08-13
中文翻译：
加蓬-刚果-卡宾达南部边缘近端地区阿普第安埃桑加-洛埃姆蒸发岩的古地理和构造地层演化
在早白垩世期间，在南大西洋开口处形成了大量的蒸发岩堆积。然而，这些盐类的沉积模式在西非边缘的尺度上仍然很难受到约束。本研究重点关注加蓬-刚果-卡宾达南部边缘的近端区域，基于 (i) 对穿越未变形至弱变形蒸发岩层段的 246 口井的测井解释和 (ii) 基底的结构特征。蒸发岩显示 11 个以米厚页岩层为边界的区域蒸发岩沉积旋回 (CI-CXI)。这些循环显示了交替的米级光卤石-盐石床，可以在数百公里范围内相互关联，并且 CVI、CVII、CVIIIa 和 CX 在局部速水石堆积中达到顶峰。横截面相关性和等厚图有助于了解每个旋回的古地理演化和从旋回底部相对较深到顶部非常浅的沉积环境。 CI 形成了沉积在盐下地形起伏中的易石盐沉积中心的马赛克。 CII 和 CIII 均匀沉积在高度延伸的盐水池中的平坦盆地上。从 CIV 到 CVIIIa，盐的地层结构是由来自北部的淡水流入和可能的基底运动塑造的。这种环境，加上盆地下降对近端区域的限制增加，有利于具有常年水下条件和极端盐度的沉积中心的发育，其中可以局部保存超过 70 m 的速水岩堆积。从CVIIIb到CXI，盆地恢复到平坦的沉积环境，没有发育良好的沉积中心，并且向西沉降不断增加。 从 CX 开始，海洋流入量增加，导致硫酸盐床沉积。在海洋最终入侵盆地之前，盐区被硬石膏沉积物覆盖，其中夹有碎屑和白云石。这项研究首次提供了西非边缘近端区域阿普第盐的大规模沉积构造地层背景。研究结果对阿普第阶钾镁盐资源勘探具有重要意义，为进一步研究边缘远端区域盐沉积环境奠定了基础。
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