ABSTRACT Submarine canyons play a major role in sediment transfer to the deep sea. When connected with fluvial systems, they can produce large submarine fans, whose sandy bodies have a high potential as petroleum reservoirs. Although canyon-river connections are common on passive margins during lowstands, they are rare during highstands. So far, the only canyon in the Western South Atlantic to have exhibited continuous river connection throughout the Quaternary, regardless of sea level position, is the Sao Francisco Canyon (SFC) in eastern Brazil. In this study, we used multibeam and 3D seismic records integrated with previously published data to investigate the reasons for the continuous canyon-river connection in the area of the Sao Francisco River. The SFC comprises two compartments: The Upper and the Mid Canyon. The Upper Canyon indents 15 km of the shelf and acts as an erosive feeder trunk to the Sao Francisco Submarine Fan. When it emerges on the continental slope (Mid Canyon), depositional processes dominate; the canyon now begins to meander and the thalweg is bordered by numerous terraces and levees. Five stratigraphic units have accumulated in the region of the canyon head since the beginning of the Oligocene. Initially, sedimentation had a uniform aggradational character (Unit 1), which later became more discontinuous in association with the development of reef build-ups (Unit 2) and of a low-lying area around the canyon head. This depression was partially infilled by siliciclastics (Unit 3) and later incised by a precursor of the SFC (Unit 4). The depression still has a morphological expression in the shelf nowadays as a bathymetric low (BL), within which the Sao Francisco River has built its delta (Unit 5). We hypothesize that the origin and evolution of the SFC is associated with three main factors: (i) breaching by the canyon head of thick carbonates rimming the shelf break; (ii) development of the shelf depression referred above; and (iii) major continental drainage reorganization. Continuous canyon-river connection was ensured by this shelf depression, which helped funnel siliciclastic sediments to the canyon and precluded lateral changes to the lower river trunk. Morphological features, such as axial incision of mass movement deposits is suggestive of some, although very limited, modern canyon activity. This case study contributes to a better understanding of river-basin sediment pathways, particularly on passive margins, which are of great importance for the successful exploration of submarine fan systems.

中文翻译：

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被动边缘的连续峡谷河流连接：以圣弗朗西斯科峡谷（巴西东部）为例

摘要 海底峡谷在沉积物向深海转移方面发挥着重要作用。当与河流系统连接时，它们可以产生大型海底扇，其砂体作为石油储层具有很高的潜力。尽管峡谷-河流连接在低位的被动边缘很常见，但在高位时很少见。迄今为止，南大西洋西部唯一一个在整个第四纪表现出连续河流连接的峡谷，无论海平面位置如何，是巴西东部的圣弗朗西斯科峡谷（SFC）。在这项研究中，我们使用多波束和 3D 地震记录与先前发布的数据相结合，来调查圣弗朗西斯科河地区连续峡谷-河流连接的原因。SFC 包括两个隔间：上峡谷和中峡谷。上峡谷凹进了 15 公里的陆架，并充当了圣弗朗西斯科潜艇扇的侵蚀性馈线干线。当它出现在大陆坡（中峡谷）时，沉积过程占主导地位；峡谷现在开始蜿蜒曲折，thalweg 与众多梯田和堤坝接壤。自渐新世开始以来，峡谷头地区已积累了五个地层单元。最初，沉积具有统一的加积特征（单元 1），后来随着珊瑚礁堆积（单元 2）的发展和峡谷头周围低洼区域的发展而变得更加不连续。该凹陷部分被硅质碎屑填充（第 3 单元），后来被 SFC 的前体（第 4 单元）切割。如今，该洼地在架子上仍具有一种形态学表现，即测深低 (BL)，圣弗朗西斯科河在其中建造了三角洲（第 5 单元）。我们假设 SFC 的起源和演化与三个主要因素有关：(i) 峡谷顶部破坏了大陆架断裂边缘的厚碳酸盐；(ii) 上述陆架洼地的发展；(iii) 主要的大陆排水系统重组。这种陆架洼地确保了峡谷与河流的连续连接，这有助于将硅质碎屑沉积物输送到峡谷中，并防止向下游干流发生横向变化。形态特征，例如大规模运动沉积物的轴向切口，暗示了一些虽然非常有限的现代峡谷活动。本案例研究有助于更好地了解流域沉积物路径，特别是在被动边缘，