Upper slope sand deposits: the example of Campos Basin, a latest Pleistocene-Holocene record of the interaction between alongslope and downslope currents
Upper slope sand deposits: the example of Campos Basin, a latest Pleistocene-Holocene record of the interaction between alongslope and downslope currents
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DOI：
10.1144/gsl.sp.1998.129.01.18
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发表时间：
1998
期刊：
影响因子：
--
通讯作者：
A. Viana;J. Faugères
中科院分区：
文献类型：
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作者：
A. Viana;J. Faugères
摘要上斜坡砂沉积物由分选良好的极细至粗砂组成，具有很大的经济价值。根据巴西东南部现代坎波斯盆地边缘的水文、地形和沉积学特征，提出了一个沉积模式。数据包括长期的底流测量、长短岩心、高分辨率地震测线和侧扫声纳记录。结果表明，离岸推移质转移的沙驱动的陆架底流迫使海洋和气象因素相结合，其中的“海底抛光效应”，旋转扫描的中尺度涡的海床。陆坡上沙堆积发育的主要条件是：（1）外凸陆坡形态;（2）陆坡边缘可获得桑迪沉积物;（3）陆坡边缘底流引起的陆坡砂净离架搬运;（4）存在较强的陆坡边界流。向坡面的迁移是连续的、短距离流动的沙通量。后者主要是通过陆架边缘切割而产生的，并导致在上斜坡上形成细长而狭窄的砂瓣。上缘的凸形形态控制着巴西海流（BC）的流动，可以识别出两个主要区域：一个是BC“漏斗区”，其中海流加速，另一个是BC“膨胀区”，其中海流减速。向南流动的巴西海流的改造在上斜坡（水深200至350米之间）形成砾石滞留沉积物、侵蚀冲刷和河道间沙波。下坡时，坡流强度的降低促进了平坦砂席的发育，受巴西逆流向北流动的控制。上第四系垂向相序具有由粗变细的不对称性。上更新统砂泥夹层逐渐覆盖，或与下部全新世粗至细粒干净砂形成尖锐或侵蚀性接触，这些砂逐渐向上部全新世细粒泥砂过渡。
Abstract Upper slope sand deposits constitute large accumulations of well-sorted very fine to coarse sand that are of great economic interest. A depositional model is here proposed based on hydrographic, physiographic and sedimentological characteristics of the modern Campos Basin margin, SE Brazil. Data comprise long-term bottom-current measurements, long and short cores, high-resolution seismic lines and side-scan sonar records. Results indicate that offshelf bed-load transfer of sand is driven by shelf bottom currents forced by combined oceanographic and meteorological factors, among them the ‘sea-floor polishing effect’, the rotatory sweeping of the sea bed by mesoscale eddies. The main conditions for development of upper slope sand accumulations are: (1) convex outer shelf-slope morphology; (2) sandy sediment available at the shelf edge; (3) net offshelf transport of shelf sands induced by shelf-edge bottom currents; (4) the presence of a relatively strong slope boundary current. Transfer to the slope occurs as successive, short-distance-flowing sand fluxes. The latter occur mainly through shelf-edge incisions and lead to elongated and narrow sand lobes on the upper slope. The convex morphology of the upper margin controls the Brazil Current (BC) flow and two main zones can be identified: a BC ‘funnelling zone’ where the current is accelerated, and a BC ‘expansion zone’ where the current has its speed decelerated. Reworking by the southward-flowing Brazil Current develops gravel lag deposits, erosional scours, and interchannel sand waves on the upper slope (between 200 and 350 m water depth). Downslope, the reduction of slope current intensity enhances the development of flat sand sheets, controlled by the northward flow of the Brazil Counter-Current. The upper Quaternary vertical facies succession is there characterized by a coarsening-to fining-up asymmetric sequence. Upper Pleistocene sand-mud intercalations are overlain gradually or with a sharp or erosive contact by lower Holocene coarse to finegrained clean sands that grade to upper Holocene fine-grained muddy sands toward the top.