Eustatic and tectonic change effects in the reversion of the transcontinental Amazon River drainage system

ABSTRACT: The development of the transcontinental Amazon River System involved geological events in the Andes Chain; Vaupés, Purus and Gurupá arches; sedimentary basins of the region and sea level changes. The origin and age of this river have been discussed for decades, and many ideas have been proposed, including those pertaining to it having originated in the Holocene, Pleistocene, Pliocene, Late Miocene, or even earlier times. Under this context, the geology of the sedimentary basins of northern Brazil has been analyzed from the Mesozoic time on, and some clarifications are placed on its stratigraphy. Vaupés Arch, in Colombia, was uplifted together with the Andean Mountains in the Middle Miocene time. In the Cenozoic Era, the Purus Arch has not blocked this drainage system westward to marine basins of Western South America or eastward to the Atlantic Ocean. Also the Gurupá Arch remained high up to the end of Middle Miocene, directing this drainage system westward. With the late subsidence and breaching of the Gurupá Arch and a major fall in sea level, at the beginning of the Late Miocene, the Amazon River quickly opened its pathway to the west, from the Marajó Basin, through deep headward erosion, capturing a vast drainage network from cratonic and Andean areas, which had previously been diverted towards the Caribbean Sea. During this time, the large siliciclastic influx to the Amazon Mouth (Foz do Amazonas) Basin and its fan increased, due to erosion of large tracts of South America, linking the Amazon drainage network to that of the Marajó Basin. This extensive exposure originated the Late Miocene (Tortonian) unconformity, which marks the onset of the transcontinental Amazon River flowing into the Atlantic Ocean.

摘要：跨大陆亚马孙河系的形成演化与安第斯山脉（Andes Chain）的构造活动、瓦乌佩斯拱（Vaupés Arch）、普鲁斯拱（Purus Arch）、古鲁帕拱（Gurupá Arch）、区域沉积盆地（sedimentary basins）以及海平面变化（sea level changes）密切相关。数十年来，学界围绕亚马孙河的起源与形成时代展开了广泛讨论，先后提出了全新世（Holocene）、更新世（Pleistocene）、上新世（Pliocene）、中新世晚期（Late Miocene）乃至更早的多种成因假说。在此研究背景下，本文从中生代（Mesozoic time）时期入手，对巴西北部沉积盆地的地质特征展开分析，并对其地层学（stratigraphy）相关问题进行了厘清。位于哥伦比亚的瓦乌佩斯拱于中新世中期与安第斯山脉一同抬升。新生代（Cenozoic Era）期间，普鲁斯拱并未阻碍该水系向西注入南美西部海盆、向东汇入大西洋的路径。古鲁帕拱直至中新世中期结束时仍保持隆起状态，迫使该水系向西流动。随着中新世晚期伊始古鲁帕拱的晚期沉降与断裂，叠加海平面大幅下降，亚马孙河快速开辟了向西的通道：从马拉若盆地（Marajó Basin）出发，通过强烈的溯源侵蚀，袭夺了原本向加勒比海（Caribbean Sea）汇流的克拉通区域与安第斯区域的庞大水系网络。此时期，由于南美大陆大片区域遭受侵蚀，大量硅质碎屑输入（siliciclastic influx）亚马逊河口（Foz do Amazonas）盆地及其扇体的规模显著增加，亚马孙水系由此与马拉若盆地实现连通。这一广泛的地表暴露作用形成了中新世晚期托尔通期（Tortonian）不整合面（unconformity），该界面标志着跨大陆亚马孙河正式注入大西洋的开端。