Fluvial floodplains prior to greening of the continents: Stratigraphic record, geodynamic setting, and modern analogues

Fluvial floodplains established prior to the greening of continents have long been overlooked, despite their relevance for landscape reconstructions in deep time. The record of fluvial overbank sedimentation dates back as far as the Mesoarchean, and mature assemblages of floodplain landforms had already developed at least by the early Palaeoproterozoic. In this review, a critical assessment of pre-vegetation floodplain processes and deposits is carried out through literature compilation and detailed descriptions of case studies. Pre-vegetation floodplains were variably composed of floodbasins, splay complexes (including crevasse- and distributary-channel fills and related splay lobes) and, in minor proportion, by channel levees. The hydrology of ancient floodbasin environments, mainly inferred from the occurrence or lack of evaporite features, is particularly topical and, once critically tested against other palaeo-environmental indicators, can be related to climate or catchment physiography.Pre-Silurian floodplains preferentially developed in rift basins prone to restricted drainage, where low-gradient axial depressions experienced limited stream power and accumulation of cohesive fines. Since supercontinents prone to host mature intracratonic basins first appeared in the Palaeoproterozoic, a causal relationship is established between the rise of modern-style plate tectonics and fluvial floodplains. By comparison, pre-vegetation overbank records are sparse in foreland, syn-orogenic, or passive-margin basins, where higher gradients and ocean-ward bypass of cohesive fines would have enhanced reworking by adjacent channels. These features are analogue to modern non-vegetated floodplains, with examples drawn from both arid endorheic drainages (Death Valley, California: Salar de Uyuni, Bolivia), and humid exorheic drainages (coastal plains of southern Iceland). Future developments in pre-Silurian sedimentology will help addressing lingering questions related to, for instance, the biogeomorphology of microbial mats and the morphogenetic variability between floodplains developed in different latitudinal climate belts. (C) 2018 Elsevier B.V. All rights reserved.

在大陆变绿之前形成的河流泛滥平原长期以来一直被忽视，尽管它们与远古时期的地貌重建相关。河流漫滩沉积的记录可追溯到中太古代，而泛滥平原地貌的成熟组合至少在古元古代早期就已经形成。在这篇综述中，通过文献汇编和案例研究的详细描述，对植被出现前的泛滥平原过程和沉积物进行了批判性评估。植被出现前的泛滥平原由不同的部分组成，包括洪泛盆地、决口扇复合体（包括决口河道和分流河道充填物以及相关的决口扇叶状体），以及少量的河道天然堤。古代洪泛盆地环境的水文情况（主要从蒸发岩特征的有无推断得出）是特别热门的话题，一旦与其他古环境指标进行严格对比验证，就可以与气候或流域地形相关联。志留纪之前的泛滥平原优先在容易出现排水受限的裂谷盆地中发育，在那里低坡度的轴向洼地水流能量有限，粘性细粒物质得以堆积。由于容易形成成熟克拉通内盆地的超级大陆首次出现在古元古代，因此在现代板块构造运动的兴起和河流泛滥平原之间建立了因果关系。相比之下，植被出现前的漫滩记录在前陆盆地、同造山盆地或被动大陆边缘盆地中较为稀少，在这些地方较高的坡度以及粘性细粒物质向海洋的绕流会增强相邻河道的改造作用。这些特征类似于现代无植被的泛滥平原，例如来自干旱的内流流域（美国加利福尼亚州的死亡谷；玻利维亚的乌尤尼盐沼）和湿润的外流流域（冰岛南部的沿海平原）的例子。志留纪之前沉积学的未来发展将有助于解决一些遗留问题，例如微生物席的生物地貌学以及在不同纬度气候带发育的泛滥平原之间的地貌成因差异等问题。（C）2018爱思唯尔公司。保留所有权利。