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首页> 外文期刊>Sedimentary geology >Morphology, accumulation and preservation of draa systems in a Precambrian erg (Galho do Miguel Formation, SE Brazil)
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Morphology, accumulation and preservation of draa systems in a Precambrian erg (Galho do Miguel Formation, SE Brazil)

机译：Predambrian ERG中DRAA系统的形态，积累和保存（Galho Do Miguel Clowation，Se Brazil）

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Proterozoic aeolian successions tend to exhibit a low complexity in the architectural organisation, generally related to simple dunes with low morphological diversity. Although most palaeoenvironmental reconstructions of ancient ergs confirm this scenario, the general conditions in the Precambrian landmass should have allowed the construction of larger and more complex sedimentary aeolian systems than those recorded in the Phanerozoic successions. To address this paradigm, the present work proposes to discuss the development and morphodynamics of ancient draa-scale bedforms and to understand which controlling factors produced the apparent absence of draa architecture in the Precambrian sedimentary record. For this purpose, this study examines an interval of c. 80 m of the Galho do Miguel Formation, which is a Mesoproterozoic aeolian succession composed of metasandstones showing well-preserved primary sedimentary structures and geometries with excellent vertical and horizontal exposures. A detailed sedimentological analysis allowed the recognition of six main architectural elements: (i) simple transverse dune, ( ii) simple linear dune; (iii) compound transverse draa; (iv) complex linear draa, (v) dry interdune and (vi) sand sheet. The lateral and vertical relationships between these architectural elements support the evidence of coeval spatial coexistence of distinct aeolian morphologies (transverse and linear bedforms), probably controlled by lateral variations in sand saturation and in circulation pattern of the palaeowind. The stratigraphic organisation indicates different intervals of draa construction and destruction, which appear to be associated with different sedimentary pulses. The intervals of draa construction represent the vertical succession between simple and compound dunes of similar morphologies, representing temporal variation in sedimentary availability and changes in angle of dune climbing. The erosional and irregular surface at the bottom of compound sets of cross-strata can represent periods of irregular accumulation, associated with partial draa destruction (dune cannibalisation), probably due to seasonal variations in sedimentary availability throughout construction stages. Evidence of a near-surface water table level, as small wave ripples, indicates that the position of the water table was directly linked to the sedimentation pattern throughout these intervals. This relationship implies that ground water was the main controlling factor on sand availability and wind saturation during aeolian morphodynamics, as well as on accumulation and preservation processes. Moreover, in conditions of high rates of subsidence, the thickness accumulated and preserved via water table fluctuation is incorporated in the sedimentary record; otherwise, these deposits would have eroded due to aeolian deflation or fluvial reworking. Apparently, the absence of large and complex aeolian systems in the Precambrian record can be related to the absence of continuous near-surface water table during the draa development, directly decreasing their preservation potential. (C) 2020 Elsevier B.V. All rights reserved.
机译：ElarteroZoic Aeolian演替倾向于在建筑组织中表现出低的复杂性，通常与形态多样性低的简单沙丘相关。虽然古代ERGS的大多数古环境重建证实了这一情景，但前锋陆地的一般条件应该允许建造比在竞争中记录的沉积天然系统的建设。为了解决这一范例，本工作建议讨论古老的Draa-Scale床单的开发和形态学性，并了解哪些控制因素在预先发现的德拉明沉积记录中产生了明显的德拉建筑。为此目的，本研究检查了C的间隔。 80米的Galho做了Miguel形成，这是一种由Metasandstones组成的中间型古代Aeolian连续，展示了良好的初级沉积结构和具有优异垂直和水平曝光的几何形状。详细的沉积学分析允许识别六个主要建筑元素：（i）简单的横向沙丘，（ii）简单的线性沙丘; （iii）复合横向DRAA; （iv）复杂的线性DRAA，（v）干燥晶状体和（VI）砂纸。这些建筑元素之间的横向和垂直关系支持不同的Aeolian形态（横向和线性弯曲）的群空间共存的证据，可能是由砂饱和的横向变化和腭瓷的循环模式控制。地层组织表示DRAA建筑和破坏的不同间隔，似乎与不同的沉积脉冲有关。 DRAA结构的间隔代表了类似形态的简单和复合沙丘之间的垂直连续，代表了沉积可用性的时间变化和DUNE攀登角度的变化。复合型跨层组底部的侵蚀性和不规则表面可以代表不规则的积累，与部分DRAA破坏（沙丘钙崩）相关，可能是由于整个施工阶段的沉积可用性的季节变化。近表面水位水平的证据，作为小波浪涟漪，表明水位的位置在整个间隔内直接连接到沉降模式。这种关系意味着地面水是在Aeolian形态学性的砂可用性和风饱和的主要控制因素，以及积累和保存过程。此外，在高沉降速率的条件下，通过水位波动累积和保存的厚度在沉积记录中结合在一起;否则，由于海葵放气或河流重新加工，这些存款将被侵蚀。显然，在Predambrian记录中没有大型和复杂的Aeolian系统可以与在Draa发育过程中缺乏连续近地水位的情况有关，直接降低了它们的保存潜力。 （c）2020 Elsevier B.v.保留所有权利。

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