Abstract 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.

中文翻译：

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前寒武纪 erg（Galho do Miguel Formation，巴西东南部）中 draa 系统的形态、积累和保存

摘要 元古界风积层序在建筑组织中往往表现出较低的复杂性，通常与形态多样性较低的简单沙丘有关。尽管大多数古代尔格的古环境重建证实了这种情况，但前寒武纪大陆的一般条件应该允许建造比显生宙序列中记录的更大和更复杂的沉积风成系统。为了解决这个范式，目前的工作建议讨论古代 draa 尺度床型的发展和形态动力学，并了解哪些控制因素导致前寒武纪沉积记录中 draa 结构的明显缺失。为此，本研究考察了 c 的区间。Galho do Miguel 组的 80 m，这是一个由变砂岩组成的中元古代风成序列，显示出保存完好的原始沉积结构和几何形状，具有极好的垂直和水平暴露。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) 复合横纹；(iv) 复杂的线性 draa，(v) 干燥的沙丘和 (vi) 砂层。这些建筑元素之间的横向和纵向关系支持了不同风成形态（横向和线性床形）的同时代空间共存的证据，这可能受沙子饱和度和古风循环模式的横向变化控制。地层组织表明 draa 建造和破坏的不同间隔，这似乎与不同的沉积脉冲有关。draa 构造的间隔代表了相似形态的简单沙丘和复合沙丘之间的垂直序列，代表了沉积物可用性的时间变化和沙丘爬坡角度的变化。复合地层底部的侵蚀和不规则表面可以代表不规则堆积的时期，与部分 draa 破坏（沙丘自相残杀）有关，这可能是由于整个施工阶段沉积物可用性的季节性变化。近地表地下水位水平的证据，如小波涟漪，表明地下水位的位置与整个这些间隔的沉积模式直接相关。这种关系意味着地下水是风成形态动力学期间沙子可用性和风饱和度以及积累和保存过程的主要控制因素。此外，在高沉降条件下，通过地下水位波动积累和保存的厚度被纳入沉积记录；否则，这些沉积物会因风蚀或河流改造而被侵蚀。显然，前寒武纪记录中没有大型复杂的风成系统可能与 draa 发育过程中没有连续的近地表地下水位有关，直接降低了它们的保存潜力。在高沉降率条件下，通过地下水位波动积累和保存的厚度被纳入沉积记录；否则，这些沉积物会因风蚀或河流改造而被侵蚀。显然，前寒武纪记录中没有大型复杂的风成系统可能与 draa 发育过程中没有连续的近地表地下水位有关，直接降低了它们的保存潜力。在高沉降率条件下，通过地下水位波动积累和保存的厚度被纳入沉积记录；否则，这些沉积物会因风蚀或河流改造而被侵蚀。显然，前寒武纪记录中没有大型复杂的风成系统可能与 draa 发育过程中没有连续的近地表地下水位有关，直接降低了它们的保存潜力。