Abstract The Sete Lagoas Formation (Ediacaran), located in the central part of the Sao Francisco Craton (Brazil), consists of limestones and dolostones deposited in very shallow waters in the inner part of the carbonate platform. Four breccia types occur throughout the stratigraphic succession: evaporitic breccia with tepees, flat-pebble breccia, hydrothermal breccia and brecciated stromatolites. Here we combine a detailed sedimentological and stratigraphic analyses of the flat-pebble breccia in order to determine its origin and the processes and environmental conditions that originated these brecciated facies. The studied interval consists of a 20 m thick succession of tabular beds composed of flat-pebble breccia interbedded with laminated microbialites. In these breccia beds, the clasts are usually platy or oblate with angular edges and are mainly disposed horizontally within the sedimentary bed, suggesting that they were not transported or reworked. The presence of microbialite clasts with sharp edges and vertices in the Sete Lagoas flat-pebble breccia suggests that the lithification process started very early in diagenesis and, even the sediments exposed at the bottom were, at least, partially lithified. Some breccia levels show bidirectional imbrication and clast size analyses reveal a NE-SW long-axis clast orientation whereas square clasts tend to fill the space among oriented clasts. Breccia clasts are vertically oriented and show deformation features increasing upwards, typically of deforming beds formed by ascendant expulsion of liquefied sediment. Disrupted layers or presenting folds and synsedimentary faults commonly occur confined between undeformed beds. Other evidences of liquefaction and soft-sediment deformation are the injection structures, as flame and load cast-like features, in the base of the brecciated beds. These structures commonly penetrate the upper bed and disrupt the sedimentary layer immediately above promoting local brecciation. These features are found both in modern and ancient deposits of seismic influence, which suggests a similar origin for the Sete Lagoas flat-pebble breccia. Thus, the processes that led to the formation of the studied flat-pebble breccia are interpreted as seismically triggered, since: a) the breccia beds are laterally continuous and extend for several kilometers; b) the breccia beds are restricted to a 20 m thick stratigraphic interval; c) the clasts of the breccia are the same lithology of non-deformed beds below and above the interval of breccia; d) the interbedding of breccia beds and laminated microbialite beds is recurrent; e) the breccia beds are subhorizontal and present irregular upper and lower contacts; f) the presence of liquefaction structures and dyke injection. Thus, this seismic-triggered breccia deposits represent the product of the synsedimentary tectonism occurred within the Sao Francisco Craton during the terminal Ediacaran and correspond to a very well-defined local stratigraphic marker in the Bambui Basin. The seismic activities could be related to the NW regional faults in the regional Paleoproterozoic basement of the study area, which were reactivated during the deposition of the Sete Lagoas Formation in the Ediacaran Period.

中文翻译：

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Sete Lagoas 组（巴西班布伊组）的神秘扁平卵石角砾岩：埃迪卡拉纪碳酸盐台地地震诱发变形的证据

摘要 Sete Lagoas组（埃迪卡拉纪）位于圣弗朗西斯科克拉通（巴西）中部，由沉积在碳酸盐台地内部极浅水域的石灰岩和白云岩组成。在整个地层序列中出现四种角砾岩类型：带有圆锥形圆锥体的蒸发角砾岩、扁平卵石角砾岩、热液角砾岩和角砾状叠层石。在这里，我们结合了对扁平卵石角砾岩的详细沉积学和地层分析，以确定其起源以及形成这些角砾岩相的过程和环境条件。所研究的层段由一系列 20 m 厚的板状层组成，这些层状层由与层状微生物岩互层的扁平卵石角砾岩组成。在这些角砾岩层中，碎屑通常呈片状或扁圆状，边缘有棱角，主要水平分布在沉积层内，表明它们没有被搬运或再加工。Sete Lagoas 扁平卵石角砾岩中具有锋利边缘和顶点的微生物岩碎屑的存在表明，岩化过程在成岩作用的早期就开始了，即使底部暴露的沉积物也至少部分地被岩化。一些角砾岩水平显示双向叠瓦状，碎屑尺寸分析显示 NE-SW 长轴碎屑取向，而方形碎屑倾向于填充定向碎屑之间的空间。角砾岩碎屑垂直取向并表现出向上增加的变形特征，典型的变形层是由液化沉积物上升排出形成的。破坏层或呈现褶皱和同沉积断层通常发生在未变形层之间。液化和软沉积物变形的其他证据是角砾岩床底部的注入结构，如火焰和负载铸造特征。这些构造通常穿透上层床并破坏紧邻上方的沉积层，促进局部角砾岩化。这些特征在受地震影响的现代和古代沉积物中均有发现，这表明 Sete Lagoas 扁平卵石角砾岩具有类似的起源。因此，导致所研究的扁平卵石角砾岩形成的过程被解释为地震触发，因为： a) 角砾岩层横向连续并延伸数公里；b) 角砾岩层仅限于 20 m 厚的地层区间；c) 角砾岩碎屑为角砾岩层段以下和以上未变形层的相同岩性；d) 角砾岩层和层状微生物岩层的互层是反复出现的；e) 角砾岩层近水平，上下接触不规则；f) 液化结构和岩脉注入的存在。因此，这种地震触发的角砾岩矿床代表了埃迪卡拉纪末期圣弗朗西斯科克拉通内发生的同沉积构造作用的产物，对应于班布伊盆地中一个非常明确的局部地层标记。地震活动可能与研究区古元古代区域基底的 NW 区域断层有关，这些断层在埃迪卡拉纪 Sete Lagoas 组沉积期间重新活动。