Supercritical fluvial styles and the shifting aridity in the Early Triassic: the example of the Sanga do Cabral Formation, Paraná Basin, Brazil

IF 2 4区 地球科学 Q1 GEOLOGY
Pedro L.A. Xavier, Claiton M. dos S. Scherer, Adriano Domingos dos Reis, Ezequiel Galvão de Souza, Felipe Guadagnin, Graciela Piñeiro
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引用次数: 0

Abstract

Froude-supercritical bedforms and associated sedimentary structures are formed in turbulent flows when value of the Froude Number is Fr greater than 1. They have been increasingly studied in recent years, and while they were previously considered to be of rare preservation, they have been increasingly identified in modern settings and the rock record. In alluvial systems, these structures are being recognized as characteristic of rivers with high variability of discharge, especially in arid, semiarid and subhumid tropical and subtropical climates. However, the development of facies models for such rivers remains tentative, particularly for the rock record, and with the exception of Australia, examples in Gondwana are scarce. The Early Triassic Sanga do Cabral Formation represents an arid to semiarid ephemeral fluvial system cropping out in Southern Brazil, Southwestern Gondwana. The present study reinterprets the sedimentary structures within this formation as Froude-supercritical structures, and ident ifies three Fluvial Styles (FS). FS1 predominantly consists of fine-grained massive sandstone, with interruptions of intraclastic conglomerates, and occasionally-visible faint lamination and mud intraclast levels. It is interpreted as deposited by unconfined flows in the distal portion of a fluvial system, generating hyperconcentrated flows which resulted in thin beds of fine-grained sandstone with massive structure or plane-parallel lamination, and incipient antidunes. FS2 was deposited by flash floods occurring repeatedly within a short period during a wet season. This resulted in a fining-upwards succession of intraclastic conglomerates with supercritical-flow structures, through sandstones with supercritical-flow structures, to sigmoidal cross-stratification and ripple marks with diffuse lamination. FS3 was deposited by catastrophic flash floods characterized by high discharge and flow velocity, possibly generated by erratic storms, which poured in single events. These catastrophic flows generated lar ge-scale sandy antidunes and other Froude-supercritical bedforms with mud intraclasts, which deposited sandstone in undulating laminae, and other supercritical-flow structures. These floods waned extremely rapidly, bypassing the stability field of lower-flow regime bedforms. Measurements taken from undulating stratification, interpreted as antidune deposits, allowed for the estimation of paleoflow velocity and depth. The largest antidunes had a maximum estimated wavelength of 28.92 m (with a mean of 15.4 m) and maximum estimated height of 1.42 m (with a mean of 0.85 m), resulting in an estimated paleoflow velocity of up to 6.72 ms-1 (with a mean of 4.9 ms-1) and a maximum flow depth of 1.59 m (with a mean of 0.9 m). These parameters are comparable to those observed in modern fluvial floods. This study reinforces the significance of Froude-supercritical structures in enhancing our understanding of fluvial systems characterized by high variability in discharge, allowing for a finer interpretation of their d ischarge patterns. This approach can be applied to better understand the many arid, semiarid, or strongly seasonal environments of the Early Triassic period in Gondwana, and potentially other regions and geological times.
早三叠世超临界河流样式与移动干旱化——以巴西帕拉纳盆地Sanga do Cabral组为例
当弗劳德数的值Fr大于1时,在湍流中形成弗劳德-超临界河床及其伴生的沉积构造。近年来,人们对它们进行了越来越多的研究,虽然它们以前被认为是保存罕见的,但它们在现代环境和岩石记录中得到了越来越多的确认。在冲积系统中,这些结构被认为是具有高流量变异性的河流的特征,特别是在干旱、半干旱和半湿润的热带和亚热带气候中。然而,这些河流的相模型的发展仍然是试探性的,特别是对于岩石记录,除了澳大利亚,冈瓦纳的例子很少。早三叠世Sanga do Cabral组代表了巴西南部冈瓦纳西南部一个干旱到半干旱的短暂河流系统。本研究将该地层中的沉积构造重新解释为弗劳德-超临界构造,并确定了三种河流样式。FS1主要由细粒块状砂岩组成,内有碎屑砾岩的中断,偶尔可见微弱的层状和泥质碎屑层。它被解释为由河流系统远端部分的无约束流沉积而成,产生了高浓度的流,形成了具有块状结构或平面平行层叠的细粒砂岩薄层和早期的反沙丘。FS2是由雨季短时间内反复发生的山洪沉积而成。这导致了具有超临界流构造的碎屑内砾岩,通过具有超临界流构造的砂岩,向上细化到s形交叉层状和具有弥散层状的波纹标记。FS3是由具有高流量和高流速特征的灾难性山洪沉积的,可能是由不稳定的风暴造成的,这些风暴是在单一事件中倾泻的。这些突变流形成了大规模的砂质反沙丘和其他带有泥质内碎屑的弗劳德-超临界河床,这些泥质内碎屑沉积了起伏纹层中的砂岩,以及其他超临界流构造。这些洪水消退得非常快,绕过了低流量流态河床的稳定场。从被解释为反沙丘沉积物的起伏分层中进行的测量,可以用来估计古流速和深度。最大的反沙丘的最大估计波长为28.92 m(平均15.4 m),最大估计高度为1.42 m(平均0.85 m),因此估计古流速可达6.72 ms-1(平均4.9 ms-1),最大水流深度为1.59 m(平均0.9 m),这些参数与现代河流洪水的观测参数相当。这项研究加强了弗劳德超临界结构在增强我们对以流量高变异性为特征的河流系统的理解方面的意义,允许对其流量模式进行更精细的解释。这种方法可以应用于更好地理解冈瓦纳早三叠纪时期的许多干旱、半干旱或强烈季节性环境,以及潜在的其他地区和地质时代。
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来源期刊
CiteScore
3.80
自引率
5.00%
发文量
50
审稿时长
3 months
期刊介绍: The journal is broad and international in scope and welcomes contributions that further the fundamental understanding of sedimentary processes, the origin of sedimentary deposits, the workings of sedimentary systems, and the records of earth history contained within sedimentary rocks.
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