Abstract

Froude-supercritical bedforms and associated sedimentary structures are formed in turbulent flows when the value of the Froude number is Fr > 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. This study reinterprets the sedimentary structures in this formation as Froude-supercritical structures and identifies 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 part of a fluvial system, generating hyperconcentrated flows which resulted in thin beds of fine-grained sandstone with massive structure or planar 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 large-scale sandy antidunes and other Froude-supercritical bedforms with mud intraclasts, which deposited sandstone in undulatory laminae, and other supercritical-flow structures. These floods waned extremely rapidly, bypassing the stability field of lower-flow-regime bedforms. Measurements taken from undulatory stratification, interpreted as antidune deposits, allowed 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 a finer interpretation of their discharge 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.