Changes in fluvial architecture induced by discharge variability, Jaicós Formation (Silurian-Devonian), Parnaíba Basin, Brazil

Abstract

Discharge variability-based facies models are key to understanding depositional dynamics and controlling mechanisms of fluvial accumulation. The well-exposed Silurian-Devonian Jaicós Formation, lower Parnaíba Basin, NE Brazil, allows for detailed fluvial architectural analyses in terms of variations in discharge patterns. This field-based study comprises a collection of high-resolution sedimentological, stratigraphic and paleocurrent data from 120 m of integrated logs. Eight architectural elements were recognized and fit into two distinct fluvial styles, termed fluvial units 1 and 2. Fluvial unit 1 is dominated by deposits of straight- and sinuous-crested bedforms, as constituents of downstream accretionary bars and subcritical, aggradational dunes. These elements indicate deposition in a low discharge variability perennial system, where water flow was perennial but bedforms migrated during peak-discharge events, related to a humid climate. Fluvial unit 2 abruptly overlies unit 1 and is dominated by small- to large-scale humpback dunes and antidunes deposited by unstable, transcritical and supercritical flows during non-periodic, peak-flood events. These deposits characterize an ephemeral system, in a high discharge variability regime linked to a semi-arid climate. The Jaicós fluvial system was deposited in a low subsidence basin, in low accommodation settings. Minimally affected by tectonic activity and latitudinal displacements, the Jaicós Formation displays a climate-controlled change in fluvial architecture that reflects shifts in discharge regime. These climate changes were probably triggered by global oscillations in the carbon cycle that were common during the Silurian, but most evidence is from low paleolatitude marine deposits. Thus, the abrupt fluvial architecture change in the Jaicós Formation comprises an example of climate-induced change in a hinterland fluvial system situated at moderate to high paleolatitudes in western Gondwana.