A quantitative depositional model of a large distributive fluvial system (Megafan) with terminal aeolian interaction: The Upper Jurassic Guará DFS in southwestern Gondwana

Abstract

Recent studies have shown that distributive fluvial systems are the dominant fluvial forms in modern continental sedimentary basins, thus composing a large part of the stratigraphic record. This study provides a basin-scale architectural analysis of the Guará Formation, from the Upper Jurassic record of southwestern Gondwana, and attempts to compare the formation's depositional model to those developed for distributive fluvial system (DFS) successions. This time interval is significant because it was a period of intense tectonic activity related to the Paraná–Etendeka plume and the Gondwana breakup. Quantitative analyses were performed on stratigraphic sections at 17 locations (exposing a total of 720 m of stratigraphy) located in southern Brazil and northern Uruguay, from a larger dataset of 64 locations (comprising a total of 1070 m of stratigraphy). Four facies associations were identified: perennial fluvial channel fills, ephemeral fluvial channel fills, floodplain deposits, and aeolian deposits, indicating a dryland climate. Spatial trends were analyzed along a downstream-oriented transect (NNE–SSW) across the system. Grain size, channel-body thickness, number of stories, and bar thickness decrease downstream, indicating a reduction in channel depth, flow capacity, and channelization of the fluvial system, interpreted to be associated with downstream-increasing bifurcation, infiltration, and evapotranspiration. Based on spatial trends and distribution of facies associations, the deposits are interpreted to have been accumulated from a large DFS which can be divided into four zones, from proximal to distal: Zone 1, dominated by perennial fluvial channels; Zone 2, a mixture of perennial and ephemeral channels; Zones 3 and 4, deposits situated externally of the fluvial channel belts dominated by aeolian and floodplain deposits prevailing in each zone, respectively. The Guará Formation likely records the stratigraphic signature of the largest distributive fluvial systems reconstructed from both modern and ancient datasets, and one of the first where fluvio–aeolian interaction is quantified. The Guará Formation DFS model presented herein is key to understanding paleoenvironmental, paleoclimatic, and geotectonic changes related to Gondwanan fragmentation.