Fluvial-eolian deposits of the Tacuarembó formation (Norte Basin – Uruguay): Depositional models and stratigraphic succession

Abstract

The Tacuarembó Formation (Upper Jurassic-Lower Cretaceous), subdivided into Batoví and Rivera members, crops out in the northern region of Uruguay, as a narrow N-S belt with an average width of 35 km by 115 km in length. The present work has as main objective the sedimentological and stratigraphic analysis of the Tacuarembó Formation, through the facies characterization, reconstruction of the depositional models and definition of contact relationship between the Batoví and Rivera members. Through facies analysis from the survey of columnar profiles in a 1:50 scale of forty-one outcrops, each lithofacies was described and interpreted, then grouped into different facies associations. The Batoví Member consists of sixteen lithofacies composing five facies associations: (1) eolian dunes, (2) eolian sand sheets, (3) ephemeral fluvial channels, (4) perennial braided fluvial channels and (5) sheetflood deposits. On the other hand, the Rivera Member is essentially constituted by one lithofacies and its vertical stacking composes eolian dunes facies associations. The intercalation between fluvial and eolian deposits, with the predominance of fluvial associations, suggests that the Batoví Member represents the depositional model of the distal portion of a distributary fluvial system. Contrastingly, the Rivera Member is characterized by a successive climbing of crescentic eolian dunes, defining a dry eolian system. The abrupt change of depositional systems, marked by a flat surface sometimes with clasts concentration, indicates the existence of a hiatus between the Batoví and Rivera members. Allied to this, the change in the depositional model suggests a climatic change, going from arid to semi-arid climate during the deposition of the Batoví Member, to a hyperarid climate during the deposition of the Rivera Member.