Climatic and geomorphologic cycles in a semiarid distributive fluvial system, Upper Cretaceous, Bauru Group, SE Brazil

Abstract

Studies of distributive fluvial systems and their preserved successions envisage the distribution and orientation of architectural elements to be primarily controlled by channels radiating outward from fan apices, in many cases along an elongate basin margin. Conceptual models for such systems account for the downstream dynamics of the fluvial network, but with limited consideration of temporal geomorphic variations, resulting vertical organisation of architectural elements, or of the interplay of factors controlling system dynamics. To understand the external and internal architecture of distributive fluvial systems, and the factors that influence their sequential facies organisation, a sedimentary succession of the proximal portion of an Upper Cretaceous, semiarid, distributive fluvial system, located at the north-eastern margin of the Bauru Basin (Southeast Brazil), has been analysed in detail. Three fining- and thinning-upward fluvial sequences are identified, forming an interval separated at the top and the bottom by two palaeosol profiles. Each sequence is formed of channel and floodplain deposits. Two types of channel deposits are identified. One is composed of stacked sets of small-scale dune deposits, suggesting perennial and steady fluvial regime, associated with more humid climate periods. The other is composed of large-scale sets indicative of flattened dunes associated with unsteady and fast-changing fluvial flow, formed in quasi-supercritical flow regime conditions, associated with drier climate periods. The vertical alternation of these two types of channel deposits records the accumulation of a fluvial succession that responded to high-frequency, climate-induced cyclic change in boundary conditions. Two palaeosol profiles, at the top and at the bottom of the succession, indicate temporary interruptions and cessation of the fluvial sedimentation, likely related to avulsion of the fluvial belt. Thus, the studied succession reveals high-frequency climate-induced allogenic sedimentary cycles that occur within a long-period autogenic geomorphologic-induced sedimentary cycle. This work suggests that the internal architecture of the channel deposits can be used as a climate proxy, and that climate and geomorphology act jointly as notable factors to control the vertical organisation of distributive fluvial systems.