Sedimentology of a distributive fluvial system : the Serra da Galga Formation, a new lithostratigraphic unit (Upper Cretaceous, Bauru Basin, Brazil).

Abstract

The Bauru Basin of SE Brazil is a large (ca. 370,000 km2 ) Upper Cretaceous intracratonic feature, important for its fossil remains and of particular value as a source of regional palaeoclimatic information. Historically, lithostratigraphic reconstructions have been performed mainly for successions of the central and southern parts of the basin, resulting in a lithostratigraphic scheme that is not applicable to the northernmost regions. In particular, the northeastern deposits of the Marília Formation (Serra da Galga and Ponte Alta members) reveal lithological, stratigraphic, and palaeontological differences from southeastern and northwestern counterparts (Echapor~a Member). Nevertheless, these deposits are considered as a single lithostratigraphic formation in the literature. To address this problem, this study demonstrates how the northeastern deposits of the Marília Formation do not show affinity to the rest of the unit. A more suitable lithostratigraphic model is proposed for the northeastern succession as a distinct and independent unit. Lithofacies and palaeopedological analysis, combined with lithostratigraphic mapping of the northeastern deposits, reveal 11 distinct lithofacies and three pedotypes over an area of 450 km2 . Sedimentary facies and pedotypes were assigned to six interbedded architectural elements: (a) type 1 channel fill, (b) type 2 channel fill, (c) type 3 channel fill, (d) interchannels, (e) palaeosols, and (f) calcrete beds. The succession is interpreted as a distributive fluvial system with overall direction of flow to the NNW, and which developed under the influence of a semiarid climate regime. This contrasts with deposits of the southeastern and northwestern Marília Formation, previously suggested to be of fine-grained aeolian affinity with interbedded poorly channelised deposits assigned to an aeolian sand sheet environment. By revising the existing lithostratigraphic scheme for the northeastern deposits, and contrasting them with laterally equivalent strata, this work demonstrates how the previously named Serra da Galga and Ponte Alta members reveal a unique set of lithological, architectural, and genetic signatures that permits to separate them from the Marília Formation. Finally, a new lithostratigraphic classification for the unit is proposed: the Serra da Galga Formation, whose deposition relates to an ancient distributive fluvial system.