Transition of ephemeral palustrine to aeolian deposits in a continental arid – semi-arid environment (Upper Cretaceous Bauru Basin, Brazil) Luiz Alberto Fernandes a, * , Giorgio Basilici b a Universidade Federal do Parana ´ – UFPR, Setor de Cie ˆncias da Terra - Depto. de Geologia, Centro Polite ´cnico – Jardim das Ame´ricas, Mail box 19 001, 81531-990 Curitiba Parana ´, Brazil b Universidade Estadual de Campinas – Unicamp, Instituto de Geocie ˆncias, R. Joa ˜o Pandia ´ Calo ´geras, 51, Sa ˜o Paulo, Brazil article info Article history: Received 28 December 2007 Accepted in revised form 5 December 2008 Available online 11 December 2008 Keywords: Paleosols Sand sheet Araçatuba Formation Vale do Rio do Peixe Formation Bauru Basin Upper Cretaceous abstract In this work, we examine the transition from an ephemeral palustrine to a desertic sand sheet deposi- tional system, corresponding to the Araçatuba Formation and Vale do Rio do Peixe Formation, respec- tively. These Upper Cretaceous formations form the lower part of the sedimentary sequence that fills the intracratonic Bauru Basin in the South American Platform (southeast Brazil). The transition between the two formations is through paleosols located at the top of the Araçatuba Formation, which have different spatial and temporal evolutions. In the inner, southern, and western parts of the Araçatuba Formation area, the paleosols are well developed, while in the outer, northern and eastern parts they are less developed. In the inner area, the mature paleosols are in sharp contact with the overlying deposits of the Vale do Rio do Peixe Formation, while in the outer area, immature paleosol successions in which sedimentary and pedogenetic features are interlayered represent the transition. Ghosts of sedimentary structures in the outer paleosols allow us to attribute the variations in paleosol development to different sedimentary inputs. In the outer part, a slow but continuous supply of sediment caused continuous rejuvenation of the soils, whereas in the inner portion the absence of sedimentary input allowed very mature soils to form. The transition between Araçatuba and Vale do Rio do Peixe formations may thus be explained by a sudden increase in sedimentation in the area, probably triggered by tectonic uplift in northern and eastern parts of the Bauru Basin, causing the progradation of alluvial fans of the Marı ´lia Formation, which supplied sediments to the sand sheet depositional system of the Vale do Rio do Peixe Formation. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Arid or semiarid environments show a variety of depositional systems formed under aeolian (e.g., erg, sand sheet) or non-aeolian (e.g., playa lake, sabkha, fluvial) conditions. Some studies have attributed the spatial and temporal changes of these depositional systems to control by external factors (Kocurek, 1988; Langford and Chan, 1993). The Upper Cretaceous Bauru Basin provides an opportunity to describe and evaluate one of these spatio-temporal changes from an ephemeral palustrine to an aeolian system. In this paper, we discuss the transitions between two sedimentary units, the Araçatuba (ArF) and the Vale do Rio do Peixe (VPF) formations, located in the western part of Sa ˜o Paulo State (Brazil). They record vertical and lateral transition changes from an ephemeral palustrine environment to aeolian deposits formed under arid-semiarid conditions. These two units are part of the lower depositional sequence filling the Upper Cretaceous intra- cratonic Bauru Basin (Fernandes and Coimbra, 1996). We describe the most important features of the transitional deposits and eval- uate the factors that controlled the transition. 2. Geological setting The 370,000 km 2 Bauru Basin is a Coniacian-Maastrichtian intracratonic continental basin (Fig. 1), developed on the central- southern part of the South-American Platform after the break-up of Gondwanaland (Fernandes and Coimbra, 1996). The basin substrate consists of basalts of the Lower Cretaceous Serra Geral Formation (Renne et al., 1992), from which the sedimentary sequence is separated by a regional erosive unconformity. Subsidence of this substratum (Fernandes and Coimbra, 1996) created the basin. The basin fill is essentially composed of sandy sediments, which are 100–200 m thick with a maximum of 320 m (Mezzalira et al., 1981), deposited under arid and semiarid conditions that become more desertic southwestwards. The sedimentary succession is divided * Corresponding author. E-mail addresses: lualfernandes@uol.com.br, lufernandes@ufpr.br (L.A. Fernandes). Contents lists available at ScienceDirect Cretaceous Research journal homepage: www.elsevier.com/locate/CretRes 0195-6671/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.cretres.2008.12.002 Cretaceous Research 30 (2009) 605–614