Paleohydrological significance of trace fossil distribution in Oligocene fluvial-fan-to-lacustrine systems of the Ebro Basin, Spain Jordi M. de Gibert ⁎, Alberto Sáez Departament d'Estratigrafia, Paleontologia i Geociències Marines, Universitat de Barcelona, Martí Franquès s/n, E-08028 Barcelona, Spain abstract article info Article history: Received 16 January 2008 Received in revised form 13 August 2008 Accepted 24 October 2008 Keywords: Ichnology Scoyenia ichnofacies Mermia ichnofacies Fluvial fan Lake The ichnological study of fluvial fan and lacustrine Oligocene deposits in the NE Ebro Basin has allowed the recognition of three ichnoassemblages having a strong correlation with depositional facies associations. Lacustrine deposits lack bioturbation due to the hydrologically closed character of the lake that caused hypersalinity and made it inhospitable for benthic life. Terminal lobe deposits of the fluvial fan, which were sedimented in low wetland areas around the lake, bear a moderately diverse ichnoassemblage dominated by horizontal Taenidium barretti, Cochlichnus anguineus and bird footprints (Gruipeda isp.). This assemblage can be assigned to the 'shorebird ichnofacies', a subset of the Scoyenia ichnofacies that characterizes the subaerial part of low-energy shore areas. Crevasse splay deposits in medial fluvial fan areas present a different assemblage also belonging to the Scoyenia ichnofacies. It includes most of the trace fossils found in the terminal lobes, except for the bird tracks, plus very abundant vertical burrows (Taenidium barretti and unnamed ornamented burrows). The presence of deep-tier traces of terrestrial invertebrates is consistent with the more proximal fan setting, having greater relief and consequently a lower mean groundwater position than lowland areas. Fluvial channels only bear irregularly clustered tunnel structures of unknown affinity formed after abandonment of the channel. Thus, trace fossil assemblages are mostly controlled by the position and fluctuations of water tables, which are the cause for their composite nature. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Diverse factors control trace fossil assemblages in continental settings: grain size, plant cover, climate, availability of organic matter in the sediment, etc. Among them, paleohydrological parameters have a paramount effect on ichnofaunas. Thus, water chemistry in permanently subaquatic (lacustrine) settings has a direct effect on the diversity and composition of the ichnoassemblages, while the position and oscillations of the water table in permanently or temporarily subaerial settings control the type of tracemaking animals able to inhabit the substrate (e.g., Buatois and Mángano, 2004, 2007; Hasiotis, 2007). In fact, the position of the water table is the major factor governing the three continental archetypal ichnofacies cur- rently accepted (e.g. Buatois and Mángano, 2007). Water table fluctuations are especially important in low-gradient fluvio-lacustrine systems in which relatively small oscillations, due to fluvial discharge or lake expansion, may temporarily flood extensive areas. This is the case of the transition between fluvial fans and lacustrine systems in the Ebro Basin during the Paleogene. The Oligocene fluvial fans of the eastern Ebro Basin have been the subject of a recent sedimentological study focusing on their modes of transition with contemporaneous lacustrine systems (Sáez et al., 2007). Trace fossils are abundant and diverse in certain subenviron- ments of these depositional systems, which allows for detailed ichnological study. The objectives of this paper are: (1) to fully describe for the first time the bioturbation in the deposits of medial to distal parts of the Oligocene Solsona–Sanaüja fluvial fan and their distal lacustrine equivalents; (2) to establish the correlation between trace fossil assemblages and sedimentary facies associations, contributing to the refinement of the sedimentary model proposed by Sáez et al. (2007); (3) to establish certain diagnostic features of such assemblages that may help to interpret similar sedimentary successions in core expression or small outcrops lacking information on depositional architecture; and (4) to discuss the significance of those trace fossil assemblages for the continental archetypal ichnofacies model. 2. Sedimentary setting: the Solsona–Sanaüja fluvial fan and the Noguera Lacustrine System From the Late Eocene to the Late Miocene, the depositional setting of the Ebro Basin (NE Spain) was characterized by the development of fluvial fans attached to the basin margins grading distally into low- gradient, hydrologically closed, shallow lakes located in the central basin areas (Cabrera and Sáez, 1987; Arenas and Pardo, 1999; Luzón, 2005; Nichols and Fisher, 2007; Sáez et al., 2007)(Fig. 1A). The major Palaeogeography, Palaeoclimatology, Palaeoecology 272 (2009) 162–175 ⁎ Corresponding author. E-mail addresses: jmdegibert@ub.edu (J.M. de Gibert), a.saez@ub.edu (A. Sáez). 0031-0182/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2008.10.030 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo