Journal of Paleolimnology 28: 441–456, 2002. 441  2002 Kluwer Academic Publishers. Printed in the Netherlands. Upper Oligocene-Lower Miocene shallowing-upward lacustrine sequences controlled by periodic and non-periodic processes (Ebro Basin, northeastern Spain) 1 1 1, 2 * ˜ ´ ´ ´ ´ Arantxa Luzon , Angel Gonzalez , Arsenio Munoz and Belen Sanchez-Valverde 1 Departamento de Ciencias de la Tierra, Universidad de Zaragoza. C / Pedro Cerbuna, 12. 50009 2 ´ ´ Zaragoza, Spain; Departamento de Metodos Estadısticos, Universidad de Zaragoza. C / Pedro Cerbuna, * 12. 50009 Zaragoza, Spain; Author for correspondence (e-mail: armunoz@posta.unizar.es) Received 22 February 2002; accepted in revised form 18 June 2002 Key words: Ebro Basin, Lacustrine sediments, Non-marine carbonates, Oligocene-Miocene, Precession cycle, Spain, Spectral analysis Abstract The lacustrine facies from two sections (Candasnos and Fraga) of the Oligocene-Miocene Torrente de Cinca lithostratigraphic Unit in the central part of the Ebro Basin (Spain) have been analysed to determine the influence of orbital parameters in lacustrine sedimentation. The unit is predominantly composed of limestones and marls, and represents a shallow lacustrine freshwater system. The sedimentological features of the facies studied demonstrate that the lower part of the Candasnos section represents offshore lacustrine subenvironments whereas the upper part, and the whole of the Fraga section, characterise marginal lacustrine areas. Series of stratal thickness variations of limestone, marl, and limestone / marl couplets from both sections have been analysed using spectral analysis. This shows that information corresponding to periodic cycles only appears in the offshore facies, that is to say, in the lower part of Candasnos section, and disappears in the marginal facies where non-periodic cycles exist. Furthermore, the spectral analysis of the offshore facies highlights the existence of a peak in the power spectrum with a period of around 7 (6.8 to 7.8) that can be recognised in the field as shallowing-upward lacustrine sequences. Magnetostratigraphic data from the Candasnos section allow us to establish a time span of 2,808 years for the limestone / marl couplet from the lower part of this section, and between 19,000 and 22,000 years for the periodic cycle identified, thus representing the climatic precession cycle. Shallowing sequences from marginal areas do not correspond with any periodic cycle. Introduction 2001). There are some works on cyclicity in lacustrine deposits of the Ebro Basin, close to the area studied, Milankovitch9s theory of climatic change has been although they focus on slightly earlier sediments ` confirmed by geological data largely originating in (Barbera et al. 1996). marine environments. The work of Hays et al. (1976) Dynamics in lacustrine basins isolated from marine marked the starting point for numerous studies, main- influence is controlled by tectonics and climate (Gal- ly centred on pelagic environments which are poten- loway 1989; Einsele 1992; Arenas 1993; Bohacs et al. tially the most suitable in the search for regular, 2000). Tectonics alters both sedimentary contribu- continuous sedimentation. On the other hand, there tions from the source areas and subsidence rate in the are also examples of sedimentary cycles induced by basin, whereas climate controls lacustrine sedimenta- orbital forces in continental environments, although tion through variations in water and sedimentary they are fundamentally recorded in lacustrine deposits supplies that induce physical, chemical and biological (Anderson 1982, 1984; Olsen 1984, 1986; Olsen and changes in lakes. However, in these sedimentary Kent 1996; Abdul Aziz et al. 2000; van Vugt et al. environments, climatic cyclicity can be masked by