Abstract

During Late Oligocene to Early Miocene times the Loranca Basin was filled by a thick sequence of continental deposits comprising three large depositional systems, Tórtola, Villalba de la Sierra and Valdeganga. Stratigraphic and sedimentological methods enable palaeoenvironmental reconstructions and the tectonic deformation history to be related to long-term palaeogeographic changes. Each depositional system consists of a fluvial fan and associated floodbasin and distal deposits, which were dominated by saline lakes. Syn-sedimentary alluvial fans fringed the basin margins, reflecting their differential tectonic movements. Topographic changes induced by fold generation controlled facies distribution evolution. The large fluvial fans contained many channel systems, and all the channel types identified in recent rivers are found. In proximal fan areas, braided channels predominated, but meander loops, and braided and other low-sinuosity channel types are found extensively across the fan surfaces. The stratigraphic successions are characterized by a high content of finer-grained deposits. Their sandstone composition reveals source areas composed of sedimentary rocks for all the depositional systems. Differences in composition of the main sediment supplies reflect diversity in the relative proportion of the different sedimentary rocks in the catchment areas. In contrast, variation in composition down the fans reflects local tributary mixing situations, both between fans or between more local alluvial systems.

Introduction

The Loranca Basin is located in the central part of Spain, within the Iberian Range (Fig. 1). This basin was formed during Eocene to Late Oligocene times and contains continental deposits locally folded along with the underlying Mesozoic rocks.