Abstract

Shallow carbonate lakes with ramp-type margins were developed during the deposition of the Upper Unit of the Loranca Basin in two different sedimentary realms controlled by tectonics: restricted and extensive carbonate lacustrine environments. The restricted ones, located in the central part of the basin, were created by tectonic uplifting of compressive structures (Huete anticline) that provoked a general rise of base level in the Tórtola alluvial system. The extensive lacustrine environments were developed in isolated areas as a result of topographic threshold formation which separated these lakes from the Tórtola system and caused them to form in the southern part of the basin.

Introduction

The Upper Unit is part of the fill of the Loranca Basin (Díaze-Molina et al, 1985, 1989). This Unit was deposited during the Late Oligocene and Early Miocene and is formed by two coalescing alluvial depositional systems, the Tórtola and the Villalba de la Sierra fluvial fans (Díaze-Molina et al, 1989). Stratigraphically, the Upper Unit may be divided into three stages which reflect the rate of diastrophism and changes in the base level of the fluvial systems (Díaze-Molina et al, 1989; see also Chapter C5).

In the Upper Unit, both stages 1 and 2 display some carbonate lithosomes interbedded with the siliciclastic facies. These carbonate deposits correspond to shallow lacustrine episodes developed in different sedimentary realms in the basin.