Aeolian-alluvial interactions at Formentera (Balearic Islands, Western Mediterranean): The late Pleistocence evolution of a costal system

Abstract

This study deals with the sedimentological and stratigraphic description of Pleistocene coastal deposits of Formentera Island that show evidence of interference between processes characteristic of alluvial and aeolian environments. Here we analyze the sedimentology and stratigraphy of a Pleistocene succession located at Northeast of Formentera and we constrain its chronology by optically stimulated luminescence (OSL) dating. Main sedimentary facies representing the succession of aeolian, alluvial, marine and coastal environments are described in this study. We conclude that although there were two prevailing environments such as coastal aeolian and alluvial, with different processes and resulting forms, the interaction resulted in deposits that share characteristics of both environments, as well as maintain elements inherited from each environment in terms of sedimentary supply, precipitation, runoff or aeolian transport. These coastal deposits represent a small alluvial apron covering the base of a vertical cliff. Most of the sediment that composes these deposits is marine bioclastic sand which was transported in the form of coastal dunes during periods of low sea level. Our results are a useful indicator of the geomorphological processes and changes that occurred during the Pleistocene, unraveling the environmental evolution and contributing to the growing knowledge on the Western Mediterranean aeolian-alluvial interacting environments.

Introduction

Discrepant interpretations of some Pleistocene deposits with an aeolian appearance along the coasts of the Balearic Islands, especially Mallorca and Menorca, have risen the generation of debate about the role played by the interaction between aeolian processes and those related to the formation of alluvial fans (Pavelic et al., 2011; Fornós et al., 2012; Rodríguez-López et al., 2012; Pomar, 2016; Pomar et al., 2018). Also, the role that this interaction plays in the construction and dismantling of dune fields or alluvial fans, and their paleoclimatic significance raised interest (Pomar et al., 2018, Del Valle et al., 2019a,b).

The Pleistocene sedimentary successions are characterized by the alternation between aeolian deposits, colluvial deposits, marine deposits, calcretes and palaeosols (Mckee and Ward, 1983) are a good indicator of the depositional processes variability and therefore geomorphological changes. These processes represent changes in sedimentation resulting in the successive formation of different deposits that acquire a complex architecture. They also represent an exceptional paleoclimatic record that occurred during the Quaternary (Brooke, 2001).

At the island of Formentera, these deposits have been described for the first time. They consist of sandstone bodies interpreted as aeolian deposits with some alluvial features and also alluvial/colluvial deposits with an aeolian appearance (Al-Masrahy and Mountney, 2015; East et al., 2015; Pomar et al., 2018). The description of these deposits is problematic because are composed of fine to medium bioclastic sands with a marine origin, that formed deposits with typically alluvial sedimentary structures. Most of the aeolian-alluvial interactions have been described at geomorphic settings characterized by the presence of significant reliefs able to act either as a barrier intercepting the aeolian sediment or generating important runoff processes (Pye and Tsoar, 2009; Ventra et al., 2013; Rowell et al., 2018). The coasts of Formentera are characterized by vertical and stepped cliffs of Upper Miocene formations that bear attached Pleistocene aeolian deposits and colluvial deposits.

In that sense, these deposits provide insights Pleistocene geomorphological and climatic history of the Balearic Islands and the Western Mediterranean, giving a reliable geochronology (Nielsen et al., 2004; Fornós et al., 2009; Andreucci et al., 2014; Del Valle et al., 2016; Pomar, 2016). Luminescence dating has become a basic tool for establishing chronologies of late Quaternary successions (<130ka for Quartz-OSL and <500 ka for pIRIR290) all over the world. It has been applied in many environments such as aeolian, loess, alluvial-colluvial and palaeosols (Andreucci et al., 2009; Murray and Clemmensen, 2000; Fornós et al., 2009; Del Valle et al., 2016). Optically stimulated luminescence (OSL) dating method has allowed to date directly the time of deposition or construction of the deposits, specially aeolian sediments (Leighton et al., 2014), and thereby to establish landscape evolution and their interaction of different geomorphological processes in relation to regional-scale climatic change (Fornós et al., 2009; Pomar et al., 2018; del Valle et al., 2019a, b). In this sense, OSL chronologies of this kind of deposits allow to place when the major aeolian inputs and the alluvial events took place in relation to the late Pleistocene significant climatic changes. This is an important issue regarding the aeolian-alluvial interaction processes in order to identify the timing of formation, which means they could take place at the same time, alternating or which role played the sea level fluctuations (Al-Masrahy and Mountney, 2015; Liu and Coulthard, 2015; Pomar et al., 2015).The aim of this study is to unravel the role played by the environmental changes that resulted in a landscape characterized by the interaction on marine, aeolian and alluvial processes. Here we analyze the sedimentology and stratigraphy of a Pleistocene succession located in the Northeast of Formentera island and we constrain its chronology by OSL dating. This chronological and sedimentological framework allows us to discuss the processes and paleoenvironmental conditions that have driven the coastal landscape changes during the late Pleistocene in this littoral area.