Quaternary evolution of the Golo river alluvial plain (NE Corsica, France)

Abstract

The Golo River drains a steep catchment (average gradient of 30 m km⁻¹, surface of 1214 km²) in the northeast part of Corsica Island, delivering sediments to the Ligurian Sea. In this study, we review and revise the geologic map and constrain the extent of the Golo coastal alluvial plain formations and their relative and absolute chronology. To update the surface extent of each formation, we performed a geomorphologic analysis with DEMs and satellite imagery data coupled with an extensive pedogenic and sedimentary field observations database, including a new borehole of 117,4 m depth. Additionally, we performed in-situ cosmogenic ¹⁰Be analysis from a depth-profile in the well-preserved alluvial terrace Fy2, yielding a minimum age of 70 ka for its emplacement. Our new chronology, based on cosmogenic ¹⁰Be and soil chronosequences, implies older ages than those previously obtained with luminescence methods. Soil mixing by bioturbation is proposed as a possibility to explain differences between luminescence and ¹⁰Be ages. In this scenario, ¹⁰Be dates the original deposition of the alluvial terrace, while luminescence dates a later soil development. We highlighted at least five outcropping alluvial terraces in the Golo coastal plain, which are controlled by sea-level fluctuations and were most likely deposited during past sea-level highstands (close to present-day sea-level). Moreover, we identified from a borehole more than 117 m of coarse fluvial sediments in the plain, that do not outcrop at the surface. New cosmogenic ²⁶Al/¹⁰Be burial ages suggest that this sedimentary unit results from a thick accumulation of fluvial material filling a zone significantly affected by subsidence, probably accommodated by a normal fault during the Early Quaternary.

Introduction

Coastal alluvial plains are key places for understanding sediment storage in a source-to-sink approach (e.g. Sømme et al., 2011; Blum et al., 2013). However, the consequences of aggradation/degradation cycles in coastal plains due to Quaternary sea-level oscillations make stratigraphic correlations difficult, with upstream (alluvial terraces) or downstream (shelf sediment accumulation) deposits. Since a decade, alluvial plains focused new research efforts, thanks to the development of relevant tools, such as dating methods on fluvial sequences. Moreover, these settings record paleo-environmental parameters controlling sediment fluxes, such as tectonics, climate or sea-level fluctuations (e.g. Twidale, 2004; Allen, 2008). In many cases, the extent of alluvial depositional terraces in alluvial plains is mainly based on topographic constraints, such as breaks in slope and escarpments (e.g. Conchon, 1975). Mapping the accurate lateral extension of alluvial terraces and determining their in-depth geometry remain often problematic and suffer from large uncertainties. Notably, the relevant resolution of topographic data is not always available and geophysical surveys are sometimes difficult to conduct in anthropogenic areas.

The geochemical maturity of an alluvial soil increases with age: while Holocene deposits are not weathered, soils from Pleistocene alluvial deposits of Mediterranean periglacial areas are matured, rich in clay, and may become reddish after experiencing several warm phases of Quaternary climatic cycles (e.g. Baize and Girard, 2008; Legros, 2012). Studying chronology of soil sequences on alluvial terraces in addition to geomorphic analysis is thus a very efficient way to determine relative chronology of deposits (Hubschman, 1973; Bornand, 1978; Delmas et al., 2015). Moreover, the use of terrestrial cosmogenic nuclides has proven its efficiency for absolute dating of alluvial deposits (e.g. Granger et al., 1996; Hancock et al., 1999; Brocard et al., 2003; Siame et al., 2004; Molliex et al., 2013).

The high and steep topography of Corsica leads to a strong sensitivity of sediment transfers due to regional climate changes, even for moderate variations (Kuhlemann et al., 2008a). The Golo River system (Eastern Corsica) is considered as a reactive system (Allen, 2008), characterized by a fast sediment transfer from the catchment to the sink (Sømme et al., 2011; Calves et al., 2013; Forzoni et al., 2015), and rapidly reacts to climate changes. Sediment volumes deposited in the alluvial plain and the deep-sea fan during the Holocene are compatible with sedimentary fluxes inferred from cosmogenic denudation rates in the catchment (Sømme et al., 2011; Calves et al., 2013; Molliex et al., 2017). However, some studies suggest that alluvial terraces in the plain may be diachronous and that the Golo River may have deposited sediment by aggradation along its entire profile at any time during the late Quaternary. Such climatically-driven pulses in sediment supply may hamper detailed stratigraphic correlations (Conchon, 1978; Sømme et al., 2011; Forzoni et al., 2015). The chronology of the Golo alluvial terraces was first established from relative dating: Conchon (1975, 1977) associated each terrace to a full glacial cycle. Later, luminescence methods applied on sand lenses of outcropping terraces (OSL on quartz and IRSL on feldspar) suggested younger ages for Fy1, Fy2, Fy3 terraces, in correlation with smaller climatic oscillations (Sømme et al., 2011; Skyles, 2013; Forzoni et al., 2015) (Table 1). Moreover, luminescence dating yields ages of terrace aggradation that are not always internally consistent, and do not always correspond to climatic oscillations and full glacial stages. Sediment buffering in the alluvial and coastal plain may play an important role in the transfer from the source to the sink and may alter a simple relationship between climate and deposition in the alluvial plain (Sømme et al., 2011; Calves et al., 2013). Obtain new absolute ages from the sedimentary material of the Golo alluvial plain is thus key to progress on understanding these processes.

In this paper, we present an updated geological model of the Golo alluvial plain based on new data acquired both in surface (geomorphology, pedology) and depth (borehole, Electrical Resistivity Tomography). We also revised the chronology of formations using new cosmogenic nuclides data (¹⁰Be depth profile and ²⁶Al/¹⁰Be burial ages). Finally, we discuss the Quaternary evolution of the Golo coastal alluvial plain in response to Quaternary climate and tectonic forcings.