Evolution and architecture of a West Mediterranean upper Pleistocene to Holocene coastal apron-fan system

The Quaternary deposits of tectonically stable areas are a powerful tool to investigate high-frequency climate variations (<10ka) and to distinguish allogenic and autogenic factors controlling deposition. Therefore, an Upper Pleistocene-Holocene coastal apron-fan system in north-western Sardinia (Porto Palmas, Italy) was studied to investigate the relations between climate changes, sea-level fluctuations and sediment source-supply that controlled its development. The sedimentary sequence records the strong influence of local (wet/dry) and worldwide (sea-level) environmental variations in the sedimentation and preservation of the deposits. A multi-disciplinary approach allowed subdivision of the succession into four major, unconformity-bounded stratigraphic units: U1 U2, U3 and U4. Unit U1, tentatively dated to the warm and humid Marine Isotopic Stage (MIS) 5, consists of sandy, gravelly coastal/beach deposits developed during high sea-level in low-lying areas. Unit U2 consists of debris-flow dominated fan-deposits (ca 74ka; MIS 4), preserved as partial fills of small valleys and coves. Unit U2 is mainly composed of reddish silty conglomerate to pebbly siltstones sourced from the Palaeozoic metamorphic inland hills (bedrock), superficially disintegrated during the preceding warm, vegetation-rich MIS 5. The cold and semi-arid climate strongly reduced vegetation cover along the valley flanks. Therefore, sediment gravity-flow processes, possibly activated by rainstorms, led to deposition of debris-flow dominated fans. Unit U3 consists of water-flow dominated alluvial-fan deposits (ca 47 to 23ka; MIS 3), developed on a slightly inclined coastal plain. Unit U3 is composed of sandstone and sandy conglomerate fed from two main sediment sources: metamorphic inland bedrock and Quaternary bioclastic-rich shelf-derived sands. During this cold phase, sea-level dropped sufficiently to expose bioclastic sands accumulated on the shelf. Frequent climate fluctuations favoured inland aeolian transport of sand during dry phases, followed by reworking of the aeolian bodies by flash floods during wet phases. Bedrock-derived fragments mixed with water-reworked, wind-blown sands led to the development of water-flow dominated fans. The Dansgaard-Oeschger events possibly associated with sand landward deflation and main fan formations are Dansgaard-Oeschger 13 (ca 47ka), Dansgaard-Oeschger 8 (ca 39ka) and Dansgaard-Oeschger 2 (ca 23ka). No record of sedimentation during MIS 2 was observed. Finally, bioclastic-rich aeolianites (Unit U4, ca 10 to 5ka; MIS 1), preserved on a coastal slope, were developed during the Holocene transgression (ca 10 to 5ka; MIS 1). The studied sequence shows strong similarities with those of other Mediterranean sites; it is, however, one of the few where the main MIS 4 and MIS 3 climatic fluctuations are registered in the sedimentary record. 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Content Back to results | < Previous 4 of 37 Next > Università di Sassari(opens in a new window)|View at Publisher| Export | Download | More... -------------------------------------------------------------------------------- Quaternary International Volume 328-329, Issue 1, 2014, Pages 3-20 -------------------------------------------------------------------------------- Middle Pleistocene to Holocene coastal evolution of NW Sardinia (Mediterranean Sea, Italy) (Article) Pascucci, V.a , Sechi, D.a, Andreucci, S.b a Dipartimento di Scienze Natura e del Territorio, Università di Sassari, Via Piandanna 4, Sassari 07100, Italy b Dipartimento di Scienze Chimiche e Geologiche, Università di Cagliari, Italy -------------------------------------------------------------------------------- View references (71) Abstract The Middle Pleistocene-Holocene Sardinian stratigraphy has been revised to identify the roles played by sea and climate changes in controlling sedimentation. The succession, dated through Optically Stimulated Luminescence (quartz, K-feldspar) and 14C (the most recent) was grouped into eight major stratigraphic units mainly represented by coastal dunes, shallow marine, and alluvial systems. These units range in age from MIS 8 (300 ka) to MIS1 (6 ka).Little information is available for the period between MIS 8 and 6. The available data, however, allow some comments on climate changes over the last 125 ka. The last interglacial (MIS 5) is subdivided into five substages (e-a). MIS5e (Eemian) is considered the climate optimum of this interglacial, sea level was 4-6 m higher than today, and climate conditions were warmer and more humid. MIS 5c is the second high stand peak. Sea level was about 1.5 m above the modern level, and paleoclimatic and paleogeographic conditions were similar to the present. During MIS 4, at the beginning of the glacial phase, a climatic deterioration occurred. Temperatures dropped by 6C° relative to MIS 5e in about 5 ka, and sea level fell about 60 m. This led to a progressive disruption of inland vegetation cover and to repeated valley slope denudation. The sea-level fall created the accommodation space for fan development in sheltered and cliffed areas, where debris flows developed and filled almost completely the terminal parts of the narrow coastal valleys/coves. In wider areas, however, coastal dune fields developed. Contemporaneous presence of alluvial fan and dune systems is associated with the local morphology that could mitigate or amplify moist conditions within a generally arid environment.During MIS 3, several climate fluctuations occurred (wet/arid, D/O-H events). However, the shallow Sardinian shelf was continuously sufficiently exposed to become the source area of bioclastic sands. These sands were blown inland, and formed extensive dunefield systems. Dunes dominated the west coasts of Sardinia under cold and relatively dry conditions. In some areas, these dunefield systems were eventually almost completely dismantled and reworked into water-flow dominated alluvial fans by catastrophic rainfall events. Thus, the local climate quickly switched from arid to humid conditions. No record of the last Glacial Maximum deposits, MIS 2, was observed along the NW Sardinian coast. During the Holocene transgression (MIS1) coastal dune systems developed and progressively migrated inland, and temperature rose to reach the present-day condition.