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GeoActa Special Publication 1 2008


Acquisto online

GeoActa
an international Journal of Earth Sciences


Luisa Sabato, Marcello Tropeano
Dipartimento di Geologia e Geofisica and Centro Interdipartimentale di Ricerca per la Valutazione e Mitigazione del Rischio Sismico e Vulcanico, Università di Bari, Via Orabona 4, 70125, Bari, Italy. E-mail: l.sabato@geo.uniba.it, m.tropeano@geo.uniba.it

The Holocene coastal alluvial fan of the Saraceno Fiumara (Calabria, Southern Italy): highstand filling of an incised valley



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Abstract

This work relates the main features of the Saraceno Fiumara coastal alluvial fan to Holocene relative sea-level changes, integrating detailed facies analysis with the concepts of sequence stratigraphy. “Fiumara” is the name used to indicate rivers that typically flow through the mountain chains of Calabria and Sicily (southern Italy). These streams are ephemeral, torrent-like, very steep and short, and drain only mountainous areas.
The active fan system records two main stages of development: a present-day entrenching stage subsequent to a previous aggrading stage; this evolution attributes a telescopic style to the fan. The studied depositional system represents an incised-valley fill: indeed, under relative sea-level fall and lowstand conditions (before alluvial fan aggradation), the Saraceno Fiumara led to the incision of a deep valley in which the Holocene coastal alluvial fan developed.
The present-day fan channels dissect the deposits of the aggrading stage, but are strongly modified by human activities. In the aggrading stage deposits, seven facies associations organized into a fining- to coarsening-upward, 12 m thick succession have been recognized. The latter consists of massive and unsorted gravel at the base (debris flow-dominated fan), capped by more organized and fine facies (stream flow-dominated fan) that grade again to massive and unsorted gravel (debris flow-dominated fan). The prevalence of unconfined debris flows and hyperconcentrated- flow deposits in the lower and upper part of the succession could be related to semi-arid climates, while the occurrence of stream-flood deposits, exposed in the middle part of the sections, could be related to a wetter climate. Since neither climate changes nor hydrodynamic processes produced changes in the style of the fan (aggradation vs dissection), facies changes could be basically explained in terms of interference between regional uplift, glacio-hydro-isostasy, and eustasy during the Holocene.
In absence of other chronostratigraphic constraints (neither geochronological nor biostratigraphic and/or geoarchaeological data are available at present), sedimentological data combined with sequence-stratigraphic concepts may suggest some ideas about age and development of the alluvial fan. The development of the still-active entrenching (telescopic) stage indicates a slowly drop of relative sea-level that, according to regional geological data, started not before 3 ky BP. The preceding aggrading stage could not start before 6.5 ky BP, corresponding to the time when the rate of relative sea-level rise was decreasing up to a brief still-stand. According to such a timing, the evolutionary stages of the fan are developed during the latest part of the late Quaternary Transgressive Systems Tract (TST) and mainly during the present Highstand Systems Tract (HST), although they simulate a late TST+HST followed by an early Falling-Stage Systems Tract.

Keywords: Coastal alluvial fan, Fiumara, Incised-valley fill, Holocene, Calabria, Southern Italy.