GNGTS 2014 - Atti del 33° Convegno Nazionale

12 GNGTS 2014 S essione 3.1 families. The constructional (allochtonous) systems include the fans, the aprons and the basin floor channels. They have been built from sediments supplied from delta, shore zone, shelf or glacial systems. The facies architecture of the allochtonous systems is mainly determined by the sediment texture and the pattern of supply at the shelf margin. The point sources of supply create the fans. The line sources have created strike-elongated prisms of slope sediments, called the slope aprons. The shelf margin deltas provide a particularly common intermediate source geometry, which forms onlapping delta-fed aprons. Another type is represented by the autochtonous system, including retrogressive aprons, canyon fills and megaslump complexes, recording the slope reworking and sedimentation. The variability in the form and growth of sediment waves on turbidite channel levees has been deeply investigated (Normark et al. , 2002). Fine-grained sediment waves have been observed in many modern turbidite systems, generally restricted to the overbank depositional elements. The sediment waves have developed on six submarine fan systems and have been compared by using seismic reflection data coupled with sediment cores. Geological data have documented the upslope migration of the wave forms, with thicker and coarser beds deposited on the up-current flanks of the waves. Some wave fields are orthogonal to channel trend and were initiated by large flows whose direction was controlled by upflow morphology, whereas fields subparallel to the channel levees resulted from local spillover. Other studies have been carried out on the youngest channel-levee systems of the Bengal Fan, resulting from digital sediment echosounder data (Hubscher et al. , 1997). Channel levee-systems represent the main architectural elements of submarine fans. Some channel-levee systems of the Ischia continental slope will be shown on seismic profiles. As a result of large input of sediments, the accumulated sediments may be considered as a high resolution record of the climatic history of the earth. The depositional structures reflect all the processes that affect sediment transfer from the hinterland towards the fan, e.g. the sea level and the climatic changes, the mountain uplift and the monsoon activity. The acoustic strata patterns and the downslope development of the channel levee system were examined with the parametric sediment echosounder Parasound. The determination of the age of the sedimentary strata shows turbiditic activity during sea level rise and highstand. The initial formation of the system in the middle fan occurred in the late glacial and outer levee growth stopped with glacial termination. Several vertical, aggradational sediments constitute the inner levees created in the Holocene. The formation of the inner levee segments indicates the construction of a wide channel in discrete phases. The top of the segments form topographic pinnacles, explaining the morphology of other channel-levee systems from other fans. Some cross sections from the lower fan reveal lenticular channel-levee systems with a common reflection characteristics. Prograding distinct reflections on the outer sides of the upper levees terminate with a downlap against an unconformity, which separates the upper part of the overbank deposits froma reflecting lower part. Examples of modern and ancient turbidite systems have been compared and the related problems and concepts have been examined (Mutti and Normark, 1987). The example, selected for the comparison represent depositional systems similar in such characteristics as the type of basin, the size of sediment source, the physical and temporal scales and the stage of development. A conceptual framework for comparing modern and ancient turbidite systems has been presented. Four basic types of turbidite basins have been defined based on size, mobility of the crust, effects of syndepositional tectonic activity and volume of sediment available in the source areas. The difference in physical scale and the great dissimilarities in the type of data available are particularly important in the comparison of modern and ancient deposits. Comparisons have been done for basin-fill sequences or complexes (1 st order), for individual fan systems (2 nd order), for stages of growth within an individual system (3 rd order) or for the scales of specific elements (facies associations and component substages) within a system, i.e. lobes, channel deposits, overbank deposits (4st order; Mutti and Normark, 1987). Individual fan elements have been defined to provide criteria applicable to both modern and ancient settings. These elements are channels, overbank deposits,