Stratigraphic architecture of deep sea depositional systems in the Southern Tyrrhenian sea: some examples in the Ischia and Stromboli volcanic islands (Southern Italy)

The stratigraphic architecture of deep sea depositional systems has been discussed in detail by Galloway (1998). Some examples in the Ischia offshore are here shown and represented. The submarine slope and base of slope depositional systems represent a major component of marine and lacustrine basin fills, constituting primary targets for hydrocarbon exploration and development. The slope systems are characterized by seven basic facies building blocks, including the turbiditic channel fills, the turbidite lobes, the sheet turbidites, the slide, slump and debris flow sheets, lobes and tongues, the fine-grained turbidite fills and sheets, the contourite drifts and finally, the hemipelagic drapes and fills (Galloway, 1998). The grain size of the supplied sediments is a primary control in the development of the morphology in the channel-lobe systems. The grain size also controls the scale and the importance of slump and debris flow deposits. Siliciclastic slope systems have been divided into two main 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 from a 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 (1st order), for individual fan systems (2nd order), for stages of growth within an individual system (3rd 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, lobes, channel/lobe transition features and scours (major erosional non-channel features). The derived characteristics, such as the fan divisions and sedimentation models are considered as secondary points only used as necessary for the discussion. The use of morphologic terms to describe ancient deposits has been also qualified. The primary emphasis remains on detailed, complete field work both on land and at sea in order to provide the characterization of the sediments and rocks assemblages and to ensure that similar features are being compared in terms of both temporal and physical scales (Mutti and Normark, 1987). Turbidite systems and their relationships to depositional sequences have been described in detail (Mutti, 1985). Long term global sea level variations and local tectonic control form the basic framework within which turbidite sediments develop as a response to breaks in the equilibrium between shelf and basin sedimentation. An understanding of the interaction of these processes and resulting types of turbidite deposition requires a precise framework of turbidite sediments within well defined depositional sequences. The volume of the gravity flows enhances the depositional characters of the channels that progressively become the only site of sand deposition where small volume and highly confined flows lose most of their fines through overbank processes. Within the same system, a decrease in the volume of gravity flows determines different stages of growth, that are expressed by distinctive facies associations. Channel-levee complexes, terminal deep sea fans and sediment wave fields associated with the Toyama Deep Sea Channel System in the Japan Sea have been described in detail (Nakajima et al., 1998). The Toyama Deep Sea Channel in the Japan Sea is one of the most prominent deep sea channels in rifted margins. The course and morphology of the channel-fan system are mainly controlled by the basin morphology. Thick, sheet-like sediments, deposited from ponded turbidity currents have accumulated in narrow throughs, whereas extensive levees have formed in more open basins. The distribution of the sediments and the consequent morphology of the channel-levee complexes are also controlled by Coriolis force. The preferential development of the levees is attributed to the Coriolis force tilt effects in the Northern Emisphere. The distribution, form and orientation of the sediment waves are consistent with the effect and direction of inferred spill-over turbidity currents, with a consequent levee growth. The sediment transport may have ceased during the Holocene in the cut and fill tributaries developed in the Quaternary succession on the slope to the through, where a wide shelf separates the canyons from the rivers in the eastern margin of the drainage area. Important results on the stratigraphic architecture of deep sea depositional systems have been obtained from the GNV Italian project (Chiocci et al., 2003). Further constraints have been obtained from the CARG Project (Aiello et al., 2010; 2012) and from the Stromboli geophysical experiment (Castellano et al., 2008; Aiello et al., 2014). The submarine portions of the Italian volcanoes, their survey and the assessment of the potential volcanic hazards have been deeply investigated with a particular reference to the DTM generation for the Vulcano, Stromboli and the southern Ischia islands (Chiocci et al., 2003). Other tasks have included the geotechnical characterization of submarine instabilities and related subaerial phenomena, the geotechnical analysis and modeling of instability phenomena affecting the flanks of the volcanic islands. Some researches on the Ischia submerged flanks have also been carried out, coupled with the reconstruction of the evolutive processes by marine data and with the seismo-stratigraphic analysis. Other objectives have included the understanding of the geological processes active in the Italian submarine areas, the evaluation of the potential risks associated with the volcanic seamounts and with the submerged portions of the volcanic islands of the Tyrrhenian sea. Investigations on the submerged portion of the Mount Etna volcanic edifice have also been carried out in order to ascertain the presence of tectonic lineaments, both extensional and compressional, connected with those on land, due to the absence of buttress towards the sea (Chiocci et al., 2003). High-resolution seismic reflection profiles (Sparker Multitip) offshore southern Ischia island (Naples Bay) have been presented (Aiello et al., 2012). New seismo-stratigraphic evidence on buried volcanic structures and overlying Quaternary deposits of the south-eastern offshore of the Ischia Island have been discussed to highlight their implications on the marine geophysics and volcanology. The Ischia Bank is a large and flat relic volcanic edifice with steep slopes, merging on the continental shelf. The age of this monogenic volcano is unknown, lacking a direct datation of its basement. It represents the eruptive center of the pyroclastic fall cropping out onshore in the eastern sectors of the island, ranging in age from 8 to 6 ky B.P. In the eastern Ischia offshore relict volcanic edifices, mostly formed by hialoclastites, have been investigated through high-resolution seismics. They represent remnants of hydro-magmatic volcanic vents and suggest a subaqueous emplacement. Regional seismic sections in the south-eastern Ischia offshore, across buried volcanic structures, have been presented and discussed (Aiello et al., 2012). High resolution seismic data (Subbottom Chirp) coupled to high resolution Multibeam bathymetry collected in the frame of the Stromboli geophysical experiment aimed at recording seismic active data and tomography of the Stromboli island are here presented. The Stromboli geophysical experiment has been already carried out based on onshore and offshore data acquisition in order to investigate the deep structure and the location of the magma chambers of the Stromboli volcano. A new detailed swath bathymetry of Stromboli islands is here shown and discussed to reconstruct an up-to-date morpho-bathymetry and marine geology of the area, compared to the volcanologic setting of the Aeolian Arc volcanic complex (Aiello et al., 2014). Due to its high resolution the new DTM of the Stromboli island has given interesting information about the submerged structure of the volcano, particularly about the volcano-tectonic and gravitational processes involving the submarine flanks of the edifice. Several seismic units have been identified based on the geologic interpretation of Subbottom Chirp profiles recorded around the volcanic edifice and interpreted as volcanic acoustic basement pertaining to the volcano and overlying slide chaotic bodies emplaced during its complex volcano-tectonic evolution (Fig. 1). They are related to the eruptive activity of Stromboli, mainly poliphasic and to regional geological processes involving the geology of the Aeolian Arc (Aiello et al., 2014).